Postpolymerization Functionalization of Copolymers Produced from Carbon Dioxide and 2‑Vinyloxirane: Amphiphilic/Water-Soluble CO₂‑Based Polycarbonates

Abstract

Common CO₂-based polycarbonates are known to be highly hydrophobic, and this “inert” property makes them difficult for the covalent immobilization of bioactive molecules. A practical method for modifying polymers is to introduce various functional groups that permit decoration of polymer chains with bioactive substances. In this report, CO₂-based poly­(2-vinyloxirane carbonate) (PVIC) with more than 99% carbonate linkages is isolated from the CO₂/2-vinyloxirane alternating copolymerization catalyzed by the bifunctional catalyst [(1<i>R</i>,2<i>R</i>)-SalenCo­(III)­(DNP)₂] (<b>1</b>) (DNP = 2,4-dinitrophenolate) bearing a quaternary ammonium salt on the ligand framework. It was also observed that the presence of propylene oxide significantly activates 2-vinyloxirane for incorporation into the polymer chain as well as inhibits the formation of cyclic carbonate in the terpolymerization process. DSC studies demonstrate that the glass transition temperature (<i>T</i>_g) decreases with the increase in the content of vinyl groups in the polycarbonate. By way of thiol–ene coupling, showing mainly “click” characteristics and nearly quantitative yields, amphiphilic polycarbonates (PVIC-OH and PVIC-COOH) with multiple hydroxy or carboxy functionalities have been prepared, providing suitable reactivities for further modifications (ring-opening of l-aspartic acid anhydride hydrochloride salt and deprotonation by aqueous ammonium hydroxide (NH₄OH_(aq))) to successfully isolate the water-soluble CO₂-based polycarbonate PVIC-COONH₄, and the PVIC-OH-Asp polymer which shows particles dispersed in water with an average hydrodynamic diameter <i>D</i>_n = 32.2 ± 8.8 nm. It is presumed that this emerging class of amphiphilic/water-soluble polycarbonates could embody a powerful platform for bioconjugation and drug conjugation. In contrast to lower <i>T</i>_gs of PVIC, (PVIC-<i>co</i>-PC), PVIC-OH, and PVIC-COOH, the polycarbonates PVIC-OH-Asp and PVIC-COONH₄ show higher <i>T</i>_gs as a consequence of their intrinsic ionic property (ammonium salts)