Mechanism of Living Lactide Polymerization by Dinuclear Indium Catalysts and Its Impact on Isoselectivity

Abstract

A family of racemic and enantiopure indium complexes <b>1</b>–<b>11</b> bearing bulky chiral diaminoaryloxy ligands, H­(NNO<sub>R</sub>), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO<sub>R</sub>)­InX]<sub>2</sub>[μ-Y]­[μ-OEt] (<b>5</b>, R = <sup><i>t</i></sup>Bu, X = Y = Cl; <b>8</b>, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly­(lactic acid) (PLA) at room temperature. In particular, complex <b>5</b> showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex <b>8</b>. Importantly, the selectivities obtained for the ROP of racemic LA with (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> and (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>8</b> are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes

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