Degradable linear and bottlebrush thioester-functional copolymers through atom transfer radical ring-opening copolymerization of a thionolactone

The radical thiocarbonyl addition–ring-opening (TARO) copolymerization of thionolactones with vinyl comonomers affords selectively degradable thioester functional polymers promising for biomedical applications. Herein, the use of atom transfer radical polymerization (ATRP) is investigated for the first time, using dibenzo[c,e]oxepane-5(7H)-thione (DOT), Cu(I)Br, and tris[2-(dimethylamino)ethyl]amine (Me6TREN) as thionolactone, catalyst, and ligand, respectively, with the acrylate comonomers poly(ethylene glycol) methyl ether acrylate (PEGA), methyl acrylate, benzyl acrylate, and butyl acrylate. Polymerizations were impeded by a side reaction, the Cu(I)-catalyzed dethionation of DOT to its (oxo)lactone analog, which caused the loss of up to 50 mol% of DOT in the early polymerization stages and limited the final copolymer DOT content. Nonetheless, readily degradable copolymers with low dispersities (1.10 ≤ Ð ≤ 1.26) were formed using DMSO, acetonitrile, or toluene as solvent. Presuming adventitious water to be the oxygen source, the dethionation side reaction could be minimized (≥ 5 mol-% lactone) by using anhydrous polymerization conditions, which enabled the synthesis of copolymers with higher DOT content. Exploiting documented advantages of ATRP over thermally-initiated RAFT polymerization in the synthesis of brushes, water-soluble molecular brushes were prepared by grafting PEGA–DOT copolymers from a pre-made multi-ATRP initiator. Due to faster incorporation of DOT, the cleavable thioesters were located close to the junctions and enabled the fast (< 1 min) oxidative cleavage of the arms from the core to give water-soluble products using 10 mM oxone. Expanding the scope of the ATRP and TARO methods, this work presents facile access to polymer materials with tailored architectures and degradability.