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

A family of racemic and enantiopure indium complexes 1–11 bearing bulky chiral diaminoaryloxy
ligands,
H­(NNOR), were synthesized and fully characterized. Investigation
of both the mono- and the bis-alkoxy-bridged complexes [(NNOR)­InX]2[μ-Y]­[μ-OEt] (5, R = tBu, X = Y = Cl; 8, 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 5 showed living polymerization
behavior over a large molecular weight range. A detailed investigation
of catalyst stereoselectivity showed that, although (R,R/R,R)-5 is highly selective for l-LA, only atactic PLA
is obtained in the polymerization of racemic LA. No such selectivity
was observed for complex 8. Importantly, the selectivities
obtained for the ROP of racemic LA with (R,R/R,R)-5 and
(R,R/R,R)-8 are different and, along with kinetics
investigations, suggest a dinuclear propagating species for these
complexes.