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 to produce relatively monitored chain-length homopolymers and copolymers, including block, star and graft copolymers. The review, which contains more than 360 references to original works, has only to be taken as a snapshot of this rapidly developing area. Investigations summed up in this article are mainly those carried out by 'polymer synthesis schools' over the world over the past 20 years, with a derailed analysis of results obtained over the past three years. Indeed, works pertaining to the possibility of continuous radical polymerization to be achieved through the complexing and stabilization of Free radicals, often called 'living' radical polymerizations, have witnessed an explosive growth 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. Despite the inherent instability of the growing macroradicals, 'living' radical polymerizations of vinyl monomers have recently been investigated 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 characters: The first uses physical methods to prevent radicals from contacting each other, thereby preventing their termination. The process is generally conducted in a heterogeneous medium. The second involves chain-growth control by chemical stabilization of the growing macroradical in homogeneous polymerization. The third approach, which involves no living 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 the chains or by addition-fragmentation processes on macromonomers.