Initiation of ring-opening polymerization of lactide: The effect of metal alkoxide catalyst

A comprehensive computational study of the initiation stage of the ring-opening polymerization (ROP) of meso-lactide catalyzed by various metal alkoxides MOR (M = Li, Mg, Zn, OR = OiPr, OtBu) is presented. Density functional theory at B3LYP/6-31G(d,p) level is applied in examining the coordination-insertion mechanism and reactivity. The influence of tetrahydrofuran medium is simulated both as an explicit coordination and as a bulk solvent. The theoretical computations characterized energetically two possible channels for initiation of the ROP of m-lactide. These channels are associated with the possible attack of the catalyst at two enantiotopic O-acyl bonds of the lactide. The attack at the least staggered plane of the lactide molecule is energetically favored. The theoretical results reveal that the energy pathway for the initiation of ROP catalyzed by alkoxides proceeds via 3 major steps: (1) coordination-addition of the alkoxide MOR to the lactide carbonyl bond; (2) shift of the metal (one step) or rotation of the metal and alkyl (from the alkoxide) grouping in two steps; (3) acyl-oxygen cleavage that leads to ring opening of the lactide. The results reveal that the activity of the initiators follows the trend Li > Mg > Zn. Depending on the ligand, the catalytic activity of the alkoxides decreases in the order OiPr > OtBu. The relative reactivity of the alkoxides is rationalized in terms of theoretical electronic structure indices: NBO charges on the carbonyl oxygen atom and the energy of the LUMO orbital of the alkoxide catalyst.