Chromium amino-bis(phenolate) complexes for CO₂ and cyclohexene oxide copolymerization

Abstract

Utilization of CO₂ as a C1 feedstock for polycarbonate production has received considerable attention in the past decades. One of the promising methods is the catalytic copolymerization of CO₂ and epoxides to afford polycarbonates. In this thesis, Cr(III) amino-bis(phenolate) complexes were synthesized and investigated as catalysts for the copolymerization of CO₂ and cyclohexene oxide (CHO). These Cr(III) complexes were characterized by MALDI-TOF mass spectrometry, single crystal X-ray diffraction, UV-Vis spectroscopy and elemental analysis. In the presence of cocatalysts such as 4-(dimethylamino)pyridine (DMAP), bis(triphenylphosphoranylidene) iminium chloride or azide (PPNCl or PPNN3), these Cr(III) complexes showed efficient activities to selectively produce polycarbonate from CO₂ and CHO with moderate molecular weights and narrow dispersities. End-group analysis of the resulting polymers by MALDI-TOF mass spectrometry suggested both the nucleophiles from cocatalyst and the Cr(III) complex could initiate the reaction. However, DMAP was found to be a better initiator to ring-open the epoxide than the chloride from the Cr(III) complex. Monitoring the copolymerization reaction via in situ attenuated total reflectance infrared spectroscopy (ATR-IR), combined with the studies of binding of azide to Cr(III) complexes via mass spectrometry showed that the steric effect of the pendant donor group of amino-bis(phenolate) Cr(III) complex played an important role in increasing the copolymerization rate. Furthermore, polycarbonate diol from CO₂ and CHO was synthesized by a modified Cr(III) complex. The resulting polycarbonate diol was used to afford tri-block copolymers via its use as a macroinitiator in the subsequent base-catalysed ring-opening polymerization of rac-lactide. The resulting copolymers showed a decreased glass transition temperature and an increased decomposition temperature compared to the original polycarbonates.