ESR spectroscopic studies of radical polymerization

Radical polymerization, a typical chain reaction involving a short-lived and highly reactive propagating radical as the chain carrier, has been studied by various ESR spectroscopic techniques. Not all systems can be studied, as the steady state concentration of propagating radicals is not always above the limit of detection of ESR. This review deals with recent advances in the ESR spectroscopic studies of radical polymerization involving detection of a variety of radical species, such as primary radicals from initiators, primary propagating radicals, and propagating radicals. Furthermore, spin trapping techniques can be applied where direct detection of the radical species fails, and can be used to obtain structural information about radical species. Propagating radicals from a variety of monomers have been quantified during polymerizations in homogeneous systems. The absolute values of the rate coefficients of propagation and termination have been determined from ESR quantification of the propagating radical for conventional monomers, sterically congested monomers, and macromonomers. ESR techniques have been used to determine the rate coefficients from low to high conversion in bulk polymerizations of methyl methacrylate and styrene. The values of the propagation rate coefficients for some copolymerizations have been determined by ESR methods, and the validity of the penultimate model has been supported by the dependence of the apparent values on comonomer composition. ESR spectroscopic study of emulsion polymerization has also been employed to quantify the propagating radicals and to determine the corresponding rate coefficients. (C) 1999 Elsevier Science Ltd. All rights reserved.