New Zinc Dicarboxylate Catalysts for the CO₂/Propylene Oxide Copolymerization Reaction: Activity Enhancement Through Zn(II)-Ethylsulfinate Initiating Groups

A series of zinc dicarboxylates were synthesized by the reaction of diethylzinc with dicarboxylic acids. Zinc monocarboxylate monoalkyl intermediates were obtained by using a defined excess of diethylzinc over dicarboxylic acid. A subsequent insertion reaction of SO₂ into these zinc alkyl bonds resulted in a defined number of Znethylsulfinate groups, which act as active centers for the copolymerization reaction of CO₂ and propylene oxide. Corresponding ethylsulfinic acid ester end groups were detected in the poly(propylene carbonate) (PPC) products. The polymerization activity depended strongly on the number of sulfinate groups incorporated and has been significantly increased compared to conventional zinc dicarboxylates. The obtained PPCs have molecular weights (M̄_w) exceeding 80 000 g · mol^-1 and polydispersities in the range of 2.3 to 3.3.