Under solvent-free conditions (at 130 degrees C), zirconium and hafnium amine tris(phenolate) alkoxides are extremely active, well-controlled, single-site initiators for the ring-opening polymerization of rac-lactide, yielding highly heterotactic polylactide.

A series of tetradentate amine(bisphenolate) ligands have been synthesized and complexed to Ti(IV), Zr(IV), and Hf(IV) centers. Two distinct ligand sets have been synthesized both exhibiting ONNO coordination. The steric bulk of the ligands was varied to assess the impact of sterics on the catalytic performance in the ring-opening-polymerization (ROP) of cyclic esters. In all cases the metal centers are pseudo octahedral in geometry. Several group 4 amine(bisphenolate) complexes have been characterized by single-crystal X-ray diffraction. The complexes were tested for the ROP of ε-caprolactone and L/rac-lactide, with significant activity seen for complexes of all group 4 metals. For Ti(IV), complexes of sterically demanding ligands were more effective initiators, whereas sterically less demanding ligands were more successful for Zr(IV) complexes. Copolymers of poly(ε-caprolactone) and poly-l-lactide were produced utilizing the Zr(IV) initiators. The polymerization of rac-lactide was also studied allowing stereoselectivity of a range of metal−ligand combinations to be assessed. Isotactic polymer was produced with both Zr(IV) and Hf(IV) centers whereas Ti(IV) produced an atactic polymer.

A series of amine bis(phenolate) ligand precursors and their group 4 metal complexes have been prepared, characterised by single-crystal X-ray diffraction, and tested for ring opening polymerisation of epsilon-caprolactone.

Polylactide (PLA) is a degradable aliphatic polyester that is obtainable economically from biorenewable resources and is of interest both as a commodity polymer and for use in biomedical applications such as medical implants and drugdelivery systems.[1] Commercially, ring-opening polymerization (ROP) of lactide (LA) is most commonly carried out without solvent in the melt by using tin carboxylate initiators, but a lack of control over polymer microstructure limits control over physical, mechanical, and degradation properties of PLA. This drawback, coupled with concerns over the toxicity of tin, have prompted the development of benign metal alkoxides as stereoselective single-site polymerization initiators.[1] It has been shown that metal complexes of achiral ligands can lead to either highly heterotactic [(S,S,R,R) n ] or highly isotactic [(S,S) n (R,R) n ] PLA through a chain-end control mechanism (e.g., complexes I,[5a] and V[5b] ; Bz = benzyl), whereas complexes of chiral ligands can lead to isotactic stereoblock PLA through an enantiomorphic site control mechanism (e.g., complexes VI [6] and VII [7] ). Despite these impressive recent developments, major challenges remain. For example, mechanistic details are not well understood [8] and, although solvent-free melt polymerization is necessary for the practical bulk production of PLA and is highly desirable to eliminate solvent residues from biomedical-grade polymer, there is to date only one previous example of highly stereoselective solvent-free ROP of rac-LA. This advance was made by Feijen and co-workers who recently reported that complex VII afforded highly isotactic PLA in the melt at 130 8C (P i = 0.88, where P i is the probability of the formation of a new i-dyad).[7a]Herein we report the synthesis of a new germanium alkoxide supported by a C 3 -symmetric ligand and demonstrate its application to the solvent-free ROP of rac-LA, most notably, to provide for the first time highly heterotactic PLA in the bulk. Although germanium is less likely to undergo undesirable transesterification reactions than tin [9] and its organic compounds are generally nontoxic, [10] to our knowledge there are no previous reports of single-site germanium alkoxide initiators for the ROP of LA even though its potential has been demonstrated by the application of spirocyclic germanium tetraalkoxides for the ring-expansion polymerization of l-LA.[10] To provide steric and electronic control at the germanium center, we chose to use an amine (trisphenolate) ligand (LH 3 , Scheme 1). This class of ligand has recently generated considerable interest in metal coordi-