Biometal derivatives as catalysts for the ring-opening polymerization of trimethylene carbonate. Optimization of the Ca(II) salen catalyst system
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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 substituants 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 86C. 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 H = 20.1 kJ/mol and S = -128 J/(mol K). 2006 American Chemical Society.