Binary Copolymerization with Catalytic Chain Transfer. A Method for Synthesizing Macromonomers Based on Monosubstituted Monomers

With appropriate choice of conditions, copolymerization of monosubstituted monomers [CH 2 CHX: e.g., styrene, butyl acrylate (BA)] in the presence of small amounts of an α-methylvinyl monomer [CH 2 C(CH 3 )Y: e.g., α-methylstyrene (AMS), methyl methacrylate (MMA), methacrylonitrile (MAN)] and a cobaloxime as chain transfer catalyst provides a route to macromonomers that are composed largely of the monosubstituted monomer and yet have a chain end derived from the α-methylvinyl monomer (−CH 2 −C( CH 2 )Y). The various factors (temperature, concentrations, type of cobaloxime, types of monomer) that influence molecular weight and end group purity and the importance of the various side reactions that may complicate the process are described. Macromonomer purity is enhanced by increasing the concentration of the α-methylvinyl monomer and reducing the cobaloxime concentration. It also depends on the structure of the cobaloxime, increasing in the series where the ligands are derived from dimethyl glyoxime < diethyl glyoxime ∼ diphenyl glyoxime, and the α-methylvinyl monomer, increasing in the series where Y is CO 2 R < CN < Ph. For styrene−AMS copolymerization, macromonomer purity (the fraction of AMS-derived ends) was enhanced by increasing the reaction temperature. For BA−AMS copolymerization, macromonomer purity was enhanced by decreasing the reaction temperature. However, it is necessary to use a high reaction temperature to limit the extent of macromonomer copolymerization that occurs as a side reaction at high monomer conversion. High purity, AMS terminal BA macromonomers can be prepared by BA−AMS copolymerization at 125 °C with as little as 2 mol % AMS. We also show how the overall composition, molecular weight, and end group functionality of copolymers formed in the presence of a chain transfer agent can be predicted using classical statistics and point out some problems with previous treatments. Analytical expressions which describe zero conversion binary copolymerization in the presence of a transfer agent are derived and successfully applied to the above-mentioned system. The effective transfer constants of cobaloxime transfer catalysts are reported. Those observed in copolymerizations of vinyl and α-methylvinyl monomers are reduced with respect to values observed in homopolymerization of α-methylvinyl monomers because of reversible consumption of the cobalt complex as an adduct to the vinyl monomer. Transfer constants of macromonomers with AMS end groups in styrene polymerization at 120 °C are in the range 0.11−0.15. At lower temperatures (80 °C), macromonomer copolymerization dominates over chain transfer and the effective transfer constant is ∼0.