Reversibly Cross-linkable Bottlebrush Polymers as Pressure-Sensitive Adhesives

Dynamically cross-linkable bottlebrush polymer adhesives were synthesized by the grafting-from strategy through a combination of ring-opening metathesis polymerization (ROMP) and photoiniferter polymerization. A norbornene-containing trithiocarbonate was first polymerized by ROMP to form the bottlebrush polymer backbone; this was followed by blue-light-mediated photoiniferter polymerization of butyl acrylate initiated by the poly(trithiocarbonate) to form the bottlebrush polymer. This strategy afforded well-defined bottlebrush polymers with molar masses in excess of 11 000 kg/mol. For un-cross-linked bottlebrush polymers, 180° peel tests revealed a cohesive failure mode and showed similar peel strengths (∼30 g/mm) regardless of the backbone polymer degree of polymerization (DP). The bottlebrush polymers were then treated with butylamine to remove the trithiocarbonate, liberating thiols on each side-chain terminus. In the presence of oxygen, these thiols readily cross-linked via disulfide bond formation. The cross-linked bottlebrush polymers with a backbone DP of 400 showed a greater than sixfold improvement in peel strength, whereas those with a backbone DP of 100 exhibited a twofold enhancement compared with un-cross-linked samples along with a change to adhesive failure. Triphenylphosphine readily reduced the disulfide bonds, effectively removing all cross-links in the bottlebrush network and allowing for recasting of the adhesive, which showed similar adhesive and rheological properties to the original un-cross-linked samples.