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

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-cis manner whereas the meso ligand coordinates in a β-cis geometry. The Hf(IV) complexes are all active for the ROP of rac-lactide in the melt, with the fluxional meso complex affording a strong isotactic bias Pm = 0.84. As expected Hf(3)(OiPr)2 polymerised L-LA faster than rac-LA (kapp = 5.9 x 10-3 mins-1 vs 3.8 x 10-3 mins-1). For Ti(IV) complexes atactic PLA was formed. The salan pro-ligands have also been complexed to Al(III), with the complexes being characterised in the solid and solution state generating a series of novel Al-Me and Al-OiPr species. Al(1)(OiPr) was fluxional on the NMR timescale, whereas Al(3)(OiPr) was locked in solution with no exchange. Interestingly, the Al(III) complexes of 3H2 produce PLA with a very strong heterotactic bias Pr upto 0.87, whereas atactic PLA is produced with 1H2. For Al(3)(OiPr) a linear relationship is observed with Mn and conversion. Experiments with the addition of an equivalent of rac-LA to the selective initiators have also been performed and are discussed.