Effect of Quaternary Phosphonium Salts as Cocatalysts on Epoxide/CO2 Copolymerization Catalyzed by salen-Type Cr(III) Complexes

Organic quaternary phosphonium salts are used in conjunction with chromium salen complexes in the alternating copolymerization of CO2 and cyclohexene oxide (CHO) or propylene oxide (PO). A highly efficient formation of atactic polycarbonate with narrow distribution and molecular weights comparable or superior to those obtained with bis(triphenylphosphine)iminium (PPN+) salts can be achieved with phosphonium salts such as triphenyl(4-pyridinylmethyl)phosphonium chloride hydrochloride (UHFFA) and tetraphenylphosphonium chloride (TPPCl). For CHO-based copolymers, a structure-property correlation can be observed: the Tg value seems to be related both to the amount of ether linkages and to the molecular weight. In addition, in the case of CHO, FTIR/ATR spectroscopy on the pretreated catalytic system can reliably predict the success of the copolymerization before the introduction of the epoxide monomer. Equilibria between the cation and anion in the phosphonium salts under study and the competition between epoxide and X- have been shown to play a role in the activity, molecular weight, selectivity, and polymer properties. A general view of these equilibria and of the polymerization mechanism has been given as well as a rationale of differences in CHO and PO copolymerization behavior and polymer properties.