Richard Whitfield

Atom transfer radical polymerization is a versatile controlled radical polymerization methodology in which control over polymerization is achieved by shifting the activation‐deactivation equilibrium towards dormant species, leading to polymers of targeted molecular weight and dispersity. However, the chemical recycling of ATRP materials lacks this level of control, with current methodologies favoring rapid end‐group activation over deactivation, resulting in an uncontrolled depolymerization whereby polymer chains unzip completely without any molecular weight control. In this work, the first example of controlled ATRP depolymerization is realized by strategically promoting deactivation over both activation and depropagation. The controlled nature of the reaction is achieved by increasing catalyst concentrations and reducing temperatures, resulting in on‐demand and uniform shortening of the polymer chains during the depolymerization, as evidenced by a linear decrease in molecular weight with conversion. Furthermore, while the uncontrolled depolymerization of diblock copolymers results in the simultaneous release of both monomers, controlled depolymerization leads to selective regeneration of the second block, where the monomer closer to the halogen end‐group is preferentially released first, thus providing a powerful analytical handle for structural characterization. The versatility of this strategy is confirmed across various activation mechanisms, including thermal, photochemical, and initiators for continuous activator regeneration (ICAR) ATRP.

Lignin-derived polymethacrylates depolymerize under mild thermal conditions, achieving up to 99% depolymerization efficiency and significantly outperforming their petroleum-based counterparts.

Chemical recycling back to monomer represents an attractive methodology to tackle plastic waste. However, current reports focus on either thermally unstable ‘designer’ polymers or highly diluted solution depolymerizations. Here we report a simple, yet efficient, bulk main-chain scission pathway, whereby the presence of N -hydroxyphthalimides triggers a hydrogen-atom-transfer reaction on the polymer backbone, followed by the rapid and near-quantitative depolymerization of polymethacrylates. Notably, this strategy can be directly applied to crude commercial polymers, does not rely on preinstalled end-groups or sacrificial co-monomers, operates at relatively low temperatures in the absence of light irradiation and precludes the use of a solvent, therefore enabling direct isolation of pristine monomers in high yields (∼90%). The methodology is compatible with polymethacrylates synthesized by either anionic, free radical or controlled radical polymerization and can also be applied to ultrahigh-molecular-weight materials (>10 ⁶ Da).

The chemical recycling of poly(methyl methacrylate) (PMMA) typically requires energy-intensive pyrolysis (>450 °C) or specialized photochemical setups restricted by light penetration depth. Herein, we report a purely thermal, low-temperature strategy that drives the efficient depolymerization of native, unaltered PMMA at 170 °C using commercially accessible peroxides. By maintaining a low steady-state radical concentration, oligomer formation can be fully eliminated, resulting in >95% of regenerated monomer under judiciously optimized conditions. Mechanistic studies and vapor-trapping experiments implicate a dual-activation pathway, where alkoxy radicals work alongside highly reactive chlorine radicals that undergo efficient hydrogen atom transfer with the polymer backbone, resulting in chain scission and subsequent depropagation to monomer. The full omission of chlorinated solvents could also be achieved, albeit at slightly higher temperatures. Notably, the real-world utility of this system is demonstrated by the direct depolymerization of commercial Plexiglas, despite the presence of comonomers, pigments, and additives.

The concept of electrochemically mediated depolymerization is introduced as a means to initiate and regulate depolymerization by efficiently switching between active and dormant species. During the chemical recycling of ATRP-synthesized polymers, high percentages of monomers are successfully regenerated by utilizing a readily available and inexpensive iron catalyst. In the absence of electrochemical means, negligible depolymerization occurs, thus enabling temporal control. Notably, under comparable conditions, electrochemical depolymerization outperforms thermal, photo, and chemical depolymerization approaches yielding higher monomer recovery and unlocking the potential of new solvents to trigger a lower-temperature depolymerization pathway. The presented approach significantly expands the chemical recycling toolbox to include an electrochemical switch as an alternative external stimulus for depolymerization.

Although the chemical recycling of polymers synthesized by controlled radical polymerization enables the recovery of pristine monomer at low temperatures, it operates efficiently under strictly anaerobic conditions. Instead, oxygen‐tolerant depolymerizations are scarce, and are either restricted to the use of a boiling co‐solvent or are performed in closed vessels, often suffering from low conversions. Here, an open‐vessel, oxygen‐tolerant depolymerization of atom transfer radical polymerization (ATRP)‐synthesized polymers is introduced, leading to high percentages of monomer regeneration (>90% depolymerization efficiency). Dissolved oxygen is eliminated by either utilizing high catalyst loadings, or lower catalyst loadings combined with a radical initiator. Notably, the methodology is compatible with various solvents (i.e., anisole, 1,2,4‐trichlorobenzene (TCB), 1,2‐dichlorobenzene (DCB), etc.) and a range of commercially available ligands including tris 2‐(dimethylamino)ethylamine (Me ₆ TREN) and tris(2‐pyridylmethyl)amine (TPMA), as well as more inexpensive alternatives such as tris(2‐aminoethyl)amine (TREN) and N,N,N‘,N‘‘,N‘‘ ‐pentamethyldiethylenetriamine (PMDETA).

Polymeric materials have become indispensable due to their versatility and low cost, yet their environmental impact presents a significant global challenge. Traditional chemical recycling methods typically rely on heat as a stimulus; for instance, pyrolysis is a popular chemical recycling methodology which faces limitations due to high energy consumption, low product selectivity, and the generation of undesirable byproducts. In response, recent advances in the promotion of depolymerization and degradation through alternative stimuli such as light, electrochemistry, and mechanical force, have shown promising potential for more efficient and selective polymer breakdown, yielding either the starting monomers or valuable small molecules. This perspective explores key examples of these emerging strategies, highlighting their potential to improve upon current protocols and offer alternative pathways under milder conditions, while identifying significant challenges that future research must address to translate promising chemistry into viable and broadly applicable recycling strategies.

Although thermal solution RAFT depolymerization has recently emerged as an efficient chemical recycling methodology, current approaches require specialized solvents (i.e., dioxane), typically suffer from extended reaction times, and operate exclusively under highly dilute conditions (i.e., 5 mM repeat unit concentration). To circumvent these limitations, a commercial radical initiator is introduced to kinetically untrap the depolymerization and promote chain-end activation. By varying the initiator concentration, a remarkable rate acceleration (up to 72 times faster) can be observed, enabling the completion of the depolymerization within 5 min. Notably, a 20-fold increase in the repeat unit concentration did not appreciably compromise the final depolymerization yield, while very high percentages of monomer could be recovered in a wide range of solvents, including dimethyl sulfoxide, anisole, xylene, acetonitrile, toluene, and trichlorobenzene. Our findings not only offer intriguing mechanistic aspects, but also significantly expand the scope and applications of thermal RAFT depolymerization.

Polymer design requires fine control over syntheses and a thorough understanding of their macromolecular structure. Herein, near-atomic level imaging of polymers is achieved, enabling the precise determination of one of the most important macromolecular characteristics: molecular weight. By judiciously designing and synthesizing different linear metal(loid)-rich homopolymers, subnanoscale polymer imaging is achieved through annular dark field-scanning transmission electron microscopy (ADF-STEM), owing to the incorporation of high Z atoms in the side chain of the monomeric units. The molecular weight of these polymers can be precisely determined by detecting and counting their metal(loid) atoms upon ADF-STEM imaging, at sample concentrations as low as 10 μg·mL^-1. Notably, a commonly used C, H, and O-containing polymer (i.e., poly(methyl acrylate)) that was thus far inaccessible at the atomic scale is derivatized to allow for subnano-level imaging, thus expanding the scope of our approach toward the atomic-level visualization of commodity polymers.