Yan Xia

All Publications

• Hydrocarbon ladder polymers with ultrahigh permselectivity for membrane gas separations. Science (New York, N.Y.) Lai, H. W., Benedetti, F. M., Ahn, J. M., Robinson, A. M., Wang, Y., Pinnau, I., Smith, Z. P., Xia, Y. 2022; 375 (6587): 1390-1392

  Abstract

  Membranes have the potential to substantially reduce energy consumption of industrial chemical separations, but their implementation has been limited owing to a performance upper bound-the trade-off between permeability and selectivity. Although recent developments of highly permeable polymer membranes have advanced the upper bounds for various gas pairs, these polymers typically exhibit limited selectivity. We report a class of hydrocarbon ladder polymers that can achieve both high selectivity and high permeability in membrane separations for many industrially relevant gas mixtures. Additionally, their corresponding films exhibit desirable mechanical and thermal properties. Tuning of the ladder polymer backbone configuration was found to have a profound effect on separation performance and aging behavior.

  View details for DOI 10.1126/science.abl7163

  View details for PubMedID 35324307

• A versatile approach for the synthesis of degradable polymers via controlled ring-opening metathesis copolymerization. Nature chemistry Feist, J. D., Lee, D. C., Xia, Y. 2021

  Abstract

  Norbornene derivatives (NBEs) are common monomers for living ring-opening metathesis polymerization and yield polymers with low dispersities and diverse functionalities. However, the all-carbon backbone of poly-NBEs is non-degradable. Here we report a method to synthesize degradable polymers by copolymerizing 2,3-dihydrofuran with NBEs. 2,3-Dihydrofuran rapidly reacts with Grubbs catalyst to form a thermodynamically stable Ru Fischer carbene-the only detectable active Ru species during copolymerization-and the addition of NBEs becomes rate determining. This reactivity attenuates the NBE homoaddition and allows uniform incorporation of acid-degradable enol ether linkages throughout the copolymers, which enables complete polymer degradation while maintaining the favourable characteristics of living ring-opening metathesis polymerization. Copolymerization of 2,3-dihydrofuran with NBEs gives low dispersity polymers with tunable solubility, glass transition temperature and mechanical properties. These polymers can be fully degraded into small molecule or oligomeric species under mildly acidic conditions. This method can be readily adapted to traditional ring-opening metathesis polymerization of widely used NBEs to synthesize easily degradable polymers with tunable properties for various applications and for environmental sustainability.

  View details for DOI 10.1038/s41557-021-00810-2

  View details for PubMedID 34795434

• Predictably Engineering the Viscoelastic Behavior of Dynamic Hydrogels via Correlation with Molecular Parameters. Advanced materials (Deerfield Beach, Fla.) Lou, J., Friedowitz, S., Will, K., Qin, J., Xia, Y. 2021: e2104460

  Abstract

  Rational design of dynamic hydrogels with desirable viscoelastic behaviors relies on an in-depth understanding of the principles correlating molecular parameters and macroscopic properties. To quantitatively elucidate such principles, a series of dynamic covalent hydrogels crosslinked via hydrazone bonds is designed. The exchange rate of the hydrazone bond is tuned by varying the concentration of an organic catalyst, while maintaining the crosslinking density unchanged. This strategy of independently tuning exchange dynamics of crosslinks and crosslinking density allows unambiguous analysis of the viscoelastic response of the dynamic hydrogels as a function of their network parameters. It is found that the terminal relaxation time of the dynamic hydrogels is primarily determined by two factors: the exchange rate of crosslinks and the number of effective crosslinks per polymer chain, and is independent of the network architecture. Furthermore, a universal correlation is identified between the terminal relaxation time determined from stress relaxation and the exchange rate determined via reaction kinetics, which can be generalized to any viscoelastic hydrogel network, in principle. This quantitative correlation facilitates the development of dynamic hydrogels with a variable desired viscoelastic response based on molecular design.

  View details for DOI 10.1002/adma.202104460

  View details for PubMedID 34636090

• Understanding the Mechanochemistry of Ladder-Type Cyclobutane Mechanophores by Single Molecule Force Spectroscopy. Journal of the American Chemical Society Horst, M., Yang, J., Meisner, J., Kouznetsova, T. B., Martinez, T. J., Craig, S. L., Xia, Y. 2021

  Abstract

  We have recently reported a series of ladder-type cyclobutane mechanophores, polymers of which can transform from nonconjugated structures to conjugated structures and change many properties at once. These multicyclic mechanophores, namely, exo-ladderane/ene, endo-benzoladderene, and exo-bicyclohexene-peri-naphthalene, have different ring structures fused to the first cyclobutane, significantly different free energy changes for ring-opening, and different stereochemistry. To better understand their mechanochemistry, we used single molecule force spectroscopy (SMFS) to characterize their force-extension behavior and measure the threshold forces. The threshold forces correlate with the activation energy of the first bond, but not with the strain of the fused rings distal to the polymer main chain, suggesting that the activation of these ladder-type mechanophores occurs with similar early transition states, which is supported by force-modified potential energy surface calculations. We further determined the stereochemistry of the mechanically generated dienes and observed significant and variable contour length elongation for these mechanophores both experimentally and computationally. The fundamental understanding of ladder-type mechanophores will facilitate future design of multicyclic mechanophores with amplified force-response and their applications as mechanically responsive materials.

  View details for DOI 10.1021/jacs.1c05857

  View details for PubMedID 34310875

• Bicyclohexene-peri-naphthalenes: Scalable Synthesis, Diverse Functionalization, Efficient Polymerization, and Facile Mechanoactivation of Their Polymers. Journal of the American Chemical Society Yang, J., Horst, M., Werby, S. H., Cegelski, L., Burns, N. Z., Xia, Y. 2020; 142 (34): 14619–26

  Abstract

  Pursuing polymers that can transform from a nonconjugated to a conjugated state under mechanical stress to significantly change their properties, we developed a new generation of ladder-type mechanophore monomers, bicyclo[2.2.0]hex-5-ene-peri-naphthalene (BCH-Naph), that can be directly and efficiently polymerized by ring-opening metathesis polymerization (ROMP). BCH-Naphs can be synthesized in multigram quantities and functionalized with a wide range of electron-rich and electron-poor substituents, allowing tuning of the optoelectronic and physical properties of mechanically generated conjugated polymers. Efficient ROMP of BCH-Naphs yielded ultrahigh molecular weight polymechanophores with controlled MWs and low dispersity. The resulting poly(BCH-Naph)s can be mechanically activated into conjugated polymers using ultrasonication, grinding, and even simple stirring of the dilute solutions, leading to changes in absorption and fluorescence. Poly(BCH-Naph)s represent an attractive polymechanophore system to explore multifaceted mechanical response in solution and solid states, owing to the synthetic scalability, functional diversity, efficient polymerization, and facile mechanoactivation.

  View details for DOI 10.1021/jacs.0c06454

  View details for PubMedID 32786795

• Enol Ethers Are Effective Monomers for Ring-Opening Metathesis Polymerization: Synthesis of Degradable and Depolymerizable Poly(2,3-dihydrofuran). Journal of the American Chemical Society Feist, J. D., Xia, Y. 2020

  Abstract

  Enol ethers are widely used as quenching reagents for Grubbs catalysts. However, we report the surprisingly effective ring-opening metathesis polymerization (ROMP) of cyclic enol ethers, because the resulting electron-rich ruthenium alkylidene complex remains active toward metathesis of electron-rich olefins, despite its deactivation toward hydrocarbon olefins. We demonstrate the first example of ROMP of cyclic enol ethers, using 2,3-dihydrofuran as the monomer, producing a new type of degradable and depolymerizable poly(enol ether). The polymers exhibited perfect regioregularity, and their molecular weights can be regulated by the loading of Grubbs initiators or by the use of a linear vinyl ether as the chain transfer agent. We also developed protocols to deactivate the catalyst following metathesis of enol ethers and cleave the catalyst off the resulting polymers using H2O2 oxidation. The resulting poly(dihydrofuran) can be recycled to monomer via depolymerization with Grubbs catalyst or degraded to small molecules by hydrolysis under acidic conditions. This work opens exciting opportunities for a new class of ROMP monomers that lead to degradable polymers.

  View details for DOI 10.1021/jacs.9b11834

  View details for PubMedID 31880922

• Tunable Coacervation of Well-Defined Homologous Polyanions and Polycations by Local Polarity ACS CENTRAL SCIENCE Lou, J., Friedowitz, S., Qin, J., Xia, Y. 2019; 5 (3): 549–57

  Abstract

  The ionic complexation of polyelectrolytes is an important mechanism underlying many important biological processes and technical applications. The main driving force for complexation is electrostatic, which is known to be affected by the local polarity near charge centers, but the impact of which on the complexation of polyelectrolytes remains poorly explored. We developed a homologous series of well-defined polyelectrolytes with identical backbone structures, controlled molecular weights, and tunable local polarity to modulate the solvation environment near charged groups. A multitude of systematic, accurate phase diagrams were obtained by spectroscopic measurements of polymer concentrations via fluorescent labeling of polycations. These phase diagrams unambiguously revealed that the liquidlike coacervation is more stable against salt addition at reduced local polarity over a wide range of molecular weights. These trends were quantitatively captured by a theory of complexation that incorporates the effects of dispersion interactions, charge connectivity, and reversible ion-binding, providing the microscopic design rules for tuning molecular parameters and local polarity.

  View details for PubMedID 30937382

  View details for PubMedCentralID PMC6439447

• Dynamic Hyaluronan Hydrogels with Temporally Modulated High Injectability and Stability Using a Biocompatible Catalyst. Advanced materials (Deerfield Beach, Fla.) Lou, J., Liu, F., Lindsay, C. D., Chaudhuri, O., Heilshorn, S. C., Xia, Y. 2018; 30 (22): e1705215

  Abstract

  Injectable and biocompatible hydrogels have become increasingly important for cell transplantation to provide mechanical protection of cells during injection and a stable scaffold for cell adhesion post-injection. Injectable hydrogels need to be easily pushed through a syringe needle and quickly recover to a gel state, thus generally requiring noncovalent or dynamic cross-linking. However, a dilemma exists in the design of dynamic hydrogels: hydrogels with fast exchange of cross-links are easier to eject using less force, but lack long-term stability; in contrast, slow exchange of cross-links improves stability, but compromises injectability and thus the ability to protect cells under flow. A new concept to resolve this dilemma using a biocompatible catalyst to modulate the dynamic properties of hydrogels at different time points of application to have both high injectability and high stability is presented. Hyaluronic acid based hydrogels are formed through dynamic covalent hydrazone cross-linking in the presence of a biocompatible benzimidazole-based catalyst. The catalyst accelerates the formation and exchange of hydrazone bonds, enhancing injectability, but rapidly diffuses away from the hydrogel after injection to retard the exchange and improve the long-term stability for cell culture.

  View details for PubMedID 29682801

• Mechanochemical unzipping of insulating polyladderene to semiconducting polyacetylene Science Chen, Z., Mercer, J. A., Zhu, X., Romaniuk, J. A., Pfattner, R., Cegelski, L., Martinez, T. J., Burns, N. Z., Xia, Y. 2017; 357 (6350): 475-479

  Abstract

  Biological systems sense and respond to mechanical stimuli in a complex manner. In an effort to develop synthetic materials that transduce mechanical force into multifold changes in their intrinsic properties, we report on a mechanochemically responsive nonconjugated polymer that converts to a conjugated polymer via an extensive rearrangement of the macromolecular structure in response to force. Our design is based on the facile mechanochemical unzipping of polyladderene, a polymer inspired by a lipid natural product structure and prepared via direct metathesis polymerization. The resultant polyacetylene block copolymers exhibit long conjugation length and uniform trans-configuration and self-assemble into semiconducting nanowires. Calculations support a tandem unzipping mechanism of the ladderene units.

  View details for DOI 10.1126/science.aan2797

• Regioselective Synthesis of [3]Naphthylenes and Tuning of Their Antiaromaticity. Journal of the American Chemical Society Jin, Z. n., Teo, Y. C., Teat, S. J., Xia, Y. n. 2017; 139 (44): 15933–39

  Abstract

  Polycyclic conjugated hydrocarbons containing four-membered cyclobutadienoids (CBDs) are of great fundamental and technical interest due to the antiaromaticity brought by CBD circuits. However, their synthesis has been challenging, hampering the exploration and understanding of such systems. We report efficient synthesis of a series of unprecedented [3]naphthylene regioisomers in high yields, where three naphthalenoids are fused through two CBDs in linear, angular, and bent regioconnectivity. Their synthesis was enabled by exclusively regioselective catalytic arene-norbornene annulation (CANAL) between dibromonaphthalenes and benzooxanorbornadienes, followed by aromatization. [3]Naphthylene regioisomers exhibited distinct optoelectronic properties. Nucleus-independent chemical shift calculations, NMR spectroscopy, and X-ray crystallography revealed the strong effect of the fusion pattern on the local antiaromaticity and aromaticity in fused CBDs and naphthalenoids, respectively. Thus, our synthetic strategy allows facile access to extended CBD-fused π-systems with tunable local antiaromaticity and aromaticity.

  View details for PubMedID 28956438

• Efficient Synthesis of Rigid Ladder Polymers via Palladium Catalyzed Annulation. Journal of the American Chemical Society Liu, S., Jin, Z., Teo, Y. C., Xia, Y. 2014; 136 (50): 17434-17437

  Abstract

  We report a new method to synthesize rigid ladder polymers using efficient palladium catalyzed annulation reactions with low catalyst loading (1 mol %). Rigid ladder polymers with benzocyclobutene backbone linkages can be synthesized from copolymerization of readily accessible aryl dibromides and norbornadiene or polymerization of AB type monomers bearing norbornene and aryl bromide or triflate moieties. High molecular weight (10-40 kDa) rigid ladder polymers can be obtained with complete monomer conversions. Diverse monomers also gave different, fixed ladder polymer conformations. The ladder polymers exhibited excellent thermal stability, high carbonization yield, and large intrinsic porosity.

  View details for DOI 10.1021/ja5110415

  View details for PubMedID 25423254

• Catalytic arene-norbornene annulation (CANAL) ladder polymer derived carbon membranes with unparalleled hydrogen/carbon dioxide size-sieving capability JOURNAL OF MEMBRANE SCIENCE Hazazi, K., Wang, Y., Bettahalli, N., Ma, X., Xia, Y., Pinnau, I. 2022; 654

  View details for DOI 10.1016/j.memsci.2022.120548

  View details for Web of Science ID 000796540200002

• 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

• Effect of Fluoroalkylsilane Surface Functionalization on Boron Combustion. ACS applied materials & interfaces Baek, J., Jiang, Y., Demko, A. R., Jimenez-Thomas, A. R., Vallez, L., Ka, D., Xia, Y., Zheng, X. 2022

  Abstract

  Boron has been regarded as a promising high-energy fuel due to its high volumetric and gravimetric heating values. However, it remains challenging for boron to attain its theoretical heat of combustion because of the existence of its native boron oxide layer and its high melting and boiling temperatures that delay ignition and inhibit complete combustion. Boron combustion is known to be enhanced by physically adding fluorine-containing chemicals, such as fluoropolymer or metal fluorides, to remove surface boron oxides. Herein, we chemically functionalize the surface of boron particles with three different fluoroalkylsilanes: FPTS-B (F3-B), FOTS-B (F13-B), and FDTS-B (F17-B). We evaluated the ignition and combustion properties of those three functionalized boron particles as well as pristine ones. The boron particles functionalized with the longest fluorocarbon chain (F17) exhibit the most powerful energetic performance, the highest heat of combustion, and the strongest BO2 emission among all samples. These results suggest that the surface functionalization with fluoroalkylsilanes is an efficient strategy to enhance boron ignition and combustion.

  View details for DOI 10.1021/acsami.2c00347

  View details for PubMedID 35467848

• Comparative experimental and computational study of synthetic and natural bottlebrush polyelectrolyte solutions. The Journal of chemical physics Horkay, F., Chremos, A., Douglas, J. F., Jones, R., Lou, J., Xia, Y. 2021; 155 (7): 074901

  Abstract

  We systematically investigate model synthetic and natural bottlebrush polyelectrolyte solutions through an array of experimental techniques (osmometry and neutron and dynamic light scattering) along with molecular dynamics simulations to characterize and contrast their structures over a wide range of spatial and time scales. In particular, we perform measurements on solutions of aggrecan and the synthetic bottlebrush polymer, poly(sodium acrylate), and simulations of solutions of highly coarse-grained charged bottlebrush molecules having different degrees of side-branch density and inclusion of an explicit solvent and ion hydration effects. While both systems exhibit a general tendency toward supramolecular organization in solution, bottlebrush poly(sodium acrylate) solutions exhibit a distinctive "polyelectrolyte peak" in their structure factor, but no such peak is observed in aggrecan solutions. This qualitative difference in scattering properties, and thus polyelectrolyte solution organization, is attributed to a concerted effect of the bottlebrush polymer topology and the solvation of the polymer backbone and counterions. The coupling of the polyelectrolyte topological structure with the counterion distribution about the charged polymer molecules along with direct polymer segmental hydration makes their solution organization and properties "tunable," a phenomenon that has significant ramifications for biological function and disease as well as for numerous materials applications.

  View details for DOI 10.1063/5.0061649

  View details for PubMedID 34418934

• Looping-in complexation and ion partitioning in nonstoichiometric polyelectrolyte mixtures. Science advances Friedowitz, S., Lou, J., Barker, K. P., Will, K., Xia, Y., Qin, J. 2021; 7 (31)

  Abstract

  A wide variety of intracellular membraneless compartments are formed via liquid-liquid phase separation of charged proteins and nucleic acids. Understanding the stability of these compartments, while accounting for the compositional heterogeneity intrinsic to cellular environments, poses a daunting challenge. We combined experimental and theoretical efforts to study the effects of nonstoichiometric mixing on coacervation behavior and accurately measured the concentrations of polyelectrolytes and small ions in the coacervate and supernatant phases. For synthetic polyacrylamides and polypeptides/DNA, with unequal mixing stoichiometry, we report a general "looping-in" phenomenon found around physiological salt concentrations, where the polymer concentrations in the coacervate initially increase with salt addition before subsequently decreasing. This looping-in behavior is captured by a molecular model that considers reversible ion binding and electrostatic interactions. Further analysis in the low-salt regime shows that the looping-in phenomenon originates from the translational entropy of counterions that are needed to neutralize nonstoichiometric coacervates.

  View details for DOI 10.1126/sciadv.abg8654

  View details for PubMedID 34330707

• Enhancing Mechanical and Combustion Performance of Boron/Polymer Composites via Boron Particle Functionalization. ACS applied materials & interfaces Jiang, Y., Dincer Yilmaz, N. E., Barker, K. P., Baek, J., Xia, Y., Zheng, X. 2021

  Abstract

  High-speed air-breathing propulsion systems, such as solid fuel ramjets (SFRJ), are important for space exploration and national security. The development of SFRJ requires high-performance solid fuels with excellent mechanical and combustion properties. One of the current solid fuel candidates is composed of high-energy particles (e.g., boron (B)) and polymeric binder (e.g., hydroxyl-terminated polybutadiene (HTPB)). However, the opposite polarities of the boron surface and HTPB lead to poor B particle dispersion and distribution within HTPB. Herein, we demonstrate that the surface functionalization of B particles with nonpolar oleoyl chloride greatly improves the dispersion and distribution of B particles within HTPB. The improved particle dispersion is quantitatively visualized through X-ray computed tomography imaging, and the particle/matrix interaction is evaluated by dynamic mechanical analysis. The surface-functionalized B particles can be uniformly dispersed up to 40 wt % in HTPB, the highest mass loading reported to date. The surface-functionalized B (40 wt %)/HTPB composite exhibits a 63.3% higher Young's modulus, 87.5% higher tensile strength, 16.2% higher toughness, and 16.8% higher heat of combustion than pristine B (40 wt %)/HTPB. The surface functionalization of B particles provides an effective strategy for improving the efficacy and safety of B/HTPB solid fuels for future high-speed air-breathing vehicles.

  View details for DOI 10.1021/acsami.1c06727

  View details for PubMedID 34110148

• Mechanochemical generation of acid-degradable poly(enol ether)s CHEMICAL SCIENCE Yang, J., Xia, Y. 2021; 12 (12): 4389–94

  View details for DOI 10.1039/d1sc00001b

  View details for Web of Science ID 000635768300015

• Engineered Matrices Enable the Culture of Human Patient-Derived Intestinal Organoids ADVANCED SCIENCE Hunt, D. R., Klett, K. C., Mascharak, S., Wang, H. Y., Gong, D., Lou, J., Li, X., Cai, P. C., Suhar, R. A., Co, J. Y., LeSavage, B. L., Foster, A. A., Guan, Y., Amieva, M. R., Peltz, G., Xia, Y., Kuo, C. J., Heilshorn, S. C. 2021

  View details for DOI 10.1002/advs.202004705

  View details for Web of Science ID 000627768200001

• Polymerization of Cyclopropenes: Taming the Strain for the Synthesis of Controlled and Sequence-Regulated Polymers. Accounts of chemical research Elling, B. R., Su, J. K., Xia, Y. 2020

  Abstract

  ConspectusCyclopropenes (CPEs) are highly strained cyclic olefins, yet there are surprisingly limited examples leveraging their high strain energy for polymerization. In the past, attempts had been made to polymerize CPEs via cationic and insertion polymerization, but side reactions often gave uncontrolled polymers with mixed backbone structures. Ring-opening metathesis polymerization (ROMP) represents an ideal strategy for polymerizing CPEs to access new types of polymers. The proximity of substituents to the olefin in the small framework of CPEs offers a modular handle to tune the kinetic barrier to propagation by the modulation of the substituents. While the first few studies focused on the homopolymerization of simple alkyl or phenyl disubstituted CPEs, we recently explored the metathesis of a wide range of CPEs with different substituents using Grubbs catalysts and discovered surprising and diverse reactivities that are contingent on the positions, sterics, and electronics of substituents. The observed reactivities ranged from living homopolymerization to catalyst deactivation to single addition to the catalyst without homopropagation. In particular, the exclusively single addition reactivity found in two families of CPEs, with either bis(methanol ester) or phenyl and methanol ester substituents at the allylic position, is unusual for any monomer and perhaps counterintuitive for highly strained cycles. These single-addition CPEs could, however, be copolymerized with low-strain cyclic olefins to generate perfectly alternating copolymers with controlled molecular weights and low dispersity and to introduce degradable backbone linkages. A single equivalent (relative to the active chain end) of such CPEs could also be added to the active chain end of living ROMP polymers to install functional terminal groups or during living ROMP to place single units of functional moieties or side chains at any desired chain locations in narrow-disperse homopolymers and block copolymers. This account summarizes the polymerization of CPEs with a focus on our journey to uncover the rich and unique metathesis reactivities of CPEs and their utility in synthesizing well-controlled and sequence-regulated polymers. It provides the first collective structure-metathesis reactivity relationships for CPEs in the context of polymer chemistry and an understanding of the interactions between the catalyst and the substituents of appended ring-opened CPEs. It may become clear from this Account that the exploration of strained cycles in polymer chemistry can be quite fruitful in discovering new chemistry and accessing new types of polymer materials.

  View details for DOI 10.1021/acs.accounts.0c00638

  View details for PubMedID 33371668

• Mechanochemical synthesis of an elusive fluorinated polyacetylene. Nature chemistry Boswell, B. R., Mansson, C. M., Cox, J. M., Jin, Z., Romaniuk, J. A., Lindquist, K. P., Cegelski, L., Xia, Y., Lopez, S. A., Burns, N. Z. 2020

  Abstract

  Polymer mechanochemistry has traditionally been employed to study the effects of mechanical force on chemical bonds within a polymer backbone or to generate force-responsive materials. It is under-exploited for the scalable synthesis of wholly new materials by chemically transforming the polymers, especially products inaccessible by other means. Here we utilize polymer mechanochemistry to synthesize a fluorinated polyacetylene, a long-sought-after air-stable polyacetylene that has eluded synthesis by conventional means. We construct the monomer in four chemical steps on gram scale, which involves a rapid incorporation of fluorine atoms in an exotic photochemical cascade whose mechanism and exquisite stereoselectivity were informed by computation. After polymerization, force activation by ultrasonication produces a gold-coloured, semiconducting fluoropolymer. This work demonstrates that polymer mechanochemistry is a valuable synthetic tool for accessing materials on a preparative scale.

  View details for DOI 10.1038/s41557-020-00608-8

  View details for PubMedID 33349696

• Ring-Opening Metathesis Polymerization of 1,1-Disubstituted 1-Methylcyclopropenes MACROMOLECULES Su, J. K., Lee, S., Elling, B. R., Xia, Y. 2020; 53 (14): 5833–38

  View details for DOI 10.1021/acs.macromol.0c00837

  View details for Web of Science ID 000557761800019

• Catalytic Arene (oxa)Norbornene Annulation (CANAL) TRENDS IN CHEMISTRY Leibham, A. M., Xia, Y. 2020; 2 (7): 680–81

  View details for DOI 10.1016/j.trechm.2020.03.009

  View details for Web of Science ID 000540342200009

• Systematic investigation of synthetic polyelectrolyte bottlebrush solutions by neutron and dynamic light scattering, osmometry, and molecular dynamics simulation. The Journal of chemical physics Horkay, F., Chremos, A., Douglas, J. F., L Jones, R., Lou, J., Xia, Y. 2020; 152 (19): 194904

  Abstract

  There is a great interest in the synthesis and characterization of polyelectrolytes that mimic naturally occurring bottlebrush polyelectrolytes to capitalize on the unique properties of this class of macromolecules. Charged bottlebrush polymers form the protective mucus layer in the lungs, stomach, and orifices of animals and provide osmotic stabilization and lubrication to joints. In the present work, we systematically investigate bottlebrush poly(sodium acrylates) through a combination of measurements of solution properties (osmometry, small-angle neutron scattering, and dynamic light scattering) and molecular dynamics simulations, where the bottlebrush properties are compared in each case to their linear polymer counterparts. These complementary experimental and computational methods probe vastly different length- and timescales, allowing for a comprehensive characterization of the supermolecular structure and dynamics of synthetic polyelectrolyte bottlebrush molecules in solution.

  View details for DOI 10.1063/5.0007271

  View details for PubMedID 33687251

  View details for PubMedCentralID PMC7252672

• Systematic investigation of synthetic polyelectrolyte bottlebrush solutions by neutron and dynamic light scattering, osmometry, and molecular dynamics simulation JOURNAL OF CHEMICAL PHYSICS Horkay, F., Chremos, A., Douglas, J. F., Jones, R. L., Lou, J., Xia, Y. 2020; 152 (19)

  View details for DOI 10.1063/5.0007271

  View details for Web of Science ID 000537242000001

• Facile Synthesis and Study of Microporous Catalytic Arene-Norbornene Annulation-Troger's Base Ladder Polymers for Membrane Air Separation ACS MACRO LETTERS Ma, X., Lai, H. H., Wang, Y., Alhazmi, A., Xia, Y., Pinnau, I. 2020; 9 (5): 680–85

  View details for DOI 10.1021/acsmacrolett.0c00135

  View details for Web of Science ID 000537152800008

• Facile Synthesis and Study of Microporous Catalytic Arene-Norbornene Annulation-Tröger's Base Ladder Polymers for Membrane Air Separation. ACS macro letters Ma, X., Lai, H. W., Wang, Y., Alhazmi, A., Xia, Y., Pinnau, I. 2020; 9 (5): 680-685

  Abstract

  We report the facile synthesis and study of two soluble microporous ladder polymers, CANAL-TBs, by combining catalytic arene-norbornene annulation (CANAL) and Tröger's base (TB) formation. The polymers were synthesized in two steps from commercially available chemicals in high yields. CANAL-TBs easily formed mechanically robust films, were thermally stable up to 440 °C, and exhibited very high Brunauer-Teller-Emmett surface areas of 900-1000 m2 g-1. The gas separation performance of the CANAL-TBs for the O2/N2 pair is located between the 2008 and 2015 permeability/selectivity upper bounds. After 300 days of aging, CANAL-TBs still exhibited O2 permeability of 200-500 barrer with O2/N2 selectivity of about 5. The polymer with more methyl substituents exhibited higher permeability and slightly larger intersegmental spacing as revealed by WAXS, presumably due to more frustrated chain packing. The facile synthesis, excellent mechanical properties, and promising air separation performance of the CANAL-TB polymers make them attractive membrane materials for various air separation applications, such as aircraft on-board nitrogen generation and oxygen enrichment for combustion.

  View details for DOI 10.1021/acsmacrolett.0c00135

  View details for PubMedID 35648573

• Degradable Polyacetals/Ketals from Alternating Ring-Opening Metathesis Polymerization. ACS macro letters Elling, B. R., Su, J. K., Xia, Y. 2020; 9 (2): 180-184

  Abstract

  We report the synthesis of degradable polyacetals and polyketals with controlled molecular weights and low dispersities using alternating ring-opening metathesis polymerization (AROMP) of 1,1-disubstituted cyclopropenes and dioxepins. Under optimized conditions, high degrees of alternation and controlled polymerization were achieved between nonpropagating cyclopropenes and low-strain dioxepins. The high degrees of alternation allowed the resulting polymers to fully degrade into small molecules under acidic conditions at variable rates depending on the acetal/ketal structures. This synthetic strategy illustrates the use of AROMP to incorporate functionalities into both the polymer backbone as well as the side chains.

  View details for DOI 10.1021/acsmacrolett.9b00936

  View details for PubMedID 35638680

• Arm-degradable star polymers with crosslinked ladder-motif cores as a route to soluble microporous nanoparticles POLYMER CHEMISTRY Teo, Y., Lai, H. H., Xia, Y. 2020; 11 (2): 265–69

  View details for DOI 10.1039/c9py01060b

  View details for Web of Science ID 000505568400008

• The cascade unzipping of ladderane reveals dynamic effects in mechanochemistry. Nature chemistry Chen, Z., Zhu, X., Yang, J., Mercer, J. A., Burns, N. Z., Martinez, T. J., Xia, Y. 2020

  Abstract

  Force can induce remarkable non-destructive transformations along a polymer, but we have a limited understanding of the energy transduction and product distribution in tandem mechanochemical reactions. Ladderanes consist of multiple fused cyclobutane rings and have recently been used as monomeric motifs to develop polymers that drastically change their properties in response to force. Here we show that [4]-ladderane always exhibits 'all-or-none' cascade mechanoactivations and the same stereochemical distribution of the generated dienes under various conditions and within different polymer backbones. Transition state theory fails to capture the reaction kinetics and explain the observed stereochemical distributions. Ab initio steered molecular dynamics reveals unique non-equilibrium dynamic effects: energy transduction from the first cycloreversion substantially accelerates the second cycloreversion, and bifurcation on the force-modified potential energy surface leads to the product distributions. Our findings illustrate the rich chemistry in closely coupled multi-mechanophores and an exciting potential for effective energy transduction in tandem mechanochemical reactions.

  View details for DOI 10.1038/s41557-019-0396-5

  View details for PubMedID 31907403

• Precise Placement of Single Monomer Units in Living Ring-Opening Metathesis Polymerization CHEM Elling, B. R., Su, J. K., Feist, J. D., Xia, Y. 2019; 5 (10): 2691–2701

  View details for DOI 10.1016/j.chempr.2019.07.017

  View details for Web of Science ID 000489631400018

• Tuning the Reactivity of Cyclopropenes from Living ROMP to Single Addition and AROMP via Simple Substituents. Angewandte Chemie (International ed. in English) Su, J. K., Jin, Z., Zhang, R., Lu, G., Liu, P., Xia, Y. 2019

  Abstract

  Ring-opening metathesis polymerization (ROMP) has become one of the most important living polymerizations. Despite being the most strained cyclic olefin, cyclopropenes (CPEs) remain underexplored for ROMP. We discovered that the simple swap of 1-methyl to 1-phenyl on 1-(benzoyloxymethyl)-CPEs elicited strikingly different modes of reactivity, switching from living polymerization to selective single addition and living alternating ROMP, respectively, to yield well-controlled polymers in both cases. This pronounced difference in reactivity stemmed from differences in steric repulsions at the Ru alkylidene after CPE ring-opening. Possible olefin or oxygen chelation from ring-opened CPE substituents was also observed to significantly affect the rate of propagation. These results demonstrate the versatility of CPEs as a new class of monomers for ROMP, provide mechanistic insights for designing new monomers with rare single addition reactivity, and generate a new functionalizable alternating copolymer scaffold with controlled molecular weight and low dispersity.

  View details for DOI 10.1002/anie.201909688

  View details for PubMedID 31571344

• Tuning the Molecular Weights, Chain Packing, and Gas-Transport Properties of CANAL Ladder Polymers by Short Alkyl Substitutions MACROMOLECULES Lai, H. H., Benedetti, F. M., Jin, Z., Teo, Y., Wu, A. X., De Angelis, M., Smith, Z. P., Xia, Y. 2019; 52 (16): 6294–6302

  View details for DOI 10.1021/acs.macromol.9b01155

  View details for Web of Science ID 000483437500027

• Benzoladderene Mechanophores: Synthesis, Polymerization, and Mechanochemical Transformation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yang, J., Horst, M., Romaniuk, J. H., Jin, Z., Cegelski, L., Xia, Y. 2019; 141 (16): 6479–83

  View details for DOI 10.1021/jacs.9b01736

  View details for Web of Science ID 000466053400009

• Varying PEG density to control stress relaxation in alginate-PEG hydrogels for 3D cell culture studies BIOMATERIALS Nam, S., Stowers, R., Lou, J., Xia, Y., Chaudhuri, O. 2019; 200: 15–24

  View details for DOI 10.1016/j.biomaterials.2019.02.004

  View details for Web of Science ID 000461128000002

• Microporous Polyimides from Ladder Diamines Synthesized by Facile Catalytic Arene-Norbornene Annulation as High-Performance Membranes for Gas Separation CHEMISTRY OF MATERIALS Abdulhamid, M. A., Lai, H. H., Wang, Y., Jin, Z., Teo, Y., Ma, X., Pinnau, I., Xia, Y. 2019; 31 (5): 1767–74

  View details for DOI 10.1021/acs.chemmater.8b05359

  View details for Web of Science ID 000461532700036

• Dinaphthobenzo[1,2:4,5]dicyclobutadiene: Antiaromatic and Orthogonally Tunable Electronics and Packing ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Jin, Z., Yao, Z., Barker, K. P., Pei, J., Xia, Y. 2019; 58 (7): 2034–39

  View details for DOI 10.1002/anie.201812581

  View details for Web of Science ID 000458828000026

• Varying PEG density to control stress relaxation in alginate-PEG hydrogels for 3D cell culture studies. Biomaterials Nam, S., Stowers, R., Lou, J., Xia, Y., Chaudhuri, O. 2019; 200: 15–24

  Abstract

  Hydrogels are commonly used as artificial extracellular matrices for 3D cell culture and for tissue engineering. Viscoelastic hydrogels with tunable stress relaxation have recently been developed, and stress relaxation in the hydrogels has been found to play a key role in regulating cell behaviors such as differentiation, spreading, and proliferation. Here we report a simple but precise materials approach to tuning stress relaxation of alginate hydrogels with polyethylene glycol (PEG) covalently grafted onto the alginate. Hydrogel relaxation was modulated independent of the initial elastic modulus by varying molecular weight and concentration of PEG along with calcium crosslinking of the alginate. Increased concentration and molecular weight of the PEG resulted in faster stress relaxation, a higher loss modulus, and increased creep. Interestingly, we found that stress relaxation of the hydrogels is determined by the total mass amount of PEG in the hydrogel, and not the molecular weight or concentration of PEG chains alone. We then evaluated the utility of these hydrogels for 3D cell culture. Faster relaxation in RGD-coupled alginate-PEG hydrogels led to increased spreading and proliferation of fibroblasts, and enhanced osteogenic differentiation of mesenchymal stem cells (MSCs). Thus, this work establishes a new materials approach to tuning stress relaxation in alginate hydrogels for 3D cell culture.

  View details for PubMedID 30743050

• Facile Synthesis of Macromonomers via ATRP-Nitroxide Radical Coupling and Well-Controlled Brush Block Copolymers MACROMOLECULES Teo, Y., Xia, Y. 2019; 52 (1): 81–87

  View details for DOI 10.1021/acs.macromol.8b02446

  View details for Web of Science ID 000455557900008

• Dinaphthobenzo[1,2:4,5]dicyclobutadiene with Strong Antiaromaticity and Orthogonally Tunable Electronics and Packing. Angewandte Chemie (International ed. in English) Jin, Z., Yao, Z., Barker, K. P., Pei, J., Xia, Y. 2018

  Abstract

  Polycyclic conjugated hydrocarbons containing antiaromatic four-membered cyclobutadienoids are of great fundamental and technical interest. However, their challenging synthesis has hampered the exploration and understanding of such systems. We report herein a modular and efficient synthesis of novel CBD-containing acene analogues, dinaphthobenzo[1,2:4,5]dicyclobutadiene (DNBDCs) with orthogonally tunable electronic properties and molecular packing. Our design also features strong antiaromaticity of CBD units, as revealed by nucleus-independent chemical shift and anisotropy of the induced current density calculations as well as X-ray crystallography. Tuning the size of silyl substituents resulted in the most favorable "brick-layer" packing for triisobutylsilyl-DNBDC and a charge mobility of up to 0.52 cm2 V-1 s-1 in field-effect transistors.

  View details for PubMedID 30565363

• Iterative Synthesis of Edge-Bent [3]Naphthylene SYNLETT Jin, Z., Teo, Y., Teat, S. J., Xia, Y. 2018; 29 (19): 2547–51

  View details for DOI 10.1055/s-0037-1610261

  View details for Web of Science ID 000451037800014

• Synthesis and Mechanochemical Activation of Ladderene-Norbornene Block Copolymers. Journal of the American Chemical Society Su, J. K., Feist, J. D., Yang, J., Mercer, J. A., Romaniuk, J. A., Chen, Z., Cegelski, L., Burns, N. Z., Xia, Y. 2018; 140 (39): 12388–91

  Abstract

  We have recently reported a polymechanophore system, polyladderene (PLDE), which dramatically transforms into polyacetylene (PA) upon mechanical stimulation. Herein, we optimized conditions to synthesize unprecedented block copolymers (BCPs) containing a force-responsive block by sequential ring-opening metathesis polymerization of different norbornenes and bromoladderene. Successful extension from PLDE to other blocks required careful timing and low temperatures to preserve the reactivity of the PLDE-appended catalyst. The PLDE-containing BCPs were sonochemically activated into visually soluble PA with a maximum absorption lambda ≥ 600 nm and unique absorption patterns corresponding to noncontinuous activation of ladderene units. Access to polymechanophore BCPs paves the way for new stress-responsive materials with solution and solid state self-assembly behaviors and incorporation of polymechanophores into other materials.

  View details for PubMedID 30229652

• Efficient and Facile End Group Control of Living Ring-Opening Metathesis Polymers via Single Addition of Functional Cyclopropenes. ACS macro letters Elling, B. R., Xia, Y. 2018; 7 (6): 656-661

  Abstract

  Living ROMP has become an important technique for preparing well-controlled, highly functional polymers; however, installing functional groups at the end of living ROMP polymers is not as straightforward as ROMP itself. We report a simple, efficient strategy to introduce functionalities at the chain end of living polynorbornenes via highly selective single addition of disubstituted 1,1-cyclopropenes (CPEs) with no homopropagation. Unlike many other methods for ROMP chain end functionalization, our method does not result in catalyst termination, allowing for further functionalization after CPE addition. The remarkable reactivity of such CPEs allowed for quantitative chain end functionalization to install a variety of useful functionalities, including halides, aldehydes, ketones, amines, and dyes, without using a large excess of CPEs. These polymer chain ends can be readily modified using a range of postpolymerization modifications.

  View details for DOI 10.1021/acsmacrolett.8b00347

  View details for PubMedID 35632973

• Synthesis of Cyclobutadienoid-Fused Phenazines with Strongly Modulated Degrees of Antiaromaticity. Organic letters Teo, Y. C., Jin, Z., Xia, Y. 2018; 20 (11): 3300–3304

  Abstract

  The streamlined synthesis of a series of regioisomeric azaacene analogues containing fused phenazine and antiaromatic cyclobutadienoids (CBDs), using a catalytic arene-oxanorbornene annulation, followed by aromatization is reported. Controlling the fusion patterns allowed strong modulation of local antiaromaticity. Enhancing antiaromaticity in these regioisomeric azaacenes led to stabilized LUMO, reduced band gap, and quenched fluorescence. This synthetic strategy provides a facile means to fuse CBDs with variable degrees of antiaromaticity onto N-heteroarenes to tune their optoelectronic properties.

  View details for PubMedID 29781279

• Efficient and Facile End Group Control of Living Ring-Opening Metathesis Polymers via Single Addition of Functional Cyclopropenes ACS MACRO LETTERS Elling, B. R., Xia, Y. 2018; 7 (6): 656–61

  View details for DOI 10.1021/acsmacrolett.8b00347

  View details for Web of Science ID 000448490000012

• A bright organic NIR-II nanofluorophore for three-dimensional imaging into biological tissues NATURE COMMUNICATIONS Wan, H., Yue, J., Zhu, S., Uno, T., Zhang, X., Yang, Q., Yu, K., Hong, G., Wang, J., Li, L., Ma, Z., Gao, H., Zhong, Y., Su, J., Antaris, A. L., Xia, Y., Luo, J., Liang, Y., Dai, H. 2018; 9: 1171

  Abstract

  Fluorescence imaging of biological systems in the second near-infrared (NIR-II, 1000-1700 nm) window has shown promise of high spatial resolution, low background, and deep tissue penetration owing to low autofluorescence and suppressed scattering of long wavelength photons. Here we develop a bright organic nanofluorophore (named p-FE) for high-performance biological imaging in the NIR-II window. The bright NIR-II >1100 nm fluorescence emission from p-FE affords non-invasive in vivo tracking of blood flow in mouse brain vessels. Excitingly, p-FE enables one-photon based, three-dimensional (3D) confocal imaging of vasculatures in fixed mouse brain tissue with a layer-by-layer imaging depth up to ~1.3 mm and sub-10 µm high spatial resolution. We also perform in vivo two-color fluorescence imaging in the NIR-II window by utilizing p-FE as a vasculature imaging agent emitting between 1100 and 1300 nm and single-walled carbon nanotubes (CNTs) emitting above 1500 nm to highlight tumors in mice.

  View details for PubMedID 29563581

• Stress Relaxing Hyaluronic Acid-Collagen Hydrogels Promote Cell Spreading, Fiber Remodeling, and Focal Adhesion Formation in 3D Cell Culture Biomaterials Lou*, J., Stowers*, R., Nam, S., Xia, Y., Chaudhuri, O. 2018; 154: 213-222

  View details for DOI 10.1016/j.biomaterials.2017.11.004

• Functionalized Rigid Ladder Polymers from Catalytic Arene-Norbornene Annulation Polymerization. ACS macro letters Lai, H. W., Teo, Y. C., Xia, Y. 2017; 6 (12): 1357-1361

  Abstract

  Rigid ladder polymers represent a unique polymer architecture but have limited synthetic accessibility and structural diversity. Using catalytic arene-norbornene annulation (CANAL) polymerization, we synthesized ladder polymers consisting of rigid and kinked norbornyl benzocyclobutene backbones and bearing various functional groups, such as alcohol, amine, ester, carbamate, amide, benzyl bromide, azide, and heterocycles. The incorporation of functional groups was achieved by either copolymerization of functionalized ladder-type dinorbornenes or postpolymerization functionalization. Functionalization of ladder polymers allows modification of their solubility, compatibility, and other properties, expanding their utilities. These ladder polymers remain microporous and highly glassy, which are desirable for separation and high-temperature applications.

  View details for DOI 10.1021/acsmacrolett.7b00806

  View details for PubMedID 35650817

• Functionalized Rigid Ladder Polymers from Catalytic Arene-Norbornene Annulation Polymerization ACS MACRO LETTERS Lai, H. H., Teo, Y., Xia, Y. 2017; 6 (12): 1357–61

  View details for DOI 10.1021/acsmacrolett.7b00806

  View details for Web of Science ID 000418636300007

• Stress relaxing hyaluronic acid-collagen hydrogels promote cell spreading, fiber remodeling, and focal adhesion formation in 3D cell culture. Biomaterials Lou, J., Stowers, R., Nam, S., Xia, Y., Chaudhuri, O. 2017; 154: 213-222

  Abstract

  The physical and architectural cues of the extracellular matrix (ECM) play a critical role in regulating important cellular functions such as spreading, migration, proliferation, and differentiation. Natural ECM is a complex viscoelastic scaffold composed of various distinct components that are often organized into a fibrillar microstructure. Hydrogels are frequently used as synthetic ECMs for 3D cell culture, but are typically elastic, due to covalent crosslinking, and non-fibrillar. Recent work has revealed the importance of stress relaxation in viscoelastic hydrogels in regulating biological processes such as spreading and differentiation, but these studies all utilize synthetic ECM hydrogels that are non-fibrillar. Key mechanotransduction events, such as focal adhesion formation, have only been observed in fibrillar networks in 3D culture to date. Here we present an interpenetrating network (IPN) hydrogel system based on HA crosslinked with dynamic covalent bonds and collagen I that captures the viscoelasticity and fibrillarity of ECM in tissues. The IPN hydrogels exhibit two distinct processes in stress relaxation, one from collagen and the other from HA crosslinking dynamics. Stress relaxation in the IPN hydrogels can be tuned by modulating HA crosslinker affinity, molecular weight of the HA, or HA concentration. Faster relaxation in the IPN hydrogels promotes cell spreading, fiber remodeling, and focal adhesion (FA) formation - behaviors often inhibited in other hydrogel-based materials in 3D culture. This study presents a new, broadly adaptable materials platform for mimicking key ECM features of viscoelasticity and fibrillarity in hydrogels for 3D cell culture and sheds light on how these mechanical and structural cues regulate cell behavior.

  View details for DOI 10.1016/j.biomaterials.2017.11.004

  View details for PubMedID 29132046

• Synthesis of Ladder Polymers: Developments, Challenges, and Opportunities CHEMISTRY-A EUROPEAN JOURNAL Teo, Y., Lai, H. H., Xia, Y. 2017; 23 (57): 14101–12

  Abstract

  Ladder polymers are unique in that their backbones consist of fused rings with adjacent rings having two or more atoms in common. The restriction of bond rotations in rigid ladder polymers greatly limits their conformational freedom, leading to many intriguing and unique properties. As a non-traditional type of polymers, rigid ladder polymers are of great fundamental interest and technical importance as advanced materials for applications such as membrane gas separation and organic electronics. Ladder polymers can be divided into non-conjugated (with kinked conformations) and conjugated (with planar conformations) structures. Their synthesis can be broadly classified into two general strategies: direct ladder polymerization, and zipping of a linear precursor polymer. This Concept article outlines the historical development of ladder polymers and the chemical strategies used for their synthesis; highlights the challenges associated with their synthesis and characterization, and presents opportunities and outlooks for this unique and intriguing type of polymers.

  View details for PubMedID 28810077

• Streamlined Synthesis of Polycyclic Conjugated Hydrocarbons Containing Cyclobutadienoids via C-H Activated Annulation and Aromatization. Journal of the American Chemical Society Jin, Z., Teo, Y. C., Zulaybar, N. G., Smith, M. D., Xia, Y. 2017; 139 (5): 1806-1809

  Abstract

  The juxtaposition of fused cyclobutadienoid (CBD) with benzenoid creates intriguing alternating antiaromatic and aromatic conjugation. Synthetic accessibility of such molecules, however, has been challenging and limited in scope. We report a modular and streamlined synthetic strategy to access a large variety of polycyclic conjugated hydrocarbons with fused CBD. Synthesis was achieved through efficient palladium-catalyzed C-H activated annulation between abundant aryl bromides and oxanorbornenes, followed by aromatization under acidic conditions. The influence of four-membered ring was examined using spectroscopy, crystallography, and computation. This strategy will facilitate exploration on the chemical, structural, and electronic properties of such conjugated systems containing CBD.

  View details for DOI 10.1021/jacs.6b12888

  View details for PubMedID 28125224

• Ring-opening metathesis polymerization of 1,2-disubstituted cyclopropenes CHEMICAL COMMUNICATIONS Elling, B. R., Su, J. K., Xia, Y. 2016; 52 (58): 9097-9100

  Abstract

  The ring-opening metathesis polymerization (ROMP) of 1,2-disubstituted cyclopropenes (CPs) has been explored for the first time using Grubbs 3rd generation catalyst. A range of 1,2-CPs yielded polymers with controllable MWs and low dispersitities, and allowed the synthesis of block copolymers, absent from secondary metathesis. However, there existed a competing intramolecular termination pathway for these monomers, limiting the timescale for their ROMP to stay living.

  View details for DOI 10.1039/c6cc00466k

  View details for Web of Science ID 000379431000029

  View details for PubMedID 26947899

• Importance of Macromonomer Quality in the Ring-Opening Metathesis Polymerization of Macromonomers MACROMOLECULES Teo, Y. C., Xia, Y. 2015; 48 (16): 5656-5662

  View details for DOI 10.1021/acs.macromol.5b01176

  View details for Web of Science ID 000360324400026

• Living Alternating Ring-Opening Metathesis Polymerization Based on Single Monomer Additions JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Elling, B. R., Xia, Y. 2015; 137 (31): 9922-9926

  Abstract

  By judiciously modulating the ring strain and sterics, we developed a class of disubstituted cyclopropenes that selectively underwent single monomer addition in ring-opening metathesis but readily underwent alternating ring-opening metathesis polymerization with low-strain cyclic olefins in a living fashion. The substituents on cyclopropenes effectively inhibited homoaddition and prevented secondary metathesis on the polymer backbone. The resulting polymers had controllable molecular weights and end groups, very low dispersities, and high regularity in microstructure under optimized conditions. (1)H and (13)C NMR spectroscopy and MALDI-TOF MS showed a rigorously alternating sequence. Interestingly, disubstituted cyclopropenes were found to present zero-order kinetics, indicating their rapid single addition and the rate-determining ring opening of the low-strain olefin.

  View details for DOI 10.1021/jacs.5b05497

  View details for Web of Science ID 000359613300027

  View details for PubMedID 26182144

• Synthesis and Direct Imaging of Ultrahigh Molecular Weight Cyclic Brush Polymers ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Xia, Y., Boydston, A. J., Grubbs, R. H. 2011; 50 (26): 5882-5885

  View details for DOI 10.1002/anie.201101860

  View details for Web of Science ID 000292001700017

  View details for PubMedID 21591042

• Efficient Synthesis of Narrowly Dispersed Brush Copolymers and Study of Their Assemblies: The Importance of Side-Chain Arrangement JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Xia, Y., Olsen, B. D., Kornfield, J. A., Grubbs, R. H. 2009; 131 (51): 18525-18532

  Abstract

  Efficient, one-pot preparation of synthetically challenging, high molecular weight (MW), narrowly dispersed brush block copolymers and random copolymers in high conversions was achieved by ring-opening metathesis (co)polymerization (ROMP) of various macromonomers (MMs) using the highly active, fast-initiating ruthenium olefin metathesis catalyst (H(2)IMes)(pyr)(2)(Cl)(2)RuCHPh. A series of random and block copolymers were prepared from a pair of MMs containing polylactide (PLA) and poly(n-butyl acrylate) (PnBA) side chains at similar MWs. Their self-assembly in the melt state was studied by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). In brush random copolymers containing approximately equal volume fractions of PLA and PnBA, the side chains segregate into lamellae with domain spacing of 14 nm as measured by SAXS, which was in good agreement with the lamellar thickness measured by AFM. The domain spacings and order-disorder transition temperatures of brush random copolymers were insensitive to the backbone length. In contrast, brush block copolymers containing approximately equal volume fractions of these MMs self-assembled into highly ordered lamellae with domain spacing over 100 nm. Their assemblies suggested that the brush block copolymer backbone adopted an extended conformation in the ordered state.

  View details for DOI 10.1021/ja908379q

  View details for Web of Science ID 000273615800066

  View details for PubMedID 19947607

• Ring-Expansion Metathesis Polymerization: Catalyst-Dependent Polymerization Profiles JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Xia, Y., Boydston, A. J., Yao, Y., Kornfield, J. A., Gorodetskaya, I. A., Spiess, H. W., Grubbs, R. H. 2009; 131 (7): 2670-2677

  Abstract

  Ring-expansion metathesis polymerization (REMP) mediated by recently developed cyclic Ru catalysts has been studied in detail with a focus on the polymer products obtained under varied reaction conditions and catalyst architectures. Depending upon the nature of the catalyst structure, two distinct molecular weight evolutions were observed. Polymerization conducted with catalysts bearing six-carbon tethers displayed rapid polymer molecular weight growth which reached a maximum value at ca. 70% monomer conversion, resembling a chain-growth polymerization mechanism. In contrast, five-carbon-tethered catalysts led to molecular weight growth that resembled a step-growth mechanism with a steep increase occurring only after 95% monomer conversion. The underlying reason for these mechanistic differences appeared to be ready release of five-carbon-tethered catalysts from growing polymer rings, which competed significantly with propagation. Owing to reversible chain transfer and the lack of end groups in REMP, the final molecular weights of cyclic polymers was controlled by thermodynamic equilibria. Large ring sizes in the range of 60-120 kDa were observed at equilibrium for polycyclooctene and polycyclododecatriene, which were found to be independent of catalyst structure and initial monomer/catalyst ratio. While six-carbon-tethered catalysts were slowly incorporated into the formed cyclic polymer, the incorporation of five-carbon-tethered catalysts was minimal, as revealed by ICP-MS. Further polymer analysis was conducted using melt-state magic-angle spinning (13)C NMR spectroscopy of both linear and cyclic polymers, which revealed little or no chain ends for the latter topology.

  View details for DOI 10.1021/ja808296a

  View details for Web of Science ID 000263576100055

  View details for PubMedID 19199611

• Well-defined liquid crystal gels from telechelic polymers JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Xia, Y., Verduzco, R., Grubbs, R. H., Kornfield, J. A. 2008; 130 (5): 1735-1740

  Abstract

  Well-defined liquid crystal networks with controlled molecular weight between cross-links and cross-link functionality were prepared by "click" cross-linking of telechelic polymers produced by ring-opening metathesis polymerization (ROMP). The networks readily swell in a small molecule liquid crystal, 5CB, to form LC gels with high swelling ratios. These gels exhibit fast, reversible, and low-threshold optic switching under applied electric fields when they are unconstrained between electrodes. For a given electric field, the LC gels prepared from shorter telechelic polymers showed a reduced degree of switching than their counterparts made from longer polymer strands. The reported approach provides control over important parameters for LC networks, such as the length of the network strands between cross-links, cross-linker functionality, and mesogen density. Therefore, it allows a detailed study of relationships between molecular structure and macroscopic properties of these scientifically and technologically interesting networks.

  View details for DOI 10.1021/ja077192j

  View details for Web of Science ID 000253100100047

  View details for PubMedID 18197667