Bio


Designing and synthesizing exotic small and giant molecules for custom properties, Assistant Professor Yan Xia works at the interface of synthetic chemistry and materials science. His research uses a combination of catalysis, organic and polymer chemistry, and a range of advanced characterizations to create, control, and study novel (macro)molecular structures and organic materials with tailored conformations, nanostructures, properties, and functions.

Research in the Xia Group combines vigorous function-driven syntheses, rational molecular design, and in-depth understanding of (macro)molecular reactivity, property, and function. Powerful synthetic methods are the enabling force behind their development of novel organic materials. They have developed various types of chemistry to generate diverse molecular ladder materials with high microporosity, antiaromaticity, or responsive behavior; controlled polymers with defined microstructures and functionalities; and dynamic polymer networks. These new molecular materials have interesting nanostructures, optoelectronic structures, mechanical properties, stimuli-responses, and assembly behaviors, for potential applications spanning separation, electronics, and health care.

Yan Xia studied chemistry at Peking University (B.S. 2002) and McMaster University (M.S. 2005), before his doctoral research on the synthesis and study of cyclic and bottlebrush polymers at California Institute of Technology with Profs. Grubbs and Kornfield (Ph.D. 2010). Following his PhD, he spent one and a half years at Dow Chemical core R&D developing materials for electronic applications, and then performed post-doctoral research on polymer-protein conjugation and assembly at Massachusetts Institute of Technology with Prof. Olsen. He joined the chemistry faculty at Stanford in the summer of 2013 to continue his longstanding interest in developing organic materials by intimately integrating synthetic chemistry with materials science.

Academic Appointments


Honors & Awards


  • Sloan Research Fellowship, Alfred P. Sloan Foundation (2019)
  • Cottrell Scholar Award, Research Corporation for Science Advancement (2017)
  • Thieme Chemistry Journals Award, Thieme Chemistry (2017)
  • CAREER Award, National Science Foundation (2016)
  • 3M Non-Tenured Faculty Award, 3M (2016)
  • Terman Fellowship, Stanford (2014-16)
  • Army Research Office Young Investigator Award, U.S. Army Research Laboratory, Army Research Office (2015)

Professional Education


  • PhD, California Institute of Technology, Chemistry (2010)
  • MS, McMaster University, Chemistry (2005)
  • BS, Peking University, Chemistry (2002)

Current Research and Scholarly Interests


Projects at the interface of synthetic chemistry and materials science include:
1. Microporous polymer membranes for gas separations
2. Stress-responsive polymers
3. Precise control of polymer structures, architectures, and assembly
4. Dynamic polymer networks
5. Unusual conjugated pi-systems as optoelectronic materials

2019-20 Courses


Stanford Advisees


All Publications


  • 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
  • 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

  • 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

  • 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
  • Regioselective Synthesis of [3]Naphthylenes and Tuning of Their Antiaromaticity. Journal of the American Chemical Society Jin, Z., Teo, Y. C., Teat, S. J., Xia, Y. 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

  • 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

  • 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

  • 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
  • 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
  • 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
  • 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
  • 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): 656–61
  • 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

  • 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
  • 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
  • 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

  • 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: 213–22

    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 PubMedID 29132046

  • 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
  • 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