End-Functional Styrene-Maleic Anhydride Copolymers via Catalytic Chain Transfer Polymerization

Styrene-maleic anhydride copolymers have been successfully synthesized using catalytic chain transfer polymerization employing the low spin [bis(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF) complex. By partially replacing styrene with alpha-methylstyrene (while maintaining the amount of maleic anhydride at 50 mol %) over a range of ratios, it was shown that the rate of reaction and molar mass decreases with increasing alpha-methylstyrene content. The polymers were characterized using MALDI-ToF-MS and H-1-C-13 gHMQC NMR to determine the end groups, which in the presence of alpha-methylstyrene was an alpha-methylstyrene unit with a vinylic functionality. For styrene-maleic anhydride copolymers, the end group was determined to be predominantly maleic anhydride with a vinylic functionality. Considering the fact that in a styrene-maleic anhydride copolymerization the propagating radicals are predominantly of a styrenic nature, this was a very surprising result, suggesting that the maleic anhydride radicals undergo a chain transfer reaction, which is orders of magnitude faster than that of styrenic radicals. This conclusion was supported by high-level ab initio quantum chemical calculations, which showed that hydrogen abstraction from the maleic anhydride radical is 40 kJ/mol more exothermic than that from a styrene radical. The chain transfer constant of COBF was determined for the different ratios of styrene and alpha-methylstyrene. It was found to increase 2 orders of magnitude from a purely styrene-maleic anhydride to a purely alpha-methylstyrene-maleic anhydride copolymer. Diels-Alder and thiol-ene reactions were performed on the vinylic end groups as postpolymerization modification reactions, as well as graft copolymerization reactions of the macromonomers with styrene and butyl acrylate.