Photochemical reactions, particularly those involving photoinduced electron transfer processes, establish a substantial contribution to the modern synthetic chemistry, and the polymer community has been increasingly interested in exploiting and developing novel photochemical strategies. These reactions are efficiently utilized in almost every aspect of macromolecular architecture synthesis, involving initiation, control of the reaction kinetics and molecular structures, functionalization, and decoration, etc. Merging with polymerization techniques, photochemistry has opened up new intriguing and powerful avenues for macromolecular synthesis. Construction of various polymers with incredibly complex structures and specific control over the chain topology, as well as providing the opportunity to manipulate the reaction course through spatiotemporal control, are one of the unique abilities of such photochemical reactions. This review paper provides a comprehensive account of the fundamentals and applications of photoinduced electron transfer reactions in polymer synthesis. Besides traditional photopolymerization methods, namely free radical and cationic polymerizations, step-growth polymerizations involving electron transfer processes are included. In addition, controlled radical polymerization and "Click Chemistry" methods have significantly evolved over the last few decades allowing access to narrow molecular weight distributions, efficient regulation of the molecular weight and the monomer sequence and incredibly complex architectures, and polymer modifications and surface patterning are covered. Potential applications including synthesis of block and graft copolymers, polymer-metal nanocomposites, various hybrid materials and bioconjugates, and sequence defined polymers through photoinduced electron transfer reactions are also investigated in detail.

A mechanically controlled atom transfer radical polymerization (mechanoATRP) was successfully carried out in an ultrasound bath with low ppm of copper catalyst. The polymerization of methyl acrylate in the presence of CuBr 2 /tris(2pyridylmethyl)amine catalyst using ultrasound as an external stimulus was temporally controlled by switching on−off ultrasound agitation. The first order kinetics was observed during ultrasonication. The experimental molecular weights agreed well with the theoretical values and displayed narrow molecular weight distribution. The effects of various types of piezoelectric BaTiO 3 nanoparticles, loadings of nanoparticles, and targeted degrees of polymerization were studied.

Photochemistry has been playing a central role in the synthetic polymer community. Aromatic ketones, examples of which include benzophenone, thioxanthone, camphorquinone, among others, are renowned for their excellent optical characteristics and have been extensively taken advantage of photochemically induction of polymerization processes. Of particular interest is thioxanthone due to its adaptability for bearing different functionalities and applications in various modes of photopolymerization which accomplishes photoinitiation in conjunction with other co-initiator compounds; a behavior that is referred to as bi-molecular photoinitiation. In this paper, we review the photochemistry of thioxanthonebased systems and their use in different modes of photoinitiated polymerizations. Citing examples from literature, the development of various photoinitiating systems based on thioxanthones along with an understanding of their mechanistic behavior has been elucidated in advance.Scheme 1 Typical representation of the photolysis of the radical photoinitiators based on Type I (top) and Type II (bottom) systems on the example of a benzoin derivative and thioxanthone, respectively.Scheme 3 Synthesis of thioxanthones by condensation of thiosalicylic acid and aromatic compounds in the presence of concentrated sulfuric acid.

Photochemically mediated atom transfer radical polymerization of vinyl monomers is successfully activated by ecofriendly heterogeneous mesoporous graphitic carbon nitride (mpg‐C3N4). This method pertains to the use of mpg‐C3N4 as photoactivator for reduction of initially loaded copper(II) species, thus promoting the in situ formation of the copper(I) species. The controlled nature of the polymerizations in both natural sunlight and UV‐light irradiation at ambient temperature is confirmed by the good agreement of the kinetics of the ­polymerization with theoretical values. The light on–off experiments ­demonstrate that polymerizations are clearly initiated and moderated by either UV light or sunlight.

In macromolecular and material science, atom transfer radical polymerization (ATRP) has intensely influenced the research strategies facilitating fabrication of a wide range of polymers with well-defined structures and functions and their conjugation with biomolecules. Recently, the required copper (I) catalyst for ATRP process is generated by photoinduced redox reactions involving the in situ reduction of Cu(II) to Cu(I). Photochemically initiated reactions provide several distinct advantages, including temporal and spatial controls, rapid and energy efficient activation. The process is based on photoredox reactions of copper catalysts under various radiation sources with or without various photoinitiators. This Highlight focuses on the historical development, mechanistic aspects, limitations, and opportunities of photoinitiated ATRP along with selected examples.

Photoinduced atom transfer radical polymerization of methyl methacrylate initiated by in situ generation of copper (I) complex from higher oxidation state species using neat zinc oxide and iron-doped zinc oxide nanoparticles is investigated. The polymerizations proceed in a well-controlled manner under UV light at room temperature as evidenced by kinetic and light on-off experiments. The evolution of molecular weight with conversion shows good correlations between experimental and theoretical molecular weights, which confirmed good control over polymerization along with a narrow molecular weight distribution.

Using the power of light to drive controlled radical polymerizations has provided significant advances in synthesis of well-defined polymers. Photoinduced atom transfer radical polymerization (ATRP) systems often employ UV light to regenerate copper activator species to mediate the polymerization. Taking full advantage of long-wavelength visible light for ATRP would require developing appropriate photocatalytic systems that engage in photoinduced electron transfer processes with the ATRP components to generate activating species. Herein, we developed conjugated microporous polymers (CMP) as heterogeneous photocatalysts to exploit the power of visible light in promoting copper-catalyzed ATRP. The photocatalyst was designed by cross-linking phenothiazine (PTZ) as a photoactive core in the presence of dimethoxybenzene as a cross-linker via the Friedel−Crafts reaction. The resulting PTZ-CMP network showed photoactivity in the visible region due to the extended conjugation throughout the network because of the aromatic groups connecting the PTZ units. Therefore, photoinduced copper-catalyzed ATRP was performed with CMPs that regenerated activator species under green or red light irradiation to start the ATRP process. This resulted in efficient polymerization of acrylate and methacrylate monomers with high conversion and wellcontrolled molecular weight. The heterogeneous nature of the photocatalyst enabled easy separation and efficient reusability in subsequent polymerizations.

Photoinduced atom transfer radical polymerization (ATRP) has been mainly explored using copper-based catalytic systems. Recently developed iron-catalyzed photochemical ATRP employed high amounts of iron catalysts under high-energy UV light irradiation. Herein, a successful photoinduced iron-catalyzed ATRP mediated under blue light irradiation with ppm amounts (100–400 ppm) of iron­(III) bromide/tetrabutyl­ammonium bromide as the catalyst is reported. Several methacrylate monomers were polymerized with excellent control providing molecular weight in good agreement with theoretical values and low dispersity (Đ < 1.20). Near-quantitative monomer conversions (∼95%) enabled in situ chain extension and block copolymerization, indicating high retention of chain-end functionality. Notably, this system can tolerate oxygen, enabling synthesis of well-defined polymers with good chain-end functionality in the presence of air.