Telechelic free-radically initiated polymerization

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... 

DEP afforded the synthesis of telechelic oligomers [29]. With the telomerization process, DEP appeared to be the first free-radical polymerization leading to telechelic oligomers. Tobolsky [30] first stated the conditions of DEP, i.e., the half life of the growing species has to be equivalent to that of the initiator. These specific conditions result in an unusual high rate of termination, which allows for the synthesis of oligomers. The telechelic structure will be obtained by combining a termination mode exclusively by recombination with the use of a difunctional initiator. 

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. 

Alternatively, end-functionalization of growing chains can also be obtained via transfer in chain addition polymerization. For instance, dihydroxy polybutadiene telechelics (i.e. carrying one hydroxyl group at each chain-end) are industrially produced by free radical polymerization of butadiene initiated by hydrogen peroxide which simultaneously gives rise to transfer reactions ... 

The synthesis of telechelics via radical polymerization techniques has been extensively reviewed. Telechelics are typically synthesized free radically by controlling the structure of the end groups either by using a large amount of initiator (dead end polymerization), or by... 

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-... 

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). ... 

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