Role of Organic Phosphates and Phosphonates in Catalyzing Dynamic Exchange Reactions in Thiol‐Click Vitrimers

Owing to their strong Bronsted acidity, organic phosphates and phosphonates are able to catalyze dynamic transesterification reactions in hydroxyl ester networks. Compared to commonly used transesterification catalysts, they are highly soluble in a wide range of acrylate monomers and neither affect cure kinetics nor shelf-life of photocurable acrylate and thiol-acrylate resins. Additionally, they promote fast stress relaxation and, by using derivatives with functional groups, are covalently incorporated within the photopolymer network. These salient features make organic phosphates and phosphonates ideal catalysts for the design and processing of photoreactive vitrimers. Herein, the catalytic activity of selected methacrylate-functional phosphates and vinyl-functional phosphonates on dynamic transesterifications are studied comprehensively. They are incorporated in a thiol-acrylate photopolymer providing functional -OH and ester moieties. Cure kinetics and thermomechanical properties are not significantly affected by the structure and functionality of the catalyst. In contrast, time-dependent stress relaxation measurements show that relaxation time and activation energy correlate well with the logarithmic acid dissociation constants of the Bronsted acids. The better understanding of the role of organic phosphates and phosphonates expands the scope of transesterification catalysts and is particularly interesting for the design of photoreactive vitrimers, which can be additively manufactured by using vat polymerization techniques.