Free-radical polymerization in ionic liquids: The case for a protected radical

The free-radical polymerization of methyl methacrylate (MMA) in ionic liquids at low monomer concentration is reported with emphasis on elucidating the "magic" rate and molecular weight enhancement that are often observed. We show that traditional methods of molecular weight capping using dodecylmercaptan as chain transfer agent significantly reduces the molecular weight of the polymer, but to a much lesser extent than analogous reactions in xylene. Similarly, the adverse effect of elevated temperatures upon reactions of this type is much less significant for polymerizations conducted in ionic liquids than those in organic solvents. Indeed, almost quantitative yield can be obtained for polymerization at temperatures up to 120 degrees C in ionic liquid, while almost no polymer is observed in an organic solvent case due to rapid initiator burnout. These factors lead to the proposal that a "protected" radical mechanism is in operation; however, elucidation of the exact nature of this protection remains elusive. As an extension of this hypothesis, block copolymers of methyl methacrylate, grown from styrene, have been prepared and characterized by NMR, GPC-MALLS, GPEC, and DSC. The absence of poly(methyl methacrylate) homopolymer in the final product suggests that the monomer is only initiated from "protected" polystyrene macroradicals in the ionic liquid. This process cannot be reproduced in organic solvents, unless additional control agents are present. Furthermore, we report much higher molecular weights of A-B block copolymers than those previously reported in the literature.