CHAIN-GROWTH CONTROL IN FREE-RADICAL POLYMERIZATION

The present review relates generally to improved processes in which it is possible to control the growth steps of a radical polymerization t o produce relatively monitored chain-length homopolymers and copolymer s, including block, star and graft copolymers. The review, which conta ins more than 360 references to original works, has only to be taken a s a snapshot of this rapidly developing area. Investigations summed up in this article are mainly those carried out by 'polymer synthesis sc hools' over the world over the past 20 years, with a derailed analysis of results obtained over the past three years. Indeed, works pertaini ng to the possibility of continuous radical polymerization to be achie ved through the complexing and stabilization of Free radicals, often c alled 'living' radical polymerizations, have witnessed an explosive gr owth in the past decade. These latter are chain-growth polymerizations , free from chain transfer, termination and other side reactions, and are promising methods for the synthesis of well-defined polymers. Desp ite the inherent instability of the growing macroradicals, 'living' ra dical polymerizations of vinyl monomers have recently been investigate d based on the general principle of 'radicophilic stabilization of the growing macroradical'. Three approaches have been followed in efforts to develop radical polymerization systems exhibiting living character s: The first uses physical methods to prevent radicals from contacting each other, thereby preventing their termination. The process is gene rally conducted in a heterogeneous medium. The second involves chain-g rowth control by chemical stabilization of the growing macroradical in homogeneous polymerization. The third approach, which involves no liv ing process but exhibits some living characteristics, is based on the repeated reinitiation of polymer chains either by homolysis (thermo-or photolysis) of weak and/or reversible linkages that are built into th e chains or by addition-fragmentation processes on macromonomers.