Free radical polymerization telechelic polymers

The goal of producing low cost ( 1—5/lb.) acrylic block, comb, star, and telechelic polymers by GTP and anionic polymerization has not been met. Free radical polymerization of acrylics and other vinyl monomers on the other hand requires little purification of materials, works in water and other protic solvents, and is low cost. Considerable efforts are presently under way therefore to develop controlled free radical polymerization methods. 

For nitroxide-mediated radical polymerizations and in the RAFT process, the same synthetic strategy as for ATRP can be used in the synthesis of AB and ABA block copolymers. The first step is coupling a functionalized alkoxyamine with a telechelic or monofunctional nonvinylic polymer to give a macroinitiator. This macroinitiator can be used in standard controlled free-radical polymerization procedures. This approach is best illustrated by the preparation of PEO-based block copolymers [81-84]. One example is the preparation of macroinitiator LMI-7 by the reaction of a monohydroxy-terminated PEO with sodium hydride followed by reaction with the chloromethyl-substituted alkoxy amine as shown in Scheme 3.16. 

In addition to the initiator-controlled polymer functionalization, transfer reactions may result in functional polymers. In free-radical polymerization, thiols are often employed as chain transfer agents. Chain transfer reactions involving thiols proceed via atom abstraction, as illustrated in Scheme 3. Consequently, these molecules do not offer any scope for introducing functionalities at both ends. However, monofunctional telechelics have been successfully prepared by using thiols. For example, Boutevin and co-workers [39,40] introduced polymerizable vinyl groups to polyvinylchloride accord-... 

The photochemically and thermally induced iniferter properties of the tetraalkylthiuram disulfides during free radical polymerization were also exploited to end functionalize PMMA and PSt (Scheme 3.6). Table 3.4 summarizes the functional iniferters used for obtaining telechelic polymers. 

Although the telechelic functional polymers are very attractive from a fundamental point of view, their synthesis is often impossible. Much more commonly, the active groups are incorporated in the chain either by a free-radical copolymerization with a small amount of functionalized comonomer or by functionalization of the chain after polymerization in the presence of free radicals (typical of the functionalization of the polyolefins). Either method generally produces several reactive sites per chain. 

The versatility associated with nitroxide-mediated polymerizations, in terms of both monomer choice and initiator structure, also permits a wide variety of other complex macromolecular structures to be prepared. Sherrington201 and Fukuda202 have examined the preparation of branched and cross-linked structures by nitroxide-mediated processes, significantly the living nature of the polymerization permits subtlety different structures to be obtained when compared to traditional free radical processes. In addition, a versatile approach to cyclic polymers has been developed by Hemery203 that relies on the synthesis of nonsymmetrical telechelic macromolecules followed by cyclization of the mutually reactive chain ends. In a similar approach, Chaumont has prepared well-defined polymer networks by the cross-linking of telechelic macromolecules prepared by nitroxide-mediated processes with bifunctional small molecules.204... 

There are three ways that can be used to regulate the molecular weights of telechelic polymers produced by free radical methods chain transfer, control of the initiator-monomer ratio, and the temperature of polymerization. 

Advantages of end-group transformation include the ability to incorporate functionality incompatible with the polymerization procedure, to prepare halogen-free materials for subsequent reactive processing, to allow characterization of the initial copolymer prior to further functionalization, and an ability to prepare telechelic polymers, block copolymers, and materials that can be immobilized to surfaces, by a full range of substitution and addition chemistry. The use of a difunctional initiator allowed for the first time in a radical process preparation of functional homo-telechelic polymers with almost any desired chain end functionality (Scheme 33). ... 

The synthesis of low molecular weight difunctional carboxyl-terminated butadiene-based polymers via either free radical or anionic polymerization is well established. Teyssie and co-workers have converted such carboxyl-terminated polymers to salt forms, which they refer to as halato-telechelic polymers, by neutralization with metal alkoxides in appropriate solvents. The quantitative removal of low molecular weight reaction products is necessary to drive the reaction to completion and fully realize the ionomeric properties of these materials. 

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