l- and rac-lactide polymerisation using scandium and aluminium permethylindenyl complexes

The synthesis and characterisation of constrained geometry scandium and aluminium permethylindenyl complexes Me2SB(RN,I*)ScCl(THF) (R = iPr (1), nBu (2) and Ph (3)), Me2SB(iPrN,I*)Sc(O-2,6-iPr-C6H3)(THF) (4), Me2SB(iPrN,I*)Sc(O-2,4-tBu-C6H3)(THF) (5), Me2SB(nBuN,I*)Sc(O-2,6-iPr-C6H3)(THF) (6), Me2SB(PhN,I*)Sc(O-2,6-iPr-C6H3)(THF) (7), Me2SB(tBuN,I*)AlCl(THF) (8), Me2SB(tBuN,I*)Al(O-2,6-Me-C6H3)(THF) (9) and Me2SB(tBuN,I*)Al(O-2,4-tBu-C6H3)(THF) (10) are reported. All complexes were characterised by NMR spectroscopy. Solid-state structures of 2–4, 6 and 8–10 were determined by X-ray crystallography. Ring-opening polymerisation of L- and rac-lactide using all complexes with the exception of 6 show first-order dependence on monomer concentration and produced polylactide with unimodal molecular weight distribution. First-order dependence on catalyst concentration was determined from L-lactide polymerisation using 4 and 9. Moderately heterotactic polylactide (Pr = 0.53–0.68) was achieved from rac-lactide polymerisation using 4, 5, 7 and 9. The effects of the metal centre (Sc and Al), the amido substituent (iPr, tBu, nBu and Ph) and the aryloxide initiating group (O-2,6-Me-C6H3, O-2,6-iPr-C6H3 and O-2,4-tBu-C6H3) on the catalytic activity are discussed.