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1. The synthesis of cyclic polymers by olefin metathesis: Achievements and challenges

   https://doi.org/10.1002/pola.29253  

   Edwards, Julian P.; Wolf, William J.; Grubbs, Robert H. (November 2018, Journal of Polymer Science Part A: Polymer Chemistry)

   ABSTRACT Cyclic polymers have drawn considerable interest for their peculiar physical properties in comparison to linear polymers, despite their equivalent compositions. Synthetically, cyclic polymers can be accessed through either macrocyclic ring‐closure or by ring‐expansion polymerization, but the main challenge with either method is the production of highly pure cyclic polymer samples. This highlight describes advances in the area of cyclic polymer synthesis, with a particular focus on ring‐expansion metathesis polymerization. Methods for characterizing cyclic polymers and assessing their purity are also discussed in order to emphasize the need for additional robust and reliable methods for synthesizing and studying topologically complex macromolecules. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.2019,57, 228–242 

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2. A general synthesis of cyclic bottlebrush polymers with enhanced mechanical properties via graft-through ring expansion metathesis polymerization

   https://doi.org/10.1039/D4SC06050D  

   Elardo, Matthew J; Levenson, Adelaide M; Kitos_Vasconcelos, Ana Paula; Pomfret, Meredith N; Golder, Matthew R (October 2024, Chemical Science)

   Cyclic bottlebrush polymers combine the promise of densely-grafted macromolecular architectures with the mechanical advantages of cyclic polymers for diverse applications across materials science. 

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3. Recent Advances in Sequence‐Controlled Ring‐Opening Copolymerizations of Monomer Mixtures

   https://doi.org/10.1002/asia.202201147  

   Wang, Xiaoqian; Huo, Ziyu; Xie, Xiaoyu; Shanaiah, Narasimhamurthy; Tong, Rong (January 2023, Chemistry – An Asian Journal)

   Abstract Transforming renewable resources into functional and degradable polymers is driven by the ever‐increasing demand to replace unsustainable polyolefins. However, the utility of many degradable homopolymers remains limited due to their inferior properties compared to commodity polyolefins. Therefore, the synthesis of sequence‐defined copolymers from one‐pot monomer mixtures is not only conceptually appealing in chemistry, but also economically attractive by maximizing materials usage and improving polymers’ performances. Among many polymerization strategies, ring‐opening (co)polymerization of cyclic monomers enables efficient access to degradable polymers with high control on molecular weights and molecular weight distributions. Herein, we highlight recent advances in achieving one‐pot, sequence‐controlled polymerizations of cyclic monomer mixtures using a single catalytic system that combines multiple catalytic cycles. The scopes of cyclic monomers, catalysts, and polymerization mechanisms are presented for this type of sequence‐controlled ring‐opening copolymerization. 

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4. Mechanism of Spatial and Temporal Control in Precision Cyclic Vinyl Polymer Synthesis by Lewis Pair Polymerization

   https://doi.org/10.1002/anie.202116303  

   McGraw, Michael L.; Reilly, Liam T.; Clarke, Ryan W.; Cavallo, Luigi; Falivene, Laura; Chen, Eugene Y. ‐X. (February 2022, Angewandte Chemie International Edition)

   Abstract In typical cyclic polymer synthesis via ring‐closure, chain growth and cyclization events are competing with each other, thus affording cyclic polymers with uncontrolled molecular weight or ring size and high dispersity. Here we uncover a mechanism by which Lewis pair polymerization (LPP) operates on polar vinyl monomers that allows the control of where and when cyclization takes place, thereby achieving spatial and temporal control to afford precision cyclic vinyl polymers or block copolymers with predictable molecular weight and low dispersity (≈1.03). A combined experimental and theoretical study demonstrates that cyclization occurs only after all monomers have been consumed (when) via conjugate addition of the propagating chain end to the specific site of the initiating chain end (where), allowing the cyclic polymer formation steps to be regulated and executed with precision in space and time. 

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5. Functionalized Cyclic Poly(α‐Hydroxy Acids) via Controlled Ring‐Opening Polymerization of O‐Carboxyanhydrides

   https://doi.org/10.1002/anie.202423973  

   Huo, Ziyu; Xie, Xiaoyu; Mahmud, Nadim; Worch, Joshua_C; Tong, Rong (April 2025, Angewandte Chemie International Edition)

   Abstract Linear poly(α‐hydroxy acids) are important degradable polymers, and they can be efficiently prepared by ring‐opening polymerization of O‐carboxyanhydrides with pendant functional groups. However, attempts to prepare cyclic poly(α‐hydroxy acids) have been plagued by side reactions, including epimerization and uncontrolled intramolecular chain transfers or termination, that prevent the synthesis of high‐molecular‐weight stereoregular cyclic polyesters. Herein, we report a scalable method for the synthesis of high‐molecular‐weight (>100 kDa) stereoregular functionalized cyclic poly(α‐hydroxy acids) by means of controlled polymerization of O‐carboxyanhydrides using a catalytic system consisting of a lanthanum complex with a sterically bulky ligand and a manganese silylamide. Additionally, using this system, we could readily prepare cyclic block poly(α‐hydroxy acids) by means of sequential addition of O‐carboxyanhydrides. The obtained cyclic polyesters and their cyclic block copolyesters exhibit distinctive physicochemical properties—including elevated phase transition temperature, improved toughness, and reduced viscosity—compared to their linear counterparts. 

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