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1. Enhanced polymer mechanical degradation through mechanochemically unveiled lactonization

   https://doi.org/10.1038/s41467-020-18809-7  

   Lin, Yangju; Kouznetsova, Tatiana B.; Chang, Chia-Chih; Craig, Stephen L. (October 2020, Nature Communications)

   Abstract The mechanical degradation of polymers is typically limited to a single chain scission per triggering chain stretching event, and the loss of stress transfer that results from the scission limits the extent of degradation that can be achieved. Here, we report that the mechanically triggered ring-opening of a [4.2.0]bicyclooctene (BCOE) mechanophore sets up a delayed, force-free cascade lactonization that results in chain scission. Delayed chain scission allows many eventual scission events to be initiated within a single polymer chain. Ultrasonication of a 120 kDa BCOE copolymer mechanically remodels the polymer backbone, and subsequent lactonization slowly (~days) degrades the molecular weight to 4.4 kDa, > 10× smaller than control polymers in which lactonization is blocked. The force-coupled kinetics of ring-opening are probed by single molecule force spectroscopy, and mechanical degradation in the bulk is demonstrated. Delayed scission offers a strategy to enhanced mechanical degradation and programmed obsolescence in structural polymeric materials. 

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2. Polyethylene Incorporating Diels–Alder Comonomers: A “Trojan Horse” Strategy for Chemically Recyclable Polyolefins

   https://doi.org/10.1002/anie.202301927  

   Parke, Sarah M; Lopez, Jaqueline C; Cui, Shilin; LaPointe, Anne M; Coates, Geoffrey W (July 2023, Angewandte Chemie International Edition)

   Abstract Polyolefins with periodic unsaturation in the backbone chain are sought after for synthesizing chemically recyclable polymers or telechelic polyolefin macromonomers. Here we introduce a bottom‐up synthesis of unsaturated high‐density polyethylene (HDPE) via copolymerization of ethylene with dimethyl 7‐oxabicyclo[2.2.1]hepta‐2,5‐diene‐3,5‐dicarboxylate followed by post‐polymerization retro‐Diels–Alder to unveil hidden double bonds in the polymer backbone. The incorporation of this “Trojan Horse” comonomer was varied and a series of unsaturated HDPE polymers with block lengths of 1.2, 1.9, and 3.5 kDa between double bonds was synthesized. Cross metathesis of unsaturated HDPE samples with 2‐hydroxyethyl acrylate yielded telechelic ester terminated PE macromonomers suitable for the preparation of ester‐linked PE. These materials were depolymerized and repolymerized, making them suitable candidates for chemical recycling. The ester‐linked PE displayed thermal and mechanical properties comparable to commercial HDPE. 

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3. Unconventional Conjugated Polymers Derived from a Common Set of trans-Enediyne Monomers

   https://doi.org/10.1055/s-0036-1591939  

   Qin, Yang (April 2018, Synlett)

   This account describes our recent efforts in the design and synthesis of several series of unconventional conjugated polymers derived from a common set of trans-enediyne (tEDY) monomers. The journey started with a failed attempt, through acyclic diene metathesis of triene monomers, to prepare soluble polyacetylenes (PAs) having cross-conjugated side-groups on alternate double bonds along the main chain. At this seemingly dead end of the project, we found that the tEDY intermediates leading to triene monomers could undergo alkyne metathesis to generate soluble polydiacetylenes (PDAs). Such acyclic enediyne metathesis represents the first example of a solution synthesis of PDAs, in contrast to the conventional topochemical methods. By applying Glaser–Hay-type reaction conditions with selected tEDY monomers, polytriacetylenes were smoothly obtained; these possessed aromatic units directly attached to the polymer main chains, which significantly affected the electronic properties of the polymers. Furthermore, through hydroboration of the tEDY monomers, ‘boron-doped’ polyacetylenes (BDPAs) were prepared that can be considered as PAs with every fourth double bond replaced with a boron atom. These BDPAs represent the first boron main-chain conjugated polymers lacking aromatic units along the backbone, potentially enhancing electronic delocalization along the main chain. 

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4. A Thermally Stable SO ₂ -Releasing Mechanophore: Facile Activation, Single-Event Spectroscopy, and Molecular Dynamic Simulations

   https://doi.org/10.1021/jacs.4c02139  

   Sun, Yunyan; Neary, William J.; Huang, Xiao; Kouznetsova, Tatiana B.; Ouchi, Tetsu; Kevlishvili, Ilia; Wang, Kecheng; Chen, Yingying; Kulik, Heather J.; Craig, Stephen L.; et al (April 2024, Journal of the American Chemical Society)

   Polymers that release small molecules in response to mechanical force are promising candidates as next-generation on-demand delivery systems. Despite advancements in the development of mechanophores for releasing diverse payloads through careful molecular design, the availability of scaffolds capable of discharging biomedically significant cargos in substantial quantities remains scarce. In this report, we detail a nonscissile mechanophore built from an 8-thiabicyclo[3.2.1]octane 8,8-dioxide (TBO) motif that releases one equivalent of sulfur dioxide (SO2) from each repeat unit. The TBO mechanophore exhibits high thermal stability but is activated mechanochemically using solution ultrasonication in either organic solvent or aqueous media with up to 63% efficiency, equating to 206 molecules of SO2 released per 143.3 kDa chain. We quantified the mechanochemical reactivity of TBO by single-molecule force spectroscopy and resolved its single-event activation. The force-coupled rate constant for TBO opening reaches ∼9.0 s–1 at ∼1520 pN, and each reaction of a single TBO domain releases a stored length of ∼0.68 nm. We investigated the mechanism of TBO activation using ab initio steered molecular dynamic simulations and rationalized the observed stereoselectivity. These comprehensive studies of the TBO mechanophore provide a mechanically coupled mechanism of multi-SO2 release from one polymer chain, facilitating the translation of polymer mechanochemistry to potential biomedical applications. 

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5. Visualizing the Orientation of Single Polymers Induced by Spin-Coating

   https://doi.org/10.1021/acs.nanolett.2c01830  

   Chan, Jonathan M.; Wang, Muzhou (July 2022, Nano Letters)

   The orientation of chains within polymeric materials influences their electrical, mechanical, and thermal properties. While many techniques can infer the orientation distribution of a bulk ensemble, it is challenging to determine this information at the single-chain level, particularly in an environment of otherwise identical polymers. Here, we use single-molecule localization microscopy (SMLM) to visualize the directions of chains within spin-coated polymer films. We find a strong relationship between shear force and the degree and direction of orientation, and additionally, we reveal the effects of chain length and solvent evaporation rate. This work utilizes single-chain resolution to observe the important, though often overlooked, property of chain orientation in the common fabrication process of spin-coating. 

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