recent advances in microwave-assisted polymer synthesis · microwave-assisted polymer synthesis 731...

CSIRO PUBLISHING Review

www.publish.csiro.au/journals/ajc Aust. J. Chem. 2007, 60, 729–743

Recent Advances in Microwave-Assisted Polymer Synthesis

Sebastian SinnwellA,B and Helmut RitterA,C

AInstitute for Organic Chemistry and Macromolecular Chemistry, Chair for Preparative PolymerChemistry, Heinrich-Heine-University of Duesseldorf, 40225 Duesseldorf, Germany.

BPresent address: Centre for Advanced Macromolecular Design (CAMD), School of ChemicalSciences and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia.

CCorresponding author. Email: [email protected]

In the past few years the use of microwave irradiation in polymer science has become a well-established technique to driveand promote chemical reactions. The main advantages of microwave heating are a strong reduction in reaction time anda high potential to contribute to green and sustainable chemistry. This article provides a short review of recent examplesin the field of microwave-assisted polymer synthesis with special emphasis on radical polymerizations, step-growthpolymerizations, ring-opening polymerizations, and polymer modifications.

Manuscript received: 26 June 2007.Final version: 24 August 2007.

Introduction

Since the early reports, published 20 years ago, the use ofmicrowave (MW) irradiation has become a well-establishedtechnique to promote and enhance chemical reactions. In par-ticular, the past 5 years have seen a great increase in the useof this technique. Consequently, the number of publicationsconcerning the application of MWs in the fields of chemicalsynthesis is rapidly increasing. Several books and reviews havealready been published that summarise the multitude of originalpapers.[1–9]

The main advantages of MW-assisted chemistry are shorterreaction times, higher yields, and a reduction of side reactionscompared with syntheses performed under conventional heat-ing. Although most of these enhancements can be describedas thermal effects, in some cases they cannot be achieved orreproduced by conventional heating. Such ‘specific’MW effectsoften result from characteristics of the dielectric heating such asthe occurrence of an inverted temperature profile,[10] superheat-ing of solvents at ambient pressure,[11] or selective heating ofhighly absorbing compounds in less polar reaction media.[12]

The possible existence of non-thermal MW effects that could

Dr. Sebastian Sinnwell was born in 1977 in Hamburg, Germany. He studied chemistry at the University of Duesseldorf (Germany)and received his Diploma in 2004. In his Ph.D. thesis he investigated the influence of microwave irradiation on the ring-openingpolymerization of lactones and cyclic imino ethers under supervision of Professor H. Ritter (Duesseldorf, Germany). After heobtained his Ph.D. in 2007 he has started a post-doctoral project in the group of Professor C. Barner-Kowollik (Sydney, Australia).Current investigations are focused on the living/controlled radical polymerization via RAFT process.

Professor Dr. Helmut Ritter was born in November 1948. After studying chemistry at the University of Marburg (1967–1972) andcompleting his doctorate with Professor H. Ringsdorf in Mainz, he was employed as head of a laboratory at Bayer AG. He startedhis university career in 1982 as a professor for organic chemistry at the University of Wuppertal, where he got his habilitation in1989. Professor Ritter left the University of Wuppertal in 1997 and moved to the University of Mainz where he took over as chairof Polymer Chemistry. In October 2001, he was appointed at the Heinrich-Heine-University Düsseldorf, and since then he is thehead of the department for Organic Chemistry and Macromolecular Chemistry.

rationalize specific synthetic pathways observed in MW and notunder conventional heating is still a subject of debate.[13–16]

The early experiments in MW-assisted synthesis were car-ried out in domestic MW ovens typically working with a pulsedirradiation, inaccurate temperature control, inhomogeneous MWfield, and the lack of safety precautions, which often prevented anappropriate reaction control. Since the development of dedicatedMW reactors for synthesis in the 1990s it is possible to obtainmore reliable results with a high reproducibility. Principally, cur-rent MW reactors can be divided into two designs (monomodeand multimode) which differ regarding the cavity size and themanner of field orientation. The commercially available MWreactors are usually equipped with built-in magnetic stirrers anddirect temperature control with an infrared (IR) pyrometer orfibre-optical temperature sensor. It is possible to heat reactionmixtures to a desired temperature because of a continuous powerfeedback control. In some cases it is possible to work underambient pressure in an open reaction vessel.

In polymer science, the application of MWs for processdevelopment has a long tradition because this method oftenprovides suitable solutions for technical problems. The first

© CSIRO 2007 10.1071/CH07219 0004-9425/07/100729

730 S. Sinnwell and H. Ritter

example, which concerns the MW-assisted radical cross-linkingof an unsaturated polyester with styrene by Gourdenne et al.was published in 1979.[17] Another investigation of the bulkpolymerization of 2-hydroxyethyl methacrylate under MW andconventional conditions by Teffal and Gourdenne followed in1983[18] even before the first reports of the MW-assisted organicsynthesis appeared in 1986.[19,20] In current polymer science,the use of MW-assisted polymer synthesis has been largelyinvestigated by several research groups and some reviews havealready appeared.[21–26] With this contribution we will presentan overview over the past 3 years of polymerization reactionscarried out using MW reactors under well-defined experi-mental conditions. The main topics of investigations can bedivided into four areas: radical polymerizations, step-growthpolymerizations, ring-opening polymerizations, and polymermodifications.

Radical Polymerizations

The free radical polymerization is the most important indus-trial polymerization technique. Polymers such as polyethy-lene, polystyrene, poly(vinyl chloride), poly(vinyl acetate),poly(acrylonitrile), poly(methyl methacrylate) (PMMA), andpoly(tetrafluoroethylene) are produced by radical polymer-ization. In some early examples, Boey and coworkers per-formed the radical bulk polymerization of styrene,[27] methylmethacrylate (MMA),[28] and methyl acrylate[29] with 2,2′-azoisobutyronitrile (AIBN) under MW conditions (domesticMW oven, power-control). The free radical polymerization ofstyrene, vinyl acetate, MMA, and acrylonitrile in a domesticMW oven was reported by Mattos and coworkers.[30] In recentyears the free radical copolymerization, emulsion polymeriza-tions, preparation of composite materials, and controlled radicalpolymerizations have come more and more into the focus of theinvestigations.

Free Radical HomopolymerizationThe MW-assisted (domestic MW oven) polymerization of MMAin bulk in the presence of benzoyl peroxide as the initiatorwas reported by Jovanovic and Adnadjevic.[31] In a compar-ative study with experiments under conventional heating atdifferent temperatures the authors described an accelerationof the reaction rate because of a reduction of the activa-tion energy in polymerizations under MW irradiation. Thesedifferent polymerization kinetics were attributed to the rapidenergy transfer under MW conditions, which was supposed tolead to a non-equilibrium energy distribution in the system.Investigations on the MW-assisted synthesis and polymeriza-tion of a series of (meth)acrylamides were reported by Ritterand coworkers. As the first example, the direct solvent freeamidation of (meth)acrylic acid with aliphatic and aromaticamines was reported in 2004.[32] It was possible to obtain thecorresponding (meth)acrylamides in the absence of couplingagents after 30 min of MW irradiation in excellent yields (upto 96%). Further studies focussed on the MW-assisted synthe-sis of chiral (meth)acrylamides from (meth)acrylic acid and(R)-1-phenylethylamine (Scheme 1).[33,34]

N-((R)-1-phenylethyl)(meth)acrylamide was obtained ingood yields after 15 min of MW irradiation in a monomodeMW reactor. In comparison, Michael addition was observedas the preferred reaction in experiments under thermal heat-ing. The selective amide formation in MW experiments wasattributed to the highly polar intermediates (zwitterions and

R

R

R � H, CH3

OH

HO O O O O NH

zyx

N

R R R R

MW

MW, AIBN

H2N

�H2O

�H2O

�

O

O

HN

Scheme 1. Microwave (MW)-assisted synthesis of chiral (meth)acry-lamides from (meth)acrylic acid and (R)-1-phenylethylamine and one-potpolymerization in the presence of 2,2′-azoisobutyronitrile (AIBN).[33,34]

salts) which were regarded as strong MW absorbing species.The MW-assisted amidation performed in the presence ofAIBN afforded optically active polymers in a single step.Another example for the efficiency of MW-assisted amidationswas published recently: the reaction of methacrylic acid and3-(dimethylamino)-1-propylamine took place during 1 min ofMW irradiation (150◦C) and yielded 82% of the desiredamide.[35] After homopolymerization of the purified monomerin solution of toluene using AIBN as the initiator the isolatedpolymer showed lower critical solution temperature behaviourin aqueous solution. Singh et al. polymerized acrylamide inaqueous solution with potassium peroxodisulfate as the ini-tiator under MW conditions (domestic MW oven).[36] Thedependency of the monomer conversion on the MW power,irradiation time, initiator, and monomer concentration was inves-tigated. Optimal conditions (98.5% conversion) were found inMW experiments with 80% MW irradiation (98◦C) for 50 swith [K2S2O8] = 2 × 10−3 M and [acrylamide] = 0.56 M. Whenapplying identical conditions in an experiment performed ina thermostatic water bath no polymerization was observed.Fischer et al. investigated the MW-assisted polymerizationof some N-alkylacrylamides with AIBN in the presence ofa chain transfer agent.[37] The polymerizations were carriedout in bulk in a domestic MW oven (power control, 350 W,30–150 s) and for comparison in solution of methanol underreflux conditions (∼65◦C, 3.5 h) as well as in an autoclave(80–170◦C, 1 h). Besides the strong rate enhancement underMW conditions the obtained polymers’ qualities were slightlydiminished with respect to their molar mass distribution. Cor-tizo reported the polymerization of diisopropyl fumarate underMW irradiation (domestic MW oven).[38] The influence ofMW power and irradiation time on the yield and the molec-ular weight of the obtained polymers was investigated. In afurther work, Alessandrini and coworkers showed polymer-ization of different dialkyl fumarates using a domestic MWoven in the presence of benzoyl peroxide as the initiator.[39] Incomparison with experimental data from the literature an accel-eration of reaction rate was found for all studied monomers.Lu and coworkers investigated the influence of carriers (Al2O3,SiO2, and MgO) on the MW-assisted (domestic MW oven)polymerization of acrylamide and 2-ethylhexyl acrylate.[40] Itwas found that the carrier quantity strongly influenced thepolymer yield. The MW-assisted synthesis and polymeriza-tion of N-substituted maleimides was reported by Ritter andcoworkers.[41–43] In contrast to the bulk syntheses of the aromatic

Microwave-Assisted Polymer Synthesis 731

MW and oil bath,organic peroxide

initiators,DMF or tolueneO

O O

�

O

nn

mm

Scheme 2. Free radical copolymerization of styrene and MMA.[45]

N-phenylmaleimide[41] and N-benzenesulfonamide,[43] whichwere found to be accelerated under MW-conditions, the for-mation of N-(2-acetoxyethyl)maleimide in a solution of aceticanhydride could be obtained under conventional heating incomparable yields.[42]

Free Radical CopolymerizationsGreiner and coworkers investigated the free radical polymeriza-tion of styrene and copolymerization of styrene and MMA withand without MW irradiation (monomode MW reactor, temper-ature control) by using different organic peroxide initiators ina solution of toluene and N,N′-dimethylformamide (DMF).[44]

It was found that the homopolymerizations of styrene in DMFshowed a significantly higher monomer conversion under MWconditions compared with experiments performed in an oilbath. However, the conversions of polymerizations performedin toluene were not influenced by MW irradiation. In all copoly-merization experiments the comonomer incorporation seemedunaffected by the MW irradiation. To find whether the observedMW-effects are attributable to an accelerated propagation reac-tion or enhanced initiator decomposition, Stange and Greinerperformed a further study on the free radical copolymerizationof styrene and MMA.[45] In a more careful investigation, themonomer conversion as well as the conversion of three differentorganic peroxide initiators in a solution of toluene and DMF wasstudied under MW conditions and under conventional heating(Scheme 2). The authors found an enhanced monomer conver-sion in DMF with tert-butylperbenzoate under MW conditions.All other copolymerizations were unaffected by the heat source.This behaviour was attributed to an increased decompositionof the initiator. The authors explained this solvent dependencewith the heating characteristics of DMF, which heats up muchmore quickly under MW irradiation than toluene. Therefore, inDMF, more radicals were formed during an early stage of thecopolymerization.

In copolymerization experiments of styrene with MMA, andof butyl methacrylate with styrene and isoprene, Fellows foundan acceleration of the reaction rate by a factor of 1.7.[46] Anincreased radical concentration because of a rapid orientationof the AIBN fragments after decomposition was consideredas the reason for the enhanced polymerization rate. Agar-wal et al. have reported the MW-assisted (monomode MWreactor) copolymerization of 2,3,4,5,6-pentafluorostyrene withN-phenylmaleimide in solution of anisole by using AIBN as theinitiator at 70◦C.[47] The experiments under MW conditionsled to an increase of the polymerization rate but gave lowerlimiting conversion compared with thermal experiments. Aneffect of MW heating on the copolymer composition was notobserved. Lu et al. investigated the MW-assisted initiator-freecopolymerization of 2-(dimethylamino)ethyl methacrylate and1-allylthiourea (domestic MW oven).[48] The influence of irra-diation time on monomer conversion and irradiation power on

R1 R2

H �CONH2

H �CN�CH3 �COOCH3

MW

OOHO

NH2

OH

OOHO

NH2

OH

OHH

OOHO

NH

OH

OOHO

NH2

O

OHH

R2

R2

R1

R1

R2

R1

�2mm

m

n

n

Scheme 3. Microwave (MW)-assisted (domestic MW oven) polymeriza-tion of MMA,[49] acrylamide,[50] and acrylonitrile[51] onto chitosan.

the inherent viscosity was studied. The obtained copolymer wasused to prepare polymer metal complexes with Cu2+.

Preparation of Graft-CopolymersThe preparation of graft-copolymers by free radical polymer-ization of vinylic monomers in the presence of polysaccharideswas reported by several groups. Singh et al., for example, per-formed the MW-assisted (domestic MW oven) polymerizationof MMA,[49] acrylamide,[50] and acrylonitrile[51] onto chitosan(Scheme 3). In comparison with experiments under thermal heat-ing using a redox initiator the yields and grafting efficiencieswere improved under MW irradiation. Even an initiator-freepolymerization was successful under MW conditions.

Similar investigations with the typical improvements underMW conditions were performed with acrylamide[52] andacrylonitrile[53] onto guar gum, acrylonitrile[54] onto cassiasiamea seed gum, and acrylamide[55] onto potato starch. TheMW-assisted (domestic MW oven) grafting of MMA ontoκ-carrageenan was demonstrated by Siddhanta and coworkers.[56]

The influence of irradiation time, MMA/κ-carrageenan ratio,reaction temperature, and initiator concentration on MMA con-version and grafting efficiency was investigated. Sodium acry-late was polymerized under MW conditions (domestic MWoven) with potassium persulfate in the presence of corn starchand poly(ethyleneglycol) diacrylate as cross-linker by Pengand coworkers.[57] An investigation of the influence of irradi-ation power, irradiation time on swelling ratio, and solubilityof the obtained materials resulted in optimal conditions of85–90 W and 10 min. Compared with conventional heating,MW-assisted polymerization yielded products with a higherswelling ratio and lower solubility. Wang and coworkers per-formed the MW-assisted free radical cross-linking of acrylicacid and 2-(acrylamido)-2-methylpropane-1-sulfonic acid withN,N′-methylenebisacrylamide in the presence of corn starch.[58]

The obtained materials were studied with respect to their waterabsorbing properties. The authors showed that the swellingbehaviour of a material obtained after MW irradiation wasimproved compared with a material synthesized by the tradi-tional method. This finding was attributed to the occurrence of


NC

NC∆T, �N22

NN

CN

Scheme 4. Selective heating and resulting temperature gradient of ironfibres in a solution of MMA, ethylene glycol dimethacrylate, and 2,2′-azoisobutyronitrile in toluene.[60]

well-dispersed pores in the resin after MW-assisted synthesis asa result of the efficient homogeneous heating.

The scale-up of the MW-assisted (multimode MW reactor)living free radical polymerization of styrene from Rasta resinbeads is described by Pawluczyk et al.[59] It was found thatthis heterogeneous reaction was not directly scalable from reac-tion conditions determined on a lower scale. After optimization,however, 40–100 g quantities of Rasta resins with loading levels>3.8 mmol g−1 could be achieved.

Preparation of Composites by Free RadicalPolymerizationThe selective MW heating of strong MW absorbing materialsin less polar reaction media can be used to produce polymericcomposite materials. Ritter and coworkers described the prepa-ration of PMMA coated metal fibres by MW irradiation.[60] Forpreparation, iron fibres were placed in a solution of MMA, ethy-lene glycol dimethacrylate, and AIBN in toluene. The authorsdemonstrated that under MW irradiation the metal fibres heatedup much faster than the surrounding solution. Because of theresulting temperature gradient, the decomposition of the initiatorwas much faster near the metal surface than in the remain-ing solution (Scheme 4). After a certain irradiation time, themetal fibres were surrounded with a defined polymer layer. Incontrast to MW heating, classical thermal heating led to thecomplete polymerization of the bulk material. After acidolysisof the iron fibres, cross-linked PMMA with defined channel-likecavities were obtained, which were regarded to be of poten-tial interest in medical areas as artificial bone replacement oras catalysts. The MW-assisted synthesis (monomode MW reac-tor) of poly(acrylamide)–metal composites was shown by Zhuand Zhu.[61] A solution of acrylamide and metal salt (AgNO3,K2PtCl6, or CuSO4) in ethylene glycol yielded homogeneousdispersed metal nanoparticles in a poly(acrylamide) matrix after15 min of MW irradiation at 125◦C. Ag particles prepared byconventional heating showed a higher average diameter and abroadened size distribution, which was attributed to the longerreaction time of 150 min that was required to obtain the compos-ite. Similar results were recently published by the same group forthe MW-assisted preparation of poly(acrylamide)–metal sulfidecomposites.[62] Therefore, a mixture of the corresponding metalsalt, sulfur powder, and acrylamide in ethylene glycol was irradi-ated for a certain time to temperatures between 125 and 195◦C.Nanosized metal sulfide particles with a narrow size distributionhomogeneously dispersed in the poly(acrylamide) matrix wereobtained.

Emulsion PolymerizationThe application of the MW technique in emulsion poly-merization was intensively studied by several workgroups,although styrene and MMA were the monomers investi-gated most often. In general, the free radical heterophasepolymerization allows the synthesis of relatively high-molecular-weight polymers within a short reaction time. Themain advantage of emulsion polymerization is the use of waterinstead of volatile organic solvents.

Wu and Gao studied the MW-assisted (domestic MW oven)emulsion polymerization of styrene in the presence of sodiumdodecyl sulfate and potassium persulfate.[63] The investigationof the final particle size in dependence of the concentrationsof the surfactant, the initiator, and the styrene led to a modelwhich allows a simple particle size prediction. In a furtherstudy this model was expanded for the surfactant-free emul-sion polymerization of styrene in a water/acetone mixture underMW irradiation.[64] Vivaldo-Lima and coworkers developed akinetic model for the MW-assisted initiator-free emulsion poly-merization of styrene.[65] The authors could demonstrate that theinfluence of the intensity of the MW irradiation can be viewedas the influence of a chemical initiator. The predicted polymer-ization rates showed a good correspondence with experimentaldata from the literature. Holtze et al. applied short pulses of MWheating (10 s) followed by longer intervals of cooling to the emul-sion polymerization of styrene with different hydrophilic andhydrophobic initiators.[66] Under optimized conditions this strat-egy led to high molecular weights and high monomer conversionafter a short time. This finding is explained as a consequence ofthe absence of a termination reaction during the cold phase. Ina recent study, Holtze and Trauer showed the suitability of thisstrategy for continuous-flow processing that allows a treatmentof ∼10 L of emulsion per hour.[67] Zhu and coworkers performedthe nitroxide-mediated free radical miniemulsion polymeriza-tion of styrene with potassium peroxodisulfate as the initiatorunder MW conditions (monomode MW reactor) at 135◦C.[68]

In comparison with experiments under conventional heating, anincreased initiator decomposition was observed under MW con-ditions which led to an increase of monomer conversion. Theobtained molecular weights showed a linear relationship withrespect to the monomer conversion and a good correspondenceto the theoretical values.

Bao and Zhang performed surfactant-free emulsion polymer-ization of MMA under MW irradiation (domestic MW oven).[69]

In comparison with experiments performed under conventionalheating, the authors found a higher reaction rate under MWconditions. This acceleration was attributed to an activation ofMMA polymerization and enhanced initiator decomposition.Furthermore, the influence of monomer and initiator concen-tration on the particle size was studied. Similar results werefound by Palacios and coworkers by studying the MW-assisted(multimode MW reactor) emulsion polymerization of MMA.[70]

An acceleration of the polymerization rate by a factor of 137and polymer samples with a higher number-average molecu-lar weight and lower polydispersity could be obtained underMW conditions.The preparation of narrow disperse cross-linkedPMMA nanoparticles under MW conditions was demonstratedby Hawker and coworkers.[71] Under conditions of super-heatedsolvent of 25 wt.-% acetone/water a significant reduction of par-ticle size was observed. In addition, a wide range of diameterscould be obtained by varying the MW power. PMMA coat-ings on poly(N-isopropylacrylamide) particles were preparedby MW-assisted emulsion polymerization by Zhang and


coworkers.[72] The high polymerization rate of MMA underMW conditions prevented the diffusion of the MMA monomerinto the inner part of the particles and, therefore, led to thepolymerization staying on the surface.

The MW-assisted surfactant-free emulsion copolymeriza-tion of styrene and N-isopropylacrylamide was demonstratedby Xu and coworkers.[73] The particles obtained after MW-assisted polymerization were smaller and more uniform thanthose obtained with conventional heating. Similar results werepresented by the same group for a MW-assisted water/ethanoldispersion polymerization of styrene in the presence of AIBNand poly(N-vinylpyrrolidone) as stabilizer.[74]

Shi and Liu showed one example of a water-free two-phase polymerization of N-isopropylacrylamide in the presenceof AIBN as the initiator and N,N′-methylenebisacrylamide asthe cross-linker.[75] In this work poly(ethylene oxide)-600 wasused as the reaction medium and pore-forming agent. Theobtained hydrogels were studied with respect to their swellingbehaviour.

Controlled Radical PolymerizationThe controlled radical polymerizations belong to modern syn-thesis concepts. Because of the controlled/living mechanism itis possible to construct well-defined macromolecules of com-plex architecture. Controlled radical polymerizations includeatom-transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization (NMP), which proceed withreversible termination, and reversible addition–fragmentationtransfer (RAFT), which proceeds with reversible chain transfer.The MW-assisted (modified domestic MW oven)ATRP of MMAusing a system that consisted of α,α′-dichloroxylene, CuCl,and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine in bulk wasperformed by Zhu and coworkers.[76] The authors foundthat the polymerization rate could be enhanced by a fac-tor of 14 under MW conditions, which was attributed to ahigher solubility of the copper salt under MW irradiation.Conversely, Zhang and Schubert reported that the ATRP ofMMA with CuBr or CuCl/N-hexyl-2-pyridylmethynimine/ethyl2-bromoisobutyrate in solution of p-xylene showed no increasein polymerization rate when performed under MW conditions(monomode MW reactor).[77] Demonceau and coworkers inves-tigated the ATRP of MMA using RuCl2(p-cymene)(PCy3)/ethyl2-bromoisobutyrate in bulk.[78] In comparison with experimentsheated in an oil bath, the authors found an enhancement in poly-merization rate at 120◦C under MW conditions (monomodeMW reactor). In a recent study, the same group investigatedthe influence of reaction temperature and use of simultaneouscooling of the reaction vessel on the MW-assisted ATRP of thesame system.[79] It turned out that at 120◦C, MW-assisted ATRPwas three times faster than polymerization performed in an oilbath, and the controlled character of the polymerization wasmaintained. At temperatures higher than 120◦C, the polymer-izations were no longer controlled. Interestingly, when reactionmixtures were exposed to simultaneous cooling with MW irradi-ation, the controlled character of the polymerization was absenteven at lower temperatures (85–120◦C). The ATRP of acrylo-nitrile with FeBr2/isophthalic acid/2-bromopropionitrile underMW conditions (domestic MW oven) was performed by Houet al. (Scheme 5).[80] The obtained polymerization rate underMW irradiation was higher than the rate obtained when usingconventional heating under the same experimental conditions ofrefluxing tetrachloromethane.

MW and conventional heating,FeBr2, isophthalic acid,

nnN

NN

N

BrBr

Scheme 5. The atom-transfer radical polymerization (ATRP) of acrylo-nitrile with FeBr2/isophthalic acid/2-bromopropionitrile under microwave(MW) and conventional conditions.[80]

R

R

R Conversion (time)

�COOCH3

�O(CO)CH3

�C6H5

80% (20 min)�90% (15 min)

65% (400 min)

SS

S

S SS

OMW

,O O

O

AIBN

nn

Scheme 6. Microwave (MW)-enhanced reversible addition–fragmentation chain transfer (RAFT) polymerization of methyl acrylate,vinyl acetate, and styrene in minutes.[83]

In the field of MW-assisted RAFT polymerization Zhu et al.performed the bulk polymerization of styrene in the presence of2-cyanoprop-2-yl 1-dithionaphthalate with (72◦C) and without(98◦C) AIBN in a modified domestic MW oven.[81] In com-parison with polymerizations under conventional heating, a rateenhancement of 5.4 and 6.2 was observed under MW conditionswith and without AIBN, respectively. It was proven that firstorder kinetics, a linear relationship between molecular weightand conversion, a low polydispersity index (PDI), and the liv-ing character were retained using the applied conditions. Perrierand coworkers compared the reaction rate of methyl acrylate andMMA RAFT polymerization under MW (monomode MW reac-tor) and conventional heating.[82] In a system that consisted ofAIBN and ethylthiosulfanylcarbonylpropionic acid ethyl esteronly a slight increase in polymerization rate was observed underMW conditions while the reaction temperature was kept at 60◦C.In a more recent publication, the same group showed that theuse of a monomode MW reactor without controlling reactiontemperatures led to ultra-fast reaction rates of the RAFT poly-merization of MMA, vinyl acetate, and styrene (Scheme 6).[83]

In particular, methyl acrylate and vinyl acetate polymerizationswere almost quantitative after 15–20 min, whereby well-definedand well-controlled polymers were obtained.

Schubert and coworkers could successfully perform the NMPof methyl acrylate and tert-butyl acrylate at 120◦C in a solutionof dioxan under MW conditions (monomode MW reactor).[84]

Experiments using an oil bath heating source showed comparablepolymerization rates but broadened molecular weight distribu-tions for high conversion. These differences were attributed tothe uniform in situ heating mode under MW conditions.

Step-Growth Polymerizations

Several different chemical reactions may be used to synthe-size polymeric materials by step-growth polymerization wherebyusually the reactions proceed between two different functionalgroups. In the field of MW-assisted step-growth polymeriza-tions, esterification, amidation, imidation, and polymers from


OH OH

[SnCl2/p-TsOH],MW (40 W), 200°C,

30 min, reduced pressure

�(n � 1) H2OHOn

nO O

OH

Scheme 7. Microwave (MW)-assisted direct polycondensation of l-lacticacid.[86]

metal-catalyzed cross-coupling reactions, in particular, havebeen studied in recent years.

Polyesters and PolyamidesNagahata and coworkers presented the MW-assisted (monomodeMW reactor) synthesis of poly(butylenes succinate) frombutane-1,4-diol and succinic acid in the presence of 1,3-dichloro-1,1,3,3-tetrabutyldistannoxane as the catalyst.[85] The influenceof reaction time, reaction temperature, catalyst concentration,and monomer ratio on the polymer yield and the molecularweight was studied for bulk and solution polycondensation. Opti-mal conditions were found to be bulk conditions with a catalystconcentration of 2 mol-%, a reaction temperature of 220◦C, adicarboxylic acid/diol ratio of 1:1.2, and a MW irradiation timeof 20 min. A comparison with conventional heating under theseconditions showed a 10-fold increase of reaction rate under MWconditions. Similar results were found by Takeucho and cowork-ers for the direct polycondensation of l-lactic acid under MWconditions (monomode MW reactor).[86] After examination ofdifferent catalysts, the binary system SnCl2/p-TsOH was foundto be the one with the highest activity under applied conditions(Scheme 7). In this case the reaction times under MW conditionswere also found to be shortened compared with conventionalpolycondensation at the same temperature.

The preparation of a series of novel poly(ether ester)s basedon aliphatic diols, derived from isosorbide, was performed byChatti et al.[87] The polymers were obtained after 5 min of MWirradiation (monomode MW reactor) at 150 and 180◦C. Com-pared with thermal heating, it was demonstrated that the reactiontime can be reduced and degradation can be prevented underMW conditions. The reaction of sebacic acid with aliphaticaminoalcohols of different chain length under MW conditions(multimode MW reactor with variable frequency) was studiedby Borriello et al.[88] The according poly(ester amide)s wereobtained after 1 h of MW irradiation at 220◦C with a higher yieldand higher molecular weight than after 3 h of thermal heating atthe same temperature.

The polycondensation of benzoguanamine and pyromelliticdianhydride under MW conditions (domestic MW oven) wasreported by Lu and coworkers.[89] The resulting poly(amic acid)was further modified and the UV-vis as well as the non-linearoptical properties were studied. The MW-assisted (monomodeMW reactor) synthesis of polyamides from a isosorbide-baseddiamine and different diacyl chlorides was demonstrated byLoupy and coworkers.[90] The corresponding polyamides wereobtained after 6 min of MW irradiation at 200◦C in quantitativeyields and showed high glass transition temperatures (115–300◦C), melting points (239–>300◦C), and thermal stabilities.

PolyimidesPoly(succinimide) was prepared from aspartic acid by MWirradiation (domestic MW oven) under solvent-free and

catalyst-free conditions by Zhang and coworkers.[91] Ammoniawas added to a suspension of maleic anhydride in water andthe mixture was irradiated for only a few minutes. The obtainedpoly(succinimide) was then hydrolyzed under basic conditions topoly(aspartic acid). The influence of the ammonia/maleic anhy-dride ratio, MW power, and irradiation time, on the product yieldwas studied. Groth and coworkers prepared co-polyimides of dif-ferent composition from 4,4′-oxidianiline and benzene-1,2,4,5-tetracarboxylic acid/4,4′-(hexafluoroisopropylidene)diphthalicacid mixtures in a monomode MW reactor equipped with anauto-sampler.[92] After optimization, conditions of 45 min ofMW irradiation at 190◦C with a monomer ratio of 2:1:1 yielded apolymer with a molecular weight of 31 500 g mol−1 and a glasstransition temperature of 347◦C. The authors emphasized thepotential of time reduction by using an automated MW tech-nique, which facilitated the development of a polyimide materialwith high thermal and mechanical stability within weeks insteadof months.

In recent years, several poly(amide-imide)s have been pre-pared using domestic MW ovens. The investigations weremainly performed by the groups of Faghihi and Mallakpour.N,N′-(Pyromellitoyl)-bis(amino acid chloride)s from conver-sion of benzene-1,2,4,5-tetracarboxylic dianhydride with dif-ferent amino acids were converted with several aromaticdiamines[93–95] and hydantoin derivatives (Scheme 8).[96–98]

Because of the chirality of the employed amino acids, opti-cally active poly(amide-imide)s were obtained. It was shownthat the polycondensations were completed after short reactiontimes (<10 min) under MW conditions. Similar results werefound for the MW-assisted polycondensation of different N,N′-(4,4′-oxydiphthaloyl)-bis(amino acid chloride)s with aromaticamines,[99–103] of different N,N′-(4,4′-hexafluoroisopropyl-idenediphthaloyl)-bis(amino acid chloride)s with aromaticamines,[104,105] of N,N′-(4,4′-diphenylether)-bis(trimellitimideacid chlorides)s with hydantoin derivatives,[106] of N,N′-(4,4′-sulfonediphthaloyl)-bis(amino acid chloride)s with aromaticamines[107,108] as well of epiclon-based diamino acid chlo-rides with aromatic amines.[109–112] The corresponding conver-sion of N,N′-(3,3′-triphenylphosphine oxide)-bis(trimellitimideacid chloride) with different aromatic amines[113] and hydan-toin derivatives[114] yielded poly(amide-imide)s with flameretardant properties. Mallakapour and Rafiemanzelat presentedthe MW-assisted conversion of N-trimellitylimido-l-valine[115]

and bis(p-amido benzoic acid)-N-trimellitylimido-l-leucine[116]

with aliphatic and aromatic diisocyanates to yield opticallyactive poly(amide-imide)s. The influence of different sol-vents and MW irradiation time on polymer yield was stud-ied. The same authors presented the conversion of bis(p-amidobenzoic acid)-N-trimellitylimido-l-leucine with methy-lene di(phenylisocyanate) in the presence of poly(ethyleneglycol) under MW conditions. The corresponding poly(amide-imide-ether-urethane)s were obtained in a one-step[117] and in atwo-step synthesis.[118] The influence of the molecular weightof the poly(ethylene glycol), catalyst, MW power, and irradia-tion time on the polymer properties was studied. A comparisonof both methods showed that the formation of hard segmentsas oligo amide-imides in the first step, and chain extension bya polyol in the second step led to polymers with higher vis-cosities, crystallinity, thermal stability, and greater interchainhydrogen-bonding interaction.[119]

Recently, Khoee and Zamani presented the MW-assisted(domestic MW oven) synthesis of a photoactive poly(amide-imide) bearing an anthracene side group.[120] The polymer was


�(n � 1) HCl

�(n � 1) HCl

n OO

MW � n H2N�Ar�NH2

MW � n

O

O

O

O

Cl

O

O

N

O

OO

O

O n

O

ClN

H

H

H

N

NAr

OO

OO

O

n

NN N H

NHHN

OO

OR1

R1

R3

R1

R1

R1

R1 � CH2C6H5, CH(CH3)2, CH2CH(CH3)2R2, R3 � C6H5, C6H4CH3, CH3, C2H5

R1 � CH(CH3)2, CH3, CH(CH3)CH2CH3

R1

R2R3

R2

N

N

Cl

Cl

N

Scheme 8. Microwave (MW)-assisted polycondensation of N,N ′-(pyromellitoyl)-bis(amino acid chloride)s with hydantoinderivatives[96–98] and aromatic diamines.[93–95]

obtained after conversion of N,N′-(4,4′-oxy bis(1,4-phenylene))-bis(trimellitic anhydride amide) with N-(4,6-diamino-1,3,5-triazine-2-yl)anthracene-9-carboxamide. The polymer wasobtained with 80% yield after 10 min of MW irradiation. In con-trast, the reaction did not proceed under thermal conditions. Thepoly(amide-imide) was investigated with respect to its thermalstability and fluorescence behaviour.

Polymers from Metal-Catalyzed Cross-Coupling ReactionsIn organic chemistry, the performance of metal-catalyzed cross-coupling reactions is one of the most useful methods for formingcarbon-to-carbon and carbon-to-heteroatom bonds. In polymerscience, this method is used for the synthesis of conjugatedpolymers, which are of interest because of their applicabilityas electronic devices.

Scherf and coworkers presented the first MW-assisted prepa-ration of semiconducting polymers using palladium-catalyzedSuzuki and Stille reactions.[121] The ladder-type poly(arylene)-based materials were obtained after reaction in a monomode MWreactor by applying a constant power (150–300 W) with reactiontimes of 9–12 min. In comparison with the conventional syn-thesis (refluxing THF or toluene for 3 days), the reaction timecould be significantly reduced under MW conditions, althoughthe molecular weights and yields were analogous to those fromconventional synthesis. In a further work, Scherf and cowork-ers presented the MW-assisted Suzuki-type cross-coupling ofnovel naphthylene ladder polymers with different linkage andsubstitution patterns.[122] Similar to the former publication,the MW synthesis could strongly reduce the reaction time.But in two examples of sterically hindered 1,5-naphthylene-based polyketones, the desired structures were only attainablefrom the MW-assisted experiment. Tierney et al. presented theMW-assisted synthesis of polythiophenes by the Stille coupling(Scheme 9).[123] Using MW heating (monomode MW reactor,200◦C, 10 min), the polymer prepared from 5,5′-dibromo-4,4′-dioctylbithiophene and 2,5-bis(trimethylstannyl)thiophene hada higher molecular weight and lower polydispersity than thatobtained from conventional heating (132◦C, 24 h).

S

SSS

Br

Br

Br

MW,Pd0

n

n

S SSn Sn

Sn

�n

C8H17

C8H17

C8H17

C8H17

Scheme 9. Microwave (MW)-assisted synthesis of polythiophenes byStille coupling.[123]

The polycondensation of poly(m-phenylene ethylene)s underMW conditions was reported by Khan and Hecht.[124]

The authors described a new method that involved an in situactivation/coupling scheme. In comparison with thermal heat-ing, a solvent dependency of MW acceleration was found:experiments performed in benzene were not influenced by usingMW heating. The use of acetonitrile as a solvent instead of ben-zene led to a reduction of the reaction time under MW conditions.Ritter and coworkers presented the palladium-catalyzed Heck-type polycondensation of poly(phenylene vinylenes) under MWconditions (monomode MW reactor).[125] For an accurate com-parison with conventional heating, the polycondensations wereperformed in refluxing 1,4-dioxan. By comparing the yield andmolecular weight of polymer samples after different reactiontimes, the authors found only a slight acceleration under MWirradiation.

The MW-assisted (monomode MW reactor) nickel-catalyzedsynthesis of poly(biphenylmethylene)s from bistriflates ofbisphenol-type monomers was demonstrated by Carter andcoworkers.[126] The use of MW heating (10 min, 200◦C) facili-tated a dramatic reduction in reaction time compared with con-ventional heating (16–24 h, 80◦C), although the polymers were


N3

HN

ONH

O

HN

ONH

O

N

N N

HN

ONH

O

N

N N

n

n

n

MW, CuOAc, DMF, 30 min, 100°C, [M] � 250 mg mL�1

MW, CuOAc, DMF, 30 min, 100°C, [M] � 100 mg mL�1

Scheme 10. Microwave (MW)-assisted synthesis of peptide-based polymers by [2 + 3] cycloaddition backbonepolymerization.[133]

obtained with comparable yields and molecular weights by bothmethods. The synthesis of semiconducting polyfluorenes withelectrophosphorescent on-chain platinum–salen chromophoresby MW-assisted Yamamoto-type coupling was presented byScherf and coworkers.[127] It was shown that the reaction timeof 3 days under thermal heating in refluxing THF could bereduced to 12 min under MW irradiation at 150◦C in THFand 220◦C in DMF/toluene (depending on the substituentsof the used 2,7-dibromo-9H-fluorene derivatives). The poly-mers were studied with respect to their electroluminescenceefficiencies and used as phosphorescent organic light-emittingdiodes (OLED). Recently, Turner and coworkers reported thepalladium-catalyzed amination between 2,4-dimethylaniline and4,4′-dibromobiphenyl using a monomode MW reactor.[128] Thecorresponding polytriarylamines were obtained after only 5 minof MW irradiation at 100◦C. Extending the reaction time up to20 min, a reduction of molecular weight was observed, whichwas attributed to the occurrence of depolymerization reactions.

Other Step-Growth PolymerizationsThe synthesis of poly(dichlorophenylene oxide)[129] andpoly(dibromophenylene oxide)[130] in a domestic MW ovenwas presented by Kisakürek and coworkers. In both casesthe use of MW techniques could reduce the induction periodand reaction time compared with conventional methods. Troevand coworkers performed the transesterification of dimethylhydrogen phosphonate with poly(ethylene glycol) under MWconditions (monomode MW reactor).[131] After 55 min reactionunder MW conditions at different temperatures (140–190◦C), apoly(oxyethylene hydrogen phosphate) with a higher molecularweight was obtained after 9.5 h of conventional heating at tem-peratures between 130 and 185◦C. The synthesis of polyureasand polythioureas under MW conditions (domestic MW oven)was studied by Banihashemi et al.[132] The reaction between ureaor thiourea with different aliphatic and aromatic diamines in thepresence of p-toluenesulfonic acid as the catalyst in solution ofN,N-dimethylacetamide gave ∼90% polymer yield after 15 minof MW irradiation (200 W for 7 min and 400 W for 8 min).Because of the fast synthesis, a rapid optimization of reactionconditions was possible. Recently, Liskamp and coworkers pre-sented the MW-assisted synthesis of peptide-based polymersby MW-assisted cycloaddition backbone polymerization.[133]

The authors describe the application of an azide- and alkynyl-terminated dipeptide that consisted of phenylalanine and alanine

as monomer for the intermolecular copper-catalyzed 1,3-dipolar[2 + 3] cycloaddition reaction. Depending on the reaction con-ditions, linear polymers or medium-sized macrocycles wereobtained (Scheme 10). In comparison with thermal heating, MWconditions resulted in an increase of polymer length and reducedthe reaction time considerably.

Ring-Opening Polymerizations

The ring-opening polymerization (ROP) of cyclic monomerssuch as cyclic ethers, acetals, amides (lactams), esters (lac-tones), and siloxanes is of commercial interest in several sys-tems, including the polymerizations of ethylene oxide, trioxane,ε-caprolactam, and octamethylcyclotetrasiloxane. In the field ofMW-assisted polymerizations, the ROP of cyclic esters and ofcyclic imino ethers (mainly 2-oxazolines) are the particular focusof the research.

Ring-Opening Polymerizations of Cyclic EstersPolyesters polymerized from lactones or lactides attract muchinterest because of their biocompatibility,[134,135] thermal prop-erties, and their potential for chemical functionalization. Inthe past decade several publications concerned kinetic stud-ies of the metal-catalyzed ROP of ε-caprolactone under MWconditions.[136–144] Most of them describe a significant reductionin reaction time for polymerizations performed under MW con-ditions. In a recently published investigation, Liu and coworkersperformed the MW-assisted (multimode MW reactor) ROP ofε-caprolactone catalyzed by Sn(Oct)2 at different MW powersand compared the obtained reaction rates with polymerizationsperformed in a specially designed glass ampoule placed in apreheated salt bath.[145] It turned out that only for a reactiontemperature of 180◦C could a rate enhancement be observedunder MW conditions. Surprisingly, for polymerization temper-atures higher than 180◦C the rate constants were even higher inexperiments under thermal heating than those under MW irra-diation. Ritter and coworkers presented the MW-assisted directsynthesis of polyester macromonomers.[146] Thus, the ROP ofε-caprolactone was performed in the presence of (meth)acrylicacid and Sn(Oct)2 as the catalyst under MW irradiation. Becauseof the rapid non-contact heating under MW conditions, fast opti-mization of the synthesis to realise short reaction times was pos-sible. However, a comparison with classical thermal activationshowed no significant acceleration effect under MW conditions.Gong et al. studied the ZnO-catalyzed ROP of ε-caprolactone in


O

O

OO

O

OOHO

2

OO

OO

H

H

HH

O

p

q p–q

p–q

OH

OH

MW (680 W), 210°C,360 min

MW (680 W),50 min

n

m

n2n

n

m

Scheme 11. Catalyst-free microwave (MW)-assisted ring-opening polymerization of ε-caprolactone in the presence ofpoly(ethylene glycol)[153] and poly(vinyl alcohol).[154]

HS

HS

HOS

O

O

OO

O

H

MW 29% yield

Oil bath 18% yield

H

OHO

O� n

n

n

86%

14% 22%

78%

Scheme 12. Influence on the chemoselectivity of the enzymatic ring-opening polymerization of ε-caprolactone with2-mercaptoethanol as the initiator.[156]

the presence of ionic liquids. Because of the strong absorption ofthe ionic liquid, high heating rates and equilibrium temperatureswere obtained which led up to 65% yield after only 30 min. Fangand coworkers demonstrated the MW-assisted (domestic MWoven) ROP of ε-caprolactone onto amino-protected chitosan inthe presence of Sn(Oct)2 as the catalyst.[147] The synthesizedcomb-like chitosan-graft-poly(ε-caprolactone) copolymers wereobtained after short reaction times and had higher graft densitiescompared with those obtained from thermal heating. Recently,Gong and coworkers reported the MW-assisted preparation ofpoly(ε-caprolactone)/clay nanocomposites by the MW-assistedROP of ε-caprolactone in the presence of clay and Sn(Oct)2 asthe catalyst.[148] In comparison with experiments under conven-tional heating, an improved conversion was found under MWconditions. The obtained composite materials were studied withrespect to their morphology and thermal properties.

The metal-free carboxylic acid-catalyzed MW-assisted ROPof ε-caprolactone was presented by Liu and coworkers.[149–152]

In the latest example, mixtures of ε-caprolactone and benzoicacid were heated under MW irradiation (domestic MW oven).The influence of the reaction temperature on the molecularweight and the heating characteristics were studied.

Yu and Liu showed the catalyst-free MW-assisted(domestic MW oven) ROP of ε-caprolactone in the presenceof poly(ethylene glycol)[153] and poly(vinyl alcohol).[154]

The former yielded well-defined poly(ε-caprolactone)-block-poly(ethylene glycol)-block-poly(ε-caprolactone) tri-blockcopolymers and the latter comb-like poly(vinyl alcohol)-graft-poly(ε-caprolactone) copolymers (Scheme 11). The abilityof poly(ε-caprolactone)-block-poly(ethylene glycol)-block-poly(ε-caprolactone) tri-block copolymers to release ibuprofen as amodel drug was investigated.

Kerep and Ritter investigated the influence of MW-irradiation on the enzymatic ROP using Novozym 435 (immo-bilized lipase).[155,156] A comparison with conventional heatingwas performed at conditions of refluxing solvents. It turned outthat positive and negative MW-effects could be observed. In par-ticular, boiling toluene as a medium led to a strong deteriorationof the polymerization. Contrarily, a slight improvement couldbe observed when boiling diethyl ether was used as the sol-vent. Dependencies on the irradiation power were not found.In a further study the same authors investigated the change inchemoselectivity of the enzymatic ROP of ε-caprolactone with2-mercaptoethanol as the initiator. The findings displayed inScheme 12 exhibit a different selectivity under MW-irradiationwhere the formation of a higher amount of polyester with a ter-minal SH moiety was observed. This was accompanied by anincrease of the total polyester yield.

The MW-assisted ROP of d,l-lactide in the presence ofSn(Oct)2 was performed by Liu and coworkers in a domes-tic MW oven.[157] It was demonstrated that the ROP proceedsquickly (10 min) at different power levels. The molecular massof the obtained polymers showed a strong dependency of theapplied MW power, which was attributed to degradation reac-tions at higher power levels. Peng and coworkers polymerizedd,l-lactide in the presence of Sn(Oct)2 under MW irradiationat atmospheric pressure.[158] The effect of the application ofdifferent heating media, monomer purity, Sn(Oct)2 concentra-tion, and the application of reduced pressure on the molecularweight and the polymer yield was studied. In comparison withthermal heating, MW-assisted polymerization showed a higherthermal resistance and an enhanced reaction rate. Gong andcoworkers polymerized l-lactide in the presence of Sn(Oct)2by using poly(ethylene glycol)[159] and methoxy poly(ethylene


N

N

NN

N

NN

N

N

Nu Nu

R

R

R

R

R

R

R

R

R

O

n � 2

n � 1 n

O

O

O

O

OO

O

O

E

E

E

E

E�

��

� �

Scheme 13. Mechanism of the cationic ring-opening polymerization of2-oxazolines to yield poly(N-acylethylenimine)s.

glycol)[160] as macroinitiator. The ROPs were performed ina monomode MW reactor and yielded poly(l-lactide)-block-poly(ethylene glycol)-block-poly(l-lactide) tri-block copoly-mers and methoxy poly(ethylene glycol)-block-poly(l-lactide)di-block copolymers, respectively. In both cases, 20 min of MWirradiation at 100◦C led to high conversions (mostly >90%)using different monomer/initiator ratios and different molecularweights of the poly(ethylene glycol). A comparison with poly-merizations heated in an oil bath was performed with methoxypoly(ethylene glycol) as the initiator where a much slower chaingrowth was observed. The MW-assisted (domestic MW oven)ROP of p-dioxanone was shown by Wang and coworkers.[161]

The heating characteristics at different MW power levels werestudied. It was found that by applying a high MW power(>270 W) and long reaction times (>25 min at 270 W) a decreaseof yield and molecular weight takes place. This behaviour wasattributed to the depolymerization that takes place under theseconditions. In comparison with thermal heating, the ROP ofp-dioxanone was found to be accelerated under MW irradiation.

Ring-Opening Polymerization of Cyclic Imino EthersThe polymerization of substituted 2-oxazolines with cationic ini-tiators proceeds via a ring-opening isomerization mechanismand gives poly(N-acylethylenimine)s (Scheme 13). Becauseof the requirement of relatively high reaction temperaturesand a polar propagating mechanism the polymerization of2-oxazolines provides an ideal candidate for the successfulapplication of the MW technique.

The MW-assisted cationic ROP of various 2-oxazolines hasbeen intensively studied in the past 3 years. In 2004, Schubert andcoworkers reported about the MW-assisted (monomode reactor)polymerization of 2-ethyl-2-oxazoline with methyl tosylate inoverheated acetonitrile for the first time.[162] The influence of thereaction temperature on the reaction rate and the monomer con-centration (up to bulk polymerization) on the molecular weightdistribution was investigated. The authors ascribed the increasedreaction rate to thermal effects. This conclusion was based onthe determined activation energy, which was in agreement withliterature values from studies under thermal conditions. Fur-ther investigations of the polymerization kinetics of 2-methyl-,2-ethyl-, 2-nonyl-, and 2-phenyl-2-oxazoline under MW irra-diation followed.[163] It was shown that first order kinetics ofmonomer consumption and the living character of the poly-merization were conserved over a broad temperature range(up to 200◦C) for all monomers. As a result of the fast andefficient non-contact heating it was possible to reduce the solventamount, whereas a narrow distribution of molecular weight wasmaintained.

Comparative studies of the MW-assisted cationic ROP of2-phenyl-2-oxazoline with conversions under conventional heat-ing were performed by Sinnwell and Ritter.[164] Experimentsperformed both in overheated acetonitrile at 125◦C and in reflux-ing butyronitrile at ambient pressure (123◦C) showed an accel-eration of the reaction rate under MW irradiation (monomodereactor), which was attributed to the strong MW absorption ofthe ionic propagating polymer. In a further investigation the sameauthors polymerized 2-phenyl-5,6-dihydro-4H-1,3-oxazine, asix-membered cyclic imino ether, in refluxing butyronitrile atambient pressure by using different initiators (methyl tosylateand butyl iodide).[165] It was found that independently from theused initiator, the polymerization rate was enhanced by a fac-tor of 1.8 under MW irradiation. The authors suggested that theobserved accelerations were a result of the heating characteristicsof the dielectric heating. Schubert et al.[166] performed the MW-assisted polymerization of 2-phenyl-2-oxazoline under closedvessel conditions (117◦C) and a nearly identical polymerizationrate as reported by Ritter et al.[164] was observed. Importantly,nearly the same rate was also obtained when the polymerizationwas carried out by conventional heating applying reflux con-ditions, which indicates the absence of any non-thermal MWeffects. Further comparisons with thermal polymerizations of2-ethyl- and 2-phenyl-2-oxazoline at 120 and 140◦C performedin pressurized reactors showed the same reaction rates underboth MW and thermal conditions.[167]

Regardless of this discussion, a series of publications reportabout the advantages of a MW-assisted ROP of 2-oxazolines.The MW technique was used for the rapid preparation of new2-oxazoline (co)polymers. Schubert and coworkers, for exam-ple, investigated the solvent dependency of the reaction rateof the polymerization of 2-nonyl-2-oxazoline,[168] the poly-merization of a series of 2-alkyl-2-oxazolines,[169] and of afatty acid-based (from soybeans) 2-oxazoline[170] under MWconditions. All polymerizations could be performed in shortreaction times because of the fast and effective dielectric heat-ing. Furthermore, the successful synthesis of a 42-memberedlibrary of 2-oxazoline diblock copolymers,[171] which was inves-tigated according the structure–property relationship of themorphology of spin-coated films,[172] and the preparation ofdiblock copolymers from the soybean-based 2-oxazoline andits micelle formation, were described.[173,174] The synthesisof 30 triblock terpolymers from four different monomers waspossible for the first time because of an improved livingnessunder MW conditions.[175] Recently, even the preparation ofwell-defined tetrablock ter- and quaterpolymers was possible aresult of the improvements under MW conditions.[176] The MW-assisted statistical copolymerization of 2-phenyl-2-oxazolinewith 2-methyl- or 2-ethyl-2-oxazoline was described to yieldquasi-diblock copolymers.[177] Another example of the sta-tistical copolymerization of 2-(pent-4-ynyl)-2-oxazoline with2-methyl- or 2-ethyl-2-oxazoline with methyl triflate under MWconditions was described by Luxenhofer and Jordan.[178] Theclassical polymerization procedure of 70 h at 85◦C was improvedunder MW conditions (monomode MW reactor) where, after20 min at 135◦C (150 W), the copolymer was obtained withan excellent yield (94%) and a narrow molar mass distribution(PDI = 1.06). The polymer bond alkyne side function was usedfor conversion to triazoles in a further step.

The possibilities of up-scaling the cationic ROP of 2-ethyl-2-oxazoline were investigated by Schubert and coworkers byboth a batch-type[179] and a continuous-flow approach.[180]

In the batch-type experiments, a 4 M monomer solution in


acetonitrile was polymerized on a 1 mL (monomode reactor),50 mL (monomode reactor), and 250 mL (multimode reac-tor) scale. The obtained gel permeation chromatography tracesdemonstrate the high reproducibility of the polymerization inde-pendent of the set-up without further optimization. To studythe MW scale-up under continuous-flow conditions, four dif-ferent set-ups were used, which consisted of two different flowcoils and a small tube with flow rates of 0.01–20 mL min−1

in a monomode reactor and a continuous stirred tank reactorwith flow rates of 12–130 mL min−1 in a multimode reactor.Compared with bath-type experiments, the molecular weightdistributions were broadened, which was attributed to differentresidence times of the four set-ups. Further improvements werefound by using ionic liquids as solvents for the MW-assistedcationic ROP of 2-ethyl-2-oxazoline (monomode reactor).[181]

The use of 1-butyl-3-methyl-imidazolium trifluorophosphatefacilitated a shorter reaction time because of a lower activationenergy while maintaining the living character of the polymer-ization. The strong and efficient MW absorption of the ionicliquid and its effortless recycling were given as opportunitiesfor the development of ‘green’ MW technologies. This ideawas developed further by using water-soluble ionic liquids forthe MW-assisted polymerization of 2-phenyl-2-oxazoline and2-(3,5-difluorophenyl)-2-oxazoline (monomode reactor) whichyielded hydrophobic polymers.[182] Even though the obtainedPDI values of polymers were partially higher then expected,this method enabled a convenient isolation of the polymers andrecovery of the ionic liquid because of its water solubility.

Polymer Modifications

The modification of polymers plays an important role in currentpolymer research. In particular, the metal catalyzed ‘click’ reac-tion of azides and alkynes is a very promising method to obtaincomplex polymer architectures with a high efficiency.[183,184]

The conversion of other functional groups, for example, car-boxylic or hydroxy groups by amidation or esterification, alsoprovides suitable procedures for a successful polymer analo-gous reaction. The following section will discuss the use of MWtechniques in the field of polymer modification.

As mentioned above, the 1,3-dipolar cycloaddition reactionof substituted azides and alkynes is a widely applied methodin polymer science because of its high efficiency and excel-lent tolerance of functional groups and solvents. Kappe andVan der Eycken could show on low-molecular-weight systemsthat the reaction time of the cycloaddition could be signif-icantly reduced under MW conditions.[185,186] Consistently,Liskamp and coworkers reported the MW-assisted ‘click’-reaction between azido peptides and dendrimeric alkynes.[187]

Compared with the classic synthesis where yields between43–45% were obtained, the yield could be increased to 96% after10 min of MW irradiation at 100◦C. Similar effects were reportedby Hawker and coworkers.[188] They could perform the quan-titative functionalization of alkyne-terminated 3,5-dioxybenzylether dendrimers with p-(azidomethyl)benzoic acid methylesterafter 10 min of MW irradiation at 140◦C with Cu(PPh3)3Br ascatalyst. The synthesis of a star-shaped poly(ε-caprolactone) bya ‘click’ reaction is described by Schubert and coworkers.[189]

They used an acetylene-functionalized poly(ε-caprolactone),prepared by the 5-hexyn-1-ol initiated ROP of ε-caprolactone,and heptakis-azido-β-cyclodextrin for the cycloaddition in thepresence of sodium ascorbate and copper sulfate (Scheme 14).

N3

OO O

O

O

O

OO

NNN

CuSO4, Na-ascorbate,MW, 100°C, 900 s

n

n

OHOH

OH

OHHO

HO7

7

7

�

Scheme 14. Microwave (MW)-assisted ‘click’ reaction of acetylene-functionalized poly(ε-caprolactone) and heptakis-azido-β-cyclodextrin.[189]

m

MW (300 W), 90 min,toluene

MW (300 W), 5 min,HOTs

H2N

m H2OO

O O

O

OO

OH

HO

HOR1

R1

R1 R2 R3

R3

R3R2

R2

OH

H H H

H �OCH3 H�CH3�CH3 �CH3

HN�

m� m H2O

n

n

n m

m

m

�

�

Scheme 15. Microwave (MW)-assisted direct hydroxyalkylamidation andesterification of poly(ethylene-co-acrylic acid).[190,191]

After 15 min of MW irradiation at 100◦C a complete conversionof the azido groups was achieved.

The use of poly(ethylene-co-acrylic acid) (PEAA) as a suit-able base material for chemical modifications under MW con-ditions (monomode MW reactor) was reported by Sinnwell andRitter. The direct hydroxyalkylamidation of PEAA under MWconditions was carried out with 2-(2-hydroxyethoxy)ethylaminein the presence of toluene as solvent (Scheme 15).[190]

The dependence of the conversion on the reaction time and thetemperature was investigated. A comparison with conventionalheating showed a slight improvement under MW conditions.With the application of the obtained hydroxyalkylamide-functionalized poly(ethylene) as a polyinitiator for the ROPof ε-caprolactone it was possible to form well-defined comb-like structures of poly(ethylene)-graft-poly(ε-caprolactone). Afurther investigation concerned the MW-assisted polymer-analogous esterification of PEAA with dissimilar phenols(Scheme 15).[191] The synthetic procedure was performed usingan excess of phenols as a pseudo-solvent in the presence ofp-toluenesulfonic acid as the catalyst.The MW experiments werecompared with conventional thermal heating in an oil bath. Theinfrared spectroscopy (FT-IR) spectroscopic analyses showed thefull conversion of the acid group for all three phenols under MWconditions after 5 min of reaction time. In contrast, after reac-tions under thermal conditions, the acid remained and only minorconversions could be determined.


HO O

R

HO HNR

O

OO,R � ,

94 NH

NH

O

MW (75 W),20 min,∼220°C

H2N� m� m H2On

p m

Scheme 16. Synthesis of the hydrophobically modified poly(acrylic acid)sby a polymer analogous reaction of poly(acrylic acid) with different aminesunder microwave (MW) irradiation.[192]

Hydrophobically modified poly(acrylic acid) was preparedby Ritter and coworkers.[192] After irradiating a mixture ofpoly(acrylic acid) and aminoalkyl-functionalized adamantanesfor 20 min, polymers with adamantyl side chains with differ-ent alkyl lengths were prepared (Scheme 16). Aqueous solutions(2 g L−1) of the resulting polymers showed gelation behaviourwith a different solution viscosity depending on the length of thehydrophobic side chain. It was shown that the properties of thehydrogels could be influenced by addition of cyclodextrin.

Cai and coworkers obtained C60 end-capped polystyreneand PMMA after MW-assisted fullerenation of the accordingbromo-double-terminated polymers with C60.[193] The bromo-double-terminated polymers were obtained from the ATRP ofstyrene and MMA usingα,α′-dibromo-p-xylene as a bifunctionalinitiator under conventional heating. The C60 end-capped poly-mers were obtained after 15 (PMMA) and 20 min (polystyrene)of MW irradiation (modified domestic MW oven). Comparedwith the conventional heating, an increase of the reaction rateunder MW conditions was observed. Varma and Nadagoudacould prepare metallic and bimetallic cross-linked poly(vinylalcohol) nanocomposites under MW conditions (monomodeMW reactor).[194] The materials were obtained by reduction ofthe according metal salt in aqueous solution in the presence ofpoly(vinyl alcohol) after 1 h of MW irradiation at 100◦C. Themorphologies of the composites were investigated by scanningelectron microscopy and showed the occurrence of nanospheres,nanodendrites, and nanocubes. More recently, the same authorspresented the MW-assisted synthesis of cross-linked poly(vinylalcohol) nanocomposites comprising single-walled carbon nan-otubes, multi-walled carbon nanotubes, and fullerenes.[195] Thematerials were obtained by a similar procedure as reported in theprevious work. For single-walled carbon nanotubes it was possi-ble to indicate from transmission electron micrographs that withincreasing an concentration of nanotubes, an increasing deposi-tion of cross-linked poly(vinyl alcohol) on the nanotube surfacetook place.

Summary and Perspectives

In this review, the recent developments of MW-assisted synthesisin polymer chemistry are outlined. The variety of publicationsin this field shows that MW techniques can be successfully usedfor nearly all types of polymerization reactions. Comparativestudies between reactions under MW conditions and conversionsperformed with a conventional heat source often resulted in thefinding of rate enhancements under MW conditions. And eventhough not all publications report about this acceleration effect,the authors agree that the use of a MW technique provides afast, efficient, homogeneous, and convenient heating method that

facilitates the optimization of reaction conditions in a short time.In this context, the examination of applicability of high reactiontemperatures plays the most important role.

Although the reliability and reproducibility of the exper-iments have especially benefited from the development ofdedicated MW reactors since the late 1990s, several laborato-ries still use domestic MW ovens, which often do not allowaccurate reaction control. This may certainly be a result of thestill relatively high costs of laboratory MW equipment, which isprobably the major drawback of this technology. But consider-ing the establishment of MW techniques in the recent years, itis expect that less expensive equipment will be available in thefuture and its application will become favoured against domesticMW ovens.

Because of limitations in reaction volume, in most of thecited examples only small-scale (typically ≤10 mL) syntheseswere performed. Nevertheless in some recently published works,the possibilities of scale-up were investigated. Therefore, bothcontinuous-flow and batch-type processing have been proven aspromising trends to use the benefits of a MW-assisted polymersynthesis on a kilogram scale. Further improvements were foundby using ionic liquids as solvents for MW-assisted polymeriza-tions.The advantages of a high polarity and high MW absorption,high thermal stability, non-flammability, high boiling points, andunusual solubility properties were given as opportunities for thedevelopment of ‘green’ MW technologies.

The use of MW techniques has demonstrated many promisingresults and is, therefore, expected to become a standard toolin polymer laboratories within a few years. If the MW-assistedpolymer synthesis will find industrial applications in addition iscurrently uncertain. Here more investigations are required thatshow the possible cost advantages of the MW technology.

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