Modular liquid crystal elastomer synthesis: Acid catalyzed oxa-Michael addition

Abstract

Incorporating functional groups into liq. crystal elastomer (LCE) networks can help amplify microscopic properties to the macroscopic scale. In 2015, the Yakacki group at the University of Colorado at Denver reported the synthesis of a tailorable and programmable LCE using a two-stage thiol acrylate Michael addn. and photopolymn. This synthesis is highly modular; nearly any diacrylate, and any di- and tetra-thiol can be used to make an elastomer so new functional groups may be introduced to the system by adding new thiol- or acrylate-functionalized monomer to tailor the LCE's properties. Thiols, however, are difficult to synthesize and di- and tetra-functional thiols are hard to find com. Most low mol. wt. thiols also have an extremely low odor threshold (around 0.011 ppm). In contrast, however, terminal alcs. are effectively odorless, more shelf-stable, and available com. in much larger varieties. Taking advantage of the oxa-Michael addn. mechanism, these terminal alcs. may provide a viable substitute for the thiol monomers to create a system with even greater modularity. Here we report that through a qual. screening process, triflic and tosyl acid catalysts were found to successfully catalyze the polymn. and crosslinking of a model diacrylate with di- and tetra-functional terminal alcs. Both traditional heating and microwave radiation were able to activate the reaction. FTIR spectroscopy is being used to first confirm the oxa-Michael reaction, and to monitor reaction kinetics over time.