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A combined experimental and computational study of the reactivities of seven commonly used Michael acceptors paired with two thiols within the framework of photobase-catalyzed thiol-Michael reactions is reported. The rate coefficients of the propagation ( k P ), reverse propagation ( k -P ), chain-transfer ( k CT ), and overall reaction ( k overall ) were experimentally determined and compared with the well-accepted electrophilicity parameters of Mayr and Parr, and DFT-calculated energetics. Both Mayr's and Parr's electrophilicity parameters predict the reactivities of these structurally varying vinyl functional groups well, covering a range of overall reaction rate coefficients from 0.5 to 6.2 s −1 . To gain insight into the individual steps, the relative energies have been calculated using DFT for each of the stationary points along this step-growth reaction between ethanethiol and the seven alkenes. The free energies of the individual steps reveal the underlying factors that control the reaction barriers for propagation and chain transfer. Both the propagation and chain transfer steps are under kinetic control. These results serve as a useful guide for Michael acceptor selection to design and predict thiol-Michael-based materials with appropriate kinetic and material properties. 

Recycling of polyurethanes is largely infeasible due to the harsh reprocessing conditions and associated risks of side reactions and degradation whereas polymer networks incorporating dynamic covalent bonds represent an attractive approach to the design of recyclable materials. Here, we report findings on the dynamic nature of thiourethanes, and their application as a new class of recyclable analogs of urethane materials. A series of small molecule experiments was initially conducted to determine the equilibrium constant and exchange reaction kinetic constant for the thiol–isocyanate reaction. Furthermore, incorporating those thiourethane moieties into a cross-linked network resulted in thermoset materials that are readily depolymerized to liquid oligomers. The resultant oligomers can be re-crosslinked to thiourethanes without any loss of performance nor change in mechanical properties (peak stress of 25 MPa with max strain of 200%). Moreover, the recycled thiol oligomers from thiourethane network polymers could potentially be transformed into other materials with mechanical properties that exceed those of the initial, pristine thiourethane materials. Overall, the ease with which these polythiourethanes are polymerized, recycled and reformulated gives a new direction and hope in the design of sustainable polymers.