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

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 crossexchange 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 degrees 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.