comments on “living polymerization: rationale for uniform terminology” by darling et al

Comments on “Living Polymerization: Rationale forUniform Terminology” by Darling et al.

Comments byProfessor Michael Szwarc, F.R.S.

1176 Santa Luisa Drive, Solana Beach,California 92075

One of the aims of nomenclature is to convey toreaders or listeners an idea that arose in one’smind, if possible in one word.

The novel characteristic of a living polymer isits capacity to retain for a relatively long time itsability to grow whenever a suitable monomer issupplied. In these terms, living polymers resem-ble biologically living systems that grow whenfood is available. The word living conveys thisidea.

Living polymerization is the polymerizationthat yields living polymers whether the initiationis fast or slow. Of course, if one desires to producepolymers of narrow molecular weight distribu-tions, one has to rapidly initiate the polymeriza-tion and ensure that the initiation is completed ina short time. However, this is a technical aspect ofthe problem, and it should not be included in thedefinition of living polymerization.

In full agreement with the authors, I find theword controlled unfortunate in the definition ofliving polymerization. It does not convey the char-acteristic feature of living polymerization, and, aspointed out by the authors, we control variousaspects of polymerization by various techniques.For example, we control the temperature of exo-thermic polymerization by performing it in emul-sion and the viscosity of rubber or nylon by addingappropriate ingredients. For a more detailed dis-cussion of the discovery of living polymerizationand my most recent definition, see ref. 1.

REFERENCES AND NOTES

1. Szwarc, M. J Polym Sci Part A: Polym Chem 1998,36, ix.

Comments byProfessor Takuzo Aida

Department of Chemistry andBiotechnology, Graduate School ofEngineering, University of Tokyo, 7-3-1Hongo, Bunkyo, Tokyo 113-8656, Japan

Whatever the original definition, living poly-merization is now the most convenient and widelyaccepted term for clearly expressing that a workor an article is intended to control the molecularweight of a polymer rather than the comonomersequence, stereosequence, or other structural pa-rameters. In a recent article,1 Szwarc stated thata living polymer should retain its activity forchain growth whenever new monomer is supplied.

A problem, to be discussed here, is what weshould call or how we should classify the followingexamples (extreme cases):

Case 1: Polymerization that can produce agrowing polymer with a very long lifetimebut a broad molecular weight distributiondue to a slow initiation.

Case 2: Polymerization that can produce apolymer with a narrow molecular weightdistribution (Mw/Mn , 1.1) and a desiredmolecular weight but involves an equilibra-tion between active and inactive species as aresult of a rapid, reversible termination orchain-transfer reaction.

In Case 1, each polymer grows in a living fash-ion, but the polymerization should not be calledliving according to the original definition of livingpolymerization, which requires the initiation to befaster than the propagation. In Case 2, thepolymerization should not be called living if theoriginal definition of living polymerization is rig-orously applied. However, the polymerizationallows the formation of block copolymers and end-functionalized polymers and displays all the char-acteristics of the classical living anionic polymer-ization of styrene. Case 2 includes living radicalpolymerization, involving atom transfer radicalpolymerization (ATRP), reversible addition frag-mentation chain transfer (RAFT), and nitroxide-

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 1710–1752 (2000)© 2000 John Wiley & Sons, Inc.

1710

mediated radical polymerization, and also immor-tal polymerization with aluminum porphyrins.

To my understanding, it may now be accept-able for the polymerizations in Case 2 to be calledliving polymerization. ATRP and RAFT are thenames that are given on the basis of the chemis-try of the polymerization mechanisms. Immortalpolymerization is a catalytic version of living po-lymerization. Although the polymerizations inCase 2 involve inactive species such as alkyl ha-lides and alcohols at the polymer termini, thesecan, however, be reversibly converted into thecorresponding active species when particular ac-tivators (catalysts) are present, so these speciesare not actually dead but dormant. The originaldefinition of living polymerization was madewithout the consideration of such an equilibrationbetween active and inactive species, but it in-cludes a consideration of the reactivities of freeions and ion pairs. It is likely that the equilibra-tion between active and inactive species is merelyon an extreme of the equilibration between freeions and ion pairs (particularly when the equili-bration is rapid).

In conclusion, we should be more generous inusing (or use together with individual names) theterm living polymerization for Case 2. Otherwise,we have to create many new, complicated namesthat would lower the efficiency of electronicsearching.



Comments byProfessor Allan J. Amass

Specialty Polymer Research Group,Aston University, CEAC, Aston Triangle,Birmingham B4 7ET, United Kingdom

Thank you for the opportunity to comment onthe article by Darling et al. on the nature anddefinition of living polymerization. Personally, Ido not wish to see any reference to the breadth ofthe molecular weight distribution of the productin the definition of living polymerization. The for-mation of narrow molecular weight distributionpolymers is a special case of living systems, one inwhich the rate of initiation is fast compared to therate of propagation. The polymerization of buta-

diene initiated by n-butyl lithium in a hydrocar-bon solvent shows most of the properties of aliving system, but, because the rate of initiation isslow, the molecular weight distribution is rela-tively broad and certainly not described as Pois-son-like. The polymerization, however, shows allthe other attributes of a living system. When dis-cussing the nature of living polymerizations withmy students, I am inclined to define a living sys-tem as one in which, once a propagating chain iscreated, the ability of that center to propagate isretained in that particular polymer chain. Fur-thermore, if the polymerization is living, the spe-cies will not enter into any other reactions thatwill destroy the integrity of the chain and preventit from participating in further propagation reac-tions. This then permits reactions to take place,provided they exist, that enable a dormant chainto be converted to an active state and back againto the dormant state. After all, the anionic poly-merization of styrene in tetrahydrofuran initiatedby sodium compounds involves the establishmentof an equilibrium between free ions and solvent-separated ion pairs. The activities of the speciesdiffer markedly, and one could argue that thepropagation effectively takes place while any par-ticular chain is in the free-ion state and the ionpair accounts for very little of the activity. Theequilibrium reaction for their interconversion ex-ists, however.

Comments by Dr. George G. BarclayShipley Company, 455 Forest Street,Marlboro, Massachusetts 01752

Practical terminology is extremely importantin scientific literature because it allows the defi-nition and discussion of complex subject areaswithout the need for elaborate descriptions. How-ever, care is required when defining nomencla-ture to avoid confusion. To date, the controversysurrounding the use of the term living polymer-ization in relation to the recent developments innitroxide-mediated radical polymerization andatom transfer radical polymerization has beenextremely ambiguous.

In organic chemistry, descriptive terms aregiven to general classes of reactions, for example,reduction, which covers the use of a broad spec-trum of reducing reagents such as metal hydridesand catalytic hydrogenation. Depending on thereducing reagent and functional group, various

LIVING OR CONTROLLED? 1711

reaction yields and chemoselectivities can beachieved. However, it is generally accepted that,independent of the yield and chemoselectivity,the reaction being carried out is unambiguously areduction.

Living polymerization, as discussed in this ar-ticle, covers a general class of polymerizationtechniques that undergo a chain-growth processin which the end groups are controlled, enablingchain growth “whenever additional monomer issupplied.” This definition covers a number of tech-niques that provide end-group control, regardlessof the chemoselectivity, yield, and mechanism ofthe chain-growth process. The controversy sur-rounding the use and broadening of the term liv-ing polymerization concerns the degree to whichthe chain-growth process is controlled, in termsof narrow polydispersity (1.0–1.1), molecularweight control, and the elimination of irreversibletermination processes. However, as the authorsrecognized, these stipulations are the ideal re-sults of a perfect living polymerization, and suchidealism should not limit the use of the termliving polymerization. The definition of living po-lymerization described in this article is consistentwith the accepted practice in organic chemistry ofnaming general classes of reactions, and as such,I strongly support the authors in their efforts torelax and broaden the use of this terminology.

Comments byProfessor Bernard Boutevin

Laboratory of Applied Chemistry,University of Science and Technics inLanguedoc, Superior National School ofChemistry, 8 rue de l’Ecole Normale,F-34053, Montpellier Cedex 1, France

The purpose of this article is to try to rational-ize the terms used by researchers in the field ofpolymerization. Particularly, the emergence ofradical polymerization in the synthesis of con-trolled architecture polymers has been intenselydebated to know if it may be considered living. InFrance too, where I am involved in research withother laboratories interested in this field, we alsohave had lots of discussions, and since the begin-ning (in 1994), I have always firmly defended theterm living radical polymerization. Besides, Ihave noticed that the detractors are always (oralmost always) former anionists who want to keep

a prerogative and a pure and noble character tothis type of polymerization.

As for me, having only used radical polymer-ization during my career (30 years), I immedi-ately admitted that rendering the transfer (I havepublished 150 articles about “dead” telomeriza-tion), or the termination, reversible was a suffi-cient contribution to call that living polymeriza-tion. Moreover, the term controlled is totally in-appropriate, just as stated in the article, becausecontrolled means to me that one is able to foreseethe reaction by modeling and, consequently, isable to give a kinetic scheme close to reality. Thatis what I was able to do in dead telomerizationand what gave conclusive results. It is a control,but it is not a living character.

In addition, I would say that nothing is perfectin this world, and if it is possible to achieve aliving polymerization of styrene by an anionicprocess, what about methyl methacrylate (MMA)and—a fortiori—acrylates?

Thus, it is not possible to reserve a term exclu-sively for one method of polymerization, the sty-rene one, which occurs in drastic conditions.

Finally, nobody has missed the similarity be-tween free or associated ion pairs and counter-radicals, and it is clear that the small differencescurrently observed (i.e., at our level of knowledge)between ion pairs (anionic) from very weak en-ergy bonds (styrene–nitroxide), complexes withmetals through halogens (atom transfer radicalpolymerization), or simply metal complexes(Ziegler–Natta or metallocene) will not take longto break into pieces when theoretical chemistry isable to better describe the reaction intermediates.

In short, I completely agree with your articleand your idea to resume using the single expres-sion radical living polymerization (particularlyradical). Then, each researcher will need to eval-uate the percentage of living character for eachsystem, that is, evaluate the amount of side reac-tions (and their nature) that decrease this per-centage, but he should be conscious that today’simperfections may evolve quickly and become asclose to perfection as cannot be imagined now.

For example, we may cite the anionic MMApolymerization that, because of the relentlesswork of some researchers (such as P. Teyssie), hasreached some outstanding results.1

As far as I am concerned, I can tell the case ofredox catalyzed telomerization, which remained adead process for 30 years and which, thanks tothe occurrence of an amine (bipyridine), became

1712 LIVING OR CONTROLLED?

living (see the work of Matyjaszewski,2

Sawamoto,3 etc.).Thus, it may be concluded that when a poly-

merization takes place with an increase in thechain length with the monomer conversion, it pos-sesses a living character and may be labeled liv-ing radical polymerization.

A big effort remains to be made on a kineticlevel to give true models to evaluate the percent-age of living character; for example, the work ofFukuda may be cited.4 In the case of nitroxides,he evaluated the purity of the obtained polymers.


1. Jerome, R.; Teyssie, Ph.; Vuillemin, B.; Zundel, T.;Zune, C. J Polym Sci Part A: Polym Chem 1999,37, 1.

2. Wang, J.-S.; Matyjaszewski, K. J Am Chem Soc1995, 117, 5614.

3. Kato, M.; Kamigaito, M.; Sawamoto, M.; Higa-shimura, T. Macromolecules 1995, 28, 1721.

4. Fukuda, T.; Goto, A. Macromol Rapid Commun1997, 18, 683.

Comments byProfessor Rebecca Braslau

Department of Chemistry &Biochemistry, University of California atSanta Cruz, Santa Cruz, California 95064

In regard to the article entitled “Living Poly-merization: Rationale for Uniform Terminology,”I find the authors’ arguments for generalizing theterminology of living polymerization to bethoughtful and practical. I need to caution you,the editors, that I am a newcomer to the field ofpolymerization and, therefore, do not feel that Ihave the overview and experience in the field totake part in your experiment in contributing tothe printed discussion on this nomenclature is-sue. Nonetheless, I find the arguments made inthis article to be thoughtful, reasonable, and ra-tional. I agree with the argument that a processor reaction name should not be dependent on theside reactions or on an analytical measure (e.g.,polydispersity). I also agree that there is alreadya general understanding by chemists of the termliving polymerization, and I support the applica-tion of Occam’s razor in trimming back the foliageof newly sprouting terminology that is appearingas alternatives to living polymerization. This is

particularly true because most of the alternativeterms do not convey a more specific or succinctsubbranch or meaning.

One modification that I would like to see is tohave the article end in a clear, concise conclusionthat states the authors proposed definition for theterm living polymerization.

Comments byProfessor William J. Brittain

Department of Polymer Science,University of Akron, Akron, Ohio 44325-3909

I appreciate the concerns of Darling et al. inraising the use of the word living in polymerscience meetings and publications. Among syn-thetic polymer chemists, this word is probablyused (and perhaps, misused) in the greatestvariety of connotations. When teaching gradu-ate students, I feel the obligation to follow theuse of the word living with a definition of how aparticular author applied the term. At the endof the day, this may provide the resolution tothe issues raised by Darling et al. As practitio-ners of the art of polymer synthesis, we have anobligation to the field to justify our use of theword living by substantiating this claim with aset of experimental measurements. If an authorclaims a living process, it is the assessment ofthe proof offered that determines syntheticvalue.

I feel that a multiplicity of terms such as pseu-do-living, quasi-living, controlled, and living ob-scure results more than they illuminate them.However, I do not agree with the analogy that aliving polymerization is analogous to a namedorganic reaction. Named organic reactions aremore specific. Living polymerizations can poten-tially refer to a variety of bond-forming and bond-breaking reactions. A difference between organicand polymer chemistry is that the former is con-cerned primarily with the success of single-stepreactions. The goal of polymer chemistry is high-yield, sequential reactions. The use of living is ameasure of the success of conducting a series ofsequential reactions rather than a specific moni-ker for a bond-forming mechanism.

For me, there are three influential articles inthe field of living polymerization. The first is thatof Szwarc,1 in which the term living was firstcoined. The anionic polymerization of styrene re-


mains one of the best polymerization systems thatcan be described as living. However, the reality ofmost polymer syntheses is that chain-transferand chain-termination reactions limit achievablemolecular weights and monomer conversions.Radical polymerizations are among the most chal-lenging systems because spontaneous, bimolecu-lar termination is omnipresent.

The legacy of physical organic chemistry is theelucidation of mechanisms through the kineticsand identification of intermediates and side reac-tions. In this sense, I appreciate the article byMatyjaszewski2 that divides polymerizations intoClasses 1–6 according to the ratios of ktr/kp andkt/kp. Matyjaszewski’s definition follows the clas-sic tenets of physical organic chemistry by recog-nizing the important role of kinetics in mechanis-tic studies. This kinetic ranking of polymeriza-tions also acknowledges the fact that nopolymerization is devoid of chain transfer or ter-mination for an indefinite period of time. Theoriginal definition of Szwarc may have been toodemanding. Few polymerizations will satisfy themost rigorous definition of living; it is the time-scale of termination and chain transfer that dis-tinguishes polymerizations. Unfortunately, thepaucity of reliable kinetic data for polymeriza-tions limits the utility of Matyjaszewski’s rankingsystem.

The article by Quirk and Lee3 describes a set ofexperimental criteria for living polymerizations.This article has been the most influential on theexperimental work in my research group. Thegoals of resumptive monomer consumption, blockcopolymer formation, and chain-end functional-ization (among other experimental criteria), asdescribed by Quirk and Lee, are practical andrelevant metrics of a living polymerization. Be-cause polymer chemistry is strongly connected toapplied science and technology, it is importantthat living polymerizations produce a high per-centage of chains that are capable of furthergrowth. Their article describes experiments thatmeasure the contribution of termination andchain-transfer reactions. The quantification ofside reactions follows the spirit of physical or-ganic chemistry, where a reaction is not onlycharacterized by its yield but also by the natureof the side reactions that consume startingmaterials.

We should not relax the traditional standardsused to characterize a living polymerization. Myrecommendation to the polymer community is touse the word living to describe a polymerization

in which there is not one, but a multiplicity ofexperimental measurements that characterizethe stability of the chain end. It is incumbent onall polymer chemists who author scientific litera-ture and review the work of their peers to care-fully use and assess the applicability of the wordliving.


1. Szwarc, M.; Levy M.; Milkovich, R. J Am Chem Soc1956, 78, 2656.

2. Matyjaszewski, K. Macromolecules 1993, 26, 1787.3. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359.

Comments byProfessor William K. Busfield andDr. Ian D. Jenkins

School of Science, Griffith University,Nathan, Brisbane, Queensland 4111,Australia

We are in agreement with the majority of theconcerns and discussion presented in the article.Of the two suggested names, controlled and liv-ing, we certainly favor the latter, as do the au-thors of the article. A common problem with boththese names, however, is that they have everydaymeanings that do not coincide exactly with whatthey represent in this context. Thus, their use canbe confusing to those not expert in the field. Forexample, we have occasionally asked students, ina third-year undergraduate examination on poly-mer science, to write “a short account” or “noteson” living polymerization. Several students overthe years have given a free-radical polymeriza-tion (with normal termination) as an example,citing the growing polymer radical as the livingpolymer. Admittedly, these are students whohave not listened carefully or read widely, but it isdifficult to deny them marks for their misconcep-tion. In the article, the authors contrast the cur-rent confusion regarding nomenclature in the liv-ing radical field with the comparative serenity inthe field of Diels–Alder reactions. A major reasonsurely relates to the choice of a specific and there-fore nonconfusing name.

We propose that if it is possible to change thename, it would be entirely appropriate to honorthe pioneering work of Szwarc and call the reac-tion Szwarc polymerization. It may be helpful tohave the interim name Szwarc-living polymeriza-


tion to provide a link with traditional practiceover the past 40 years, with the intention of drop-ping the living tag in the longer term. Szwarc wasthe first to recognize that the combination of fastpropagation with no termination step in anionicsystems led to what he called polymer architec-ture. With his coworkers, he convincingly demon-strated that polymers with very narrow molecu-lar weight distributions, specific end groups, andplanned blocks of different monomer units couldbe made by the anionic living polymer technique.His work spawned a flurry of research activity onanionic and, eventually, cationic systems duringthe 1960s. The objectives were little differentfrom those of what we might call the second phaseof research activity of the late 1980s and 1990sinto living polymerization utilizing cleverly de-signed free-radical systems.

In summary, our proposal is that any polymer-ization designed to minimize the occurrence ofirreversible polymer chain termination andthereby produce polymers with predictable molec-ular weights, narrow polydispersities, and capa-bilities for chain extension should be named aSzwarc polymerization.

Comments by Dr. Stanley BywaterNational Research Council of Canada,Ottawa, Canada K1A 0R6

I do not wish to make formal comments, but inmy view the Szwarc definitions of living polymer-ization, which mean that each individual polymergrows until the monomer is exhausted withouttransfer or termination, should only be used ifrigorous experiments show that this is true (in myview, to Mn . 105). No quasi-living, controlled-living, or other hyphenated varieties or “living”terms should be used, but more specific namessuch as atom transfer radical should be used forthese systems.

Comments byProfessor Francesco Ciardelli

Department of Chemistry and IndustrialChemistry, University of Pisa, ViaRisorgimento, 35, 56126 Pisa, Italy

I find the article by Darling et al. very useful,as some confusion about the real significance ofliving polymerization has always existed and has

become particularly relevant in recent yearsthanks to developments in cationic, radical, andmetal-catalyzed processes.

The first point necessary to make is the cleardistinction between living polymerization and liv-ing macromolecule. The former refers to the pro-cess, the latter to the polymer chain:

1. A living process is observed when no ter-mination reactions exist but does not ex-clude transfer reactions; this is typicallyobserved in transition-metal-catalyzedpolymerization. As a consequence, theamount of polymer increases after a secondaddition of monomer, but the molecularweight does not.

2. If both termination and transfer reactionsare lacking, both the process and the poly-mer chain are living, and this is the casethat should be called living polymerization.In this case, the addition of monomerbrings an increase in molecular weight,block copolymer can be formed, and thepolymer is monodisperse in its molecularweight.

3. A complex situation may arise if two mech-anisms are occurring in the system, oneproducing living macromolecules and theother dead macromolecules. In this case,the degree of living should be evaluated,but the product formed is different fromthat expected from the real living situationindicated in the previous paragraph.

I hope this very short comment can be useful insome way.

Comments byProfessor James V. Crivello

Department of Chemistry, RensselaerPolytechnic Institute, Troy, New York12180

The comments of the authors of this articleconcerning the current fragmented terminologyused in the literature with respect to livingfree-radical polymerizations are entirely appro-priate. As a teacher, I find the present nomen-clature in this field to be especially confusing tostudents who see the literature replete with amultiplicity of highly specialized terms appliedto seemingly very similar types of polymeriza-


tions. The development of any new field of sci-ence also brings with it an evolutionary aspectof the nomenclature that is used to describe it.Initially, the terminology applied is complex,but it becomes simplified with time as generalconcepts and unifying principles emerge to de-scribe the entire field. It appears that this is thepresent situation with living free-radical poly-merizations.

One is reminded of a similar situation in theearly 1970s in which the classical/nonclassicalcarbenium ion controversy raged in the ranks oforganic chemists. Then as now, specific termswere being invented and applied to the extremecases of what turned out in the end to be a con-tinuum of species present in these reactions. Dar-ling et al. correctly reminded us that there is nosuch a thing as a perfect living polymerization.This is because in polymer chemistry, we are al-ways dealing with a large assembly of moleculesundergoing reaction. Statistically, if a side reac-tion can occur that results in the permanent ter-mination of a growing polymer chain, it will takeplace. I would agree that if a given polymer form-ing a reaction meets the minimal criteria, such asvery low levels of chain transfer and termination,narrow molecular weight distribution, and theability to chain-extend, then it should be charac-terized as simply a living polymerization. Thisdesignation should be applied regardless ofwhether the mechanism involves free-radical, cat-ionic, anionic, or coordination intermediates. Itshould also be pointed out that living polymeriza-tions are observed only in those systems in whichchain termination and transfer are minimized ei-ther by the inherent low reactivity of the activespecies or by the deliberate suppression of itsreactivity by one means or another. Generally,the specialized terms that Darling et al. cited intheir article are used by various authors to de-scribe the specific manner in which the reactivityof a free-radical species has been suppressed.

For these reasons, this commentator would ad-vocate the use of the uniform term living free-radical polymerization in the titles and keywordsof articles in this field. As always, investigatorshave the freedom to provide detailed mechanisticdescriptions in their articles, in which they canpoint out the peculiarities of a specific systemthat serve to distinguish it from other types ofliving polymerizations.

These considerations aside, the argumentsthat the Darling et al. advance for the establish-ment of a uniform terminology are valid and

should really be addressed by the scientific com-munity actively engaged in this field of research.There are currently a considerable number ofmeetings and conferences on living polymeriza-tions that could serve as forums for the polymerresearch community to come together and decidethis issue.

However, rarely are such matters decided byconsensus. Ultimately, the question of terminol-ogy will be solved, as it has in the past, by thefuture authors of textbooks of polymer chemistry.In the meantime, the multiplication of terms usedin this field may well continue.

Comments byProfessors Takeshi Endo1 andIkuyoshi Tomita2

1Research Laboratory of ResourcesUtilization, Tokyo Institute ofTechnology, 4259 Nagatsuta-cho,Midori-ku, Yokohama 226-8503, Japan2Department of Electronic Chemistry,Interdisciplinary Graduate School ofScience and Engineering, Tokyo Instituteof Technology, 4259 Nagatsuta-cho,Midori-ku, Yokohama 226-8503, Japan

We think that it is a good idea to have an occa-sion to discuss the terminological aspect in the jour-nal. We have had a feeling that our definition ofliving polymerization is a little different from that ofmany researchers who are interested in or areworking in this field. Within our understanding, thedefinition of living polymerization seems to be toostrict, which may cause confusion among the re-searchers.

If we want to satisfy the proposed definition ofliving polymerization, we must be very careful touse the term living polymerization. The research-ers should devote a lot of time to determiningwhether their systems follow a living polymeriza-tion by definition. Apparently, we cannot use theterm living polymerization before a detailed studyif we pay much attention to the minor aspectsspecified in the definition. Even if we carry out acareful study and detect no symptoms of unfavor-able side reactions and mechanisms, still we can-not say that further studies with more advancedinstruments might not expose any proofs to re-phrase them as nonliving systems.

We believe that the important aspects of liv-ing polymerization are molecular weight con-


trol, narrow polydispersity, and a capacity forperfect end functionalization and block copoly-merization. From this viewpoint, a mechanisticdiscussion of whether they imply reaction stepsunfavorable for living polymerization might bea secondary concern if the systems satisfy theaforementioned requirements. Most probably,the definition has been fixed through the quiteextensive research related to this field that mayhave, however, built up a tower of ivory. Wemight be allowed to use the term in a morerelaxed way, although we should discuss thismore carefully before modifying the definition ofliving polymerization.

Because the problem of searching ChemicalAbstracts is not specific to this field, it may notbe necessary to make haste in fixing the borderof living polymerization. Further discussion bymany researchers from different fields isneeded before we redefine living polymeriza-tion.

Comments byProfessor Rudolf Faust

Department of Chemistry, University ofMassachusetts at Lowell, One UniversityAvenue, Lowell, Massachusetts 01854

The article entitled “Living Polymerization:Rational for Uniform Terminology” by Darlinget al. has been kindly provided by the editors ofthe Journal of Polymer Science to allow theviews of other polymer chemists on the subjectto be stated and published together with theoriginal article.

Following the discovery of living anionic,ring-opening, cationic, and coordination poly-merizations, new methods have been reportedrecently for the synthesis of well-defined mac-romolecules with controlled molecular weightand relatively low polydispersity by radical po-lymerization. The article discusses the termi-nology of living polymerization in light of thesenew developments and proposes that these rad-ical polymerizations should be called living(even when chain-breaking reactions undoubt-edly occur) instead of controlled. I strongly dis-agree with this proposition. It is certainly truethat a lack of common language may hinder theprogress of any scientific field. Therefore, it isimportant to seek a nomenclature that preciselydefines terms and conditions without any ambi-

guity. However, the frequent misuse (abuse) ofan existing definition is not a reason to com-pletely abandon the definition.

It is striking that the precise definition of liv-ing polymerization—chain polymerization pro-ceeding in the absence of irreversible chain-breaking reactions (transfer and termination)—isnot recited in the article. “Significantly faster (?)propagation in comparison with termination” and“polydispersity of living polymerization should beless than 1.1” are not necessary or sufficient cri-teria. This is exactly what leads to many misusesin the literature. Proper tools to diagnose livingpolymerization have been reported and analyzedin terms of their accuracy. Contrary to the opinionof the authors, living polymerization is not animpossible ideal in ionic and some coordinationpolymerization, but it certainly does not meanimmortality, as was pointed out by Szwarc.1

The analogy of name reactions is faulty.However we may compare the Diels–Alder re-action to nitroxide-mediated polymerization,atom transfer radical polymerization, revers-ible addition fragmentation chain transfer, andso forth, these are indeed named on the basis oftheir different mechanisms, regardless of theiryields. Additionally, they may be called livingwhen they satisfy the criteria (the absence ofirreversible transfer and termination) of livingpolymerization. It is true that the absence ofchain-breaking reactions is dependent on ourdetection limit. Therefore, it is more appropri-ate to state that irreversible transfer and ter-mination could not be detected from the diag-nostic plots instead of saying that the absenceof termination and transfer was determinedfrom the diagnostic plots. It is important torecognize, however, that increasing the mono-mer/initiator ratio, that is, the theoretical mo-lecular weight, can facilitate the detection. Thishas been discussed frequently, and a criticalevaluation has been given. Unfortunately, thereseems to be a general lack of interest in deter-mining the limitations of any given technique.

The proposed relaxation of the term living toinclude processes that yield living polymerswould lead to confusion because, on the basis ofthe new definition (if the yield is unimportant),almost all chain polymerizations could be calledliving. The following example illustrates thispoint. Cationic polymerization dominated by achain transfer via b-proton elimination in theabsence of irreversible termination yields poly-mers with unsaturated chain ends and uncon-


trolled (in the sense that we do not have control)molecular weights. It is clear that this conven-tional polymerization should not be called liv-ing. According to the new definition, however,this process yields living polymers because ofthe following equilibrium at complete monomerconsumption: P5 1 H1 4 3 P1 (where P rep-resent the polymer chain).

In conclusion, I cannot support the proposedchange in the terminology of living polymeriza-tion. Rather, scientists should be urged to use theold one rigorously.



Comments byProfessor Hanns Fischer

Institute of Physical Chemistry,University of Zuerich,Winterthurerstrasse 190,CH 8057 Zuerich, Switzerland

Important inventions often result from intrigu-ing but imprecise suppositions, and the recentlyinvented radical polymerizations yielding poly-mers which are “able to grow whenever additionalmonomer is supplied,” that is, living polymers,confirm this rule. In my opinion, an initially lowlevel of understanding of the underlying basickinetics has caused the introduction of the con-fusing multitude of adjectives for the polymeriza-tion process that the authors now correctly de-plore. Such adjectives should characterize the de-tails, but the synonymous use of controlled,living, pseudo-living, living/controlled, regulated,and so on is indeed anything but helpful. I agreethat a uniform terminology is needed. If one ac-cepts the aforementioned definition of a livingpolymer, it is quite natural to call a radical pro-cess leading to such a polymer a living polymer-ization, as the authors suggest. To distinguish itfrom other living (anionic, cationic, etc.) polymer-izations, the extension to living radical polymer-ization is needed. At this point, my commentcould end, were it not for some points raised in therationale that deserve further clarification.

First, the authors refer to the classical pictureof a living polymerization (ref. 1 of the article) anduse its terminology repeatedly in the text. In this

classic picture, there is no (or only truly negligi-ble) termination by the bimolecular self-reactionof the transient propagating species, and allchains start simultaneously at the beginning ofthe process. Such a process is also called polymer-ization without termination.1 I wish to emphasizestrongly that the classic picture cannot be used toexplain a living radical polymerization. The rea-son is very simple: In a radical polymerization,the conversion rate of the monomer is propor-tional to the concentration of the transient prop-agating radicals. Whenever there are transientradicals, there will be bimolecular termination,and the radical concentration depends on the ter-mination rate constant, so this rate constant alsoinfluences the rate of conversion.

Second, it must be recognized that there aretwo very different kinetic principles that lead toliving radical polymerizations. The authors do notdistinguish between them, but they have mecha-nistically little in common, and both do not followthe classic picture. One is the persistent radicaleffect.2–4 It operates in polymerizations mediatedby persistent radicals, such as nitroxides, or bymetal complexes that bind transient radicals re-versibly (cobalt complexes) and in the atom trans-fer radical polymerization technique. From thebeginning of a polymerization and continuouslythereafter, the bimolecular self-termination of thetransient (propagating) radicals by coupling ordisproportionation causes a buildup of persistentspecies with time. These react reversibly with thepropagating growing transient radicals to the liv-ing (dormant) chains. As the concentration of thepersistent species increases, the cross reaction ofthe transient propagating radicals with the per-sistent species becomes faster than the bimolecu-lar termination of the transients. Hence, the liv-ing (dormant) products accumulate more than thedead termination products. The self-reaction ofthe propagating radicals itself sets this scenariofor control and causes the simple chemical self-regulation. Therefore, it is kinetically importantand cannot be called negligible, as the authors do,although the products are often minor fractions,at least for successful experimental conditions.Moreover, when the monomer is exhausted, thepropagation automatically ceases, but the termi-nation does not, so at sufficiently long times, allpolymer chains die. However, the initiation mustbe fast compared to the total duration of the pro-cess, and in this respect one feature of the classicpicture of a living polymerization is retained. Theother process involves a degenerate transfer of


atoms or groups between propagating radical anddormant chains. This homogenizes the chain-length distribution, and reversible addition frag-mentation chain transfer is a very successful ex-ample. In contrast to the case of the persistentradical effect, the initiation is not fast comparedto the overall reaction but is maintained at a verylow and constant level during the whole polymer-ization, again at variance with the classic picture.Yet the radical concentration is kept very low.Therefore, there is always a little bimolecularself-reaction of the transient propagating radi-cals, but the polymerization rate still depends onthe termination constant. Obviously, in bothcases one should never speak of a negligible ter-mination reaction and at best of a negligible ter-mination product formation. Certainly, in duetime adjectives specifying the two quite differentmechanisms of living radical polymerization willemerge, and other mechanisms may be invented.

Third, the analogy with the Diels–Alder reac-tion terminology is not really correct or complete.Organic chemists normally isolate their stableproducts and correctly call a reaction leading to atypical Diels–Alder product a Diels–Alder reac-tion, admittedly even regardless of the yield.However, the living (better dormant) productfraction of a living radical polymerization is sel-dom isolated or exactly determined. Also, aspointed out previously, the living (or dormant)product prepared by mechanisms involving thepersistent radical effect is inherently unstable, atleast in principle and without special posttreat-ment. Its fraction first increases but decreasescontinuously at long times because of the lastingunavoidable radical self-termination. In practice,it may even drop to zero during the monomerconversion, whereas the polydispersity remainsnarrow and the molecular weight is controlled bythe converted monomer/initiator ratio. Hence, Iagree with the authors that the term living rad-ical polymerization may be used regardless of theyield of the process because under unfavorablecircumstances a living radical polymerizationmay (paradoxically) give only final dead polymer.However, besides the other important criteria forliving polymerizations, such as low polydispersityand control of the molecular weight, workers us-ing the term should always establish that theirliving radical polymerization has produced a liv-ing polymer at least at some stage of conversion.

Finally and astonishingly, the authors stateboldly that “the phenomenon of living polymeriza-tion will be observed whenever propagation is

significantly faster than any process for irrevers-ible termination.” If this were true, the new phe-nomenon of living radical polymerization wouldhave been observed long ago. I am not an experi-enced polymer scientist, but I do teach my stu-dents that the much higher rate (not rate con-stant) of propagation compared to termination isthe essential feature of all successful polymeriza-tions. Am I mistaken, or do the authors wish tocall now all polymerizations living?


1. Young, R. J.; Lovell, P. A. Introduction to Polymers,2nd ed.; Chapman & Hall: London, 1991; p 78.

2. Fischer, H. J Am Chem Soc 1986, 108, 3925.3. Kothe, T.; Marque, S.; Martschke, R.; Popov, M.;

Fischer, H. J Chem Soc 1998, 2, 1553.4. Fischer, H. J Polym Sci Part A: Polym Chem 1999,

37, 1885.

Comments byProfessor Takeshi Fukuda

Institute for Chemical Research, KyotoUniversity, Uji, Kyoto 611-0011, Japan

So-called living radical polymerization nowseems to have acquired a good, general concept. Itincludes some kind of reversible activation of po-tentially active or dormant species, whereas otherfeatures of radical polymerization are essentiallyunchanged (or at least there has been no clearevidence against this). Namely, chain-breakingreactions such as termination and irreversiblechain transfer can occur as they do in conven-tional radical polymerization. This is why manyauthors, including myself, when preparing an ar-ticle or a formal lecture, hesitate or even object tousing the term living radical polymerization. Nowthat its nature is generally well understood, I,being tired of the multiple terminology, wouldlike to welcome a unified name, living, “living”,controlled, controlled/“living”, or whatever, pro-vided that it is generally approved.

I find the proposal of Darling et al. particularlyagreeable. The Szwarc definition of living poly-mers, that they are “able to grow whenever addi-tional monomer is supplied,” would smoothly ap-ply to the growable species in the radical system,which are in either an active or a dormant form.Then, their proposal that living polymerizations“be defined as those processes that yield living


polymers” would also smoothly apply to the radi-cal system because it states nothing about theyield. (To rule out from this definition those pro-cesses that yield living polymers only at the finalstage, for example, conventional chain-transferprocesses yielding terminally functional poly-mers, it might be slightly modified to somethinglike “living polymerizations are defined as thoseprocesses that are mediated by and yield livingpolymers.”) The nature of radical polymerizationis mentioned in their proviso, “It should, however,also be recognized that . . . are negligible.” Livingradical polymerization defined in this way wouldinvite little confusion. Above all, it is simple.

I hope that this experiment by the editors willbring about a good yield, in any form.

Comments byProfessor H. James Harwood

Department of Polymer Science,University of Akron, Akron, Ohio 44325

Both the article by Darling et al.1 and the mac-romolecular nomenclature note2 to which it re-sponds represent attempts to standardize nomen-clature for polymerization processes that exhibitliving polymerization character as initially de-scribed by Szwarc3 and that meet, to varying de-grees, the criteria defined for living polymeriza-tion by Quirk and Lee.4

If an accepted nomenclature can be adopted,bibliographic indexing and searching in this areawill be simplified, and authors will be able tocommunicate with improved clarity. There mayeven be legal consequences (e.g., patent validity)associated with the nomenclature adopted.

In my opinion, no polymerization can be re-garded as being perfectly living, and the relaxeddefinition of Szwarc5 discussed by Darling et al.seems very reasonable. A problem arises concern-ing what is a desired maximum size and what canbe considered a negligible degree of terminationor (irreversible) chain transfer. It seems that itwill be necessary to be flexible and pragmatic inthese matters.

The pragmatic features that most people willassociate with living polymerization behavior are(1) that the growth of individual polymer chainsoccurs throughout a polymerization process and(2) that a large proportion of the polymer chainends are active or capable of being activated atthe end of the process and can be involved in

further polymerization of monomers or can par-ticipate in chemical reactions that enable theirends to be functionalized. Polymer chemists takeadvantage of these features to prepare block co-polymers, tapered copolymers, polymers withstar, radical block, and dendritic architectures,polymers with narrow molecular weight distribu-tions, polymers with useful end-group functional-ity, and so forth. In very favorable situations,reactant concentrations, solvents, temperatures,counterions, catalysts, and modifiers can be var-ied to control the structures of the polymers pro-duced, but this can be true whether or not apolymerization system exhibits the pragmaticfeatures of living polymerizations. For example,molecular weight can be controlled in some livingpolymerization processes by varying the mono-mer/initiator ratios but not in all of them. I do notbelieve that initiation rates must equal or prefer-ably exceed propagation rates for a system to beconsidered living. Furthermore, I do not believethat the word control should be used to define anyparticular set of characteristics of a polymeriza-tion process. However, I find that controlled poly-merization is helpful when used to emphasize aprocess, living or nonliving, where the choice ofthe amounts and types of reactants enables one toprepare polymers with predictable structural fea-tures.

Terms such as quasi-living, pseudo-living, con-trolled living, apparently living, and “living” havebeen used by investigators to refer to polymeriza-tion systems that exhibit the aforementionedpragmatic features. These qualifying terms areused to indicate that the mechanisms of many ofthese polymerizations involve equilibria betweenactive and dormant chain ends as opposed tomechanisms of many more classical living poly-merization systems where the chain ends arethought to be permanently active (probably onlytrue for anionic and cationic processes that in-volve anions or cations of relatively low nucleo-philic or electrophilic character, as in the ring-opening polymerizations of cyclic ethers, esters,and amides).

Not to muddy the water, but if it is importantto distinguish between polymerizations that do ordo not involve equilibria between active and dor-mant polymer chain ends, the former might betermed resurrecting, resurrectional, restorable, orrevivable polymerizations to emphasize their ter-mination–reinitiation features; I do not think itdesirable to do so. Instead, I would like to refer toall polymerizations that exhibit the pragmatic


features discussed previously as living polymer-izations and to include, where necessary, modify-ing names or acronyms that indicate what typesof processes are involved. Most readers encoun-tering terms such as living anionic polymeriza-tion, living cationic polymerization, living ATRP(atom transfer radical polymerization) polymer-ization, living RAFT (reversible addition frag-mentation chain transfer) polymerization, livingGTP (group transfer polymerization) polymeriza-tion, and living ROMP (ring opening metathesispolymerization) polymerization will have someknowledge about the particular side reactions as-sociated with these processes and their impor-tance. Terms need to be agreed on for living po-lymerizations that involve reversible interactionsof propagating radicals with stable (persistent)radicals (FRP, SFRP, ?) or organometallic com-pounds such as cobaloxime and for other ways ofobtaining living polymerization behavior in radi-cal systems (reversible additions of radicals tocompounds that form stable radicals is a possibil-ity that may eventually be demonstrated).6 Byusing such modifying names or acronyms, writerswill automatically provide subclassifications forthe systems they are discussing. Agreement needonly be reached on what modifying terms shouldbe used. When one wishes to refer generically toliving systems that involve active–dormantchain-end equilibria, the use of living seems ac-ceptable.

I do not believe it is desirable to incorporateinformation about initiation or irreversible termi-nation processes into any definitions of living po-lymerization because they would have to be arbi-trarily quantitative. This would then bring in thesubject of control, which would also require arbi-trary quantitative definitions and would lead tounnecessary complication. I believe that differentaspects of control will be associated with eachliving or “living” polymerization system and thatthese will become apparent as workers read aboutor use a particular system. We do not need todevelop definitions for controlled polymerizations.

I do not think that the nomenclature for livingpolymerization systems needs to be made morecomplicated than what has been proposed.


1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev,A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson,

R. R., Jr.; Moad, G.; Rizzardo, E. J Polym Sci PartA: Polym Chem 2000, 38, 1706.

2. Matyjaszewski, K.; Muller, A. H. E. Polym Prep(Am Chem Soc Div Polym Chem) 1997, 38, 6.

3. Szwarc, M. Nature 1956, 178, 1168.4. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359.5. Szwarc, M. J Polym Sci Part A: Polym Chem 1998,

36, ix.6. Harwood, H. J.; Christov, L.; Guo, M.; Holland,

T. V.; Huckstep, A. Y.; Jones, D. H.; Medsker, R. E.;Rinaldi, P. L.; Saito, T.; Tung, D. S. MacromolSymp 1996, 111, 26–28.

Comments byProfessor Koichi Hatada

Department of Chemistry, Faculty ofEngineering Science, Osaka University,Toyonaka, Osaka 560, Japan

I read the article “Living Polymerization: Ra-tionale for Uniform Terminology” by Darling et al.with great interest. Certainly, many polymeriza-tions have been reported to be living since Dr.Szwarc published his article on the anionic poly-merization of styrene in the absence of termina-tion and chain-transfer reactions and named thereaction living polymerization, and variousnames such as living cationic, living ring-open-ing, and living radical polymerization have beenproposed. In addition, new names such as quasi-living, pseudo-living, and apparently living havealso been proposed. Now it is time to think deeplyabout the terms relating to living polymerizationto aid communication and avoid confusion.

The IUPAC definition of living polymerizationis “a chain polymerization from which chaintransfer and chain termination are absent.”1 Notethat in many cases, the rate of chain initiation isfast compared to the rate of chain propagation, sothat the number of kinetic chain carriers is essen-tially constant throughout the polymerization.

This is the term defined on the basis of themechanism of the polymerization reaction. Con-sequently, a certain polymerization can be re-ferred to as a living polymerization when it occurswithout termination and chain-transfer reac-tions. If desired, the term living polymerizationmay be qualified by a self-explanatory adjective oran adjective based on the well-established defini-tion: anionic living polymerization, cationic livingpolymerization, radical living polymerization, ste-reospecific living polymerization, and so forth.The IUPAC definition of radical polymerization is


a chain polymerization in which the kinetic chaincarriers are radicals. Therefore, radical living po-lymerization is a chain polymerization in whichthe kinetic chain carriers are radical and chaintransfer and termination are absent. Living rad-ical polymerization should mean the same, butthe preference of living radical polymerizationand radical living polymerization is to be dis-cussed.

Now the definition of living radical polymeriza-tion or radical living polymerization is very clear.It is important that the polymer chemist whowants to refer to his or her polymerization asliving polymerization clarify the mechanism ofreaction and carefully decide whether or not themechanism fits the definition.

These are very important procedures. The evi-dence of no termination and no chain transfer canbe usually obtained by a detailed analysis of theproducts, for example, an NMR analysis of a poly-mer with the aid of synthetic means or contriv-ance. Narrowness of the molecular weight distri-bution (MWD) is neither a necessary conditionnor a sufficient condition for living polymeriza-tion; living polymerization sometimes gives apolymer with a broad MWD, and nonliving poly-merization sometimes gives a polymer with a nar-row MWD.

Some polymer chemists disagree that polymer-ization with reversible termination is living, prob-ably because active species are in a dynamic equi-librium with inactive or dormant species.2 I agreethat such a polymerization is living as long as allthe chains can add monomer molecules in a suf-ficiently long time. I think that the IUPAC defi-nition of living polymerization does not requirethe chain carriers to be activated all the time;they can be reversibly deactivated for some time.I myself sleep every day, but no one opposes thatI am living. However, I do not like to define newterms such as quasi-living and pseudo-living po-lymerizations for the polymerization.

The existence of the equilibrium between reac-tive species and dormant species does not affectthe main course of the chain reaction as long asthe reactive sites are regenerated from the dor-mant species.

The modification of basic terms by prefixes oradjectives that are not self-explanatory some-times causes confusion or misunderstanding. Liv-ing polymerization with reversible deactivation isenough if they want to qualify it, as Matyjasze-wski and Muller mentioned in their article.3

Another important problem is what we dowhen our polymerization is not completely living.The IUPAC recommendation1 advises us to re-solve such a problem in the following way. Topresent a clear concept, idealized definitions arenecessarily adopted. Deviations from idealityusually arise with real polymers and reactions.Although such deviations are not explicitly takeninto account in the definitions, the recommendednomenclature can usefully be applied to the pre-dominant features of real polymers or reactions.If necessary, self-explanatory adjectives such asessentially, almost completely, or highly can beused for qualification.

So if the polymerization includes chain trans-fer and termination but in small proportions com-pared to propagation, it can be called essentially,almost completely, or predominantly living. Ofcourse, the deviation from ideality should besmall enough, and I cannot agree with the pro-posal that the process that yields a living polymeris living polymerization even if chain transfer andtermination occur considerably. In such a case,you might mention that a part of the propagatingspecies polymerizes monomers in a living mannerbut the reaction as a whole is not living. Chaintransfer and termination are not simple side re-actions. The absence of these side reactions is anabsolute necessity for living polymerization.

I agree with Darling et al. not to use the termcontrolled for apparently living or “living”. Mostof the chemists carry out the reactions to be con-trolled in every meaning. So the adjective con-trolled is too broad and not self-explanatory, andwe cannot imagine anything specified from theterm controlled polymerization. Without the ad-jective controlled, we can describe such a living-like polymerization with appropriate modifiers,for instance, “predominantly living polymeriza-tion with a small extent of irreversible termina-tion.”

Finally, I must tell you that “the use of theterm controlled radical polymerization instead ofliving radical polymerization” is not an IUPACrecommendation. This is a misunderstanding byDarling et al. The term controlled may be a pro-posal by Matyjaszewski and Muller.3

The author would like to thank Dr. Teiji Tsuruta,Professor Emeritus of the University of Tokyo, andProfessor Tatsuki Kitayama for their helpful discus-sions.



1. IUPAC. Pure Appl Chem 1996, 68, 2287–2311.2. Living polymerization is defined for chain polymer-

ization. Chain polymerization is defined in the IU-PAC document1 as “a chain reaction in which thegrowth of a polymer chain proceeds exclusively byreaction(s) between monomer(s) and reactive site(s)on the polymer chain with regeneration of the re-active site(s) at the end of each growth step”

reactive site 3 ,chain growth. 321

dormant speciesregeneration of 3 ,chain growth. 3

reactive site21

dormant species

3. See ref. 1 in Darling et al.

Comments by Dr. Craig HawkerIBM Almaden Research Center, 650Harry Road, San Jose, California 95120-6099

As with many scientific disciplines, the adventof new techniques and approaches almost invari-ably results in a reevaluation of existing termi-nology and beliefs. Polymer science is not im-mune, with this discussion concerning livingpolymerizations and the newer approaches tofree-radical polymerization in particular being aprime candidate. As pointed out in previous arti-cle by Darling et al.1 and recently by Matyjas-zewski and Muller,2 considerable confusionreigns in the area of living polymerizations; can afree-radical polymerization ever be a living proce-dure or is it something else? If it is somethingelse, then what is it and what do you call it?

As both sets of authors correctly pointed out,living free-radical polymerizations, whether me-diated by metal complexes [atom transfer radicalpolymerization (ATRP)],3–6 nitroxides,7–12 orsuch invariably suffer from termination reac-tions. Therefore, in the strictest sense, they arenot true living systems. Numerous authors haverecognized this point as the field has grown, and agreater understanding of the underlying pro-cesses has developed.13–16 To accommodate this,various terms such as pseudo living, living, andcontrolled have been put forth in the literature,and this profusion of terms has caused consider-able confusion and debate. Unfortunately, a strictadherence to guidelines is problematic because it

can be argued that no polymerization can ever betruly living; for example, the total exclusion ofcontaminants, terminating moieties, and so forthfrom an anionic procedure is a goal that can neverbe achieved. A good analogy for not using thesestrict and unobtainable definitions is the field ofdendrimers. Until the recent advent of advancedmass spectrometry techniques, it was difficult todetect subtle defects in the overall branchedstructure of dendrimers; as a consequence, a per-fect structure was generally assumed or expected.However, MALDI-TOF experiments17 have clearlyshown that divergently grown dendrimers18 areprone to defects, and even the most carefullygrown convergent structures18 may also containdefects. Should we refrain from using the termdendrimer17–21 for divergently grown structuresand only use it for well-defined convergent struc-tures,22 or should we not use the term dendrimerat all because it is extremely difficult, if not im-possible, to obtain a strictly monodisperse samplewith perfect branching? I think the answer isdefinitely no; both sets of compounds approachvery closely the goal of an idealized dendriticstructure and experimentally have been shown tohave polydispersities of less than 1.01 and inmany cases less than 1.001. Although they are notperfect dendrimers, they are dendrimers.

As Szwarc23 and Darling et al.1 pointed out,living polymers are distinguished from dead poly-mers by being “able to grow whenever additionalmonomer is supplied.” From this perspective,polymer chains prepared under living free-radicalmethods are far from being dead; they can beisolated, characterized, stored, and so forth andthen simply reactivated by the addition of mono-mer under the appropriate conditions. Althoughit can be argued that some condensation proce-dures can also be reactivated, the combination ofthis feature and the ability to prepare narrowpolydispersity materials and control the molecu-lar weight, macromolecular architecture, endgroups, and block purity for living free-radicalprocedures does justify the use of the term livingpolymerization. As has been shown by a numberof authors,24–26 the ability to prepare linear andstar polymers with controlled molecular weightsand polydispersities of 1.03–1.08 make these ma-terials similar in numerous respects to those pre-pared by more acknowledged living processessuch as anionic procedures. In addition, the fieldof living polymerization is still in its infancy, andas improvements and new concepts emerge, thecharacteristics associated with livingness will


certainly improve. As Oscar Wilde emphasized inThe Importance of Being Earnest, “The truth israrely pure, and never simple.”


1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev,A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson,R. R.; Moad, G.; Rizzardo, E. J Polym Sci Part A:Polym Chem 2000, 38, 1706.

2. Matyjaszewski, K.; Muller, A. Polym Prep (AmChem Soc Div Polym Chem) 1997, 38, 6.

3. Wang, J. S.; Matyjaszewski, K. J Am Chem Soc1995, 117, 5614.

4. Kato, M.; Kamigaito, M.; Sawamoto, M.; Higash-imura, T. Macromolecules 1995, 28, 1721.

5. Percec, V.; Barboiu, B. Macromolecules 1995, 28,7970.

6. Granel, C.; DuBois, P.; Jerome, R.; Teyssie, P. Mac-romolecules 1996, 29, 8576.

7. Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.;Hamer, G. K. Macromolecules 1993, 26, 2987.

8. Hawker, C. J. J Am Chem Soc 1994, 116, 11314.9. Li, I. Q.; Howell, B. A.; Koster, R. A.; Priddy, D. B.

Macromolecules 1996, 29, 8554.10. Fukuda, T.; Terauchi, T.; Goto, A.; Ohno, K.; Tsujii,

Y.; Yamada, B. Macromolecules 1996, 29, 6393.11. Moad, G.; Rizzardo, E. Macromolecules 1995, 28,

8722.12. Puts, R. D.; Sogah, D. Y. Macromolecules 1996, 29,

3323.13. Fischer, H. Macromolecules 1997, 30, 5666.14. Hawker, C. J. Acc Chem Res 1997, 30, 373.15. Colombani, D. Prog Polym Sci 1997, 22, 1649.16. Matyjaszewski, K. Controlled Radical Polymeriza-

tion; ACS Symposium Series 685; Oxford Univer-sity Press: New York, 1998.

17. Leon, J. W.; Frechet, J. M. J. Polym Bull 1995, 35,449.

18. Hummelen, J. C.; van Dongen, J. L. J.; Meijer,E. W. Chem Eur J 1997, 3, 1489.

19. Gooden, J. K.; Gross, M. L.; Mueller, A.; Stefa-nescu, A. D.; Wooley, K. L. J Am Chem Soc 1998,120, 10180.

20. Durst, H. D.; Tomalia, D. A. Top Curr Chem 1993,165, 193.

21. Newkome, G. R.; Moorefield, C. N.; Vogtle, F. Den-dritic Molecules; VCH: Weinheim, 1996.

22. Hawker, C. J.; Frechet, J. M. J. J Am Chem Soc1990, 112, 7638.


24. Patten, T. E.; Xia, J.; Abernathy, T.; Matyjasze-wski, K. Science 1996, 272, 866.

25. Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker,C. J. J Am Chem Soc 1999, 121, 40.

26. Hawker, C. J.; Zech, C. Unpublished results.

Comments byProfessor Allan S. Hay

Department of Chemistry, McGillUniversity, 801 Sherbrooke Street West,Montreal, Quebec, Canada H3A 2K6

One of the requirements of an article is a titleand abstract that contain enough information fora potential reader to identify it as an article ofinterest that he would like to learn more about byretrieving and reading. In organic chemistry, asthe authors note, terms such as the Diels–Alderreaction are invaluable as a communication tool.An even better example is the Friedel–Crafts re-action, with the many catalysts and reactantsthat have been used.

The plethora of terminologies that have arisento describe various forms of living polymerizationreactions and the concomitant duplication pre-sents a problem in identifying articles of interest,especially if mechanical searching is being used,because these methods may rely principally onkeywords.

I propose that the writers imagine they arewriting a comprehensive article on living poly-merization reactions and prepare titles for an in-dex in which the headings and subheadingswould then necessarily provide a recommendedterminology for the specific area under the um-brella of living polymerization reactions.

Comments byProfessor Toshinobu Higashimura

Department of Materials Science, Facultyof Engineering, University of ShigaPrefecture, 2500 Hassaka-cho, Hikone,Shiga 522-8533, Japan

Darling et al.1 submitted a constructive com-ment on living polymerization terminology. In myview, the confusion about the terminology stemsfrom the following two points.

Living Polymerization Versus ControlledPolymerization: As Szwarc, the pioneer of livingpolymerization, stated distinctively in one of hisbooks, the reaction is originally defined as apolymerization where “propagation proceeds withexclusion of termination and chain-transfer.”2 Be-cause such an ideal polymerization is, practicallyspeaking, least likely to exist, he later proposed a“relaxed” and pragmatic definition that “we shallrefer to polymers as living if their end-groups


retain the propensity of growth for at least as longa period as needed for the completion of an in-tended synthesis, or any other desired task.”2

However, Matyjaszewski,3 taking a stand for theearlier, strict definition, claimed that one should notcall a polymerization living that is recognized toinvolve chain-breaking reactions, for example,those that fail to give high molecular weight poly-mers. He accordingly proposed two terms, con-trolled polymerization for a process that at leastpermits polymers of controlled structures and con-trolled/living polymerization for a reaction wherecontrolled polymers can form with an unknown con-tribution of chain-breaking reactions. As Darling etal.1 pointed out, however, the term controlled poly-merization has been used for different meaningsand is, therefore, unsuitable for defining anotherpolymerization of particular characteristics such asthose that we are now discussing.

From these arguments, I think Szwarc’spragmatic definition is more appropriate; namely,the term living polymerization should refer to aprocess that produces polymers with the charac-teristics of the strictly defined living polymers,rather than be confined to chain polymerizationsfor which the absence of termination and chaintransfer is actually and unambiguously proved(practically, however, this proof seems almost im-possible to obtain).

Terminology Based on PolymerizationTechniques—A Proposal: Another confusionregarding the living polymerization terminology,in my view, is the use of a variety of terms thatbear particular polymerization techniques, suchas atom transfer radical polymerization andiniferter method. If these terms are employed torefer to polymerizations that are thought to pro-duce living polymers (as defined previously), theparticular polymerization technique of interestshould be attached to the term living polymeriza-tion, for example, living polymerization by theatom transfer method. This terminology is exactlythe same in principle as, for example, “substitu-tion reaction, SN1,” by which a particular substi-tution process is distinguished by reference to itsreaction mechanism. This technique-based termi-nology may also circumvent the problem in com-putational literature searching with which Dar-ling et al. expressed some concern.

For referring to polymerization reactions thatlead to polymers with the characteristics of livingpolymers, I would like to recommend attaching aphrase for a relevant polymerization technique tothe term living polymerization, so as to avoid the

confusion in terminology, to make literaturesearches flawless, and in turn to promote creativeresearch in polymer chemistry.


1. Darling, T. R.; Davis, T. P.; Fryd, M.; Gridnev,A. A.; Haddleton, D. M.; Ittel, S. D.; Matheson,R. R., Jr.; Moad, G.; Rizzardo, E. J Polym Sci PartA: Polym Chem 2000, 38, 1706.

2. Szwarc, M.; van Beylen, M. Ionic Polymerizationand Living Polymers; Chapman & Hall: New York,1993; p 12.

3. Cationic Polymerization: Mechanisms, Synthesis,and Applications; Matyjaszewski, K., Ed.; MarcelDekker: New York, 1996; p 266.

Comments byProfessor Hartwig Hocker

Textile and Macromolecular Chemistry,University of Technology, RWTH Aachen,Veltmanplatz, W-5100 Aachen, Germany

Living polymerization has become a fashion-able expression. It seems that a new polymeriza-tion reaction is generally considered to have ahigher value if it is called living, although poly-mers obtained by polymerization reactions thatcannot be considered living may be better thanthe living ones. Therefore, a uniform terminologyis very much required.

A living polymerization should yield livingpolymers (or their dormant precursors). It mustbe recognized, however, that pseudo first-orderkinetics (the constant concentration of livingends) are realized, along with a degree of poly-merization that is proportional to the conversion,the proportionality constant being [M0]/c*, where[M0] is the initial monomer concentration and c*

is the concentration of active species. This impliesthat the molecular weight distribution is narrow,that is, Pw ' Pn. This condition certainly is notfulfilled when Pw/Pn is equal to 1.5 because thisvalue is reached with radical polymerization andtermination via combination reactions. It defi-nitely should be lower than 1.2.

In conclusion, there always has been controversyover the term living because, for example, a livingcell is susceptible to apoptosis, except for a cancercell. If the term living is accepted, however, theliving system should be defined as it is herein.


Comments byProfessor Thieo E. Hogen-Esch

Loker Hydrocarbon Research Institute,University of Southern California,Los Angeles, California 90089

As polymer chemists who are speaking the lan-guage of polymer chemistry, it would seem tofollow that we should agree on its vocabulary. Itoo am troubled by the current lack of consensuson the meaning of living, quasi-living, and pseu-do-living polymerizations and believe that suchterms may be contributing to some of the confu-sion in this area. As I understand it, the authorspropose that (1) the term living polymerization beredefined as “those processes that yield livingpolymers (or their dormant precursors)” and (2)the terms controlled radical and controlled/livingradical polymerizations, proposed in their ref. 1,should be avoided. I disagree with their first pointand agree with their second, but for somewhatdifferent reasons.

The original definition of an ideal living poly-merization as proceeding without termination ortransfer was useful and appropriate.1 However,let us not forget that the word living, howevercolorful and apt, is metaphorical. Like the wordpregnant, the term is also absolute, and it is rea-sonable to infer that a polymerization is eitherliving or not. As originally pointed out bySzwarc,1a polymerizations proceeding withouttermination or transfer regardless of timescaleare probably experimentally unattainable. Fur-thermore, this definition carries specific mecha-nistic/kinetic implications, that is, the absence ofirreversible termination or other side reactionsthat deactivate or otherwise compromise thestructure of the chain.

I agree with Darling et al. that it is desirable toretain the term living. However, I do not believethat their definition, “those processes that yieldliving polymers or their dormant precursors,” isappropriate, as it uses the very term living that itaims to define. A better definition and one thatclosely follows the original and subsequent defi-nitions would be “polymerizations proceedingwithout termination or transfer on the polymer-ization timescale.” This definition leaves out thephrase “or any other desired task” (discussedlater).1b The clause “on the polymerization time-scale” appears to considerably relax the strin-gency of the original definition and give it addi-

tional and appropriate flexibility. The reasons forthis proposed modification are detailed later.

The nature of the original definition of livingpolymerization is a mechanistic and thus theo-retical concept. It seems to make sense thatthe definition should remain mechanistic andshould not include experimental criteria as well.However, the degree to which a living polymer-ization conforms to the ideal model can be exper-imentally tested through a comprehensive list ofcriteria.1,2

Thus, I believe that it would be most practicalto apply the term living only to the polymeriza-tion and the inherent and unavoidable termina-tion, transfer, or other side reactions of the prop-agating chain end. For instance, the ability tocarry out subsequent reactions such as end func-tionalization, block copolymerization, formationof stars, and other reactions of this type is anexcellent criterion for the degree of conformitywith the ideal model. However, the success incarrying out these subsequent reactions, at leastin principle, depends on their rates relative to therate of spontaneous degradation of the activechain end. Thus, we may find ourselves in a po-sition of stating that a polymerization is living forone type of subsequent chain-end reaction but notfor another. The shelf life of a living polymershould not be used for its definition but should beused as a criterion only.

A similar argument can be made with respectto initiation as initiation and polymerization aredifferent reactions and have different rate con-stants. For instance, the alkyllithium-initiatedpolymerization of styrene or isoprene in a hydro-carbon solvent closely approximates an ideal liv-ing polymerization.1,2 The initiation by the lessnucleophilic 1,1-diphenylhexyllithium or an evenslower initiator would give a wider molecularweight distribution polymer, especially if the in-tended number-average molecular weight is low.2

Could it then be said that the former polymeriza-tion is living and the latter is not? Depolymeriza-tion reactions and equilibria between active anddormant chains occurring on the polymerizationtimescale also are known to lead to wider molec-ular weight distributions. Do all of these polymer-izations then cease to be living, even though thechains do not terminate or give chain transfer?

For this and other reasons, I do not believe thatthe terms quasi- and pseudo-living polymeriza-tions, describing polymerizations proceedingthrough interconverting reactive and unreactive(or less reactive) species, are useful because most


living polymerizations are covered by that defini-tion. For instance, the living anionic polymer-ization of dienes or styrene initiated by alkyl-lithiums in hydrocarbons involves unreactive ordormant alkyllithium aggregates in rapid equilib-rium with reactive polydienyllithium chains. Intetrahydrofuran, some anionic polymerizations ofstyrene are dominated by tiny amounts of highlyreactive free ions in equilibrium with relativelyunreactive ion pairs.1,2

Indeed, it could be said that the presence of un-reactive intermediates that rapidly interconvertwith the active chain end greatly contributes to theliving character of many polymerizations. Thus, thelow concentration of the active chain ends in theliving radical polymerizations also greatly reducesintermolecular deactivation reactions and is an es-sential element in conferring living character to thispolymerization.3 There are several other recent re-ports on the living anionic polymerization of(meth)acrylates in which dormant species are pos-tulated or demonstrated.4–6 The situation is quitesimilar in some cationic polymerizations in whichseveral species with widely different reactivitieshave been demonstrated.7

I agree with Darling et al. that controlled-liv-ing and similar terms are not desirable andshould be avoided. The words controlled and con-trol, meaning to exercise restraint or directionover,

8

are neither metaphorical nor absolute, andthe expression degree of control, as opposed todegree of livingness, is internally consistent. Inthis case, the word control appears to be linked tothe process leading to an intended outcome of thepolymerization. This control may include, but isnot limited to, molecular structure, molecularweight, molecular weight distribution, stereo-chemistry, end functionalization, and other prop-erties but does not have to conform to all, or evenmost, of these aspects or to the criteria for livingpolymerizations. For instance, a polymerizationmay be controlled by the formation of a polymerwith a particular number-average molecularweight without the polymerization process neces-sarily being living.1,2

In conclusion, definitions should not be changedwithout compelling reasons. “If it ain’t broke, don’tfix it” would seem to be good advice in this area also.Keeping the definition of living polymerization sim-ple but flexible and restricting it to polymerization,chain transfer, or termination would avoid muchconfusion and proliferation of terms.


1. (a) Szwarc, M. Nature 1956 178, 1168; (b) Szwarc,M. Ionic Polymerizations Fundamentals; Hanser:New York, 1996; (c) Szwarc, M.; Van Beylen, M.Ionic Polymerization and Living Polymers; Chap-man & Hall: New York, 1993; p 12–15.

2. Hsieh, H. L.; Quirk, R. P. Anionic Polymerization:Principles and Practical Applications; Marcel Dek-ker: New York, 1996; p 2.

3. Matyjaszewski, K.; Wang, J.-S. J Am Chem Soc1995, 117, 5614.

4. (a) Webster, O.; Hertler, W.; Sogah, D.; Farnham,W.; Rajan Babu, T. J Am Chem Soc 1983, 105,5706; (b) Quirk, R. P.; Bidinger, G. P. J Polym Bull1989, 22, 63.

5. (a) Zagala, A. P.; Hogen-Esch, T. E. Macromole-cules 1996, 29, 3038; (b) Baskaran, D.; Mueller,A. H. E.; Kolshorn, H.; Zagala, A. P.; Hogen-Esch,T. E. Macromolecules 1997, 30, 6695.

6. Ishizone, T.; Yoshimura, K.; Yanase, E.; Naka-hama, S. Macromolecules 1999, 32, 955.

7. Miyamoto, M.; Sawamoto, M.; Higashimura, T.Macromolecules 1984, 17, 265.

8. Webster’s New Universal Unabridged Dictionary;Barnes & Noble: New York, 1994.

Comments byProfessor Bob A. Howell

Center for Applications in PolymerScience, Department of Chemistry,Central Michigan University, Mt.Pleasant, Michigan 48859

Over the past several years, attempts to moder-ate the reactivity of the propagating species in rad-ical polymerization have been multifold. The focusof these efforts has been the elimination of un-wanted side reactions (radical coupling, dispropor-tionation, radical transfer). In the absence of suchreactions that lead to polydispersity broadening andthe formation of polymer defect structures, the po-lymerization should be much more well-behavedthan traditional radical polymerization. It has beennoted that for “systems that proceed in the absenceof termination and chain transfer, polymers can beprepared with control of molecular weight, molecu-lar weight distribution, copolymer composition andmicrostructure, tacticity, chain-end and in-chainfunctional groups, architecture, and morphology.”1

Success has been achieved to a greater or lesserdegree depending on the exact approach used. How-ever, none have been completely successful; that is,none has provided an ideal polymerization. All ofthese processes involve the presence of a capping


agent (nitroxyl radical, halogen atom, organometal-lic moiety, or other molecular fragment) to reactreversibly with the propagating radical to generatea nonradical species that is unreactive. In thisbound state, the propagating radical is stable ordormant and unable to participate in termination orchain-transfer processes. Chain extension is con-trolled by the position of equilibrium between thedormant and active or free propagating radical. Un-der appropriate conditions, very good mediation ofthe radical polymerization may be achieved, andsome of the characteristics of living polymerizationmay be observed, such as ready chain extension toform block polymers, narrow polydispersity, molec-ular weight control (reflecting the monomer/initia-tor molar ratio for systems in which preformed ini-tiators/mediators are used), and end-group purity.However, none of these reactions meet rigorous cri-teria for living polymerization.

These have been defined as follows:2

1. The polymerization proceeds until all themonomer has been consumed; the furtheraddition of monomer results in continuedpolymerization.

2. The number-average molecular weight, Mn(or Xn, the number-average degree of poly-merization), is a linear function of conver-sion.

3. The number of polymer molecules (and ac-tive centers) is a constant, which is sensi-bly independent of conversion

4. The molecular weight of the polymer canbe controlled by the stoichiometry of thereaction.

5. Narrow molecular weight distributionpolymers are produced.

6. Block copolymers can be prepared by se-quential monomer addition.

7. Chain-end functionalized polymers can beprepared in quantitative yields.

8. Linearity of a kinetic plot of the rate ofpropagation as a function of time.

9. Linear dependence of the degree of poly-merization as a function of time.

To fairly be termed living, a polymerizationmust meet all these criteria because some aresensitive to certain characteristics of living po-lymerization whereas others are sensitive toother features. Because none of the reportedmediated radical polymerizations may be legit-imately described as living but, at the sametime, have some of the characteristics of living

polymerization, a great variety of terminologyhas sprung up to describe these processes, in-cluding pseudo-living and quasi-living. Suchdescriptions often reflect a desire to include liv-ing in the descriptive terminology rather than aregard for accuracy in description or precisionof language. More recently, specific descriptorssuch as nitroxyl-mediated radical polymeriza-tion (NMRP) and atom transfer radical poly-merization (ATRP) have been used in an at-tempt to introduce a greater degree of precisionin the description of the polymerization reac-tion. This has largely been successful. Neitherof these processes is living, but both possesscertain characteristics of living polymerizationthat vary with the exact nature of the polymer-ization and the reaction conditions. Becausethese processes have been rather widely ex-plored and their scope and limitations largelyestablished, the terms NMRP and ATRP arefunctionally descriptive; that is, to what theyrefer is well understood. Neither contains anydirect reference to the extent of the livingness ofthe process that it describes; in either case, thisis variable depending on the reaction condi-tions, is reflected experimentally, and is im-plicit in the descriptor. For both these cases andfor others in which a mediating agent ispresent, the propagating radical is neither freenor living. However, because of the presence ofthe mediating species, unwanted side reactionsare suppressed (to a greater or lesser degreedepending on reaction conditions), and the re-sulting polymer is more nearly ideal than thatwhich can be obtained in the absence of themediating agent. Therefore, it is perfectly ap-propriate to refer to these processes as medi-ated radical polymerization (MRP) rather thanas controlled radical polymerization (the con-trol is not complete in any case and is variablefrom case to case) or living radical polymeriza-tion (all mediated radical polymerizations ex-hibit some features of living polymerization, butnone display all such characteristics). The termMRP is not only descriptive of a wide range ofradical polymerizations carried out in the pres-ence of agents to modify the reactivity of thepropagating species but avoids the confusionassociated with the terms living free-radical,living radical, and controlled radical polymer-ization.

The goal of any nomenclature/terminologyshould be clarity. To broaden the definition ofliving to include a variety of mediated radical


processes serves only to destroy the utility of theterm living. This term, as it has been, should bereserved for processes that truly meet the criteriafor livingness. It was argued by Darling et al. thatsuch a broadening of the term living is consistentwith Occam’s razor. Nothing could be furtherfrom the case. Rather than providing a simplifi-cation, it would only introduce confusion into anarea currently well-defined, that is, living poly-merization.

Darling et al. suggested that reactions shouldbe classified on the basis of mechanism and thenproceeded to use the Diels–Alder reaction (a re-action, as they correctly noted, that may occur bymultiple mechanisms) as an example. The termDiels–Alder simply refers to the conversion of adiene and an appropriate monoene to an unsat-urated cyclic adduct, as first observed by Dielsand Alder, without regard to mechanism. Themore specific way to refer to these processeswould be to do so by reference to mechanism, thatis, as a concerted 4 1 2 cycloaddition reaction, acycloaddition involving a diradical intermediate,or a cycloaddition as a consequence of ligand re-organization at a metal center. The term Diels–Alder simply refers to the nature of the productformed and not to the process by which it wasformed. As a consequence, it is a far less usefulterm than concerted 4 1 2 cycloaddition. No onewould argue that radical polymerization con-ducted in the presence of a mediating agentshould be referred to simply as polymerization(somewhat akin to the use of Diels–Alder to de-scribe cycloaddition). However, it can be de-scribed in a much more definitive way based onhow it occurs, that is, based on mechanism. It isnot living polymerization or controlled polymer-ization. It may exhibit some characteristics of ei-ther. To avoid confusion, to introduce clarity, andto maintain mechanisms as the basis for descrip-tive terminology, this process may best be de-scribed as MRP.


1. Hsieh, H. L; Quirk R. P. Anionic Polymerization:Principles and Practical Applications; Marcel Dek-ker: New York, 1996; p 2.

2. Hsieh, H. L; Quirk R. P. Anionic Polymerization:Principles and Practical Applications; Marcel Dek-ker: New York, 1996; p 76–77.

Comments byProfessor Shohei Inoue

Department of Industrial Chemistry,Faculty of Engineering, ScienceUniversity of Tokyo, Kagurazaka,Shinjuku, Tokyo 162-8601, Japan

The authors’ proposal is quite reasonable. Themeaning of the term living polymerization shouldbe relaxed and be defined as those processes thatgive living polymers. Living polymers are definedas those being able to grow whenever additionalmonomer is supplied, in contrast to dead poly-mers. The word controlled is too broad to definepolymerization. There can be even a mechanicalway to control the polymerization process. A def-inition should not be dependent on a detectionlimit that can change with the progress of meth-ods.

However, no one can restrict the proposal ofnew terminology because a scientist has a right toemphasize his or her identity. It is, of course, alsopossible for other scientists to either agree with oroppose any proposal. There is a long history ofsimilar discussions about language. Opinionshave been often expressed by those worried aboutthe confusion of a language they would correct.Even so, languages have gradually changed andwill continue to change in the future. Terminologyin chemistry cannot be an exception. A rigorousdefinition does not make much sense.

Comments byProfessor Aubrey D. Jenkins

School of Chemistry, Physics, andEnvironmental Science, University ofSussex, Brighton, BN1 9QJ,United Kingdom

Although there is an erroneous reference inthis article to “a recent IUPAC nomenclaturenote,” it is astonishing to find no further allusionto IUPAC, in particular to the work of the IUPACCommission on Macromolecular Nomenclature,the body charged with the responsibility of rec-ommending the nomenclature and terminology tobe employed internationally in polymer science.The fact that some of the authors participate inIUPAC work, within the Macromolecular Divi-sion of IUPAC, adds to the surprise.

In view of the importance, stressed by Darlinget al.,1 of using terms uniformly throughout the


polymer science community, the first thing to doin any discussion of this kind is to refer back tothe nomenclature already established in, for ex-ample, the Glossary of Basic Terms in PolymerScience2 and the Compendium of Chemical Ter-minology.3 The only way to avoid chaos in nomen-clature is to build on the existing body of termi-nology, rather than to invent new terms ad hoc.

In the Glossary,2 Definition 3.21, we find thatliving polymerization is “a chain polymerizationfrom which chain transfer and chain terminationare absent.” Turning to the Compendium,3

(chain) termination is defined as “the steps in achain reaction in which reactive intermediatesare destroyed or rendered inactive, thus endingthe chain.”

The very last phrase in the latter definition isthe one most pertinent to this discussion. If it isheld that the intention was to convey a sense ofending the chain permanently, then there is noproblem in using the term living radical poly-merization to describe the kinds of system inquestion, where the reactive intermediates re-vive, perhaps many times, after temporary qui-escence. This is the view that I would person-ally advocate.

Having said that, it could be argued that itwould be useful to add an adjective that distin-guishes such a system from the type, well-knownin anionic polymerization, in which the chains arenot interrupted at all. The simplest suggestion Ican make for this purpose is to use either inter-rupted living radical polymerization or intermit-tent living radical polymerization (in either case,ILRP). I believe that these expressions accuratelyand succinctly convey the desired impression, incontrast to controlled, which tells us nothingabout what is controlled or how.



2. IUPAC Commission on Macromolecular Nomencla-ture. Pure Appl Chem 1996, 68, 2287.

3. McNaught, A. D.; Wilkinson, A. Compendium ofChemical Terminology; Blackwell: Oxford, En-gland, 1997; p 415.

Comments byProfessor Robert Jerome

Director, Center for Education &Research on Macromolecules,University of Liege, Sart-Tilman,B6 4000 Liege, Belgium

Although the article entitled “Living polymer-ization: Rationale for Uniform Terminology” byDarling et al. aims at clarifying the concept oflivingness, I do have the feeling that at the end ofa discussion that is basically not convincing, thereader is left unsatisfied by the final proposal.

I do not understand at all the authors who par-allel the problem of defining a living polymerizationand the fact that “one of the basic principles ofchemistry is to name reactions on the basis of theirmechanisms, not their yields.” Polymerizations areindeed clearly named on the basis of their mecha-nisms (anionic, cationic, etc.), whereas livingnessrefers to characteristic features shared by differentmechanisms.

When the authors propose to define living po-lymerizations as those processes that yield livingpolymers (or their dormant precursors), where isthe clarification, with no definition being pro-posed for living polymers ?

The authors also argue that “a definitionshould not be dependent on the limitations ofcurrent technology (a detection limit).” Never-theless, anyone who is conducting polymeriza-tion experiments needs criteria to which he(she) can refer to conclude to the livingness ofthe polymerization or not. This need for ac-cepted criteria and for experimental measure-ments (techniques) to quantify them is by farthe most important issue. When the authorsadmit that various characteristics of the finalpolymers are associated with livingness (poly-dispersity, molecular weight, end functionality,etc.), they indirectly address this basic ques-tion, although they make no proposal to answerit (which is a gap to be filled).

A definition is completely useless as long as thereis no agreement on how to handle it and draw firmconclusions from experimental observations.

Finally, I never considered narrow polydispersityan attribute of living polymerization. The relativerate constant of the initiation compared to the prop-agation does not prevent chains from growingwhenever additional monomer is supplied, if I referto the distinction of living polymers from dead poly-mers, according to ref. 4 of the article.


Shortly, I would propose that living polymer-ization is any process that is free from irreversiblechain-breaking reactions, whatever the relativerate constants of the constitutive processes. Inpractice, this definition will be relaxed (to use thesame term as the authors in their proposal) de-pending on the experimental criteria that each ofus accept to use for drawing conclusions. Oncethese criteria are clearly defined, it will appearthat the notion of yield (I suppose that the au-thors mean monomer conversion) is importantbecause it might happen that the criteria arefulfilled up to a certain monomer conversion andthat increasingly more important deviations ap-pear at longer reaction times. Finally, that thepolydispersity is narrow or broad (depending onthe relative rate constant for initiation and prop-agation) merely makes livingness very well suitedor not to the macromolecular engineering of thepolymers under consideration. The problem of theinitiator efficiency (number of chains formed perinitiator) should be treated in the same way, par-ticularly if this efficiency increases with monomerconversion. So, my proposal is (1) to use a defini-tion that is very close to an early definition, (2) toemphasize the need for experimental criteria onthe basis of which this (or any other) definitionwill be considered fulfilled or not, and (3) to con-sider that in some instances livingness might benot suited to macromolecular engineering.

Although at this point I do not embark myselfin establishing a list of criteria for livingness (I dobelieve that those ones that are most often usedby many authors, such as the dependence of Mn

on conversion, the dependence of ln([M]o/[M]) ontime, and the dependence of (Mn)exp on (Mn)theor

are a valuable basis), I hope that these consider-ations will be of some help in the rationale foruniform terminology, something most of us con-sider highly desirable.

Comments byProfessor Mikiharu Kamachi

Department of Applied Chemistry andPhysics, Fukui University of Technology,6-3-1 Gakuen, Fukui 910-0028, Japan

Twenty years ago, we determined the propaga-tion rate constants for several monomers by usingthe rotating sector method. In the course of thisstudy, we needed to determine the initiation rateunder the same conditions as the determination

of the propagation rate. Usually, we determinedthe initiation rate by using benzoquinone, DPPH,verdazyl, and TEMPOL (1-hydroxy-1,1,6,6-tetra-methyl piperinin-1-oyl). TEMPOL quantitativelycatches up the initiating radical via a couplingreaction below 60 °C, whereas the other inhibitorshave extra reaction sites such as nitro and car-bonyl groups.1 Accordingly, we used TEMPOL asthe best inhibitor for the determination of theinitiation rate. From my experience, I think thatthe polymerization with nitroxyl radical is tem-perature-controlled radical polymerization; poly-merization with reversible activation and deacti-vation takes place at temperatures greater than100 °C. Moreover, I think the concentration ofnitroxyl radical is also an important factor in theachievement of well-controlled radical polymer-ization with reversible activation and deactiva-tion. Living polymerization with an anionic initi-ator, which was used by Professor Szwarc, usu-ally does not contain irreversible terminationprocesses. Accordingly, I think controlled radicalpolymerization is more realistic than other termi-nology, such as living polymerization, for unionterminology.


1. Kamachi, M.; Honda, Y. Department of Macromo-lecular Science, Osaka University. Unpublishedpaper, 1983.

Comments byProfessor Konstantin S. Kazanskii

Laboratory of Hydrophilic PolymerMaterials, Polymer and CompositeDepartment, N. N. Semenov Institute ofChemical Physics, Russian Academy ofSciences, ul. Kosygina 4, Moscow 117977,Russia

THE BEST IS THE ENEMY TO THE GOOD

It is well-known that the notion of living polymer-ization relates not to any particular type of chem-ical reaction but rather to a specific regime ofchain polymerization processes. The determiningfeature of this regime is that within the timesufficient to attain a definite synthetic aim, chain-termination or chain-transfer reactions do notseem to take place. As a rule, it does not strictly


mean that the mentioned reactions are really ab-sent in the given polymerizing system but thatthey involve only an insignificant portion of thepropagating macromolecules. Thus, being gener-ated simultaneously, these species are living; thatis, they retain the activity of their terminalgroups, which in turn allows the polymer chain tobe extended with the same or another monomerand also allows any reactions of these groups to becarried out (e.g., functionalization, branching,and crosslinking).

The number of nines needed to characterize theextent of livingness of a growing polymer on thewhole (0.99, 0.999, 0.9999, or even more) dependson the aim of the synthesis or the pedantry of anyparticular chemist. From this pragmatic point ofview, the absolute livingness without any limita-tions in time becomes an unattainable ideal, sonobody gives even a finger for its reality. This isquite reasonably noted by Darling et al.1

In regard to the experimental foundations ofliving polymerization, the first appeared verylong ago. It is worth remembering the work ofPerry and Hibbert2 on the anionic polymerizationof ethylene oxide, where the stepwise increasingof the molecular weight at the successive portion-ing of the monomer was clearly shown for the firsttime. These data subsequently inspired Flory togive his classical treatment of the statistics ofterminationless polymerization.3

The explosion of the 1950s and 1960s, mainlyin the study of the anionic polymerization of sty-rene, completed the formulation of the term livingpolymerization and the understanding of all itspreparative advantages in designing a variety ofpolymeric systems. This mighty wave overflowedfirst cationic and, quite recently, radical polymer-ization, with a vast literature on this problemresulting.

At the same time, the limitations of the preciseuse of this term and the synthetic approach itselfare becoming more and more numerous, givingrise to the effort of improving the terminology.The most useful alternative terms are living andcontrolled, as is quite clear from Darling et al.1

Which of them is the broader and more reason-able? Which reflects to a greater extent the fun-damentals of the process and which relates tosome particulars? In this respect, my vote is def-initely for the first of these terms, and I am goingto argue for this choice.

My own experience in the field of living poly-merization is restricted entirely to the anionicpolymerization of ethylene oxide, which is obvi-

ously one of the most pure examples of livingpolymerization. In fact, poly(ethylene oxide) ofpractically any predetermined molecular weightcan be easily prepared with the appropriatechoice of initiator and solvent.4,5 Despite somereview publications,6 there are not even signs oftermination or transfer reactions in the ethyleneoxide anionic polymerization in bulk or tetrahy-drofuran. Thus, if some kinetic aspects are nottaken into account, it is an ideal living polymer-ization with all the consequences. Naturally, itseems to be completely controlled as well.

Nevertheless, if anybody intends to obtainpoly(ethylene oxide) of a molecular weight as highas 107, he obviously will meet serious problems orcompletely fail, because in this case the purity ofthe system must be assured on the 1024 M level.The same problem occurs if quantitative function-alization of the polymer is needed. The analysisand experiments show that it cannot be donewithout the expected molecular weight of thepolymer being taken into account, and for certaincases, the value of 105 seems to be an essentiallimit.7 However, in both these examples, purelyliving polymerization simply gets out of control.So, the livingness by itself as a determining fea-ture of this polymerization gives no guaranteethat it can be controlled in all conditions. Is thisthe reason to consider this reaction as livingwithin some range of conditions and nonlivingoutside this range because of purely technicalproblems? The answer is obviously no.

The circumstances responsible for the con-trolled state of any living polymerization can bearbitrarily subdivided into internal and externalones. The first group of factors is related exclu-sively to the nature of the active species, mono-mer, and polymer units. The existence of anyreactions between these three main componentsof the polymerizing system (irreversible termina-tion, chain transfer, and depropagation), exceptfor chain growth, usually excludes the possibilityof living polymerization. Some of these unfavor-able internal factors can be more or less avoidedby the appropriate choice of temperature andtimescale. The majority of us know that livingpolymerization is quite real even in the case of thethermodynamic reversibility of the process.Therefore, we have the elaborated theory andpractice of polymerization control, the character-istic times of the said reactions being decisive.

As for so-called external factors (solvents, ad-ditives, purity of the whole system, and all ma-nipulations with living polymer), the tactics to


keep them under control are quite different butare often quite successful.

The problem of applying the habitual terminol-ogy of living polymers to radical polymerizationappears more complicated and gives rise to agreat variety of terms.1 Nevertheless, the situa-tion here is quite similar to many other livingpolymerizations, although the inexorable ten-dency of living radicals to bimolecular termina-tion makes it necessary to control more stronglythe lifetime of the radicals in a living state. Thereis also the seriously elaborated methodology toavoid this internal factor of instability. In manycases, this tactic is quite successful.

It seems clear that there are enough reasons toconsider the term living as more fundamental,whereas the term controlled is more particular inits meaning. Moreover, the term controlled lookslike a question, expecting an explanation, of howthis process is controlled. Therefore, I believe thefirst of these terms must retain its customaryplace in the nomenclature of polymer science, re-gardless of the term controlled.

TWO ADDITIONAL NOTES INCONCLUSION

The term living polymerization should be kept forone more reason. This helpful and lasting termcarries a memory of the romantic period, havingfreed polymer chemists from many earlier restric-tions of their fantasy. In any event, a host ofdistinguishing achievements of polymer chemis-try is the result of this freedom.

Finally, we can draw a purely illustrative anal-ogy to human life. Really, why should we deprivea man of the attribute living, even being aware ofhis mortality? There are also a lot of internal andexternal factors of livingness that result in a life-time distribution that is analogous to chain-length distribution. However, within a mean life-time, every man has a right to plan all his affairs,and his relatives and colleagues may expect ofhim all the activity inherent in the notion of liv-ing. Is this not true?



2. Hibbert, H.; Perry, S. J Am Chem Soc 1940, 62,2599.

3. Flory, P. J. J Am Chem Soc 1940, 62, 1561.4. Kazanskii, K. S.; Solovyanov, A. A.; Entelis, S. G.

Eur Polym J 1971, 7, 1421.5. Dubrovskii, S. A.; Kumpanenko, I. V.; Goldberg,

V. M.; Kazanskii, K. S. Vysokomol Soedin A 1975,17, 2733.

6. Powell, G. M.; Bailey, F. E. In Encyclopedia ofChemical Technology; Standen, A., Ed.; Inter-science: New York, 1960; 2nd Suppl. Vol., p 597.

7. Kazanskii, K. S.; Ptitsyna, N. V. Makromol Chem1989, 190, 255.

Comments byProfessor Joseph P. Kennedy

Maurice Morton Institute of PolymerScience, University of Akron, 302 EastBuchtel Avenue, Akron, Ohio 44325-3909

I do not agree with the proposed relaxation “ofthe term living polymerization [to] be defined asthose processes that yield living polymers.” I pre-fer to use the classical definition of living poly-merization, that is, polymerizations in whichRtr,M 5 Rt 5 0. The requirement that Ri begreater than Rp is of secondary significance; if Ri

is greater than Rp, molecular weight distributioncontrol is possible (and a narrow molecularweight distribution is useful for the diagnosis oflivingness), but this inequality is not part of therigorous definition. After all, polymerizationswith a relatively low Ri can still be living.

Comments by Dr. Young H. KimDuPont Central Research &Development, Experimental StationE328/261, Wilmington, Delaware 19880-0328

What criteria need to be met for a polymeriza-tion to be worthy of being classified as living po-lymerization and what we can conveniently call atype of polymerization reaction that may renderliving polymerization are two different issues.

Living Polymerization: Like Szwarc’s origi-nal definition, the only criteria for living polymer-ization should be that the polymer chain ends arestable enough at the complete consumption ornear complete consumption of monomers so that anew supply of monomers can continue the poly-merization on all the resting ends of the polymer


chains. To make all the polymer chains to beinvolved in the growth of the chain, there shouldbe no initiator left at the point of incrementalmonomer addition. These requirements are com-monly confirmed by the first reaction order depen-dence of monomers for living polymerization. Ifthe reaction rate of the initiation reaction is fasterthan that of the polymer propagation, the result-ing polymer will have a narrow molecular weightdistribution, which is another common require-ment for living polymerization.

Now, let us think about why living polymeriza-tion has attracted so much fascination. There aretwo important advantages that living polymeriza-tion can offer better than any other polymeriza-tion. One is rendering polymers with a narrowmolecular weight distribution, and the other iscomplete chain-end functionalization or forma-tion of block copolymers. In comparing these twovirtues that living polymerization renders, thelatter attribute is far more important than thefirst attribute because the advantage of narrowmolecular weight distribution in the materialproperties, process, or both has not been unam-biguously demonstrated yet. Nevertheless, in re-ality the quantity of polymers required in com-merce needing these attributes is very small. Allthe research and hype on living polymerizationseem to be out of proportion compared to otherpolymeric materials of far more importance. Inas-much as the objective of the polymerization reac-tion is to obtain complete end-group functional-ization, many polymerization reactions that maynot fit the rigorous definition of living polymeriza-tion can provide excellent means to achieve thisgoal. TEMPO-mediated vinyl free-radical poly-merization and some Lewis-base-mediated vinylether cationic polymerizations are examples.

The definition of living polymerization does notneed to be refined from what Szwarc’s defined,and we do not need to use new terminology todescribe the degree of the perfection of livingness.An ideal living polymer should have a moietyfrom the initiator, and the other end has to havereactive ends for the polymerization at the time ofthe complete consumption of monomer. Therecould be many analytical methods to confirm thisrequirement, but an analytical method should notbe used to define the livingness of the polymer-ization reaction.

Nomenclature: Whether a particular poly-merization is living or not, the naming of thereaction is a quite different issue. Most people inthe field of polymer science would recognize that

group transfer polymerization (GTP) is a livingpolymerization, even though it is not spelled outas living GTP. If naming a particular reactionhelps communication between scientists, itshould be employed. It has been common practiceto use such a name for many years. Hundreds ofso-called named reactions have been a marvel oforganic chemistry for centuries. Certain polymerreactions involving a common technique or mech-anism can be named for convenience for scientificcommunication. It is up to a scientist to learn andunderstand what the scope and limitation of anamed polymerization are, if it is worthwhile.Leave it to the free-market economy; it works.

Comments byProfessor Bert Klumperman

Laboratory for Polymer Chemistry,Eindhoven University, P.O. Box 513,5600 MB Eindhoven, The Netherlands

It is beyond doubt that the term living poly-merization is loaded with controversy. In my opin-ion, it is a good idea to strive toward uniformterminology. The situation with living radical po-lymerization is probably the most complex. Thetechnique is derived from free-radical polymeriza-tion, where attempts are undertaken to suppresschain-breaking reactions. This background couldcall for a name like living free-radical polymeriza-tion, but this would needlessly further complicatethe terminology. In some cases [e.g., in atomtransfer radical polymerization (ATRP)], it is un-clear whether free diffusion of the radicals takesplace. In trying to use names that are as detailedas possible with respect to the mechanism, therewill be no uniformity, and searching for examplesin Chemical Abstracts will be difficult.

The suggestion of Darling et al. to simplify theterminology is the only way to prevent an unlim-ited growth of names for comparable techniques.The use of living radical polymerization seems tobe the best approach. The definition of living po-lymerization would then be a chain polymeriza-tion in which chain-breaking reactions are mini-mized. The degree to which the living character ofthe polymerization is achieved can be reported toshow the level of success in performing a livingpolymerization.

To recognize the underlying mechanism, itwould be advantageous to mention the living rad-ical polymerization method in the keywords of a


publication [e.g., ATRP, reversible addition frag-mentation chain transfer (RAFT), and nitroxide-mediated]. This would greatly facilitate searchesin databases such as Chemical Abstracts.

Comments byProfessor Shiro Kobayashi

Department of Materials Chemistry,Graduate School of Engineering, KyotoUniversity, Kyoto, 606-8501, Japan

This article describes the authors’ proposal forthe definition of living polymerization, which hasrecently been used in various ways depending onthe results obtained by the many research groups.It is quite understandable that when researchersfind something new in chemistry, they often pro-pose a new terminology to stress their findings.This has resulted in confusion in the terminologytypically associated with living polymerization. Itis also recognized that these authors tried to rem-edy the confused usage concerning living poly-merization. They proposed the following as a uni-form terminology, that “the meaning of the termliving polymerization be relaxed and defined asthose processes that yield living polymers (ortheir dormant precursors). The term living poly-merization should be used regardless of the yieldof the process.”

According to the definition of the IUPAC No-menclature Committee, living polymerization is“a chain polymerization from which chain trans-fer and chain termination is absent.”1 Note thatin some living polymerizations, reversible deacti-vation of chain carriers may occur. However, inwhat case should we use living polymerization forpractical publication in academic articles?

Now, I partially agree with this proposal forthe following reasons. I think that the authors’terminology is too relaxed. If the proposed termi-nology is applied, almost all processes can becalled living polymerization because almost allpolymerization reactions yield living polymers tosome extent, even in a very low yield. In manycases, polydispersity (Mw/Mn) should becomewider. This does not reflect actually expected liv-ing polymerization.

The authors referred, in regard to the yield ofthe reaction, to the Diels–Alder reaction as anamed reaction. I do not think this is an appro-priate analogy. In the case of name reactions,naming the reaction is quite clear. Typically, Diel-

s–Alder reactions, Friedel–Crafts reactions, andso forth are specifically confirmed by their reac-tion pattern, mechanism, product structures, orsome combination of these. Other examples areWittig reactions and Mannich reactions, whichare defined simply by the starting reagents andproduct structures.

In contrast, living polymerization is a generalconcept to explain a phenomenon of polymerizationreactions, involving living propagating species, thatbrings about molecular weight control, narrow poly-dispersity, end-group control, and the ability tochain-extend. This concept concerns the phenome-non observed in various polymerization reactions,including not only different reaction mechanisms ofanionic, cationic, radical, transition-metal-cata-lyzed, and enzyme-catalyzed processes but also dif-ferent reaction types of vinyl, olefin, acetylene,diene, and ring-opening polymerizations. In thesecomplex circumstances, it is very difficult to defineliving polymerization as a general uniform termi-nology. This situation is like “there is no generalrule without some exceptions.”

I think we should return to the original conceptof Szwarc. To practically characterize the gener-ally accepted living polymerization for morequantitative consideration, we need two factors,the yield of living polymers and the polydisper-sity, as minimum requirements. In the perfectcase, initiation is much faster than propagation,without termination and chain transfer. The yieldof living polymers is 100%. Then, polydispersitymust be 1.0. However, nothing is actually perfect.The confusion in terminology stems from the factthat researchers have different rigidities for us-ing a polydispersity value. Sometimes, living po-lymerization is claimed even when the value isgreater than 1.5, where livingness is too broadlyunderstood. Therefore, we require criteria. Iwould like to propose that the process be calledliving polymerization when the yield of livingpolymers, including their dormant species, isgreater than 95% and when the polydispersity isless than 1.2. These values of 95% and 1.2 needfurther arguments, but I think these may be prac-tically meaningful and reasonable values. A pro-cess showing the respective values 95% . yield$ 90% and 1.2 , Mw/Mn # 1.5 is then to be calledliving-like polymerization. Again, the values of90% and 1.5 need to be argued.


1. IUPAC. Pure Appl Chem 1996, 68, 2287–2311.


Comments byProfessor Mikhail Lachinov

Department of Chemistry, Moscow MV,Lomonosov State University, VorobevyGory, Moscow 119899, Russia

I agree with the authors of this article on theexisting problem, concerning the terminology of acommon name for several radical polymerizationreactions, that recently has attracted wide atten-tion. This problem exists not only from an author’spoint of view but also from the point of view ofpolymer chemistry teaching. After many authorsinvestigated several radical polymerization reac-tions, each with its own special name, includinginiferter polymerization, nitroxide-mediated poly-merization, atom transfer radical polymerization(ATRP), and reversible addition fragmentationchain transfer (RAFT), we no longer had questionsabout the names of the reactions (they all are quitecorrect). The question is what common name to givethem if they have common properties. In fact, theydo have properties that permit us to control thekinetic parameters of polymerization to a higherextent than in the case of classic radical polymer-ization and to obtain well-defined products. There-fore, we can say that the IUPAC working partyrecommendation is correct enough.

The second question is whether we can name allthese radical reactions as living polymerizations?All of these reactions have the same features, suchas the growth of molecular weight with conversionand a low polydispersity of produced polymers, thatare common for the case of living polymerization.However, in most cases these reactions have moreor less dead chains that result from inevitable bi-molecular termination, chain transfer to monomer,or both. Therefore, I suppose that not all of thesereactions satisfy Szwarc’s definition of living chains.Nevertheless, we should find ways to have real liv-ing radical polymerization.

Comments byProfessor Toshio Masuda

Department of Polymer Chemistry,Graduate School of Engineering, KyotoUniversity, Kyoto 606-8501, Japan

I read your letter and the article entitled “Liv-ing Polymerization: Rationale for Uniform Termi-nology” by Darling et al. I basically agree with theidea of the authors.

As Szwarc aptly stated, living polymers arepolymers that are able to grow whenever addi-tional monomer is supplied, and living polymer-izations are polymerizations that involve livingpolymers. As there are no perfect stereospecificpolymerizations, perfect living polymerizations,where no dead polymer is formed at all, do notexist either. Hence, if the aforementioned defini-tion is applicable and a polymerization exhibitsall or most of the characteristics of livingnesssuch as low polydispersity, end functionalizationby terminating agents, and formation of blockcopolymers, then the polymerization can be re-garded as a living polymerization. If such charac-teristics are not very evident, the polymerizationshould be called a polymerization involving long-lived species or a quasi-living polymerization. Ialso think that the name controlled polymeriza-tion does not make sense in itself and is not di-rectly related to living polymerization.

Here I touch on our studies. We have been work-ing on the living polymerization of substituted acet-ylenes by using MoOCl4-based multicomponent cat-alysts (for a recent review, see ref. 1). For instance,a catalyst composed of MoOCl4, n-Bu4Sn, and eth-anol polymerizes a variety of substituted acetylenes(e.g., 1-chloro-1-octyne, o-CF3-phenylacetylene, andtert-butylacetylene) in a living fashion to providepolymers with narrow molecular weight distribu-tions (polydispersity ratio 5 1.02–1.10). Further-more, block copolymers can be easily obtained fromthese monomers. These findings demonstrate thatthese polymerizations are living polymerizations.One problem in our systems is that the initiationreaction is not quantitative. However, this does notkeep it from being a living polymerization and israther convenient for obtaining high molecular weightliving polymers from a practical point of view.


1. Masuda, T.; Hayano, S.; Iwawaki, E.; Nomura, R. JMol Catal 1998, 133, 213–220.

Comments byProfessor Lon J. Mathias

Department of Polymer Science,University of Southern Mississippi, P.O.Box 10076, Hattiesburg, Mississippi39406-0076

Words are fascinating creatures: they beguileand delight, sadden and disgust, define and ob-


scure, reveal and hide. Words are the mechanismof conscious thought and the basis for action andcreative endeavor. Words can define and explain,generalize and differentiate. Sometimes, thesame word means very different things to differ-ent people.

To discuss Darling et al.’s article correctly, wemust understand the purpose and function ofwords: they help us to think, act, and communi-cate. As scientists, we seek objective facts, con-cepts, relationships, and definitions. We wouldlike our words to be precise and controlled, sayingfor us exactly what we mean and no more. How-ever, language is a dynamic, changing part ofhuman experience, as much in science as in anyother human activity. Because the words of sci-ence are of human origin and subject to humanmanipulation, they undergo continuous modifica-tion through use. The word living itself is anexample of such change: it once applied only tobiological systems, to plants and animals, butnow has been adapted to chemical reactions pos-sessing certain characteristics of biological life.

This article, on the uses and misuses of theterm living in describing the behavior of polymer-izations, raises issues regarding generality andspecificity. Does the term refer to the ideal of anindefinite lifetime for a propagating chain end, asmentioned by the authors of the article? Thiswould be a characteristic that did not pertain toits original use concerning plants and animals, forwhich termination by death was inherent in theword’s meaning. Should it now include reversibletermination, controlled addition, assisted inser-tion, and systems that look as if they are livingbut are not according to some specific detail of aspecific definition? Does it matter? If it does, howand why does it matter?

Words, especially scientific words, should helpus to do our work better. So what does the wordliving communicate to each of us? How does ithelp us think more clearly or creatively and doour work more productively? Well, depending onthe context and background, it can mean differentthings. More to the point, its meaning can changeand, in fact, must change as our knowledge andunderstanding of what we use the word forchanges. A word such as living represents a partof chemical reality in some objective way. How-ever, our understanding of that reality evolvesover time, both in what and how we see it, and inits creative extension: we learn more because wecan see more, and think more clearly, about thatobjective reality. As we combine this knowledge

with other knowledge, we extend and develop anddiscover, and soon we have a new reality to rep-resent with the words we have. The words mustchange to reflect this new reality, to generalizeour understanding to other systems, and to allowbetter communication of what we are seeing andthinking to others.

Let us deal with specifics. If we standardize theuse of living now, will that change what is alreadywritten? Should we change the historical legacyin light of our current knowledge? Of course not,for this would hide the human aspect of science:we learn and grow. Should we mandate a moreprecise (or more general) use of the term now andin the future? Should we ban or limit the use ofthe other terms discussed in the article so that weall “speak the same language?” These beg the realquestions that need to be asked: what are the usesthese terms enjoy? Why are the multiple termswith variations of specific details used? The ques-tion is not of how to restrict and limit, or expandand generalize; the question is what terms anddefinitions do we need to effectively think and actand communicate.

Words change how we think in both subtle andbroad ways. Take the term phase transfer catalysis.Before this term was coined, interfacial chemistrywas done on a routine basis: the Schotten–Bau-menn synthesis of amides from amines and acidchlorides was known to be catalyzed by tertiaryamines. However, it was not until someone actuallycoined the term, gave a broad definition to the con-cept of a catalyst for the phase transport process,that we were able to communicate the idea freelyand apply the method more generally. That is, wehad to have a new way of thinking about the processto use it better. So is this the problem with whichthe article deals, that we need a new term to facil-itate our thinking about the general concept? In aword, no. The concept is there, it is broadly used,but the problem is that of the evolution of the defi-nition and the reality to which it applies. As thereality increased in breadth and complexity, adjec-tives were added and new terms were developed toincrease the communication of the unique specificsof the individual system being described. Is thisbad? In fact, it is both good and necessary. Withinthe broad concept and the general words used todescribe it are specifics that are unique, that encom-pass a smaller range of characteristics. There isrichness in these variations, an ability to help usthink and act and talk about the unique within thegeneral. It is not an either/or situation but a both.We need the specific and the general definitions.


The latter helps us to extrapolate beyond what weknow now, the former helps us to understand andapply the unique aspects to real situations. Al-though I cherish being human and honor my familyname for what it gives me of my ancestry, my givennames make me unique as a man and an individual.

So should we broaden or limit the definition ofliving? Yes and no. Both will inevitably happen asthe needs of people who use the terms evolve. Indiscussing the definitions and the details, we in-crease our understanding of what the terms meanand how they can be used as words in our thoughtprocesses to extend knowledge and create or dis-cover new systems and applications. “The onlyconstant is change,” and this applies especially towords and how we use them. Sure, we can defineand use the word living more carefully, but thatdefinition will in turn have to be modified, lim-ited, or expanded, as our knowledge increases andour understanding expands. By all means, dis-cuss and experiment with the word and its defi-nitions, but it is the usage of the word that willcontrol its change. Embrace that change, enjoy it,discuss it as needed; just do not argue about it.

Comments byProfessor K. Matyjaszewski

Department of Chemistry, CarnegieMellon University, 4400 Fifth Avenue,Pittsburgh, Pennsylvania 15213-3890

When a field becomes mature, such as con-trolled (or living) radical polymerization (CLRP),the appropriate definitions should be discussed bythe IUPAC Nomenclature Committee rather thanin a polymer journal.

Living polymerization is more difficult to de-fine than living polymers, as originally proposedby Szwarc, because it is easier to determine theboundaries of a material than those of a processthat has a kinetic meaning.1

The IUPAC Nomenclature Committee pro-poses to define living polymerization as a chain-growth process without chain-breaking reactions.My understanding is that this refers to the ab-sence of irreversible chain-breaking reactions;thus, living polymerization is compatible with re-versible deactivation (also known as reversibletermination) and with reversible transfer (such asdegenerative transfer).

The authors of the submitted article do notprovide any definition or criteria that a radical

process should fulfill to become living. It is notenough to say that atom transfer radical polymer-ization (ATRP), nitroxide-mediated polymeriza-tion (NMP), and reversible addition fragmenta-tion chain transfer (RAFT) are living radical po-lymerizations. As an example, the NMP of methylmethacrylate is a typical nonliving process, butthe NMP of styrene may, under some conditions,fulfill criteria of CLRP.

We disagree with the authors’ statement thatliving polymerization is mechanistically differentfrom conventional radical polymerization and donot understand the analogy to Diels–Alder chem-istry. The step of the conversion of monomer intopolymeric unit is mechanistically exactly thesame in both CLRP and conventional radical po-lymerizations, as confirmed by the same stereo-,regio-, and chemoselectivities. Several years ago,we stressed the identical mechanism of conven-tional and living carbocationic polymerization,contrary to some other proposals at that time.2

We think that an identical situation occurs inCLRP. Because living polymerization does not de-fine a mechanism of the polymerization in theorganic chemist’s sense but only the ratio of ratesand rate constants of the involved reactions, it isvery important to quantify the rate constants ofall chain-breaking reactions.

It has to be recognized that if the rate of con-ventional polymerization and CLRP is the same,the concentration of chains irreversibly termi-nated is also approximately the same because therate of propagation is first order and the rate oftermination is second order in respect to radicalconcentrations. Thus, from the point of view of therates and rate constants of termination, there iscurrently no improvement in chemoselectivitiesfor polymerizations proceeding with the samerates. Only when CLRP is slower than conven-tional radical polymerization is the terminationrate reduced. However, although the absoluteconcentrations of dead chains are similar in con-ventional and CLRP systems occurring with thesame rate, nevertheless, in conventional systemspractically all chains are dead, whereas in CLRPsystems their proportion is small in comparison todominating dormant chains.

However, it is possible to imagine that trueliving radical polymerizations can be developed inthe future. In these systems, the rate and rateconstant of termination could become signifi-cantly lower than in conventional processes. Thiscan happen when growing radicals are complexedby a specific reagent, as previously reported for


phosphoric acid,3 or propagation may occur in aconfined space (e.g., clathrates or zeolites).4 Inboth cases, the rate constants of termination be-tween two growing radicals could be significantlyreduced because of steric or electronic effects.

To summarize, the extension of the definitionof living polymerization to currently developedradical systems is not appropriate. Also, the re-laxation of the term living polymerization maylead to a situation in which any chain-growthsystem will become defined as living, especiallywhen no definition and criteria are provided. Fi-nally, we would suggest that all researchers inthe CLRP field make continuous efforts in mea-suring the contribution of chain-breaking reac-tions and providing data on end functionalitiesthat, in addition to the ratios of expected andmeasured molecular weights and polydispersi-ties, are very important, especially for block copo-lymerization and in the synthesis of well-definedpolymers.


1. It has to be remembered that polymers with de-grees of polymerization predetermined by the ratioof the concentrations of the reacted monomer to theintroduced initiator and with low polydispersitiescan be obtained in living polymerizations onlywhen the rate of initiation becomes comparable orlarger than that of propagation.

2. Matyjaszewski, K.; Sigwalt, P. Polym Int 1994,35, 1.

3. Kabanov, V. A. J Polym Sci Polym Symp 1975, 50,71.

4. Farina, M.; Silvestro, G. D. Chem Commun 1976,842.

Comments byProfessor E. W. (Bert) Meijer

Laboratory of Macromolecular andOrganic Chemistry, EindhovenUniversity of Technology, P.O. Box 513,5600 MB Eindhoven, The Netherlands

LIVING SCIENCES DO NOT USE AUNIFORM TERMINOLOGY FOR EVENTS

It was with pleasure that I accepted the invitationby the editors to write a short personal commenton the contribution of a group of scientists con-

cerning living polymerization and its terminol-ogy. I am not an expert in that particular field,nor I am very fond of my grammar, especially in aforeign language, but I am intrigued by the pro-cess of rationalization for uniform terminology. Iwould like to express my thoughts here, by whichit should become clear that using different termi-nologies is one of the most prominent exposures ofliving science and we are very well equipped tolive with that. One can even argue that as soon aswe all agree on a generally accepted nomencla-ture for events such as reaction types, mecha-nisms, phenomena, effects, and fields of chemis-try, most aspects of that topic are known andhardly of interest for further investigations. Ob-viously, the opposite is true for topics such theIUPAC nomenclature for compounds or the val-ues for fundamental constants; without a uniformterminology, we are desperate.

TOTAL SYNTHESIS OF NATURALPRODUCTS

The total synthesis of natural products is a fieldin chemistry that probably everyone knows of andrelates to a uniform terminology: you synthesize amolecule that is present in nature from scratch.However, what is allowed as your starting com-pound? Something that is not natural? Or can onefinal reaction on another natural product alsoresult in a total synthesis? The debates on theterminology of total synthesis are nicely describedby the Johnson in his recently published book, AFifty-Year Love Affair with Organic Chemistry.1

Following the terminology of Cornforth and Rob-inson, we use the term formal total synthesis for asynthesis that depends on relays through inter-mediates that were supplied (for further steps) bythe degradation of natural products. Woodward,however, never used the word formal; he refusedto recognize that there was any difference be-tween a synthesis that was carried all the waythrough from simple chemicals and that relied onrelays, citing Johnson from his book. We probablyall agree with Johnson, that when a significantnumber of the steps of a synthesis are performedwith material derived from the natural product(by a process that is called partial synthesis), it isnot right to call your final product totally syn-thetic material. Obviously, the interpretation of “asignificant number of the steps” is very personal,but it does not influence the use of the term totalsynthesis. The moral of this example of total syn-


thesis stresses the importance of a personal inter-pretation of a terminology that has become uni-form with time.

STEREOCHEMISTRY

Stereochemistry is probably the area of chemistryin which debates on chemical terminology are themost hectic. Some people like to use terms such ashomochiral, chiral center, and optical yield,whereas others will never use these words andprefer enantiomerically pure, stereocenter, and en-antiomeric excess. You should use the R and Snomenclature in the case of molecules with ste-reocenters and P and M nomenclature for helices,whereas D and L configurations are useful forcarbohydrates. However, it is obvious to many ofus that it is very easy to make mistakes in thisarea, and we like to consult specific articles to usethe most adequate terms. Again, you will see thatthe nomenclature for molecules is uniform,whereas the nomenclature of events or phenom-ena is under debate, with different schools sup-porting different terminologies.

I personally encountered one of those prob-lems at the famous Burgenstock Conference onStereochemistry in 1996.2 In my lecture, I usedthe term cryptochirality for a series of chiraldendritic macromolecules that did not show anyoptical activity, despite the fact that the mole-cules were enantiomerically pure. It was Dunitzof the ETH Zurich who, in the discussion, cor-rected me: “Young fellow, I like to remind youthat an object is either chiral or a geometricalproperty.” I used that word according to thedefinitions of Mislow,3 another eminent stereo-chemist, after an extremely fruitful correspon-dence on the question of whether a certain den-drimer is more chiral than another. It was onlyafter the meeting that I found out that cryp-tochirality is only one of the many examples of adifference in preferred terminology between theZurich and Princeton schools of stereochemis-try. Because the chirality in our molecules isreally hidden, I liked and still like the termcryptochirality, which probably is wrong, but itnicely makes use of the wealth of a language toexpress something that is otherwise very diffi-cult to describe.

LIVING POLYMERIZATION

The question of whether a living polymerizationis living, quasi-living, or pseudo-living and the

fact that authors like to call it living or controlledshows that many aspects of the different radicalpolymerizations still need to be worked out, orthat some investigators expect to make signifi-cant progress on the livingness of this type ofpolymerization. I would like to give everyone thefreedom to add those words to their polymeriza-tions, to use what they think expresses their ideasmost accurately. At this time, counting the num-ber of articles with the different names can becompared to a popularity poll before a champion-ship. However, for knowing the name of thechampion, we have to wait until all the games areplayed. Therefore, I am interested in seeing theoutcome in the debates on the terminology forliving polymerization, especially because this pleafor uniformity is enriched with the discriminationbetween living and ideal living in ref. 1 of Darlinget al.


1. Johnson, W. C. A Fifty-Year Love Affair with Or-ganic Chemistry; American Chemical Society:Washington, DC, 1998; p 54.

2. Hunziker, J. Chimia 1996, 50, 282.3. Peerlings, H. W. I.; Strijik, M. P.; Meijer, E. W.

Chirality 1998, 10, 46.4. Mislow, K.; Bickart, P. Isr J Chem 1977, 15, 1.

Comments byProfessor Martin Moeller

Organic Chemistry Division III,University of Ulm, Albert-Einstein Allee11, D-89081 Ulm, Germany

Terminology questions are certainly an impor-tant issue that has to be discussed broadly and indetail. In most cases, the result will be some com-promise, and I think the discussion should be veryobjective and dispassionate.

Personally, I agree with the authors. The ma-jority of the people in this field use the term. Theyunderstand that it means sufficient control ofchain breakage to control molecular weight andend-group functionality and to prepare block co-polymers. To me, living implies mortality, and Ithink the major question is addressed very wellby the statement of Szwarc, provided we do nottalk about oligomers only.


Comments byProfessor Klaus Mullen andDr. Markus Klapper

Max-Planck Institute for PolymerScience, Ackermannweg 10, D-55128Mainz, Germany

We fully concur with the authors that, in re-spect to a common language in research, anagreement for definitions and naming should bereached in this area. However, these definitionshave to be clear and unambiguous.

Recently, Matyaszewski and Muller1 publisheda similar article, also cited by the authors, aboutthe naming of controlled or living polymeriza-tions. We believe this article clearly mentions allthe necessary criteria concerning the designationof a controlled or living process. The kinetic as-pects are especially well discussed and explained.Thus, one should clearly distinguish between liv-ing or controlled radical polymerization; clear def-initions are already given.

A further relaxation of the word living is un-necessary and even deleterious. Language givesus the chance to express things in an exact way.Clear definitions are especially desirable and nec-essary in science. Living suggests the perfectionof the reaction. When side reactions occur, such aschain-breaking reactions, one should use theword controlled. Why not express a different be-havior with a different word? From the kineticpoint of view, the controlled radical polymeriza-tion corresponds to a free-radical polymerizationat low free-radical concentrations, but nobodywould name the normal free-radical polymeriza-tion living.

In conclusion, a clear definition is desirable,and in our opinion one should focus on livingpolymerization and controlled (radical) polymer-ization as defined by Matyjaszewski and A.H. E.Muller.1 Expressions such as quasi, pseudo, andcontrolled living should no longer be used, as theyare imprecise and ambiguous. Controlled is thebest descriptive word because of the principal ef-fect, that the polymerization mechanism is notreally affected even when the polymerization be-havior is controlled by concentrations and reac-tivity.


1. Matyjaszewski, K.; Muller, A. H. E. Polymer Prepr(Am Chem Soc Div Polym Chem) 1997, 38, 6.

Comments byProfessor R. J. M. Nolte

Department of Organic Chemistry,NSR Center, University of Nijmegen,Toernooiveld 1, 6525 ED Nijmegen,The Netherlands

The naming problem, raised by Darling et al.,of when a polymerization reaction should becalled living, is reminiscent of similar nomencla-ture problems in organic chemistry. For manyyears, synthetic organic chemists have been de-scribing chemical transformations in complex re-action sequences without much proof of mecha-nisms. Natural product synthesis has not reallysuffered from this lack of mechanistic informa-tion, and the same can be said for materials sci-ence, although this field is much younger and stillin an emerging state. Synthetic results are oftenpublished because authors would like to informthe scientific community that they have preparednew compounds or have improved existing syn-thetic methodologies. In many cases, they are notconcerned with the precise reaction mechanismsby which the products have been synthesized,unless they have encountered serious problemsthat have hampered further progress. Becausethe situation in the field of polymer synthesis isnot much different from that in organic synthesis[scribitur ad narrandum non ad probandum (onewrites to narrate, not to prove)], we should notworry too much about whether a radical polymer-ization reaction is strictly living or not. The prod-ucts and the procedures by which they are pre-pared, as efficiently as possible, are far more im-portant. I am in favor of the terminology proposedby the authors. We should take living as easily aspossible; life is complicated enough.

Comments byProfessor Oskar Nuyken

Lehrstuhl fur Makromolekulare Stoffe,Technische Universitat MunchenLichtenbergstrasse 4D-85747 Garching, Germany

Although I fully agree that there are too manymisleading terms in the literature concerning liv-ingness, I do not completely agree with the au-thors view in which they argue with Diels–Alderreactions. Nobody would call a polymerizationcontrolled if he had found 5% polymer only !!


Personally, I share M. Swarc’s view. If a poly-mer is able to grow when more monomer is added,its molar mass is controlled by [M] : [I], it has anarrow molar mass distribution, its molar massincreases with conversion, and we find head andend group control, I would call it living. It wouldnot bother me whether the active species is stableover the whole reaction or whether we have re-versible termination (indeed, we included the re-versible termination into the inifer mechanismalready in 1982;1 shortly later2,3 we found theexperimental evidence for that ). I am pleased tosee this reversible termination now included inalmost every mechanism.


1. Nuyken, O.; Pask, S. D.; Vischer, A. MakromolChem 1983, 184, 553.

2. Pask, S. D.; Nuyken, O.; Vicher, A.; Walter, M. InCationic Polymerization and Related Processes;Goethals, E. J., Ed.; Academic: London, 1984; p. 25.

3. Nuyken, O.; Pask, S. D.; Vischer, A.; Walter, M. InCationic Polymerization and Related Processes;Goethals, E. J., Ed.; Academic: London, 1984; p. 35.

Comments byProfessor Yoshio Okamoto

Department of Applied Chemistry,Graduate School of Engineering, NagoyaUniversity, Furo-Cho, Chikusa-Ku,Nagoya 464-8603, Japan

It is a good time to define the meaning of theterm living polymerization in a form that we canaccept and properly use in the field of polymerscience without confusion. Generally, nomencla-ture (naming) for a reaction (polymer) can begiven for an ideal example. Therefore, living po-lymerization may be defined as a polymerizationthat yields an ideal living polymer as described inref. 4 in the article by Darling et al. However, asthe authors mentioned, actual reactions far fromthe ideal case are often still called Diels–Alderreactions for convenience. This situation appearsin most naming. For example, although isotacticpolymer is defined for an ideal (100%) isotacticpolymer, we often use the term isotactic polymerfor a polymer with a low isotacticity. Analogously,the term living polymerization may be conve-niently used for a polymerization with a lowerdegree of livingness. However, the degree of liv-

ingness or its reasoning should be explained in aproper manner, for instance, by polydispersity.

Comments byProfessor Takayuki Otsuand Professor Akikazu Matsumoto

Department of Applied Chemistry,Faculty of Engineering, Osaka CityUniversity, Sugimoto, Sumiyoshi-ku,Osaka 558-8585, Japan

The proposal for the use of uniform terminologyfor living polymerization is important, but its exe-cution is not easy. In a nomenclature note in 1997,1

it was proposed that several polymerization sys-tems, including radical polymerization, be calledcontrolled or controlled/“living”, but not living po-lymerization. However, the term controlled hasbeen used everywhere in the field of polymer chem-istry, and the use of a quotation mark is nothingmore than a compromise. The other terms based onpolymerization techniques, such as atom transferradical polymerization and reversible addition frag-mentation chain transfer, can represent the featureof each polymerization, but they are not all-inclu-sive names. Radical polymerization should be dis-tinguished from any other living polymerizationsthat proceed via an ionic or coordination mecha-nism, even if the radical polymerization is fully con-trolled without any detectable chain-breaking reac-tions because of the features of the polymerizationthat are different from the other living polymeriza-tions. That is, the propagating species is neutraland forms dormant (covalent) species in competi-tion with the bimolecular termination. In 1982,2 oneof the authors proposed the concept of iniferter forthe design of polymer chain-end structures, and itsimultaneously was applied to a new model for liv-ing radical polymerization. Fortunately, many peo-ple favor the term living radical polymerization inmany scenes. Therefore, we think that the use ofliving radical polymerization, which includes all(and unavoidable) characteristics of radical poly-merization, is the best choice for escaping the con-fusion of the terminology at this time.


1. Matyjaszewski, K.; Muller, A. H. E. Am Chem SocDive Polym Chem Polym Preps 1997, 38(1), 6.

2. Otsu, T.; Yoshida, M.; Tazaki, T. Makromol Chem,Rapid Commun 1982, 3, 133.


Comments byProfessor Timothy E. Patten

Department of Chemistry, University ofCalifornia, Davis, One Shields Avenue,Davis, California 95616-5295

The authors ask whether we should broadenand apply the term living to nitroxyl-radical-me-diated polymerizations, atom transfer radical po-lymerization (ATRP), reversible addition frag-mentation chain transfer, and other such classesof radical polymerizations in lieu of controlled (orcontrolled/“living”). I think that the key questionunderlying the discussion of this matter is, “Can apolymerization be termed living if irreversibletermination is a fundamental component of themechanism of polymerization?”

A living polymerization in the strictest sense ofthe definition is an impossible ideal to achieve. Theintermediates used in such polymerizations are suf-ficiently reactive or unstable that, given enoughtime, irreversible termination or transfer will occurto some fraction of the active chain ends. Thus,although irreversible chain-breaking reactions arenot part of the polymerization mechanism, theiroccurrence is imminent. Nevertheless, experimen-tal conditions usually can be found, such that dur-ing the timescale of the polymerization, these chain-breaking reactions are negligible. Such is the casefor living anionic, cationic, and transition-metal co-ordination polymerizations. The situation with po-lymerizations proceeding via free-radical interme-diates that exhibit molecular weight control, how-ever, is quite different. In these polymerizations,irreversible termination is an integral part of themechanism of polymerization.

As discussed by Fischer,1 molecular weight con-trol in these radical polymerizations is derived fromthe persistent radical effect. In the initial stages ofthe polymerization, the concentrations of transient(propagating chains) and persistent radicals areformed at equal rates. Because the propagating rad-icals can undergo fast termination via coupling, dis-proportionation, or both, their concentration de-creases, and the concentration of the persistent rad-ical builds up. Eventually, the concentration ofpersistent radicals will be sufficiently large that therate at which the propagating radicals react withthe persistent radicals in a deactivation (or revers-ible termination) step is much faster than the rateat which the propagating radicals react with oneanother in an irreversible termination step. Thus,irreversible termination plays an important regula-

tory role during the initial stages of the polymeriza-tion. It is also important to note that a true steadystate is not reached in these polymerizations andthat the concentrations of propagating and persis-tent radicals are not constant throughout the poly-merization. The concentration of propagatingradicals will decrease with time, whereas the con-centration of persistent radicals will increasewith time. Thus, irreversible termination occursthroughout the course of the polymerization. Al-though the extent of termination is small for a typ-ical free-radical polymerization technique exhibit-ing molecular weight control, it can have measur-able impacts on the polymerizations. For example,in ATRP, in which multiple chains are grown froma single core,2 the small but finite probability thattwo chain ends can terminate via coupling is mag-nified by the fact that a single macromolecule iscomposed of many of these growing chains linked tothe same core. In these polymerizations, couplingproducts can comprise a significant fraction of thesample if the conversion of the polymerization issufficiently high. Thus, irreversible termination isan integral part of the mechanism of these polymer-izations and can have a measurable impact on theresulting macromolecule.

The term living polymerization is a constructcreated and applied to the subset of polymeriza-tion methods that meet the abstract criterion ofan absence of chain-breaking reactions via a set ofexperimentally verifiable criteria. Expanding thisterm to encompass experimental methods thatare living during the timescale of the polymeriza-tion but may exhibit the inevitable irreversibletermination transfer at longer times because theend groups are not indefinitely stable, as pro-posed by Szwarc,3 is a practical adjustment. Forpolymerizations that exhibit molecular weightcontrol but have irreversible termination as anintegral part of the mechanism of polymerization,another term really needs to be defined, such ascontrolled.4 An advantage to using this new termis that it encompasses polymerizations that aredemonstrably not living yet display molecularweight control under certain experimental condi-tions and can be useful for preparing block copol-ymers and other such macromolecules.

To conclude, is this discussion over living andcontrolled terminologies for polymerization meth-ods a matter of semantics, or does this distinctionserve a useful purpose? I think that it makes auseful distinction in terms of the polymerizationmechanism. Others may disagree. Either way,


this debate should continue until a workable so-lution is agreed on for the benefit of everyone.


1. Fischer, H. Macromolecules 1997, 30, 5666–5672.2. Angot, S.; Murthy, S.; Taton, D.; Gnanou, Y. Mac-

romolecules 1998, 31, 7218–7225.3. Szwarc, M. J Polym Sci Part A: Polym Chem 1998,

36, ix–xv.4. Matyjaszewski, K.; Muller, A. H. E. Polym Prepr

(Am Chem Soc Div Polym Chem) 1997, 38, 6–9.

Comments byProfessor Stan Penczek

Center of Molecular and MacromolecularStudies, Polish Academy of Sciences,Sienkiewicza 112, 90-363 Lodz, Poland

The terms living polymerization and controlledpolymerization are being defined by the Nomen-clature Commission of the Macromolecular Divi-sion of IUPAC. At the time when the documentKinetic and Thermodynamics of Polymerizationwas mostly completed, this commission waschaired by Stepto (Hess is now its Chairman);Marechal and I have been assigned to coordinatethe work of the commission on this document(Baron, Hatada, Hess, Jenkins, Jones, Kahovec,Kubisa, Wilks, and Vohlidal are involved). Theentire document (94 entries), before becoming anofficial IUPAC document, will be published as aproposition open for discussion. In this document,the two terms discussed by Darling et al. havebeen defined as follows: Living polymerization: Achain polymerization from which chain transferand chain termination are absent. Note that insome living polymerizations, reversible deactiva-tion of chain carriers may occur. Controlled poly-merization: A polymerization process allowing thepreparation of macromolecules with a predeter-mined Mn, polydispersity index, and end groups.Note the following: (1) Controlled polymerizationmay but does not have to be living. (2) Controlledpolymerization with a known transfer, termina-tion step, or both should not be called living (e.g.,a controlled radical polymerization involving ter-mination: a stable radical modified or atom trans-fer polymerization, a controlled cationic polymer-ization involving transfer, etc.).

This article discusses the terminology problem,namely, whether recently developed radical poly-

mers should be called living or controlled. Theauthors strongly advocate the first term. I do notthink this is correct. These novel radical polymer-izations involve two kinds of terminations, butonly one is reversible; the other is irreversible.Thus, the process is inherently nonliving: activespecies disappear during the polymerization pro-cess. The higher the target ^Mn& is, the higher theproportion of the irreversible processes is. There-fore, until someone comes along with a real dis-covery that will allow sufficiently increasing thekp/kt/ktr ratio (no transfer and termination couldbe detected), these polymerizations should just becalled controlled (if control is indeed achieved).

Thus, for example, “Controlled ATRP of …”,“Persistent Radical Controlled Polymerization of…”, and “Nitroxyl-Radical Controlled …” are cor-rect titles. If control is attempted but notachieved, the title should specify the methodused. In this way, we may have “Controlled ATRPof …” and “ATRP of …”

Comments byProfessor Nicolai A. Plate andProfessor Arkady D. Litmanovich

Russian Academy of Sciences, AVTOPCHIEV, Petrochemical SynthesisInstitute, Moscow 117912, Russia

First of all, we would like to note that theauthors of the article put forth the actual termi-nological problem and considered it comprehen-sively. Indeed, 4 decades ago, a sense of the termliving invoked by Szwarc1 was absolutely clear:an active anionic end of the growing chain re-mained alive any time provided the reaction me-dium was pure enough. As a consequence, it waspossible to carry out postpolymerization of newportions of the monomer or to prepare a blockcopolymer, both of a narrow molecular weightdistribution. Eventually, many types of polymer-ization processes were discovered for which nocanonical termination step was observed andwhich were used to design a variety of macromo-lecular structures. In particular, with free-radicalliving polymerization, it is possible to obtain notonly linear block polymers and random copoly-mers but also structures such as hyperbranchedpolymers and hybrid dendritic–linear polymers.2

This situation raised a multiplicity of terms, aspointed out in the article.


Problems of terminology are solved via agree-ment. We agree with the authors of the articlethat the term controlled is too broad. Also, weagree with the principle of naming reactions “onthe basis of their mechanism, not their yields.”

The essence of the mechanism of the processesunder consideration is the absence of irreversibletermination of the growing chains. For such pro-cesses, living appears the most relevant term.

Certainly, to describe any concrete reaction,one needs some additional definitions, say, livingring-opening isomerization polymerization.3 It isin the order of things.

So let the term living polymerization remainalive.


1. Szwarc, M. Nature 1956, 178, 1168.2. Malmstrom, E. E.; Hawker, C. Macromol Chem

Phys 1998, 199, 923.3. Miyamoto, M.; Watanabe, T.; Kimura, Y. Macromol

Chem Phys 1998, 199, 2237.

Comments byProfessor Roderic P. Quirk

Maurice Morton Institute of PolymerScience, University of Akron,Akron, Ohio 44325-3909

The enunciation of the concept of living poly-merization by Szwarc, Levy, and Milkovich1

(SLM) in 1956 has had a profound influence onpolymer science in general and on synthetic poly-mer science specifically. They simply stated that aliving polymer is produced from a chain-reactionpolymerization that does not have a termination(or transfer) step.2 The importance of living poly-merization is that it provides methodologies forthe synthesis of polymers with well-defined struc-tures and low degrees of compositional heteroge-neity.3,4

In their pioneering article, Szwarc et al.1 rec-ognized and described some of the most importantsynthetic consequences of a living polymerization;that is, chains grow until all of the monomer isconsumed (except when near the ceiling temper-ature), the subsequent addition of monomer in-creases the molecular weight (all chains continueto grow),5 and block copolymers can be formed bysequential monomer addition. In the interveningyears, the synthetic consequences of living poly-

merizations have been refined and expanded toinclude (1) the prediction and control of the num-ber-average molecular weight, (2) the control ofthe molecular weight distribution when the rateof initiation is competitive with or faster than therate of propagation,5 (3) the control of branchingarchitecture by linking reactions, and (4) the con-trol of functional end groups by controlled termi-nation with functionalizing agents.3

Thus, the definition of living polymerization asenunciated by Szwarc et al.1 is not an esoteric,unrealizable ideal. On the contrary, it is a funda-mental concept in polymer science that has spe-cific, useful, general synthetic and mechanisticconsequences. For a living polymerization, it ispossible to prepare polymers with predictable,well-defined structures and with low degrees ofcompositional heterogeneity. When terminationand transfer occur, these reactions lead to compo-sitional heterogeneity and a loss of the ability toprepare polymers with well-defined structures.

A further attribute of the SLM definition of aliving polymerization is that it can be applied,and synthetic consequences can be realized, forchain polymerizations regardless of mechanistictype. Thus, in addition to the classic living anionicpolymerization described by Szwarc and cowork-ers,1–3 living polymerizations have been reportedfor cationic,6 Ziegler–Natta,7 ring-opening met-athesis,8 and radical9 chain-reaction polymeriza-tions. For each of these mechanistic types, to theextent that the designation of living is correct, thesynthetic consequences of a living polymerizationfor the precision synthesis of well-defined poly-mers has been realized. These developments fur-ther validate the correctness and usefulness ofthe original definition of living polymerization.For example, for any living polymerization (re-gardless of the nature of the chain-carrying spe-cies), it is possible to prepare polymers with con-trolled, well-defined structures and with low de-grees of compositional heterogeneity.

Darling et al. have proposed that “the meaningof the term living polymerization be further re-laxed and be defined as those processes that yieldliving polymers (or their dormant precursors) …and that ”the term living polymerization shouldbe used regardless of the yield of the process.“ Inother words, these authors propose that if anyliving polymer chains are formed, this processshould be defined as living. The authors go on tostate that ”it should, however, also be recognizedthat the various characteristics associated withlivingness … will be maximized when irreversible


termination processes such as chain transfer orself reaction … are negligible.“ Thus, these au-thors propose to generalize and trivialize the def-inition of living polymerization to include nonliv-ing systems because it will now include systemsin which transfer and termination occur (”regard-less of the yield“) and for which the characteris-tics associated with livingness will not necessar-ily be a consequence of the process.

Polymer science, with its underlying theoriesand definitions, must be capable of change to ac-commodate new data and new ideas. However,these changes should be based on careful, skepti-cal analysis and the quest for clarity, correctness,and rationality. In this respect, it is important toevaluate the rationale proposed by Darling et al.to justify changing the definition of living poly-merization to include nonliving systems.

1. The authors state, “The terms controlled po-lymerization, living, living/controlled, pseudo-liv-ing, living polymerization with reversible deacti-vation, and others are scattered throughout theliterature. Lack of a common language createsconfusion, wastes time and journal space, and hasthe potential to inhibit computational literaturesearching.

Rebuttal: The fact that the term living is mis-used and that people propose ambiguous terms todescribe systems with less than living character-istics is not a compelling argument for abandon-ing the precise definition of living. Efficient andrapid searching of the chemical literature hasalways been a challenge, but the proposal to in-clude nonliving systems under the umbrella ofliving for expediency of literature searching is aquestionable goal.

2. With respect to a recent proposal to use theterm controlled radical polymerization, the au-thors state that “it seeks to reserve the term liv-ing for an impossible ideal (immortality), therebyremoving a valuable term from the vocabulary ofthe polymer scientist.”

Rebuttal: The term living is not an impossibleideal. For example, in the alkyllithium-initiatedpolymerization of styrenes and dienes in a hydro-carbon solution under high-vacuum conditions, itis possible to prepare polymers with predictableMn’s, narrow Mw/Mn’s, block copolymers withoutcontamination for the first block segment, star-branched polymers with well-defined arms, andnarrow molecular weight distributions and withquantitative end-group functionality. Whetherthe term controlled is a useful term for certainsystems is neither critical nor relevant to the

proposed trivializing of the term living as pro-posed by Darling et al.

3. The authors incorrectly state that “the at-tributes of living polymerization are well-knownand include molecular weight control, narrowpolydispersity, end-group control, and the abilityto chain extend.”

Rebuttal: As delineated in numerous learneddiscussions on the subject of living polymerizationand reviewed in depth by Henderson andSzwarc10 in 1968, a narrow molecular weight dis-tribution should not be used as a criterion forliving polymerizations because it is neither a nec-essary nor sufficient condition to indicate whethera given polymerization is living.5

4. The authors state that “a definition shouldnot be dependent on the limitations of currenttechnology (a detection limit) or be based on sub-jective argument (an expectation).” They go on tosay that “one criterion that has been suggested isthat the polydispersity of living polymerizationshould be less than 1.1. The 1.1 limit was intro-duced when the determination of polydispersitiesless than 1.1 was not possible by the analyticaltools then available.”

Rebuttal: Once again it should be noted thatnarrow molecular weight distribution is not agenerally useful criterion for a living polymeriza-tion (see the rebuttal to 3).5 Furthermore, thedescription of a polydispersity less than 1.1 wasnot proposed as a criterion of livingness by Fet-ters11 but as a useful guide to distinguish narrowdistribution polymers; to wit, if Mw/Mn is greaterthan 1.1, it should not be classified as narrow.This had nothing to do with the state of the art forthe determination of narrow molecular weightdistributions.

5.The authors state that “one of the basic prin-ciples of chemistry is to name reactions on thebasis of their mechanisms, not their yields” and“there is little precedent for assigning reactionnames on the basis of a perceived absence … ofside reactions.”

Rebuttal: The term living polymerization hasvery specific implications with respect to themechanism of polymerization. A mechanism isdefined as “a detailed description of the path-way leading from the reactants to the productsof a reaction, including as complete a character-ization as possible of the composition, struc-ture, and other properties of the reaction inter-mediates and transition states.”12 The authorsare confusing named reactions (which usuallyhave historical rather than mechanistic signif-


icance) with mechanisms. The Diels–Alder re-action is not a mechanism; the designation as a412 cycloaddition reaction describes a mecha-nism. Living polymerization has mechanisticsignificance because it describes the pathwayfrom reactants (initiator, monomer) to products(living polymers that retain their active chainends) without the intervention of terminationand transfer steps. As such, it does not describethe mechanism of the formation of terminatedor transfer products. Thus, this distinction be-tween named reactions and mechanisms is con-fusing and does not serve to enlighten the is-sues at hand.

6. The authors propose that “the meaning ofthe term living polymerization be relaxed and bedefined as those processes that yield living poly-mers (or their dormant precursors). The term liv-ing polymerization should be used regardless ofthe yield of the process.”

Rebuttal: First, one cannot define living po-lymerizations by stating that they yield livingpolymers. This tautology does not address thequestion of what living means. If this relaxationof the term living were universally adopted, theterm living polymerization would be trivializedand meaningless. What would be the value ofthis designation? It would not have mechanisticor synthetic consequences that could be rigor-ously investigated, characterized, and under-stood.

Darling et al. have not presented a convincingcase for changing the meaning of the term livingpolymerization to include polymerizations inwhich nonliving polymer chains are formed. Thedesignation as a living polymerization specificallyrefers to a chain-reaction polymerization proceed-ing in the absence of chain-termination andchain-transfer reactions. This mechanistic desig-nation has implicit kinetic and synthetic conse-quences that can be tested and verified experi-mentally. This definition has been in place forapproximately 43 years and has served to codifywidely diverse processes and successfully stimu-late research to find living processes involvingdifferent types of chain-carrying reaction inter-mediates and design new well-defined moleculararchitectures.


1. Szwarc, M.; Levy, M; Milkovich, R. J Am Chem Soc1956, 78, 2656.

2. Szwarc, M. Nature 1956, 178, 1168.3. Hsieh, H.; Quirk, R. P. Anionic Polymerization:

Principles and Practical Applications; Marcel Dek-ker: New York, 1996.

4. Webster, O. W. Science 1991, 251, 887.5. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359.6. Cationic Polymerizations: Mechanisms, Synthesis,

and Applications; Matyjaszewski, K., Ed.; MarcelDekker: New York, 1996.

7. Doi, Y.; Keii, T. Adv Polym Sci 1986, 73/74, 201.8. Grubbs, R.; Risse, W.; Novak, B. M. Adv Polym Sci

1992, 102, 47.9. Controlled Radical Polymerization; Matyjaszewski,

K., Ed.; ACS Symposium Series 685; AmericanChemical Society: Washington, DC, 1998.

10. Henderson, J. F.; Szwarc, M. J Polym Sci MacromolRev 1968, 3, 317.

11. Fetters, L. J. In Encyclopedia of Polymer Scienceand Engineering; Kroschwitz, J. I., Ed.; Wiley In-terscience: New York, 1987; Vol. 10; p 19.

12. Glossary of Terms Used on Physical OrganicChemistry. Pure Appl Chem 1979, 51, 1725.

Comments byDr. Joseph C. Salamone

Vice President, Chemical Research,Global Scientific Affairs, Baush & Lomb,1400 North Goodman Street,Rochester, New York 14609

Professor Emeritus, University ofMassachusetts, Lowell, Lowell,Massachusetts 01854

Difficulties in terminology have long been as-sociated with the polymer field, and the topic un-der consideration, namely, how to classify a rad-ical polymerization reaction that can exhibit liv-ing characteristics, is a continued manifestationof our attempts to classify various polymerizationreactions. Consider, for example, the difficultiesin polymer nomenclature. If IUPAC- or CAS-based nomenclatures were used, particularly forcomplicated structures, few scientists would un-derstand the polymer’s structure without thestructure being represented schematically. If thisis not difficult in itself, other polymer nomencla-tures may use abbreviations or source-basednames. Thus, the current state of polymer nomen-clature leaves much to be desired in understand-ing the structure of the polymer under consider-ation.

Related circumstances are often present inmany polymerization reactions. In the area ofcoordination polymerization, reactions are often


initially categorized as one type of process, only tohave this designation altered over time, as moredata become available.

While preparing the Polymeric Materials Ency-clopedia,1 I encountered the same difficulty dis-cussed by Darling et al., namely, how to classifyradical polymerization that is considered living.In general, the polymer community has a ten-dency to use the simplest terminology to describea system, probably because of the complexitiesinvolved in using precise designations. The origi-nal concept of living polymerization was based onno termination and that chain growth could con-tinue when the initial monomer was exhausted ifadditional (polymerizable) monomer was added.Other concepts, such as initiation and propaga-tion rates, kinetic and thermodynamic parame-ters, and molecular weights and molecular weightdistributions, were added to clarify the character-istics of living polymerization.

However, times change, and all fields of scienceevolve, with the continued generation of new con-cepts. As scientists, we must be cognizant of thesechanges, and, when appropriate, these changesmust be accommodated. For a polymerization re-action where fresh monomer is added to a pre-sumably dead polymer and the polymerizationcontinues, this reaction should be classified as aliving polymerization. Ultimately, what is the dif-ference between a living ion-pair chain end and aliving, covalently bonded chain end? In this re-viewer’s opinion, reversible termination leadingto continued chain growth in a radical polymer-ization should be classified as a living radicalpolymerization.


1. Polymeric Materials Encyclopedia; Salamone,J. C., Ed.; CRC: Boca Raton, FL, 1996.

Comments byProfessor Mitsuo Sawamoto

Department of Polymer Chemistry,Graduate School of Engineering, KyotoUniversity, Kyoto 606-8501, Japan

As most of us understand, living polymeriza-tion is defined as chain-growth polymerizationwithout termination, chain transfer, and otherchain-breaking reactions. With this definition

granted, overall I agree with the proposal by Dar-ling et al. that the term living polymerizationshould be used without seemingly ambiguousvariants. For this uniform terminology to be prop-erly executed, of course, we must carefullypresent solid and reproducible experimental re-sults that confirm the occurrence of such polymer-izations, and my agreement is based on the as-sumption that, whenever we use the term living,the reactions thus called are experimentally dem-onstrated to clearly fulfill the definition of livingpolymerization.

Since the beginning of the 1980s,1 an increas-ing number of new polymerizations have beenreported that exhibit phenomenology that con-form with the definition of living polymerization.A common characteristics of these, including ourcationic2 and metal-mediated radical3 living poly-merizations, is that they involve dormant species,covalent precursors that generate the true grow-ing species via catalysis or a physical stimulussuch as heat and light. Dormant species are indynamic equilibria with the true growing end,and thus the formation of the latter from a dor-mant end is reversible. The backward reaction,that is, the generation of a dormant species froma growing end, corresponds to a termination re-action in conventional chain-growth polymeriza-tions, and this seems to be the reason for thespreading use of various terms related to livingpolymerizations. In my view, however, if the in-terconversion between dormant and true growingspecies is reversible, the involvement of dormantspecies does not deny the use of the term livingpolymerization.

Herein I would like to emphasize that to claimany living polymerization, we must present solidand reproducible experimental facts to confirmthat it fulfills the definition of living polymeriza-tion. For this, the following set of results may bepresented:

1. A linear increase of the number-average mo-lecular weights (Mn) in direct proportion tomonomer conversion or polymer yield.

2. A further increase in Mn, again proportionalto conversion or yield, when a fresh feed ofmonomer is added to a completely polymer-ized reaction mixture.

3. Molecular weight distributions (mostly sizeexclusion chromatograms) before and aftersuch sequential monomer addition, as in Re-sult 2, that shift to higher molecular


weights without leaving any fractions indic-ative of growth-inactive polymers.

Results 1 and 2 demonstrate the absence ofchain transfer (but not necessarily the absence oftermination), whereas Result 3 visually demon-strates the absence of both chain transfer andtermination. The absence of termination may alsobe shown in other ways, such as linear first-orderplots for monomer consumption and quantitativeanalysis of end groups by NMR.

In any case, a single result usually cannot dem-onstrate unequivocally the absence of chaintransfer and termination, and we need these com-bined sets of results for experimental verificationof living polymerization. In particular, when cou-pled with the molecular weight changes againstconversion, molecular weight distribution data, ifproperly obtained, are quite informative in show-ing the absence of chain-breaking reactions dur-ing the course of polymerizations. Unfortunately,there seems to be some confusion or misunder-standing in some of the published articles inwhich, for example, very narrow molecularweight distributions alone or molecular weightdata without molecular weight distribution evi-dence are often regarded as sufficient to claimliving polymerizations. As often pointed out,abuse of the term living should be avoided to keepthe important research activity in this field ac-cepted by the majority of scientists, particularlythose working outside the field of polymer chem-istry.


1. Webster, O. W. Science 1991, 251, 887.2. Sawamoto, M. Prog Polym Sci 1991, 16, 111.3. Sawamoto, M.; Kamigaito, M. CHEMTECH 1999,

29, 30.

Comments byProfessor Masayoshi Tabata

Department of Molecular Chemistry,Graduate School of Engineering,Hakkaido University, Sapporo 060-8628,Japan

Since Szwarc discovered the living anionic po-lymerization of styrene and suggested that livingpolymers grow to a desired maximum associatedwith a negligible degree of termination or chain

transfer, many terms have been proposed to de-scribe various types of living polymerizations.Living polymerizations are methods of precise po-lymerizations in which some controversies arealso involved, especially in the case of the recentliving radical polymerization, which makes themeaning of living polymerization vague. There-fore, as the authors proposed, it seems that amore uniform terminology is needed among poly-mer chemists regarding living polymerization.

Therefore, I want to propose that the term liv-ing polymerization be used when the polymeriza-tion is characterized under conditions such as anarrow polydispersity (Mn/Mw , 1.01), a molecu-lar weight control of more than three times, amolecular weight of more than 10,000, and theformation of a block polymer together with a morespecific description of the relevant polymerization[e.g., nitroxide-mediated polymerization (NMP),atom transfer radical polymerization, and revers-ible addition fragmentation chain transfer(RAFT)], excluding the words controlled, control,quasi, and pseudo, regardless of the polymeriza-tion yield.

Comments byProfessor Teiji Tsuruta

University of Tokyo, 1-1-1-609 Shimoda-cho, Kohoku-ku, Yokohama 223-0064,Japan

I basically agree, with some reservations, withDarling et al. that the meaning of the term livingpolymerization should be further relaxed and de-fined as those processes that yield living polymers(or their dormant precursors).

Darling et al.’s definition of living polymeriza-tion has the same logical structure as that ofstereospecific polymerization. An IUPAC nomen-clature document states that “stereospecific poly-merization” is “polymerization in which a tacticpolymer formed.1” Here, it is to be noted that theIUPAC document defines tactic polymer indepen-dently, and then defines stereospecific polymeriza-tion by using the term tactic polymer to avoidcircular logic.

Darling et al. stated that living polymers aredistinguished from dead polymers, being “able togrow whenever additional monomer is supplied.”However, this is not enough because by consensusa living polymer is formed by chain polymeriza-tion.2 Darling et al. must strictly define living


polymer first, then define living polymerization byusing the term living polymer.

The comparative discussion of living polymer-ization and Diels–Alder reactions cannot be a ra-tionale for Darling et al.’s proposal. In this dis-cussion, we have two contrasting reactions: livingand nonliving polymerizations. For example, if20% of a polymerization process takes place un-der a living mechanism, the whole polymerizationcannot be called living polymerization because80% of the polymerization was a nonliving poly-merization. The same logic may be applied to, forexample, Markovnikov and anti-Markovnikov ad-dition reactions. A reaction which obeys Mark-ovnikov’s rule 20% cannot be called a Markovni-kov-type addition reaction because an anti-Mark-ovnikov-type reaction obviously predominates.

The applicability of the term living polymeriza-tion to the whole polymerization system dependson the living/nonliving ratio in the relevant poly-merization system, but I do not think it wise toconsider “threshold numerals for living/nonlivingratio” for qualifying the polymerization as living.Every experienced polymer scientists (see thepreamble of ref. 2) should know the ratio. How-ever, it is safe to say, for instance, that livingpolymerization takes place to an extent of 20% inthis reaction system.


1. IUPAC. Basic Definitions of Terms Relating toPolymers. Pure Appl Chem 1974, 40, 477.

2. IUPAC. Glossary of Basic Terms in Polymer Sci-ence. Pure Appl Chem 1996, 68, 2287.

Comments by Howard W. Turner,Ralph B. Nielsen, Adam L. Safir,Gerrit Klaerner, andChristopher Goh

Symyx Technologies Inc., 3100 CentralExpressway, Santa Clara, California95051

The article by Darling et al. recommends theuniform application of the term living polymeriza-tion to a very broad range of radical polymeriza-tion processes and mechanisms, regardless of themechanistic details or the yield of living chainsfrom a particular process. Although we appreciatethe authors’ concerns, we do not agree with their

recommendation. Collapsing language describingfundamentally different processes into a singleterm, living polymerization, is an oversimplifica-tion and, worse, a distortion of the concepts cur-rently denoted by the word living.

The concept of living polymerization alreadyhas a simple, well-established meaning. It is anideal and not a mechanism. Like the abstractconcepts of high yield, selective, rapid, and simple,it is subjective but measurable. Many differentpolymerizations can be made to function as living,with significantly different mechanisms evenwithin broad categories such as anionic, cationic,group transfer, and radical. Livingness denotesthe fraction of polymer chains that grow, withouttermination, transfer, or other side reactions,from the beginning to the end of a polymerizationprocess. A polymerization is practically livingwhen a sufficient fraction of chains needed toimpart the desired polymer properties survivewhile growing to the target molecular weight ofthe process, all of which are subjective criteria.Also, Darling et al. are inconsistent in arguingthat “one of the basic principles of chemistry is toname reactions on the basis of their mechanisms”while recommending that many mechanisms becombined under the name living. To call a processliving based on mechanism, “whether the yield is99.99% or 50%, or even 20%” as the authors sug-gest, is like referring to a low-yield high-yieldreaction, defined as high-yield by its mechanism.

Most scientists already apply Occam’s razornaturally and do not create terms unnecessarily.The existing terminology related to controlled po-lymerization processes is evidence of a rich andvaried scientific landscape. Communication inthis field will not be improved by the oversimpli-fication or broadening of the definition of livinguntil it is effectively meaningless.

Comments byProfessor Robert M. Waymouth

Department of Chemistry, StanfordUniversity, Stanford, California94305-5080

The article by Darling et al provides a usefulcommentary on the nomenclature of living poly-merizations, particularly as it applies to free-rad-ical polymerizations. There is little substantiveadded here beyond what Quirk1 argued persua-sively in his discussion of the experimental crite-


ria for living polymerization. I would find this auseful comment to stimulate dialogue to come tosome consensus on a consistent terminology forthese classes of polymerization reactions. I foundthe arguments reasoned and the authors’ positiondefensible.

The editors experiment to provide a series ofcommentaries on various sides of the issues is aquite reasonable one.


1. Quirk, R. P.; Lee, B. Polym Int 1992, 27, 359.

Comments by Dr. Owen WebsterCentral Research & Development, E. I.du Pont de Nemours & Company,DuPont Experimental Station,Wilmington, Delaware 19898

I agree with the overall suggestion that the termliving with respect to polymerization processes in-clude all methods in which, after monomer deple-tion, the addition of new monomer starts chaingrowth again. These systems allow one to makeblock and star polymers and to control end function-ality and molecular weight. By and large, the ma-jority of workers in the field use the term living inthis respect when communicating orally. The writ-ten language should follow suit.

Along with the term living, we will still needother modifiers, such as living free-radical, anionic,and cationic polymerization. To narrow down thefield further, terms such as stable free radical poly-merization (SFRP), atom transfer radical polymer-ization (ATRP), reversible addition fragmentationchain transfer (RAFT), and group transfer polymer-ization (GTP) should be used.

In general, languages evolve by usage, and at-tempts at control will be short-lived.

Comments byProfessor Karen L. Wooley

Department of Chemistry, WashingtonUniversity, One Brookings Drive, St.Louis, Missouri 63130-4899

The confusion that has arisen from the vari-able terminology found for radical polymeriza-tions that approach or exhibit living character isthe basis for the authors’ proposed relaxation of

the term living polymerization. It is agreed thatthe diversity of nomenclature, ranging from pseu-do-living to controlled polymerization, is less thanideal for those examples that do not meet thebasic criteria established for living polymeriza-tion and that uniformity is needed. However,broadening the term living to all conditions thatlead to some fraction of chains exhibiting livingcharacter is not an acceptable solution.

The statement buried within ref. 7 of the arti-cle, suggesting that the polydispersity criterion bereduced to less than 1.01, is extreme and likelybeyond the human error associated with the ex-perimental preparation of the polymers. Thisstatement is made to demonstrate that the crite-ria for the evaluation of livingness should not bedependent on the detection limits of current tech-nology; however, a rationale based on molecularweight dispersity is overemphasized throughoutthe article. Moreover, a single criterion cannot bethe basis for the determination of a living poly-merization process, and there is no reason to ne-glect or abandon the well-established identifyingproperties, including molecular weight controland narrow molecular weight distribution result-ing from initiation rates that far exceed propaga-tion rates, a linear increase in the degree of poly-merization as a function of monomer conversion,and the remaining reactivity of the chain end(s)following monomer consumption because of thelimitation of irreversible termination reactions,which allows for end-group compositional controland the ability to chain-extend for the productionof block copolymers or other macromolecular ar-chitectures. The most interesting and useful ap-plications of living polymerizations do not relysolely on the narrow molecular weight distribu-tion that is the focus of the article (e.g., ref. 7). Inaddition, narrow molecular weight distributionsare not necessarily limited to true living polymer-izations that yield chains with ends of specific andequal reactivity, as its value primarily relies onthe relative kinetics of the initiation, propagation,and irreversible termination reactions (ki @ kp@ kt). A situation can be imagined in which sidereactions leading to irreversible chain termina-tion occur on a timescale preventing convenientpostpolymerization chain-end manipulation, eventhough the polymer chain growth appears frommolecular weight and molecular weight distribu-tion control to result from a living process. Fur-thermore, ref. 3 of the article states that slowinitiation does not by itself cause chain death—this is obvious. Although slow initiation relative


to propagation will lead to an increase in themolecular weight distribution, the yield of livingpolymer chain ends can be quantitative if the rateof propagation is still much faster than the ratesfor irreversible termination reactions.

The discussion regarding named reactions in or-ganic chemistry is presented in support of the argu-ment that “a new name should not be defined whenit is realized that some side reaction also occurs.”Although this is a valid point, especially with re-spect to technique-specific nomenclature, the termliving indicates that side reactions are limited.

The article presents a problem in terminology,but it does not offer a viable solution. The proposedsolution is to use the term living polymerization forall processes that yield living polymers (or theirdormant precursors) regardless of the yields of theprocesses. This will most certainly lead to a broaduse of the term living polymerization beyond thosecases that actually exhibit living polymerization be-havior, which will become a nuisance for those re-searchers who are in search of chemistries that canefficiently generate polymer chains containingchain ends that are accessible for clean and com-plete employment in the construction of more elab-orate compositions and structures. It is not suffi-cient to recognize that “the various characteristicsassociated with livingness … will be maximizedwhen irreversible termination processes … are neg-ligible,” while still assigning the term living poly-merization regardless of the yield of the process.Perhaps a better solution would be settlement on asingle term, such as pseudo-living or quasi-living,for those polymerizations that give either less thanoptimal yields of polymer chains with remainingchain-end reactivity or that give polymers with lessthan ideal control over molecular weight properties.

Comments byProfessor Hajime Yasuda

Department of Applied Chemistry,Faculty of Engineering, HiroshimaUniversity, Higashi-Hiroshima 739-8527,Japan

The term living polymerization is used widely.However, the use of this term has spread to poly-

merization that is actually nonliving. I suggestusing this word more correctly:

1. Because living polymerization means thelack of termination and chain-transfer re-actions, the conversion or polymer yieldshould be more than 98% in short or longpolymerization times.

2. The molecular weight distribution (Mw/Mn)should be less than 1.05 or at least lessthan 1.15 when Mn (not Mw) is greaterthan 10,000. When Mw/Mn is greater than1.15, the word pseudo-living or controlledpolymerization should be used. When Mn isgreater than 100,000, Mw/Mn should beless than 1.25.

3. Because living polymerization means notermination or no chain transfer, Mn

should increase to more than 100,000. Oli-gomerization exhibiting an Mn of less than10,000 with a rather small polydispersityshould be called controlled or pseudo livingpolymerization.

4. Living polymerization indicates that Mn

should increase linearly with conversion.This should be confirmed experimentally.

5. The actual meaning of living polymeriza-tion lies in the possibility of performing ABdiblock copolymerization or ABA or ABCtriblock copolymerization. In these cases,the catalytic efficiency or initiator effi-ciency should be greater than 75%. Wehave realized greater than 80% catalyticefficiency in the rare-earth-metal-initiatedpolymerization of methyl methacrylate(MMA)1 and block copolymerization.2


1. Yasuda, H.; Yamamoto, H.; Yamashita, M.; Yokota,K.; Nakamura, A.; Miyake, S.; Kai, Y.; Kanehisa,N. Macromolecules 1993, 26, 7134.

2. Ihara, E.; Morimoto, M.; Yasuda, H. Macromole-cules 1995, 28, 7886.


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