Ring-Opening Polymerization of Trimethylene Carbonate Using Aluminum(III) and Tin(IV) Salen Chloride Catalysts

Abstract

Aluminum and tin salen complexes have been shown to effectively catalyze the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) to polycarbonate. The most active salen derivative in each instance contained a phenylene backbone with chloro substituents in the 3,5-positions of the phenolate rings, with the aluminum derivatives being significantly more active than their tin(IV) counterparts. Importantly, the resultant polycarbonate was shown by 1H NMR to be void of ether linkages. The reaction was demonstrated to proceed via a mechanism first order in both [catalyst] and [monomer] and to involve TMC ring-opening by way of acyl oxygen bond cleavage. Consistent with a reaction pathway involving an insertion of the monomer into the metal−nucleophile bond (e.g., Al−Cl or Sn−Cl), the activation parameters were determined to be ΔH⧧ = 51 kJ/mol and ΔS⧧ = −141 J/(mol deg).