Low temperature thermal RAFT depolymerization: the effect of Z-group substituents on molecular weight control and yield

Abstract

The labile end-groups inherent to many controlled radical polymerization methodologies, including atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain-transfer (RAFT) polymerization, can trigger the efficient chemical recycling of polymethacrylates yielding high percentages of pristine monomer. Yet, current thermal solution ATRP and RAFT depolymerization strategies require relatively high temperatures (i.e. 120–170 °C) to proceed, with slower depolymerization rates, and moderate yields often reported under milder reaction conditions (i.e. lower temperatures). In this work, we seek to promote the low temperature RAFT depolymerization of polymethacrylates via regulating the Z-group substitution of dithiobenzoate. While electron-withdrawing meta and para substituents, including trifluoromethyl (CF₃) and trifluoromethoxy (OCF₃), compromised the percentage of monomer recovery at 90 °C (e.g. 18% of conversion), instead the incorporation of electron-donating groups in the benzene ring, such as methoxy (OMe) and tertiary butoxy (OtBu), had a remarkable effect leading to up to four times higher conversions (e.g. 75%). Notably, electron-withdrawing Z-groups imposed control over depolymerization, reflected in the gradual decrease of the molecular weight during the reaction, as opposed to electron-donating groups which underwent a more uncontrolled depolymerization pathway. Density Functional Theory (DFT) calculations revealed accelerated bond fragmentation for electron-donating Z-groups, further supporting our findings. Taken altogether, this work highlights the importance of RAFT agent selection to either lower the reaction's temperature while maintaining high conversions, or induce control over the depolymerization.