Controlled Polymerization of dl-Lactide and ε-Caprolactone by Structurally Well-Defined Alkoxo-Bridged Di- and Triyttrium(III) Complexes

We have prepared the yttrium(III) alkoxides [(L Me2 )(HL Me2 )Y 3 (OCH 2 C 6 H 5 ) 4 ] ( 3) and [( S -L Me )Y] 2 ·2(HOCH 2 C 6 H 5 ) ( 1 ·2BnOH) through treatment of two previously developed complexes with benzyl alcohol. Complex 3 possesses a novel cluster structure that appears to be retained in solution and is capable of polymerizing both dl -lactide (LA) and ε-caprolactone (CL). The polymerizations are controlled as evidenced by the linear nature of molecular weight versus conversion plots and [M]/[ 3 ] ratios. The kinetics of LA polymerization using 3 are first-order with respect to the monomer at low [LA]/[Y] ratios. The polymerization of CL is zero-order in monomer. Complex 1 ·2BnOH possesses a dimeric structure previously seen in both the solid and solution state. However, the added presence of benzyl alcohol moieties in 1 ·2BnOH increases the rate of LA polymerization and allows for control over the molecular weights of isolated polylactide, a feature unseen in earlier reported dimers. The complex 1 ·2BnOH is not active for the polymerization of CL.