We report a wavelength-selective polymerization process controlled by visible/UV light, whereby a base is generated for anion-mediated thiol–Michael polymerization reaction upon exposure at one wavelength (400–500 nm), while radicals are subsequently generated for a second stage radical polymerization at a second, independent wavelength (365 nm). Dual wavelength, light controlled sequential polymerization not only provides a relatively soft intermediate polymer that facilitates optimum processing and modification under visible light exposure but also enables a highly cross-linked, rigid final material after the UV-induced second stage radical polymerization. A photobase generator, NPPOC-TMG, and a photo-radical initiator, Irgacure 2959, were selected as the appropriate initiator pair for sequential thiol–Michael polymerization and acrylate homopolymerization. FT-IR and rheological tests were utilized to monitor the dual cure photo-polymerization process, and mechanical performance of the polymer was characterized at each distinct stage by dynamic mechanical analysis (DMA). By demonstrating complete light control in another sequential polymerization system (thiol–Michael and thiol–ene hybrid polymerization), this initiator pair exhibits great potential to regulate many other coupled anion and radical hybrid polymerizations in both a sequential and controllable manner.

The effect of thiol substitution in radical thiol–ene reactions has been studied by using model, monofunctional thiols as well as multifunctional thiol monomers along with the assessment of their subsequent polymerization reactions and polymer mechanical behavior. FT-IR was used to monitor the polymerization rate and quantify the overall conversion. While the total conversion was observed to range from 70% to 100%, the polymerization rate was found to decrease by as much as 10-fold as the thiol substitution was changed from primary to tertiary. Analogous multi-thiol monomers of similar structure but varying substitution were synthesized to observe the effect of substitution type on polymerization kinetics and polymer behavior. Methylation at the α-carbon was varied from primary to tertiary to observe these differences. Mechanical properties were assessed by using dynamic mechanical analysis and water sorption experiments, where the glass transition temperatures were found to be within 1–2 °C as thiol substitution varied. Furthermore, primary thiol films absorbed 1–3% more water than secondary thiol films. Resin shelf stability experiments were performed by using rheometry to measure storage time-dependent viscosity changes, and it was found that secondary thiol films remained relatively stable for up to 100 times longer than their primary counterparts. It was concluded that while there are differences under relatively slow initiation conditions, at typical initiation rates all three thiol substitutions may be made to react at similar rates for both monofunctional and polymeric systems.

The kinetic effects of the substitution and functionality of the thiol in thiol-Michael reactions were investigated using model monofunctional thiols and multifunctional thiols used in various cross-linking polymerizations. The differences in kinetic rates and final conversions were observed via Fourier transform infrared spectroscopy. The shelf life of these polymers and their mechanical properties were analyzed using a rheometer to measure viscosity changes over time. It was concluded that for monofunctional systems, the reaction rate is dependent on both electronic and steric interactions. For systems with a propagation rate-limiting step (propionate), the secondary thiol was faster than the primary thiol due to increased reactivity of the thiolate anion, by as much as much as a 60% increase in the rate. However, more sterically hindered internal alkenes resulted in primary and secondary rates about equal to each other. For systems with a chain transfer-limiting step (alkyl thiol), the rate was dependent on the pK a of the thiol and ease of deprotonation; in these cases, the primary thiol was the fastest. Though primary and secondary thiols had relatively mild differences in rates, reactions of tertiary thiols were slower than either of the others. For polymerizing systems using multifunctional thiols, the results varied depending on the substitution and functionality. When reacting with a difunctional alkene, the secondary thiol was 74–95% faster than the primary thiol, depending on the type of thiol assessed, and as the functionality of the alkene increased, the rates became more comparable. In the tetrafunctional alkene systems, the primary thiol was 57% faster than the secondary thiol. The shelf life of the systems produced varied results. Typically, in systems with the difunctional thiol, the primary thiol formulation was significantly less stable and gelled more rapidly than the resin with the corresponding secondary thiol. However, in the tetrafunctional thiol systems, the resin containing the secondary thiol gelled more rapidly than that containing the primary thiol. All systems typically gelled within 30 days regardless of substitution, although no additional formulation adjustments were made to stabilize any of these systems beyond changing the thiol structure.

The thiol–thioester reaction has emerged as a promising method for developing covalent adaptable networks (CANs) due to its ability to exchange rapidly under low temperature conditions in a number of solvents, orthogonality among other functional groups, and tunability. Here, the effects of thiol substitution (i.e., primary vs secondary) were assessed with respect to their reactivity in two dynamic thioester reactions: the thiol–thioester exchange and the reversible thiol–anhydride addition. Model NMR experiments were conducted using small-molecule compounds to observe how polymers of similar components would behave in thiol–thioester exchange. It was determined that the K eq was near unity for mixtures of primary thiols and secondary thioesters, and vice versa, in both a polar solvent, DMSO-d 6, and at most slightly favors primary thioesters in a relatively nonpolar solvent, CDCl3. Dielectric spectroscopy and stress relaxation experiments were used to determine the relaxation times and activation energies of the two thioester-containing networks: Thiol-ene networks, which undergo thioester exchange, displayed activation energies of 73 and 71 kJ/mol from dielectric measurements and 36 and 53 kJ/mol from stress relaxation for the primary and secondary thiols, respectively. Thiol–anhydride-ene networks, which undergo both thioester exchange and reversible thiol–anhydride addition, displayed activation energies of 94 and 114 kJ/mol from dielectric and 111 and 139 kJ/mol from stress relaxation for primary and secondary thiols, respectively. In both types of networks, the secondary thioester-based networks demonstrated slower dynamics as compared to the same primary network by at least one order of magnitude. In the anhydride network, the secondary thiol also biased the dynamics toward reversible addition.

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