Reversibly\nCross-linkable Bottlebrush Polymers as\nPressure-Sensitive Adhesives

Abstract

Dynamically cross-linkable bottlebrush\npolymer adhesives were synthesized\nby the grafting-from strategy through a combination of ring-opening\nmetathesis polymerization (ROMP) and photoiniferter polymerization.\nA norbornene-containing trithiocarbonate was first polymerized by\nROMP to form the bottlebrush polymer backbone; this was followed by\nblue-light-mediated photoiniferter polymerization of butyl acrylate\ninitiated by the poly­(trithiocarbonate) to form the bottlebrush polymer.\nThis strategy afforded well-defined bottlebrush polymers with molar\nmasses in excess of 11 000 kg/mol. For un-cross-linked bottlebrush\npolymers, 180° peel tests revealed a cohesive failure mode and\nshowed similar peel strengths (∼30 g/mm) regardless of the\nbackbone polymer degree of polymerization (DP). The bottlebrush polymers\nwere then treated with butylamine to remove the trithiocarbonate,\nliberating thiols on each side-chain terminus. In the presence of\noxygen, these thiols readily cross-linked via disulfide bond formation.\nThe cross-linked bottlebrush polymers with a backbone DP of 400 showed\na greater than sixfold improvement in peel strength, whereas those\nwith a backbone DP of 100 exhibited a twofold enhancement compared\nwith un-cross-linked samples along with a change to adhesive failure.\nTriphenylphosphine readily reduced the disulfide bonds, effectively\nremoving all cross-links in the bottlebrush network and allowing for\nrecasting of the adhesive, which showed similar adhesive and rheological\nproperties to the original un-cross-linked samples.