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