The key attribute of the thiol-Michael addition reaction that makes it a prized tool in materials science is its modular “click” nature, which allows for the implementation of this highly efficient, “green” reaction in applications that vary from small molecule synthesis to in situ polymer modifications in biological systems to the surface functionalization of material coatings. Over the past few decades, interest in the thiol-Michael addition reaction has increased dramatically, as is evidenced by the number of studies that have been dedicated to elucidating different aspects of the reaction that range from an in-depth analysis aimed at understanding the mechanistic pathways of the reaction to synthetic studies that have examined modifying molecular structures with the aim of yielding highly efficient thiol-Michael reaction monomers. This review examines the reaction mechanisms, the substrates and catalysts used in the reaction, and the subsequent implementation of the thiol-Michael reaction in materials science over the years, with particular emphasis on the recent developments in the arena over the past decade.

A Diels–Alder (DA) network containing dissolved multiacrylate monomers is demonstrated as a novel two-stage reactive polymer network, with a potential application in self-supporting stereolithography. Initially, a thermoreversible Diels–Alder “scaffold” network is formed, containing unreacted acrylate monomers and photoinitiator. During photopatterning with light at 15 mW/cm2 from a 365 nm source for 16 s of exposure at either ambient temperature or 70 °C, both acrylates and unreacted maleimides polymerize to form a permanent, covalently cross-linked network structure that simultaneously maintains the thermoreversible characteristics afforded by the underlying DA network. Light exposure of a DA network containing between 25 and 50 wt % acrylate monomer resulted in a sharp increase in cross-link density and a 60 °C jump in glass transition temperature of the material. As a result of the temperature-dependent DA equilibrium, the temperature of the film during light exposure has dramatic effects on the resulting acrylate conversion (as measured by FT-IR) and mechanical behavior (as measured by DMA) of the complex dynamic network structure. For example, despite the irreversible acrylate network, the rubbery modulus of the material decreases above the glass transition temperature due to the presence of the dynamic thermosensitive DA network. The shape of the modulus curve was also affected by the ratio of DA monomers to acrylate monomers; higher DA monomer content resulted in greater temperature sensitivity of the rubbery modulus in light-exposed films. 3D structures with feature sizes ranging from 50 to 500 μm were produced in geometries such as stacked rectangles and “logpile” structures. In the unexposed regions, free acrylate and maleimide groups were shown to tolerate temperatures as high as 120 °C with no premature gel formation observed. Removal of unexposed material during the development step was achieved at 120 °C, where the Diels–Alder equilibrium shifts toward the furan and maleimide reactants and the network depolymerizes. Finally, a process was developed for the fabrication of 3D microstructures via layer-by-layer photopatterning. The process is highly repeatable and results in complete elimination of unexposed regions. Additionally, excess quantities of the unexposed mixture may be stored at 4 °C for at least several weeks and then reused by heating to 120 °C to fully depolymerize the DA network, subsequently using the liquid mixture to make films.

A novel, mild, and high yielding synthesis is developed for trithiocarbonate and allyl sulphide-containing AFT (addition–fragmentation termination) monomers. The image illustrates the novel core, linker, and end groups of the monomer array crosslinked by PETMP, through which polymer films of low stress and capable of photoplasticity may be formed.

With the advent of systematically designed controllable reversible addition–fragmentation termination (CRAFT) compounds, we have identified structure–property relationships related to the RAFT compositional structure as it impacts photoplasticity in covalent adaptable networks (CANs). In this study, we have expanded the range and functional capabilities of addition–fragmentation capable network forming monomers by synthesizing and evaluating systematically varying CRAFT monomers with the general formula ABCBA. Subsequent assessment of the impact of these monomers on photoplasticity and stress relaxation was performed. Structural variation of the A and B segments, in particular, imparts increased efficiency and efficacy in stress relaxation and photoplasticity. The CRAFT monomers employed have highly efficient stress relaxation properties demonstrating stress reduction of up to 54% and 75%, respectively, in postpolymerization network photoplasticity experiments. Furthermore, polymerization stress reduction in purely acrylate and acrylate–thiol networks with CRAFT monomers shows a remarkably enhanced efficacy with the inclusion of relatively small amounts of the monomers. With a loading of only 1.5 wt % of the alkene trithiocarbonate monomer in each system more than 75% stress reduction was achieved.

Photomediated addition-fragmentation chemistry is applied to demonstrate the precisely controlled diffusion of chemical species through polymer networks. Fluorescent groups connected to polymer networks by allyl sulfide moieties become mobile upon irradiation with UV light due to radical-mediated addition-fragmentation bond exchange. Photoinduced transport through the bulk, into solution, and across film interfaces is demonstrated.

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The effects of the addition of small amounts of multifunctional monomers that contain functional groups capable of undergoing addition-fragmentation during radical polymerizations are investigated. Specifically, up to 6 wt % of phenyl trithiocarbonate (TTC)-containing diacrylate was added to conventional thiol-multiacrylate photopolymerizations where its addition led to up to 60% reduction in polymerization-induced shrinkage stress. The higher levels of TTC achieve the lowest stress though they also significantly depress the polymerization rate. Using up to 0.5 wt % phenyl TTC successfully reduces the stress by nearly 20%, demonstrating the effectiveness of the phenyl TTC, while minimizing the influence that the RAFT activity of the TTC unit has on the polymerization rate. When the polymerization rates of the TTC-containing resins are increased by changing the incident light intensity, complete acrylate conversion is achieved and the stress remains up to 40% lower in the TTC-containing resins.