Alternating Ring-Opening Polymerization of Cyclohexene Oxide and Anhydrides: Effect of Catalyst, Cocatalyst, and Anhydride Structure

Ring-opening copolymerization of cyclohexene oxide with alicyclic anhydrides containing different ring strain (succinic anhydride, cyclopropane-1,2-dicarboxylic acid anhydride, and phthalic anhydride) was performed applying metal salen chloride complexes, (salen)MCl (M = Al, Cr, Co; salen = N,N-bis(3,5-di-tert-butylsalicylidene)diimine) with different metals and ligand diimine backbones. While some of the bulk copolymerizations afforded poly(ester-co-ether)s, all solution polymerizations produced perfect alternating copolymers. The chromium catalysts performed best while the aluminum catalysts were the least active ones. For each metal, the salophen complexes yielded the best performing catalyst. A variety of cocatalysts have been employed: bis(triphenylphosphoranylidene)ammonium chloride, N-heterocyclic nucleophiles including 4-(dimethylamino)pyridine, N-methylimidazole, and 1,5,7-triazabicyclododecene and the phosphines trimesitylphosphine, tris(2,4,6-trimethoxyphenyl)phosphine, tricyclohexylphosphine to triphenylphsophine. Of all cocatalysts, bis(triphenylphosphoranylidene)ammonium chloride was found to be the most efficient cocatalyst in combination with salophenCrCl for the copolymerization of cyclohexene oxide with phthalic anhydride, and 1 equiv was enough to reach optimum activity. N-Heterocyclic nucleophiles showed the lowest activity. Of the three anhydrides used, phthalic anhydride is the most reactive giving the highest conversions and the highest molecular weight products.