Thermally controlled silicone functionalization using selective Huisgen reactions

The thermal azide–alkyne cycloaddition using electron deficient alkynes has previously been applied to the room temperature crosslinking of polysiloxanes. However, only propiolates were used as the alkyne partner to functionalize polysiloxanes under metal-free conditions. Herein, we extend the methodology to alkynes with differing electronic demands and therefore different reactivities, which opens the possibility for selective silicone functionalization using sequential reactions simply by changing temperatures. To obtain a more thorough understanding of the effect of the alkyne electron density on the cycloaddition, differential scanning calorimetry was used to determine the temperature at which triazole formation is initiated for a variety of alkynes. The study showed acetylene dicarboxylates to have the lowest onset cycloaddition temperature of those studied, 37°C. By contrast, propargyl esters only began to react at 101°C, with other carbonyl-modified alkynes showing intermediate reactivity. Benzyl azide was used to demonstrate that the silicone copolymers bearing pendant propiolate alkynes could be reacted preferentially at lower temperatures, but only just, over the more electron rich terminal propiolamide alkynes. By contrast, the reaction of silicone propiolates with benzyl azide occurred nearly specifically in the presence of terminal propargyl alkynes on a polysiloxane copolymer containing both groups.