Development of a photo-degradable polyester resulting from the homopolymerization of o-hydroxycinnamic acid

Abstract

Recently hydroxycinnamic acids have received increased attention due to their ability to form photo-responsive materials and also their natural abundance providing sustainable origins. However, of this class, o-hydroxycinnamic acid has been underexplored as a raw material for the development of stimuli-responsive polymers. Herein, this limitation is addressed through the development of a new photo-degradable polyester resulting from the homopolymerization of o-hydroxycinnamic acid. This material was synthesized in a single step via a condensation polymerization reaction utilizing an acid chloride intermediate. Once synthesized, the resulting poly(o-hydroxycinnamic acid) was exposed to UV light in solution, which initiates backbone cleavage through an intramolecular transesterification reaction, as identified through proton NMR chemical shifts. In addition, this transformation was characterized from a macromolecular standpoint using GPC, allowing for non-selective hydrolysis to be ruled out as a degradation pathway. Two control molecules were also synthesized, exposed to UV light, and analyzed via HPLC, identifying that degradation for this polymer occurs most rapidly at the chain-end instead of through internal backbone cleavage. With this information at hand, an “on”/”off” photo-regulation experiment was carried out, demonstrating that control can be expressed over the degradation rate of this material. The results outlined herein demonstrate that upon exposure to 300 nm light, poly(o-hydroxycinnamic acid) undergoes controlled degradation through a photo-regulated intramolecular cyclization reaction, leading to the formation of coumarin.