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Thermal Metamorphosis in (Meth)acrylate Photopolymers: Stress Relaxation, Reshaping, and Second-Stage Reaction

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journal contribution
posted on 2019-10-18, 17:48 authored by Maciej Podgórski, Brady T. Worrell, Jasmine Sinha, Matthew K. McBride, Christopher N. Bowman
Thermally transformable/responsive (meth)­acrylate photopolymer networks were constructed from commercial (meth)­acrylate esters and synthetic di- and mono­(meth)­acylate monomers bearing thioester functionalities. The thermal responsiveness, here self-limited exchange, relied on the catalytic metamorphosis of thioesters into esters with the concomitant depletion of hydroxyls and subsequent generation of free thiols. The thioester–hydroxyl cross-exchange was demonstrated in network systems with interchain thioesters as well as in networks with side-chain pendant thioacetyls. The interchain metamorphosis resulted in close to 80% conversion of thioesters into esters when 2 equiv of hydroxyl groups was initially present. In practical terms, such an outcome enabled efficient stress relaxation (60%) and good shape adaptation (90% shape fixity) in 1 h at 105 °C. On the other hand, side-chain S → O acyl transfer reactions were found to vary in efficiency depending on the vicinity of thioesters and hydroxyls. When in close vicinity, efficient noncatalytic exchange was observed in materials with 1-to-1 thioester-to-hydroxyl ratios nearing 60%. The benefits of generating free thiols postpolymerization were further explored in enhanced two-stage curing systems where a subsequent thiol–ene photopolymerization was demonstrated at ambient as well as at elevated temperatures.

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