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1. Controlled Ring-Opening Polymerization of O-Carboxyanhydrides to Synthesize Functionalized Poly(α-Hydroxy Acids)

   https://doi.org/10.1055/s-0040-1722698  

   Wang, Xiaoqian; Chin, Ai Lin; Tong, Rong (January 2021, Organic Materials)

   null (Ed.)

   Poly(α-hydroxy acids), as a family of biodegradable polyesters, are valuable materials due to their broad applications in packaging, agriculture, and biomedical engineering. Herein we highlight and explore recent advances of catalysts in controlled ring-opening polymerization of O-carboxyanhydrides towards functionalized poly(α-hydroxy acids), especially metal catalyst-mediated controlled polymerization. Limitations of current polymerization strategies of O-carboxyanhydrides are discussed. 

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2. Photocatalyst-Independent Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides: Stereocontrol and Mechanism

   https://doi.org/10.1039/D0SC05550F  

   Zhong, Yongliang; Feng, Quanyou; wang, xiaoqian; Yang, Lei; Korovich, Andrew G; Madsen, Louis A; Tong, Rong (January 2021, Chemical Science)

   null (Ed.)

   Photoredox ring-opening polymerization of O -carboxyanhydrides allows for the synthesis of polyesters with precisely controlled molecular weights, molecular weight distributions, and tacticities. While powerful, obviating the use of precious metal-based photocatalysts would be attractive from the perspective of simplifying the protocol and enabling unexpected reactivity. Herein, we report the Co and Zn catalysts that are activated by external light to mediate efficient ring-opening polymerization of O -carboxyanhydrides, without the use of exogenous precious metal-based photocatalysts. Our methods allow for the synthesis of isotactic polyesters with high molecular weights (>200 kDa) and narrow molecular weight distributions ( M w / M n < 1.1). Mechanistic studies indicate that light activates the oxidative status of Co III intermediate that is generated from the regioselective ring-opening of the O -carboxyanhydride. We also demonstrate that the use of Zn or Hf complexes together with Co can allow for stereoselective photoredox ring-opening polymerizations of multiple racemic O -carboxyanhydrides to synthesize syndiotactic and stereoblock copolymers, which vary widely in their glass transition temperatures. 

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3. Synthesis of polypeptides via bioinspired polymerization of in situ purified N -carboxyanhydrides

   https://doi.org/10.1073/pnas.1901442116  

   Song, Ziyuan; Fu, Hailin; Wang, Jiang; Hui, Jingshu; Xue, Tianrui; Pacheco, Lazaro A.; Yan, Haoyuan; Baumgartner, Ryan; Wang, Zhiyu; Xia, Yingchun; et al (May 2019, Proceedings of the National Academy of Sciences)

   Ribozymes synthesize proteins in a highly regulated local environment to minimize side reactions caused by various competing species. In contrast, it is challenging to prepare synthetic polypeptides from the polymerization of N -carboxyanhydrides (NCAs) in the presence of water and impurities, which induce monomer degradations and chain terminations, respectively. Inspired by natural protein synthesis, we herein report the preparation of well-defined polypeptides in the presence of competing species, by using a water/dichloromethane biphasic system with macroinitiators anchored at the interface. The impurities are extracted into the aqueous phase in situ, and the localized macroinitiators allow for NCA polymerization at a rate which outpaces water-induced side reactions. Our polymerization strategy streamlines the process from amino acids toward high molecular weight polypeptides with low dispersity by circumventing the tedious NCA purification and the demands for air-free conditions, enabling low-cost, large-scale production of polypeptides that has potential to change the paradigm of polypeptide-based biomaterials. 

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4. Crystalline aliphatic polyesters from eight‐membered cyclic (di)esters

   https://doi.org/10.1002/pol.20220418  

   Tang, Xiaoyan; Shi, Changxia; Zhang, Zhen; Chen, Eugene Y. ‐X. (October 2022, Journal of Polymer Science)

   Abstract Ring‐opening polymerization (ROP) of lactones or cyclic (di)esters is a powerful method to produce well‐defined, high‐molecular‐weight (bio)degradable aliphatic polyesters. While the ROP of lactones of various ring sizes has been extensively studied, the ROP of the simplest eight‐membered lactone, 7‐heptanolactone (7‐HL), has not been reported using metal‐based catalysts. Accordingly, this contribution reports the ROP of 7‐HL via metal‐catalyzed coordinative‐insertion polymerization to the corresponding high‐molecular‐weight polyester, poly(7‐hydroxyheptanoate) (P7HHp). The resulting P7HHp is a semi‐crystalline material, with aT_mof 68 °C, which is ~10 °C higher than poly(ε‐caprolactone) derived from the seven‐membered lactone. Mechanical testing showed that P7HHp is a hard and tough plastic, with elongation at break >670%. P7HHp‐based polyesters with higherT_mvalues have been achieved through stereoselective copolymerization of 7‐HL with an eight‐membered cyclic diester, racemic dimethyl diolide (rac‐8DL^Me), known to lead to highT_mpoly(3‐hydroxyburtyrate) (P3HB). Notably, catalyst's strong kinetic preference for polymerizingrac‐8DL^Meover 7‐HL in the 1/1 comonomer mixture rendered the formation of di‐block copolymer P3HB‐b‐P7HHp, showing two crystalline domains withT_m1 ~ 65 °C andT_m2 ~ 160 °C. Semi‐crystalline random copolymers withT_mup to 164 °C have also been obtained by adjusting copolymerization conditions. Mechanical testing showed that P3HB‐b‐P7HHp can synergistically combine the high modulus of isotactic P3HB with the high ductility of P7HHp. 

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5. Accelerated polymerization of N-carboxyanhydrides catalyzed by crown ether

   https://doi.org/10.1038/s41467-020-20724-w  

   Xia, Yingchun; Song, Ziyuan; Tan, Zhengzhong; Xue, Tianrui; Wei, Shiqi; Zhu, Lingyang; Yang, Yingfeng; Fu, Hailin; Jiang, Yunjiang; Lin, Yao; et al (February 2021, Nature Communications)

   Abstract The recent advances in accelerated polymerization ofN-carboxyanhydrides (NCAs) enriched the toolbox to prepare well-defined polypeptide materials. Herein we report the use of crown ether (CE) to catalyze the polymerization of NCA initiated by conventional primary amine initiators in solvents with low polarity and low hydrogen-bonding ability. The cyclic structure of the CE played a crucial role in the catalysis, with 18-crown-6 enabling the fastest polymerization kinetics. The fast polymerization kinetics outpaced common side reactions, enabling the preparation of well-defined polypeptides using an α-helical macroinitiator. Experimental results as well as the simulation methods suggested that CE changed the binding geometry between NCA and propagating amino chain-end, which promoted the molecular interactions and lowered the activation energy for ring-opening reactions of NCAs. This work not only provides an efficient strategy to prepare well-defined polypeptides with functionalized C-termini, but also guides the design of catalysts for NCA polymerization. 

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