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><sub>obs</sub>) and to the PLA heterotacticity
(<i>P</i><sub>s</sub> = 0.87). Generally, the initiators
have high rates, excellent polymerization control, and a tolerance
to low loadings
