| DESCRIPTION |
Among the most versatile living radical polymerization techniques is a process known as reversible addition-fragmentation chain transfer (RAFT). In this process, a propagating polymer radical (Pm*) adds to a dormant species (Pn-X), where X is a RAFT chain transfer agent, to generate a new radical (Pm-X*-Pn). This intermediate radical fragments either into a new propagating radical Pn* and a new dormant species Pm-X, or back to Pm* and Pn-X. The RAFT agent establishes a dynamic addition-fragmentation equilibrium by transferring activity between the propagating radicals and the dormant species. The polymerization can be reactivated by the addition of a more radical initiator and monomer. WHAT DISTINGUISHES RAFT polymerization from all other methods of controlled/living free-radical polymerization is that it can be used with a variety of monomer types, including some which have been notoriously difficult, such as unprotected (meth)acrylic acids, acrylamides, vinyl acetate, and N-vinyl pyrrolidone. The procedure is one of the most forgiving, allowing a wide range of polymerization conditions to be used," he remarked. "For example, it has been successfully performed over the temperature range 20 to150 °C in a broad range of solvents, including water. RAFT polymerization is also unique in allowing polymerization conditions close to those used in conventional radical polymerization." The RAFT process provides controlled-molecular-weight polymers with polydispersities of less than 1.2 and sometimes less than 1.1. Polydispersity is a measure of the range of molecular weights of the constituent molecules of the polymer. A monodisperse polymer is composed of molecules of uniform molecular weight
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