Control of Molecular Weights, Polydispersities and Functionalities inControlled/'Living' Polymerizations. Comparison of Carbocationic and Radical Systems

摘要

For a long time carbocationic and radical polymerizations have been considered asvery difficult if not impossible to control at a level attainable for anionic polymerization of styrene and dienes. Usually polymers with unpredictable molecular weights, broad polydispersities and uncontrolled end functionalities have been obtained in both systems. Different reasons, related to the very fundamental nature of carbocations and radicals, are responsible for such a behavior. Carbocations react extremely rapidly with alkenes and readily participate in transfer reactions by loss of beta-protons, especially in the presence of basic impurities. On the other hand, radicals recombine and disproportionate with rates close to diffusion controlled limits. Thus, in a typical carbocationic and radical polymerization, concentration of growing specie (carbocations or radicals) must be very low, otherwise cationic polymerization would be too fast and/or radicals would recombine too quickly.