Holographic Photopolymers via Two-Stage Orthogonal Thiol-Click Chemistries Leveraging Kinetic Selectivity

Leveraging the kinetic selectivity of various thiol-based chemistries, sequential thiol-Michael and thiol–ene reactions were applied semiorthogonally toward holographic recording, thereby expanding the available toolbox for developing thiol–ene-based optical recording media. In a unique ternary mixture, thiol glycolates are highly favored kinetically due to the higher stability and therefore enhanced reactivity of the thiolate anion as compared with aliphatic thiols in the thiol-Michael reaction. The thiol glycolate is base-catalyzed to react with a Michael acceptor, i.e., an electron-deficient double bond, to form the first-stage matrix, leaving most of the aliphatic thiol unreacted and available for the successive thiol–ene photopolymerization. Through product ratios obtained from 1H NMR, the high kinetic selectivity was demonstrated in small molecule model studies, in which a significant excess loading of aliphatic thiol monomer was utilized (up to five-fold excess of thiol functional groups). Furthermore, the two-stage behavior was evaluated in a bulk material system comprised of multifunctional monomers through photorheology. The resultant films, which are robust elastomers, exhibit high spatiotemporal control in photopatterning. Taking advantage of the decoupled choice of thiol monomers to realize a higher theoretical refractive index contrast between two stages, transmission holographic gratings were recorded in similar formulations, yielding a peak-to-mean refractive index contrast of 0.0064 with high fidelity of spatial resolution even at a size scale of 620 nm period.