Synthesis of Ultrahigh Molecular Weight PLAs Using a Phenoxy-Imine Al(III) Complex

Abstract

l- and d-lactide polymerization kinetics using phenoxy-imine ligands of the type Me2Al[O-2-tert-Bu-6-(C6F5N=CH)C6H3] in the presence of n-butanol and benzyl alcohol by ring-opening polymerization into polylactide are investigated. Effects of initiator concentration, catalyst concentration, polymerization temperature, and time on the molecular weight of poly-l-lactide are also investigated. Purification and drying of l-lactide are found to significantly influence the polymerization kinetics and the final molecular weight achieved. Ultrahigh molecular weight poly(l-lactic acid) PLLA (Mw = 1.4 × 106 g/mol with A= 1.8) and ultrahigh molecular weight poly(d-lactic acid) PDLA (Mw = 1.3 × 106 g/mol with A= 2.0) are obtained when polymerization is performed with a molar ratio of monomer to catalyst (LA/Al) of 8000 for 72 h at 120 °C in the presence of benzyl alcohol with conversions of 96 and 91%, respectively. We report for the first time the synthesis of ultrahigh molecular weight poly-l- and d-lactide using the Me2Al[O-2-tert-Bu-6-(C6F5N═CH)C6H3] catalyst. The identified catalyst is found to be suitable for the synthesis of a broad range of molecular weights.