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 proligands. The Hf(IV) complexes have all been characterised in the solid-state, the chiral ligands coordinate to Hf(IV) in an alpha-cis manner whereas the meso ligand coordinates in a beta-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 P-m = 0.84. As expected Hf(3)(OPr)(2) polymerised L-LA faster than rac-LA (k(app) = 5.9 x 10(-3) min(-1) vs. 3.8 x 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(1)(OPr) was fluxional on the NMR timescale, whereas Al(3)((OPr)-Pr-i) was locked in solution with no exchange. Interestingly, the Al(III) complexes of 3H(2) produce PLA with a very strong heterotactic bias P-r upto 0.87, whereas atactic PLA is produced with 1H(2). For Al(3)(OPr) a linear relationship is observed with M-n and conversion. Experiments with the addition of an equivalent of rac-LA to the selective initiators have also been performed and are discussed.