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

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.