Polyhydroxyurethanes (PHUs) have attracted increasing attention as a sustainable alternative to conventional polyurethanes (PUs). PHUs are synthesized by reacting cyclic carbonates and amines in an atom-economical, catalyst-free route under relatively mild conditions avoiding the use of harmful isocyanates. Many of the cyclic carbonates are obtained from platform chemicals such as diols in combination with dimethyl carbonate or through the incorporation of CO2 in epoxides, an environmentally important reaction. Such a large feedstock availability gives access to a library of cyclic carbonates and the opportunity to tailor PHUs depending on the properties sought. The formed pendent hydroxyl groups along the PHU backbone are sites available for subsequent reactions, yet their presence enhances the intramolecular H-bonding and limits the solubility of PHUs, which in turn hinders further post-polymerization modification or copolymerization. To approach this challenge and advance the utilization of PHUs towards more complex copolymer architectures, we utilized polymer-in-monomer solubility as a tool to synthesize graft copolymers from a tailor-made PHU. The design of the PHU structure enabled its solubility in ε-caprolactone (CL) at elevated temperature, creating ideal conditions for the graft ring-opening polymerization of CL from the pendent hydroxyl groups of the PHU. Structural characterization (1H, DOSY, 31P NMR) of the PHU-graft-PCL copolymers proved the efficiency of the strategy as solely copolymers were formed, and all of the hydroxyl groups reacted. The molar mass of the grafted PCL chains was modulated simply by controlling the CL:OH ratio, impacting the thermal properties of the copolymers. Our strategy demonstrates that a simple approach can overcome common hurdles and provide intriguing opportunities to design sustainable polymers.

Polyhydroxyurethane-graft-poly(ε-caprolactone) copolymers: Designing complex polymer architectures through sustainable routes

ACS Fall 2021

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