Harnessing ene-type and stereochemistry to control reaction kinetics and network architecture in thiol-ene photopolymerizations using maleate and fumarate-derived monomers

Herein we report photo-dose tunable crosslinking density in polymer networks by exploiting the relative rates of thiol-ene click chemistry and chain-growth homopolymerization in symmetric triene monomers. From biomass-derived diacids, these synthesized trienes incorporate terminal allyl ether groups and internal fumarate/maleate groups, providing varied reactivity. Through small-molecule monothiol addition, ¹H-NMR results indicate fast preferential thiol addition to terminal allyl groups and slower stereochemistry-dependent homopolymerization of fumarate/maleate groups. Incorporating these monomers with dithiols and triallyl crosslinkers allows formation of polymer networks, using both thiol-ene addition and homopolymerization as photo-crosslinking mechanisms on differing timescales. In situ photo-rheology and dynamic mechanical analysis demonstrate impacts of the mixed-mechanism on light-dependent evolution of network architectures from initial gelation to increasing crosslinking density with prolonged exposure. Ultimately, the mixed-mechanism polymerization enables grayscale patterning and 3D printing, offering potential for in situ patterning of glasslike and rubbery regions within monolithic materials.