Insertion of Degradable Thioester Linkages into Styrene and Methacrylate Polymers: Insights into the Reactivity of Thionolactones

Abstract

The thionolactone 3,3-dimethyl-2,3-dihydro-5H-benzo[e][1,4]dioxepine-5-thione (DBT) is shown to radically homopolymerize, copolymerize rapidly with acrylates and styrene, and, for the first time, copolymerize with methacrylates, introducing a degradable thioester backbone functionality. Surprisingly, the aminolysis of DBT homopolymers was accompanied by the intramolecular ether cleavage, leading to the formation of 2,2-dimethylthiirane and salicylamides. The rapid copolymerization with styrene was exploited to produce degradable copolymers through free-radical polymerization in a starve-fed semibatch setup. The higher reactivity of DBT compared to the current benchmark thionolactone dibenzo[c,e]oxepine-5(7H)-thione (DOT) was inconsistent with the expected electron-donating effect of the alkoxy substituent in DBT. Using single-crystal XRD structure analysis and DFT modeling, this study rationalized the higher reactivity of DBT by (i) better stabilization of the intermediate radical in DBT by means of a better overlap with the adjacent aromatic, which shifts the addition equilibrium to the right; (ii) an increased ring strain in DBT compared to DOT, which drives the ring-opening; and (iii) better reinitiating efficiency of the tertiary alkyl open-ring radical of DBT compared to the benzylic radical of DOT. These insights are expected to facilitate the development of further thionolactone monomers with tailored copolymerization behavior.