Benzoxazine Functionalization of CO2-Derived Block Copolymers toward High-Performance Thermosets through Cooperative Covalent Crosslinking and Hydrogen Bonding

Abstract

CO2-derived aliphatic polycarbonates have attracted considerable attention as sustainable polymeric materials; however, their relatively low glass transition temperature and limited thermal stability restrict their applications in high-performance engineering materials. Herein, a series of benzoxazine-functionalized CO2-derived block copolymers, poly(cyclohexene carbonate)-b-poly(styrene-alt-1-(3-phenyl-3,4-dihydro-2H-benzoxazin-6-yl)-1H-pyrrole-2,5-dione) (PCHC-b-PSHPMIBZ), were successfully synthesized through sequential ring-opening copolymerization (ROCOP) of cyclohexene oxide and CO2 and reversible addition–fragmentation chain-transfer (RAFT) polymerization. The thermal properties of the resulting block copolymers were systematically tailored by varying the PSHPMIBZ contents followed by thermal ring-opening polymerization (ROP). During thermal curing, the benzoxazine units simultaneously generated a covalently crosslinked network and abundant phenolic hydroxyl groups, which established extensive intermolecular and intramolecular hydrogen-bonding interactions with neighboring carbonate and imide carbonyl groups. The thermal ROP mechanism and structural evolution were comprehensively elucidated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), in situ Fourier-transform infrared (FTIR) spectroscopy, two-dimensional (2D) correlation FTIR spectroscopy, and solid-state 13C NMR spectroscopy. These complementary analyses revealed that hydrogen-bonding formation occurred immediately following benzoxazine ring opening, producing a hierarchical dual-network architecture composed of covalent crosslinks and supramolecular hydrogen bonds. Consequently, the glass transition temperature of PCHC-b-PSHPMIBZ51 increased dramatically from 115 to 199 °C, while the char yield increased from 0.9 wt % for the corresponding PCHC-b-PSHPMI51 copolymer to 25.3 wt %, and further to 38.3 wt % after post-curing at 270 °C. The cured block copolymers retained homogeneous morphology despite extensive crosslinking, demonstrating that benzoxazine functionalization provides an effective molecular design strategy for converting sustainable CO2-derived polycarbonate block copolymers into thermosetting materials with substantially elevated glass transition temperatures and enhanced thermal resistance.