Ke Zheng

All Publications

• Cascade Mechanochemical Transformation of a Benzobarrelane Polymer. Journal of the American Chemical Society Lee, D. C., Flear, E. J., Xu, R., Zheng, K., Martínez, T. J., Xia, Y. 2025

  Abstract

  Designing and understanding the reactivity of individual force-responsive molecules, mechanophores, has been a central topic in polymer mechanochemistry. However, we envision that when certain molecular structures are closely coupled along a polymer backbone, new mechanochemical reactivity that is absent from individual molecules may arise. Herein, we describe a polymer consisting of benzobarrelane repeat units that are connected via backbone alkenes, where force-induced bond scission triggers a radical cascade reaction to transform the polymer backbone structure. Despite the lack of weak covalent bonds or significant ring strain in the benzobarrelane polymer, it achieved similar degrees of mechanochemical transformation as our previously reported polyladderene systems consisting of highly strained repeat units. In contrast, no mechanochemical reaction was observed in benzobarrelane units that are distant from one another along the backbone: the mechanochemistry of barrelane requires immediately connected barrelane units. This work demonstrates the possibility of eliciting novel reactivities by creating cooperative chemical pathways across otherwise inert individual units.

  View details for DOI 10.1021/jacs.5c14494

  View details for PubMedID 41086304

• High Molecular Weight Semicrystalline Substituted Polycyclohexene From Alternating Copolymerization of Butadiene and Methacrylate and Its Ambient Depolymerization. Journal of the American Chemical Society Zheng, K., Yang, J., Luo, X., Xia, Y. 2024

  Abstract

  Cyclohexene cannot be polymerized via ring-opening polymerization under any conditions due to its lack of ring strain. A hypothetical polycyclohexene would therefore have a strong thermodynamic driving force to depolymerize to monomer if a metathesis catalyst were provided while otherwise having thermal and hydrolytic stability under normal conditions because of its hydrocarbon backbone. We envisioned access to this otherwise unattainable family of polymers via the alternating polymerization of a diene and an alkene. Ethyl aluminum chloride was found to promote highly alternating polymerization of butadiene and methacrylate when radically initiated at room temperature, resulting in formal polycyclohexene structures. Ultrahigh molecular weight (up to 1750 kDa) polymers can be synthesized at the decagram scale in high monomer conversions. The resulting presumably atactic copolymers exhibited semicrystallinity, leading to high toughness. In the presence of a small amount of the Grubbs catalyst, the generated polycyclohexene can be fully depolymerized at ambient temperatures into pure constituent cyclohexene. The strategy of using orthogonal chemistry for the polymerization and depolymerization processes allows access to polymer structures with subambient ceiling temperatures without using ultralow temperature synthesis or relying on the monomer-polymer equilibrium.

  View details for DOI 10.1021/jacs.4c09811

  View details for PubMedID 39219069

• Synthesis of Contorted Polycyclic Conjugated Hydrocarbons via Regioselective Activation of Cyclobutadienoids. Journal of the American Chemical Society Yin, X., Zheng, K., Jin, Z., Horst, M., Xia, Y. 2022

  Abstract

  Contorted carbon structures have drawn much attention in the past decade for their rich three-dimensional geometries, enhanced solubility, and tunable electronic properties. We report a modular method to synthesize contorted polycyclic conjugated hydrocarbons containing helical moieties in controlled topologies. This strategy leverages our previously reported streamlined synthesis of pi-systems containing four-membered cyclobutadienoids (CBDs), whose catalyzed cycloaddition with alkynes affords helical structures. Interestingly, we observed exclusive nonbay region regioselectivity in the C-C bond activation of CBDs in our system, which is opposite to the scarce previous examples of [N]phenylene activation that led to the formation of linear phenacene structures. The quantitative and regioselective nonbay region alkyne cycloaddition yielded a variety of helical carbon structures with their topologies predetermined by the CBD-containing precursor hydrocarbons. The cycloaddition can be inhibited by methyl substituents exocyclic to the four-membered ring, thus allowing selective activation of only certain desired CBD units while leaving the others intact. Calculation elucidated the basis for the observed regioselectivity. The described method provides a new route to multihelical aromatic hydrocarbons with complex yet defined geometries, facilitating the further exploration of such fascinating carbon structures.

  View details for DOI 10.1021/jacs.2c02457

  View details for PubMedID 35793470