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Synthesis of Nanocomposite Organic/Inorganic HybridMaterials Using Controlled/Living Radical
Jeffrey Pyun and Krzysztof Matyjaszewski*
Center for Macromolecular Engineering, Department of Chemistry, Carnegie MellonUniversity, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213
Received February 16, 2001
The preparation of hybrid organic/inorganic nanocomposites comprised of well-definedpolymers was reviewed. In particular, synthetic methods using controlled/living radicalpolymerization techniques, such as stable free-radical/nitroxide-mediated polymerizations,atom transfer radical polymerization, and reversible addition-fragmentation chain-transferpolymerization were described. The various approaches taken to prepare hybrid copolymers,nanoparticles, polymer brushes, dispersed silicate nanocomposites, and nanoporous materialswere discussed.
The synthesis of novel materials with improvedproperties and performance is a continually expandingfrontier at the interface of chemistry and materialsscience. In this pursuit, the ability to control molecularstructure on atomic and macroscopic dimensions is akey parameter in designing materials with prepro-grammed activity. A significant advance in this area hasbeen the synthesis of nanocomposites where the struc-tural order within the material can be controlled onnanometer/submicron scales. While materials possess-ing such structural complexity are common in nature,robust and versatile methods to prepare syntheticnanocomposites remains an exciting challenge that isbeing tackled by research groups around the world.1
One approach to prepare nanocomposites has been theincorporation of well-defined organic and inorganiccomponents into a singular material. In particular, theinclusion of well-defined polymers to inorganic sub-strates is of significance, because the functionality,composition, and dimensions of these macromoleculesenable the design of specific properties into the resultinghybrid.2
Well-defined organic polymers have been attached toinorganic (co)polymers, particles, surfaces, glassy net-works and interpenetrating polymer networks to pre-pare organic/inorganic hybrid materials. Additionally,polymers of controlled size, composition, and architec-ture have been used as shape templates in the synthesisof mesoporous inorganic networks (Figure 1). Polymers,such as poly(tetramethylene oxide) and poly(oxazolines),have been used to synthesize hybrid organic/inorganicnanocomposites.3 However, recent developments incontrolled/living radical polymerization (CRP) haveprovided another valuable methodology to introducewell-defined organic (co)polymers to a variety of inor-ganic substrates. The scope of this review will coverhybrid organic/inorganic nanocomposites that have been
made using CRP. For more fundamental discussions oforganic/inorganic hybrid materials, the reader is di-rected to other reviews.2-5
2. Controlled Radical Polymerization
CRP has proved to be a versatile and robust methodto prepare well-defined organic polymers. In the pastdecade, several techniques have been developed tosynthesize well-defined polymers via radical polymer-ization. A major difference between conventional radical[i.e., azobis(isobutyronitrile)- or peroxide-initiated pro-cesses] and controlled radical polymerizations is thelifetime of the propagating radical during the course ofthe reaction. In conventional radical processes, radicalsgenerated by decomposition of the initiator undergopropagation and bimolecular termination reactionswithin a second. In contrast, the lifetime of a growingradical can be extended to several hours in a CRP,enabling the preparation of polymers with predefinedmolar masses, low polydispersity, controlled composi-tions, and functionality.6,7
The mechanism invoked in CRP processes to extendthe lifetime of growing radicals utilizes a dynamicequilibration between dormant and active sites with
Figure 1. Examples of organic/inorganic hybrid materials.
3436 Chem. Mater. 2001, 13, 3436-3448
10.1021/cm011065j CCC: $20.00 2001 American Chemical SocietyPublished on Web 08/03/2001
rapid exchange between the two states. Unlike conven-tional radical processes, CRP requires the use of per-sistent radical (deactivator) species, or highly activetransfer agents to react with propagating radicals.These persistent radicals/transfer agents react withradicals (deactivation or transfer reactions with rateconstant, kd) to form the dormant species. Conversely,propagating radicals are generated from the dormantspecies by an activation reaction (with rate constant,ka).
2.1. Classification of CRP Systems. In the pastdecade, the field of CRP has seen tremendous develop-ment as evidenced by the wide range of materials thathave been prepared using these techniques. In particu-lar, three methods of considerable importance are thefollowing: stable free-radical polymerization [SFRP;e.g., nitroxide-mediated processes (NMP)], metal-cata-lyzed atom transfer radical polymerization (ATRP), anddegenerative transfer [e.g., reversible addition-frag-mentation chain transfer (RAFT)]. While these threesystems possess different components, general similari-ties in the CRP processess can be seen in the use ofinitiators, radical mediators (i.e., persistent radicals ortransfer agents), and in some cases catalysts (Scheme1). It is important to note that while SFRP and ATRPare subject to the persistent radical effect (PRE),8,9degenerative processes, such as RAFT, do not conformto the PRE model because of the transfer-dominatednature of the reaction.
2.1.2. SFRP. In this type of CRP, alkoxyamineinitiators10 (-PnX; eq 1 in Scheme 1) and nitroxidepersistent radicals (X0; eq 1 in Scheme 1) have beeneffectively used to polymerize styrenes and acrylates.In certain systems, alkoxyamines have also been gener-ated in situ by the initial use of conventional radicalinitiators (AIBN and peroxides) and nitroxide persistentradicals, which also led to a CRP process.11 A widelyused nitroxide in the polymerization of styrene (Sty) is2,2,6,6-tetramethylpiperidinyloxy (TEMPO), althoughrecently developed nitroxides can also polymerize acry-lates in a controlled fashion.12,13 The current limitationin this system lies in the inability to successfullypolymerize methacrylate monomers, because of -hy-drogen elimination to the nitroxide radicals. Addition-ally, thiuram disulfides and dithiocarbamate iniferter
systems have been used as agents for CRP with limitedsuccess.14,15
2.1.3. ATRP. In these polymerizations, radicals aregenerated by the redox reaction of alkyl halides (-PnX;eq 2 in Scheme 1) with transition-metal complexes (Y;eq 2 in Scheme 1).16-18 Radicals can then propagate butare rapidly deactivated by the oxidized form of thetransition-metal catalyst (X-Y0; eq 2 in Scheme 1).Initiators typically used are R-haloesters (e.g., ethyl2-bromoisobutyrate and methyl 2-bromopropionate) orbenzyl halides (e.g., 1-phenylethyl bromide and benzylbromide). A wide range of transition-metal complexes,such as Ru-, Cu-, and Fe-based systems, have beensuccessfully applied to ATRP. For Cu-based systems,ligands such as 2,2-bipyridine and aliphatic amineshave been employed to tune both the solubility andactivity of various ATRP catalysts. ATRP has beensuccessfully applied for the controlled polymerizationof styrenes, (meth)acrylates, (meth)acrylamides, acry-lonitrile, and 4-vinylpyridine. ATRP systems are cur-rently limited to monomers that do not strongly coor-dinate to the catalyst.18
2.1.4. Degenerative Transfer. Radical polymeriza-tions based upon a degenerative transfer system relyupon the rapid and reversible exchange of highly activetransferable groups (-X; eq 3 in Scheme 1) and growingpolymeric radicals (-Pm0 and -Pn0; eq 3 in Scheme 1).While conventional radical initiators (AIBN and perox-ides) are used in this reaction, these compounds onlyserve as a radical source to drive reversible exchangereactions between active and dormant states. Transferagents in this process contain moieties for both initiationand transfer which are generated in the presence ofradicals. Controlled radical polymerizations from de-generative transfer reations have been done using alkyliodides,19 unsaturated methacrylate esters,20 or thioestersas the transfer agents. In particular, the use of thioestersin the radical polymerization of vinyl monomers resultsin a RAFT polymerization.21 The RAFT process hasproven to be a versatile method to polymerize functionalstyrenes, (meth)acrylates, and vinyl esters.
2.2. Synthetic Methodologies To Prepare Nano-composite Hybrids Using CRP. A variety of CRPtechniques have been developed to incorporate well-defined organic polymers to inorganic substrates. A keyadvantage of CRP processes is the facile functionaliza-tion/deposition of initiator and polymerizable moietiesonto inorganic polymers and surfaces.
2.2.1. Synthesis of Hybrid Homopolymers andBlock, Graft, and Random Copolymers. In thepreparation of organic/inorganic nanocomposite materi-als, hybrid copolymers are of particular interest becauseof the inherent incompatibility of the two segments.Thus, phase separation of these segments yields avariety of controlled nanostructures depending on thedegree of incompatibility of the components, the com-position, and the degree of polymerization in the finalcopolymer.
A wide range of polymeric structures have been madeusing different combinations of organic and inorganiccomponents. In the simplest case, a vinyl monomerpossessing an inorganic moiety (R ) inorganic moiety;Scheme 2a) can be homopolymerized in the presence ofinitiator (R-X) to prepare a hybrid homopolymer with
Scheme 1. Classification and General Mechanismof CRP Methods for (1) SFRP, X ) O-NR2, (2)
ATRP, X ) Halogen, Y ) Mtn/Ln, Y0 ) X-Mtn+1/Ln,(3) Degenerative Transfer