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

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 1 H 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 Δ H ⧧ = 51 kJ/mol and Δ S ⧧ = −141 J/(mol deg).