Chiral indium alkoxide complexes as initiators for the stereoselective ring-opening polymerization of rac-lactide.

The indium complex InL(2)N'' has been prepared from the reaction of 2 equiv of ((t)Bu)(2)P(O)CH(2)CH((t)Bu)OH (HL) with InN''(3) (N'' = N(SiMe(3))(2)). This complex reacts with a further equivalent of 2,6-di-tert-butylphenol or HL to afford the adducts InL(2)(OAr) and InL(3), respectively. Confirmation that the anion L(-) exhibits "ligand self-recognition" in the formation of predominantly homochiral complexes RR-InL(2)N'' and SS-InL(2)N'' is obtained from (1)H and (31)P NMR spectroscopic data. However, the self-recognition is less effective at the indium cation, and mixtures of InL(3) complexes with different configurations are observed. Single-crystal X-ray diffraction data confirm the five-coordinate, distorted bipyramidal In center in InL(2)N'' and InL(2)(OAr) as anticipated. Selected crystals of InL(3) show two of the possible configurations: one is the fac-RRR-InL(3) complex, analogous to the lanthanide complexes LnL(3) reported previously (Ln = Y, Eu, Er, Yb); another is the alternative, homochiral mer form RRR'-InL(3). All three complexes are efficient single-component initiators for the ring-opening polymerization of rac-lactide over a wide range of temperatures and monomer-to-initiator ratios, exhibiting reasonable control over the synthesis of isotactic polylactide. Despite its poorly defined structure, InL(3) is the fastest initiator among the three complexes for the polymerization of rac-lactide, and shows the best tacticity control. The polylactide samples have high molecular weights M(n,exp) (between 44 000 and 270 000 g/mol at completion) and narrow polydispersities (as low as 1.25 at completion).