Chain-End Functionality

One of the major advantages of living free radical chemistry, when compared to other living procednres for the polymerization of vinyl monomers, is the stability of the initiating, or propagating, centers. This has enabled the development of a wide variety of functionalized unimolecular initiators for the synthesis of well-de ned linear polymers, block copolymers, and other complex macromolecnlar structures. ATRP of monomers with functionalized alkyl halide initiators leads 

Problem 11.11 Write the structures of chain end functionalized polymers obtained by ATRP of styrene and methyl acrylate from the following functionalized initiators (I)  

For styrene (St) (1) 4-bromobenzyl bromide (2) 2-bromopropionitrile (3) vinyl chloroacetate and (4) 

For methyl acrylate (MA) (1) allyl bromide (2) hydroxyethyl 2-bromopropionate (3) a-bromo-y- 

The structure of ATRP polystyrene resulting from the cited functionalized initiators are shown in Fig. 11.22 and those of ATRP px ly(methyl acrylate) that can be obtained from the cited functionalized initiators are shown in Fig. 11.23. These cases have been reported in the literature (Matyjaszewski et al., 1998). 

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Ruths M and Granick S 1998 Rate-dependent adhesion between opposed perfluoropoly (alkyl ether) layers dependence on chain-end functionality and chain length J. Rhys. Chem. B 102 6056-63... 

When a polymer is prepared by radical polymerization, the initiator derived chain-end functionality will depend on the relative significance and specificity of the various chain end forming reactions. Tlius, for the formation of telechelic polymers ... 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

Hyperbranched polymers generally have very low melt and indinsic viscosities. The large number of chain-end functional groups present in hyperbranched macromolecules have also been shown to dramatically affect physical properties... 

Izumisawa, S. and Jhon, M. S., "Stability Analysis and Molecular Simulation of Nanoscale Lubricant Films with Chain-End Functional Groups, /. Appl. Phys., 2002, Vol. 91,2002, pp. 7583-7585. 

Scheme 7 Fomation of chain-end functionalized nickelacycle initiators...

The bifunctional initiator 4-hydroxy-bulyl-2-bromoisobulyralc, HBBIB, promoted the ATRP of styrene as well as the cationic ring opening polymerization of THF [134], In the presence of Cu/CuBr2/PMDETA styrene was polymerized through the bromoisobutyrate function of HBBIB, to give PS chains end-functionalized with hydroxyl groups, PS-OH. The in situ... 

A second, and potentially more useful feature is the stability of these unimolecu-lar initiators to a wide variety of reaction and polymerization conditions which is in sharp contrast to traditional initiators for anionic procedures, such as n-butyl lithium. This allows the initiators to be fully characterized, purified and handled by normal techniques, thus simplifying the polymerization process. It also permits a variety of chemical transformations to be performed on the initiator prior to polymerization, which greatly facilitates the preparation of chain end functionalized macromolecules. For example, the chloromethyl functionalized al-koxyamine, 18, can be readily converted in high yield to the corresponding aminomethyl derivative, 19, followed by polymerization to give well-defined linear polymers, 20, with a single primary amine at the chain end (Scheme 12). 

The application of these functionalization reactions to polymers has been catalogued in the anionic polymer review literature (4-6). Unfortunately, many of the reported applications of these functionalization reactions to anionic chain-ended polymers have not been well characterized (7). In order to exploit these functionalization reactions to their potential, well-defined procedures for quantitative chain end functionalization must be available. 

The use of AB monomers for the preparation of both hyperbranched and dendritic macromolecules leads to the presence of an extremely large number of chain end functional groups for both systems. In analogy with dendrimers, the... 

Functional homopolymers can be synthesized by essentially two different methods. The first and more preferred way is to use a functional initiator which will ensure a high rate of chain end functionality. For instance, the polymerization of St initiated by a unimolecular terpyridine-functionalized nitroxide initiator yields well-defined PS homopolymers. The second technique is based on post-polymerization modifications. In this case, the reaction between mPEG and chloroterpyridine yields terpyridine-functionalized PEG building blocks, as illustrated in Scheme 13. 

A cationic mono-Cp-based cobalt complex FlO-6 can initiate the living polymerization of ethylene, which has been successfully applied to the preparation of chain-end-functionalized polymers (see Section 11.20.4.5.4). " ... 

In recent years several attempts have been made to prepare polymers possessing chain end functions capable of giving rise to free radical or to cationic sltes i This research has been mostly aimed at extending the possibilities of synthesis of block copolymers, in which only one of the blocks is obtained anionlcally. The synthesis of -hydroperoxy polymers has already been mentioned. Peroxy-or peranhydride functions have also been introduced into polymer chainsSubsequent radical polymerization of a second monomer results in block copolymers. 

Chain end functionalization of reactive carbocationic monomers, like isobutyl vinyl ether, can occur using ionic nucleophilic quenching reagents, i.e., methanol, alkyl lithium, etc. (6). 

However, chain end functionalization does not occur when these reagents are added to living polymerization of less reactive monomers such as isobutylene (7). 

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