Biometal Derivatives as Catalysts for the Ring-Opening Polymerization of Trimethylene Carbonate. Optimization of the Ca(II) Salen Catalyst System

Schiff base derivatives of the biometals (Zn, Mg, Ca) in the presence of anion initiators have been shown to be very effective catalysts for the ring-opening polymerization of trimethylene carbonate (TMC or 1,3-dioxan-2-one) to poly(TMC) devoid of oxetane linkages. The order of catalytic activity as a function of metal was found to be Ca(II) ≫ Mg(II) > Zn(II). Optimization of the calcium system was achieved utilizing a salen ligand with tert-butyl substituents in the 3,5-positions of the phenolate rings and an ethylene backbone for the diimine along with an azide ion initiator. These conditions led to a TOF of 1286 h-1 for a melt polymerization carried out at 86 °C. Solution studies in tetrachloroethane demonstrated the polymerization reaction to proceed via a mechanism first order in [monomer], [(salen)Ca], and [anion initiator] and to involve TMC ring-opening by way of acyl−oxygen bond cleavage. The activation parameters were determined to be ΔH⧧ = 20.1 kJ/mol and ΔS⧧ = −128 J/(mol K).