Overcoming aggregation in indium salen catalysts for isoselective lactide polymerization

Abstract

A methodology for controlling aggregation in highly active and isoselective indium catalysts for the ring opening polymerization of racemic lactide is reported. A series of racemic and enantiopure dinuclear indium ethoxide complexes bearing salen ligands [(ONNO_R)InOEt]₂ (R = Br, Me, admantyl, cumyl, <i>t</i>-Bu) were synthesized and fully characterized. Mononuclear analogues (ONNO_R)InOCH₂Pyr (R = Br, <i>t</i>-Bu, SiPh₃) were synthesized by controlling aggregation with the use of chelating 2-pyridinemethoxide functionality. The nuclearity of metal complexes was confirmed using PGSE NMR spectroscopy. Detailed kinetic studies show a clear initiation period for these dinuclear catalysts, which is lacking in their mononuclear analogues. The polymerization behavior of analogous dinuclear and mononuclear compounds is identical and consistent with a mononuclear propagating species. The isotacticity of the resulting polymers was investigated using direct integration and peak deconvolution methodologies and the two were compared.