Yttrium Phosphasalen Initiators for <i>rac</i>-Lactide Polymerization

Abstract

A series of highly active yttrium phosphasalen initiators for the heteroselective ring-opening polymerization of <i>rac</i>-lactide are reported. The initiators are yttrium alkoxide complexes ligated by iminophosphorane analogues of the popular “salen” ligand, termed “phosphasalens”. A series of novel phosphasalens have been synthesized, with varying substituents on the phenoxide rings and ethylene, propylene, <i>rac</i>-cyclohexylene, <i>R</i>,<i>R</i>-cyclohexylene, phenylene, and 2,2-dimethylpropylene groups linking the iminophosphorane moieties. Changing the substituents on the phosphasalen ligands results in changes to the rates of polymerization (<i>k</i>_obs) and to the PLA heterotacticity (<i>P</i>_s = 0.87). Generally, the initiators have high rates, excellent polymerization control, and a tolerance to low loadings