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.

As the demand for polymeric materials transitions towards the need for customizable, high value, specialty polymeric materials, the ability to use light to initiate various physicochemical changes in polymers represents one of the most powerful and rapidly evolving approaches. Whether for polymer formation, polymer modification, shape change, or inducing smart material responses, light has the unique capacity for enabling 4D manipulation of each of those processes. Given the simple, 3D ability to focus light on a targeted voxel and the even simpler ability to turn a light on and off to facilitate temporal control, light has been used widely in various polymer modifications. Further, in addition to the ability to enhance the control of various reactive processes, due to the much greater energy available in a photon as compared to the thermal energy available, light enables chemical processes to occur at ambient conditions that are otherwise inaccessible without heating. In particular, within the polymer chemistry field, light has been used to cause bond formation, bond degradation, and isomerization, with subsequent reactions including polymerization, polymer degradation, polymer functionalization, and responsive changes in properties of smart materials. Here, this article attempts to provide a fundamental basis for the various photochemical processes implemented in polymer systems, followed by selected examples of that implementation in various polymerization, functionalization, degradation, and other reactions.

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We present a composite material composed of dual polymer networks uniquely formed from a single reaction type and catalyst but involving monomers with dramatically different reactivities. This powerful new approach to creating polymer networks produces two narrow glass transition, homogeneous networks sequentially from a single reaction but with all monomers present and uniformly mixed prior to any polymerization. These materials exhibit a triple shape memory effect based on the dual polymer networks, which were both formed using the thiol−Michael addition reaction. Two multifunctional thiol monomers (i.e., mercaptoacetate (MA) and mercaptopropionate (MP)) and two multifunctional vinyls (i.e., vinyl sulfone (V) and acrylate (A)) were polymerized in situ using a nucleophilic initiator. The MA-V polymer network (T g = 55 °C) was generated first associated with the higher functional group reactivities followed by the formation of the MP-A network (T g = 10 °C) which was confirmed by FT-IR, SEM, DMA, and a separately prepared composite polymer consisting of MA-V particles embedded in an MP-A matrix. The triple shape memory effect was characterized using DMA, and it was demonstrated that the shapes could be programmed either by a one-step (single temperature) or a two-step method (two different temperatures). This material was able to hold its transitional shape for an extended time period (>1 h) at intermediate temperature (20 °C) between its two T g s, mainly due to narrow transitions of two separate networks. This new approach to obtain dual polymer networks with distinct transitions and characteristics is simple and robust, thus enabling applications in areas such as triple shape memory polymers, biomedical materials, and composites.

A visible-light base generating system was successfully employed in catalyzing the thiol-Michael addition reaction to yield cross-linked polymers from a stoichiometric mixture of model thiol and vinyl monomers. Implementation of the radical inhibitor TEMPO with a combination of a photosensitizer (isopropylthioxanthone, ITX) and a photobase generator (triazabicyclodecene tetraphenylborate, TBD•HBPh 4 ) resulted in suppression of radical mediated side reactions and provided stoichiometric and complete conversion of both thiol and vinyl functional groups. The new initiating system acts as an efficient visible-light photobase generator that improves the orthogonality of the thiol-Michael addition with respect to offstoichiometric radical thiol-vinyl addition/vinyl chain reactions. This approach opens up a variety of possibilities for basecatalyzed reactions in multiple applications such as coatings and biomaterials that require biocompatible, environmentally friendly, and low-energy visible-light initiation.