Specifics of catalytic chain transfer in free-radical polymerization of poly(ethylene oxide) macromonomers

The effect of system components on the efficiency of substituted isopropylpyridinato cobal(3+)oxime boron fluoride as a chain-transfer catalyst in the free-radical polymerization of poly(ethylene oxide) (PEO) methacrylate macromonomers in aqueous solution was studied. When 4,4'-azobis(4-cyanovaleric acid) was used as an initiator, the transition to the neutral buffer medium enhanced the chain-transfer phenomenon: molecular-mass distribution curves of comb-shaped soluble polymers shifted to the region of shorter networks, and the non-Gaussian behavior of polymer chains disappeared at smaller concentrations of the catalyst. Presumably, this effect is related to the partial hydrolysis and loss of activity of the transfer catalyst in acidic medium. Analogous and even more pronounced changes in the elastic behavior of networks were observed for photopolymerization initiated by 2,2-dimethoxy-2-phenylacetophenone. In both cases, the value of elastic modulus as a formal measure for the crosslink density of networks decreased. To clarify the possibility of competition between PEO chains and the chelate shelf of a Co ion, the bulk polymerization of methyl methacrylate was carried out in the presence of oligomeric PEO additives to yield PMMA with longer chains than those prepared without additives; in some cases, the resulting polymer was characterized by bimodal molecular-mass distribution. One may suppose that the both observed effects are partially responsible for the low efficiency of catalytic chain transfer in the systems of interest owing to a decrease in concentration and/or imperfection of the cobaloxime structure.