This perspective summarizes the latest exciting developments in controlled radical polymerization during the last decade (2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020). Our focus is to critically highlight strengths and weaknesses of recent achievements and portray how these discoveries have expanded the scope of tailor-made polymeric materials. Our perspective on remaining future challenges and where we expect the field to grow toward in the next decade will also be discussed.100 years have passed since the landmark publication by Hermann Staudinger in 1920, after which the field of polymer chemistry was founded. One major revolution in the field has been controlled radical polymerization (CRP), also referred to as reversibledeactivation radical polymerization (RDRP), as it grants users the ability to regulate molecular weight, dispersity (Ð), composition, architecture, and end-group fidelity of vinyl polymers. Currently, numerous CRP techniques are available, most famously atom transfer radical polymerization (ATRP), 1 nitroxide-mediated polymerization (NMP), 2 and reversible addition-fragmentation chain-transfer (RAFT) polymerization. 3 Importantly, polymers synthesized by CRP find use as emulsifiers, dispersants, electrolytes, rheology, and surface modifiers and are applied in commercial products related to home care, beauty, health, paint, energy, and electronics. 4 The focus of this perspective is to acknowledge and critically review recent developments in the area of CRP (decade 2010-2020) (Figure 1). The order in which the developments are presented is not associated to their significance or date of their original report. USING LIGHT AS AN EXTERNAL STIMULUSAmong various stimuli, light is perhaps the most attractive due to its abundance, wide availability, mild nature, low cost, and environmental benignity while it offers tremendous possibilities for temporal and spatial control. In addition, light-mediated polymerizations might offer additional opportunities as they do not require high temperatures, which may facilitate side reactions and/or depolymerization. At the same time, however, light-mediated polymerizations suffer from limited depth penetration and scalability issues, although these have been considerably addressed through the development of flow photochemistry, which allows for faster polymerizations, enhanced control over the molecular weight distributions, and the possibility to be combined with on-orin-line monitoring characterization techniques (the reader is referred to a recent review in flow chemistry polymerization). 5 Furthermore, certain wavelengths and intensities may be disadvantageous for various biological systems. Currently, there are two main developments in this area, namely photo-ATRP (including metal-mediated and metal-free ATRP) and photoinduced electron transferThe Bigger Picture Challenges and opportunities:Universal catalysts compatible with a range of monomers and stimuli are urgently required to produce materials with enhanced control over monomer seq...

The ability to reverse controlled radical polymerization and regenerate the monomer would be highly beneficial for both fundamental research and applications, yet this has remained very challenging to achieve. Herein, we report a near-quantitative (up to 92%) and catalyst-free depolymerization of various linear, bulky, cross-linked, and functional polymethacrylates made by reversible addition–fragmentation chain-transfer (RAFT) polymerization. Key to our approach is to exploit the high end-group fidelity of RAFT polymers to generate chain-end radicals at 120 °C. These radicals trigger a rapid unzipping of both conventional (e.g., poly­(methyl methacrylate)) and bulky (e.g., poly­(oligo­(ethylene glycol) methyl ether methacrylate)) polymers. Importantly, the depolymerization product can be utilized to either reconstruct the linear polymer or create an entirely new insoluble gel that can also be subjected to depolymerization. This work expands the potential of polymers made by controlled radical polymerization, pushes the boundaries of depolymerization, offers intriguing mechanistic aspects, and enables new applications.

Retrieving the starting monomer from polymers synthesized by reversible deactivation radical polymerization has recently emerged as an efficient way to increase the recyclability of such materials and potentially enable their...

Controlled polymerization methods are well-established synthetic protocols for the design and preparation of polymeric materials with a high degree of precision over molar mass and architecture. Exciting recent work has shown that the high end-group fidelity and/or functionality inherent in these techniques can enable new routes to depolymerization under relatively mild conditions. Converting polymers back to pure monomers by depolymerization is a potential solution to the environmental and ecological concerns associated with the ultimate fate of polymers. This perspective focuses on the emerging field of depolymerization from polymers synthesized by controlled polymerizations including radical, ionic, and metathesis polymerizations. We provide a critical review of current literature categorized according to polymerization technique and explore numerous concepts and ideas which could be implemented to further enhance depolymerization including lower temperature systems, catalytic depolymerization, increasing polymer scope, and controlled depolymerization.

Reversing reversible deactivation radical polymerization (RDRP) to regenerate the original monomer is an attractive prospect for both fundamental research and industry. However, current depolymerization strategies are often applied to highly heat-tolerant polymers with a specific end-group and can only be performed in a specific solvent. Herein, we depolymerize a variety of poly(methyl methacrylate) materials made by reversible addition−fragmentation chain-transfer (RAFT) polymerization and terminated by various end groups (dithiobenzoate, trithiocarbonate, and pyrazole carbodithioate). The effect of the nature of the solvent on the depolymerization conversion was also investigated, and key solvents such as dioxane, xylene, toluene, and dimethylformamide were shown to facilitate efficient depolymerization reactions. Notably, our approach could selectively regenerate pure heat-sensitive monomers (e.g., tertbutyl methacrylate and glycidyl methacrylate) in the absence of previously reported side reactions. This work pushes the boundaries of reversing RAFT polymerization and considerably expands the chemical toolbox for recovering starting materials under relatively mild conditions.