Polyurethane polymers cured via azide-alkyne cycloaddition

Abstract

Conventional thermoset polyurethane polymers are crosslinked by reaction of a polyisocyanate compound with a polyol. Herein are described alternative crosslinking polyurethanes (ACPUs) for coatings and related applications that cure by azide-alkyne cycloaddition. Commercial polyisocyanate resins including allophanate, isocyanurate, and biuret types were reacted with propargyl alcohol or 2-hydroxyethyl propiolate to yield polyurethane resins with terminal alkyne functionality. Various polyols, including polyether, polyester, and polyacrylic types were modified to convert their hydroxyl functionality to azide functionality. The best performance was obtained with an alkyne component based on Desmodur XP 2580 and an azidated polyol based on Setalux D A 870 BA. Clear, high-solids, two-component coatings were prepared with and without Cu(I) catalyst. The coating performance properties including pencil hardness, MEK double rubs, and glass transition temperature (Tg) were comparable to a conventional polyurethane control coating made from the precursor resins. Azide-alkyne formulations in the presence of copper catalyst exhibited faster curing kinetics than the polyurethane control. Propiolate-based systems showed significantly faster curing kinetics compared to the propargylated systems with or without Cu(I) catalyst. A study of azide:alkyne stoichiometry surprisingly showed that higher crosslink density of ACPUs may be obtained by formulating with 35–50 mol% excess azide component.