CO2/epoxides ring-opening copolymerization towards hydroxy-functionalized polycarbonates

Abstract

Ring-opening copolymerization (ROCOP) of CO₂ and epoxides witnesses continued interest to access sustainable polycarbonates. Introduction of an exocyclic functional group onto the epoxides enables to tune and diversify the properties of the resulting CO₂-based polycarbonates. Herein, the CO₂/benzyl glycidyl ether (BnGE) or CO₂/cyclohexene oxide (CHO) ROCOP has been performed, using a bicomponent catalyst system composed of either a {diamino-bisphenolate}MCl (Al, Fe) or {Salphen}CoCl complex or triethylborane (BEt₃) as catalyst, combined with bis(triphenylphosphoranylidene)ammonium chloride (PPNCl) as initiator. While the Al/Fe-based catalyst systems selectively returned the corresponding benzyloxymethylene five-membered cyclic carbonate (5CCOBn) with poor activity in the copolymerization of CO₂/BnGE, the {Salphen}CoCl/PPNCl and BEt₃/PPNCl systems produced poly(benzyl glycidyl ether carbonate) (PBnGEC) with high chemoselectivity (∼80% and >98%) and regioselectivity (>99% and ∼84%), featuring >99% and ∼85% of carbonate linkages, respectively. Investigation of the {Salphen}CoCl/PPNCl and BEt₃/PPNCl catalytic systems in the ROCOP of CO₂/BnGE/CHO with different comonomers loadings, enabled to prepare a series of tunable P(BnGEC-co-CHC) terpolymers with similar selectivities. Subsequent hydrogenolysis of these hydrophobic polymers using Pd/C resulted in the deprotection of the side-chain benzyloxy moieties, affording the corresponding hydrophilic P(GC-co-CHC) polymers featuring hydroxyl pendant groups; yet, significant degradation of the polycarbonate main chain was observed for hydroxyl contents >15 mol%. Depolymerization of PBnGEC, PCHC and P(BnGEC-co-CHC) using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as catalyst quantitatively returned both corresponding 5CCs. In-depth characterizations by NMR spectroscopy, mass spectrometry, SEC, TGA, and DSC analyses supported well-defined protected and deprotected terpolymers with tunable chemical and thermal properties, providing opportunities for biomedical and/or industrial outcomes.