Block and Graft Copolymers

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11CONTeNTs11.1 11.2

Block and Graft CopolymersAli E. Muftuoglu, M. Atilla Tasdelen, Yusuf Yagci, and Munmaya K. Mishra

Introduction .................................................................................................307 Synthesis of Block Copolymers ..................................................................308 11.2.1 Block Copolymers via Conventional Radical Polymerization ......309 11.2.2 Block Copolymers via Controlled Radical Polymerization Routes ............................................................................................ 310 11.2.3 Block Copolymers via Nitroxide Mediated Polymerization (NMP)............................................................................................ 314 11.2.4 Block Copolymers via Atom Transfer Radical Polymerization (ATRP) .......................................................................................... 317 11.2.5 Block Copolymers via Reversible Addition Fragmentation Chain Transfer Polymerization (RAFT) ....................................... 325 11.3 Synthesis of Graft Copolymers ................................................................... 329 11.3.1 Synthesis of Graft Copolymers via Conventional Radical Polymerization Methods................................................................ 329 11.3.1.1 Graft Copolymers by Grafting onto Method ............. 329 11.3.1.2 Graft Copolymers by Grafting from Method ............ 330 11.3.1.3 Graft Copolymer Synthesis via Macromonomers ......... 330 11.3.2 Synthesis of Graft Copolymers via Controlled/Living Radical Polymerization Methods................................................................ 333 11.4 Conclusion ................................................................................................... 337 References .............................................................................................................. 337

11.1 INTRODUCTIONSynthesis of materials with novel properties has attracted growing scientific interest in recent years. A great deal of research activity has been devoted to this fundamental issue due to endless demands of the industry for high-tech applications. A number of strategies have been sought by polymer researchers as well as many others, in need of preparing new materials displaying improved physical and chemical properties. Such improved properties might be achieved by combining a number of physical and chemical properties and can usually be adopted by either blending homopolymers307

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Handbook of Vinyl Polymers: Radical Polymerization and Technology

or by chemically linking different polymer chains. When two homopolymers are mechanically mixed, formation of an immiscible blend is often the case encountered. Microphase-separated heterogeneous mixtures arising from the incompatibility of the blended polymers can successfully be avoided by the latter strategy, which involves preparation of block and graft copolymers. Thus, engineering macromolecules of various block and graft structures appears to be an elegant approach in achieving polymers with improved physical and chemical properties. Preparation of block copolymers using living polymerization techniques has been known for quite a long time. Anionic polymerization [1], which provides endgroup control and permits the synthesis of polymers with a narrow polydispersity index, is a noteworthy example of these techniques. Yet, it does not offer full-control over the molecular weight of the synthesized macromolecules. The limited choice of monomers and the extremely severe reaction conditions lower its applicability. Recently, advances in controlled polymerization techniques [25], such as radical, cationic, metathesis, and group transfer, have allowed for the synthesis of polymers with predetermined molecular weights and low polydispersities. A variety of block copolymers has been prepared by cationic, group transfer, metallocene, and metathesis routes. Among others, controlled radical polymerization routes have been studied extensively, because it can be employed for the polymerization of numerous vinyl monomers under mild reaction conditions. These are atom transfer radical polymerization (ATRP) [68], nitroxide-mediated polymerization (NMP) [911], and reversible addition-fragmentation chain transfer polymerization (RAFT) [12, 13]. The significant advance in the block and graft copolymer synthesis has come with the advent of controlled radical polymerization (CRP) techniques [14, 15]. One way to prepare block and graft copolymers is by using main chain and side chain macroinitiators, respectively. In this method, reactive sites are produced at the chain ends or side chains, which serve as initiating moieties in the polymerization of a second monomer. In another approach, separately synthesized macromolecular chains having antagonist groups or latent active sites are made to react with each other. If these groups are located at the chain ends, the resulting polymer is a block copolymer. In a similar way, the chains bearing the reactive end-functions can be interacted with pendant moieties of a backbone yielding a graft copolymer. This chapter covers the synthetic strategies to prepare block and graft copolymers via radical polymerization routes, focusing on the latest, state-of-the-art studies.

11.2 sYNTHesIsOFBLOCKCOPOLYMeRsNormally, block copolymers cannot be synthesized by classical free radical copolymerization technique. Spontaneous block copolymer formation upon free radical copolymerization of A and B monomers would only occur when both reactivity ratios ra and r b are far larger than unity. Such system has never been found. Generally, one can get access to block copolymers by the following strategies. According to the most common methodology, monomer A is polymerized completely, after which, monomer B is introduced in the mixture and its polymerization proceeds upon initiation by the active site of the first block. This approach is referred to as sequential monomer addition in controlled/living polymerization methods. For obtaining a

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Block and Graft Copolymers

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well-defined block copolymer, the living site of the first monomer must be effective in initiating the polymerization of the second monomer. That is, simultaneous initiation of all growing B chains must be ensured and the rate of the crossover reaction must be higher than the rate of propagation of monomer B. Another route involves the use of a difunctional initiator and has been employed in the preparation of ABA symmetric triblock copolymers. In this methodology, a compound possessing two initiating sites is utilized in the formation of the middle block first, followed by the polymerization of the second monomer to synthesize the first and the third blocks. This allows the preparation of the ABA blocks in two, instead of three steps without fractionation or other purification steps. The difunctional initiator must be chosen to initiate the polymerization with the same rate from either direction. Moreover, AB diblock or ABA triblock copolymers can be prepared by coupling two appropriately end-functionalized chains of A and B homopolymers or AB blocks, respectively. In the latter case, block B should initially contain half the molecular weight of the final desired block B. The efficiency of coupling will be high if the reaction is rapid and the living diblock copolymer is used in excess.

11.2.1 BlockcopolymersviaconventionalradicalpolymerizationThe most widely employed method for the preparation of block copolymers by using conventional radical polymerization route is the macroinitiator technique. This technique has several advantages. First, macroinitiators can be fully characterized before their use in the free radical step to obtain block copolymers. Moreover, the macroinitiators can be prepared practically by all polymerization methods. In living polymerizations, it is possible to introduce the free radical initiating functionalities into polymers at both initiation and termination steps as presented in Scheme 11.1. Azo or peroxy groups are incorporated into the free-radical initiating sites of the polymers. Typical examples of such methodology by using living anionic and cationic polymerizations are given in Schemes 11.2 [16] and 11.3 [17], respectively. Tables 11.1 and 11.2 give the compilation of block copolymers prepared by using macroinitiators possessing thermolabile azo and peroxy groups, respectively.Initiation Functionalization Macroinitiator Termination Functionalization

= Initiator for Monomer 1 = Initiator for Monomer 2

= Monomer 1 = Monomer 2

sCHeMe11.1

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Handbook of Vinyl Polymers: Radical Polymerization and TechnologyO NH2 NH C O N O O CH3 CN CH3 N C CN O O R n CH3 CN

CH2 CH2 C N2

R 2n H O R n O

NH CH C NH NH C CH2 CH2 C N

CH2 CH2 C NH NH C CH NH

sCHeMe11.2

11.2.2 BlockcopolymersviacontrolledradicalpolymerizationroutesWell-defined block copolymers can be synthesized by using well-known anionic polymerization technique and the first study dates back to as early as 1956, in which styrene and isoprene were successfully blocked anionically [95]. In tailoring welldefined blocks, it is essential that the polymerization be performed in the absence of undesired transfer and termination, which can be provided quite successfully with anionic polymerization. However, it has several drawbacks, such as limited choice of monomers and the extremely severe reaction conditions. Radical polymerization is the most widely used method in the polymerization of numerous vinyl monomers without the need to provide special reaction conditions required for ionic polym