Photochemical action plots evidence UV-promoted radical ring-opening polymerisation of cyclic ketene acetals

Abstract

Radical ring-opening polymerisation (RROP) of cyclic ketene acetals (CKAs) is a powerful avenue for the synthesis of biodegradable polyesters with the potential to replace non-decomposable conventional polymers. The radical polymerisation of CKAs is – surprisingly – accelerated by UV light, yet to-date the cause of the acceleration is unknown. We herein demonstrate how highly wavelength-resolved photochemical action plots of the light-induced RROP of 2-methylene-1,3,6-trioxocane (MTC) provide key information for understanding the light-prompted acceleration. We showcase that two wavelengths, 275 and 350 nm, are critical for the acceleration, with the first one facilitating the ring-opening step of the key CKA-intermediate and the second one promoting free radical initiator decay in a wavelength-orthogonal fashion. In contrast to previous studies, we aimed at unravelling the photochemically-driven monomer conversion by performing RROP at room temperature. Computational studies on the MTC radical formed during RROP indicated the cause of the acceleration: a delocalisation of the radical within the ring, which are calculated to be excited by wavelengths close to those identified experimentally. Thus, remarkably, 275 nm light critically accelerates the rate-determining ring-opening step during RROP, suggesting that photons can be used as a traceless reagent in an unexpected fashion to expedite RROPs.