The environmental impact of plastics has intensified interest in biodegradable alternatives, such as PLA (polylactic acid), to replace conventional petrochemical-based polymers. Complexes based on zirconium and aluminium as metal centre are of interest, due to their Lewis acidity, predictable coordination geometry, non-toxicity, lack of colour, and (potential) stereocontrol at high temperatures. The square-planar coordination geometry of NONO calix[4]arenes is targeted to destabilize octahedral coordination geometries and to allow mild Lewis acidity with potential coordination numbers above 6. We target stereocontrol via the rather elusive “ligand-assisted activated monomer” mechanism, which allows stereocontrol, but avoids problems such as competition with chain-end control and lability towards protic substances typically observed for the more traditional “catalytic-site control” mechanism. This requires the presence of a basic site in proximity of the metal centre, which was introduced via different ancillary ligands alkoxides, Schiff bases and amino-acids.

Structural characterization via single crystal X-Ray diffraction revealed a high flexibility of the ligands which distorted to accommodate octahedral geometries. Nevertheless, coordination numbers greater than six were observed for zirconium complexes.

Several complexes were moderately active lactide polymerization at 140 °C. While stereocontrol was, disappointingly, moderately heterotactic, the results suggest that polymerization indeed followed a “ligand-assisted monomer activation” mechanism.

Victor Trudel-Lalande

Universite de Montreal

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