Adaptable Networks with Semiorthogonal Two-Stage Polymerizations Enabled by Sequential Photoinitiated Thiol-Ene and Disulfide-Ene Reactions

Sequential thiol-ene and disulfide-ene photopolymerizations and their utility in materials applications such as shape fixation, photolithography, and holographic recording were explored. Though thiol-ene and disulfide-ene reactions are both radical-mediated and share a common reactive group, the fact that they have several orders of magnitude difference in the reaction rate was utilized to form two-stage photopolymers with high specificity in a sequential and semiorthogonal manner. While the faster thiol-ene reaction was utilized to form a first-stage matrix, the disulfide-ene reaction was then initiated to break cross-links via disulfide cleavage and subsequently form twice as many thioether bonds as new cross-links. As such, sequential bond breakage and formation, enabling the dynamic behavior in the second stage of a single network, were explored and applied in various scenarios. Combining a remarkable difference in mechanical properties between the two stages, the dynamic photopolymer materials were capable of enabling shape fixation by initiating the second-stage polymerization while being strained and deformed. Photolithography was then utilized to quantify shape retention in deformed samples, revealing a fidelity of approximately 95% following the second-stage cure. Additionally, polarized light microscopy was used to understand better how these mechanisms affect the mechanical properties of the material when stress is applied. Finally, taking advantage of the integrated network in the two-stage system, the photopolymers were employed to record a holographic grating with a refractive index modulation (Delta n) of 0.0022 and functionally nearly zero haze.