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1. Riboflavin‐Catalyzed Photoinduced Atom Transfer Radical Polymerization

   https://doi.org/10.1002/macp.202400323  

   Coskun, Halil Ibrahim; Votruba‐Drzal, Thomas; Wu, Hanshu; Jockusch, Steffen; Yilmaz, Gorkem; Matyjaszewski, Krzysztof (April 2025, Macromolecular Chemistry and Physics)

   Abstract The photoATRP of methyl acrylate (MA) is investigated using riboflavin (RF) and CuBr₂/Me₆TREN as a dual catalyst system under green LED irradiation (λ ≈ 525 nm). Both RF and CuBr₂/Me₆TREN enhanced oxygen tolerance, enabling effective ATRP in the presence of residual oxygen. High molar mass polymers (up toM_n ≈ 129 000 g·mol⁻¹) with low dispersity (Đ≤ 1.16) are prepared, and chain‐end fidelity is confirmed through successful chain extension. The molecular masses of the obtained polymer increased linearly with conversion and showed high initiation efficiency. Mechanistic studies by laser flash photolysis reveal that the predominant activator generation mechanism is reductive quenching of RF by Me₆TREN (83%, under [CuBr₂]/[Me₆TREN] = 1/3 condition), supported by polymerization kinetics and thermodynamic calculations. 

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2. Ultra‐High Molecular Weight Polydimethylsiloxane Bottlebrushes From Norbornene‐Pyrrolidine Capped Macromonomers

   https://doi.org/10.1002/pol.20250521  

   Cater, Henry L; Liu, Yutong; Page, Zachariah A (September 2025, Journal of Polymer Science)

   ABSTRACT Bottlebrush (BB) polymers with polydimethylsiloxane (PDMS) side‐chains exhibit supersoft moduli (E < 100 kPa) without solvent, making them promising tissue‐mimetic materials for biomedical applications. While larger backbones and side‐chains enhance softness, synthetic limitations have constrained their molecular weight. We report a graft‐through method to access ultra‐high molecular weight PDMS BB polymers (~1–20 million g mol⁻¹). Central to this was the design of a norbornene‐pyrrolidine aryllithium initiator for the anionic ring‐opening of hexamethylcyclotrisiloxane (D3), yielding monofunctionalized PDMS macromonomers (MMs) with molecular weights from 1–30 kg mol⁻¹. Subsequent ring‐opening metathesis polymerization (ROMP) produced BB homopolymers within minutes (t ≤ 5 min) and with low dispersity (Đ < 1.3). The ‐C‐Si‐O‐ linkage between the backbone and side‐chains provided improved chemical stability over earlier ‐C‐O‐Si‐ analogues. These MMs were also used to create a linear‐bottlebrush‐linear block copolymer, forming a supersoft (E = 79 kPa), thermoplastic elastomer with excellent elastic recovery (~96%). This platform enables the synthesis of ultra‐high molecular weight graft (co‐)polymers, advancing the study of extreme structure–property relationships and supporting the development of novel soft materials for bio‐interfacing. 

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3. Viscoelastic Response of Dispersed Entangled Polymer Melts

   Brandon L. Peters, K. Michael (October 2020, Macromolecules)

   Polymer synthesis routes result in macromolecules with molecular weight dispersity ĐM that depends on the polymerization mechanism. The lowest dispersity polymers are those made by anionic and atom-transfer radical polymerization, which exhibit narrow distributions ĐM = Mw/Mn ∼ 1.02–1.04. Even for small dispersity, the chain length can vary by a factor of two from the average. The impact of chain length dispersity on the viscoelastic response remains an open question. Here, the effects of dispersity on stress relaxation and shear viscosity of entangled polyethylene melts are studied using molecular dynamics simulations. Melts with chain length dispersity, which follow a Schulz–Zimm (SZ) distribution with ĐM = 1.0–1.16, are studied for times up to 800 μs, longer than the terminal time. These systems are compared to those with binary and ternary distributions. The stress relaxation functions are extracted from the Green–Kubo relation and from stress relaxation following a uniaxial extension. At short and intermediate time scales, both the mean squared displacement and the stress relaxation function G(t) are independent of ĐM. At longer times, the terminal relaxation time decreases with increasing ĐM. In this time range, the faster motion of the shorter chains results in constraint release for the longer chains. 

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4. Impact of Oxygen and Sulfur Heteroatom Core Substitution on Catalyst Properties of Phenoxazines and Their Performance in Organocatalyzed Atom Transfer Radical Polymerization (O‐ATRP)

   https://doi.org/10.1002/chem.202501179  

   Lathrop, Jessica L; Portela, Brandon S; Paton, Robert S; Miyake, Garret M (July 2025, Chemistry – A European Journal)

   Abstract Phenoxazines are a successful class of organic photoredox catalysts (PCs) with tunable redox and photophysical properties. Originally, we aimed to realize more reducing phenoxazine PCs through heteroatom core substituted (HetCS) derivatives, while maintaining an efficiently oxidizing PC^·+. However, core modification with thioether or ether functionality to a PC that exhibits photoinduced intramolecular charge transfer (CT) negligibly alters the singlet excited state reduction potential (E_S1°*), while yielding a less oxidizing PC^·+(E_1/2) (E_1/2 = 0.50–0.64 V vs. SCE) compared to the noncore modified PC1(0.68 V vs. SCE). Photophysical characterization of HetCS PCs revealed that increasing electron density on the core of a CT exhibiting PC stabilizes the emissive state and PC^·+, resulting in a relatively unchangedE_S1°* compared to PC1. In contrast, modifying the core of a PC that does not exhibit CT yields a highly reducingE_S1°* (PC3= −2.48 V vs. SCE) compared to its CT equivalent (PC1d= −1.68 V vs. SCE). The impact of PC property on photocatalytic ability was evaluated through organocatalyzed atom transfer radical polymerization (O‐ATRP). HetCS PCs were able to yield poly(methyl methacrylate) with low dispersity and moderate targeted molecular weight as evaluated by initiator efficiency (I*) in DMAc (Ð= 1.20–1.26;I*= 47–57%). Ultimately, this work provides insight into how phenoxazine PC properties are altered through structural modification, which can inform future PC design. 

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5. Colloidal synthesis of low-dispersity high entropy rock-salt oxide nanoparticles

   https://doi.org/10.1039/D5CC03836G  

   Rowell, Jonathan L; Joshi, Aditya A; Tan, Haotian; Yoon, Dasol; Muller, David A; Robinson, Richard D (October 2025, Chemical Communications)

   A novel synthesis route torwards small (<5 nm), low-dispersity (<20%) high entropy and multi-metal rock-salt oxide nanoparticles is described. 

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