Metal influence on the iso- and hetero-selectivity of complexes of bipyrrolidine derived salan ligands for the polymerisation of rac-lactide

Abstract

In this paper we have prepared a series of Ti(iv), Hf(iv) and Al(iii) complexes based on bipyrrolidine salan pro-ligands. The Hf(iv) complexes have all been characterised in the solid-state, the chiral ligands coordinate to Hf(iv) in an α-<i>cis</i> manner whereas the <i>meso</i> ligand coordinates in a β-<i>cis</i> geometry. The Hf(iv) complexes are all active for the ROP of <i>rac</i>-lactide in the melt, with the fluxional <i>meso</i> complex affording a strong isotactic bias <i>P</i>_m = 0.84. As expected Hf(<b>3</b>)(OⁱPr)₂ polymerised l-LA faster than <i>rac</i>-LA (<i>k</i>_app = 5.9 × 10^-3 min^-1<i>vs.</i> 3.8 × 10^-3 min^-1). For Ti(iv) complexes atactic PLA was formed. The salan pro-ligands have also been complexed to Al(iii), and the novel Al-Me and Al-OⁱPr complexes were characterised in the solid and solution state. Al(<b>1</b>)(OⁱPr) was fluxional on the NMR timescale, whereas Al(<b>3</b>)(OⁱPr) was locked in solution with no exchange. Interestingly, the Al(iii) complexes of <b>3</b>H₂ produce PLA with a very strong heterotactic bias <i>P</i>_r upto 0.87, whereas atactic PLA is produced with <b>1</b>H₂. For Al(<b>3</b>)(OⁱPr) a linear relationship is observed with <i>M</i>_n and conversion. Experiments with the addition of an equivalent of <i>rac</i>-LA to the selective initiators have also been performed and are discussed.