The Experts below are selected from a list of 177 Experts worldwide ranked by ideXlab platform
Paula L Diaconescu - One of the best experts on this subject based on the ideXlab platform.
-
redox control of a ring opening Polymerization Catalyst
Journal of the American Chemical Society, 2011Co-Authors: Erin M Broderick, Carola S Vogel, Cuiling Xu, Jorg Sutter, Jeffrey T Miller, Karsten Meyer, Parisa Mehrkhodavandi, Paula L DiaconescuAbstract:The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening Polymerization of l-lactide. The oxidized complex was characterized by X-ray crystallography and 1H NMR, XANES, and Mossbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of Polymerization of l-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of Polymerization.
-
redox control of a Polymerization Catalyst by changing the oxidation state of the metal center
Chemical Communications, 2011Co-Authors: Erin M Broderick, Carola S Vogel, Cuiling Xu, Jorg Sutter, Jeffrey T Miller, Karsten Meyer, Neng Guo, Tianpin Wu, Thibault Cantat, Paula L DiaconescuAbstract:The activity of cerium alkoxide complexes supported by a Schiff base ligand was controlled using redox reagents during the ring-opening Polymerization of L-lactide. The rate of L-lactide Polymerization was modified by switching in situ between the cerium(III) and cerium(IV) species.
Christopher W. Bielawski - One of the best experts on this subject based on the ideXlab platform.
-
A Ring-Opening Metathesis Polymerization Catalyst That Exhibits Redox-Switchable Monomer Selectivities
Chemistry: A European Journal, 2017Co-Authors: Dominika N. Lastovickova, Huiling Shao, Gang Lu, Christopher W. BielawskiAbstract:A ring-opening metathesis Polymerization Catalyst supported by a redox-active N-heterocyclic carbene was synthesized and found to undergo reversible reduction. In its neutral form, the Catalyst polymerized 1,5-cis,cis-cyclooctadiene at a higher rate than that of a norbornene derivative; however, upon reduction, the selectivity was found to reverse. Utilizing this oxidation state dependent selectivity, a series of copolymers with controlled compositions, microstructures, and physical properties were prepared by redox-switching the Catalyst over the course of a series of Polymerization reactions.
-
graphite oxide as a dehydrative Polymerization Catalyst a one step synthesis of carbon reinforced poly phenylene methylene composites
Macromolecules, 2011Co-Authors: Daniel R Dreyer, Karalee Jarvis, Paulo J Ferreira, Christopher W. BielawskiAbstract:The synthesis and characterization of poly(phenylene methylene) (PPM) and carbon composites thereof are described. The materials were prepared using graphite oxide (GO), which was discovered to function in two distinct roles. First, the GO was found to facilitate the dehydrative Polymerization of benzyl alcohol (BnOH) to form PPM. Second, the residual carbon from the GO Catalyst, having undergone thermal deoxygenation during the Polymerization reaction, served as a graphene-like additive in the resulting composite. While pure (i.e., additive-free) PPM was found to be mechanically compliant (E′ = 40 MPa), inclusion of 0.1 wt % GO in the starting reaction mixture improved the material’s mechanical properties significantly (E′ = 320 MPa). Homogeneous dispersion of the additive in the matrix was confirmed by powder X-ray diffraction (PXRD) analysis, Raman spectroscopy, and transmission electron microscopy (TEM). The carbon additive was separated from the PPM via trituration in dichloromethane, and the GO star...
Toshiaki Taniike - One of the best experts on this subject based on the ideXlab platform.
-
High-Throughput Synthesis of Support Materials for Olefin Polymerization Catalyst
ACS Combinatorial Science, 2017Co-Authors: Patchanee Chammingkwan, Minoru Terano, Toshiaki TaniikeAbstract:Rational Catalyst design necessitates fundamental knowledge on the structure-performance relationship, while the synthetic throughput for heterogeneous Ziegler–Natta olefin Polymerization Catalysts has long prevented the acquisition of a statistical database. In this contribution, an in-house reactor system was developed to realize the parallel synthesis of support materials for Ziegler–Natta Catalysts for the first time. The developed system enabled parallel synthesis of 24 magnesium ethoxide samples with excellent reproducibility and morphological control comparable to a conventional experiment. Our demonstration revealed that the generation of diverse particle characteristics could be achieved through the addition of a third component as a structural modulator, in which the in-house parallel reactor system combined with the first principle component analysis enabled fast screening of effective modulators.
-
High-Throughput Synthesis of Support Materials for Olefin Polymerization Catalyst
ACS Combinatorial Science, 2017Co-Authors: Patchanee Chammingkwan, Minoru Terano, Toshiaki TaniikeAbstract:Rational Catalyst design necessitates fundamen-tal knowledge on the structure-performance relationship, while the synthetic throughput for heterogeneous Ziegler−Natta olefin Polymerization Catalysts has long prevented the acquisition of a statistical database. In this contribution, an in-house reactor system was developed to realize the parallel synthesis of support materials for Ziegler−Natta Catalysts for the first time. The developed system enabled parallel synthesis of 24 magnesium ethoxide samples with excellent reproducibility and morphological control comparable to a conventional experiment. Our demonstration revealed that the generation of diverse particle characteristics could be achieved through the addition of a third component as a structural modulator, in which the in-house parallel reactor system combined with the first principle component analysis enabled fast screening of effective modulators. KEYWORDS: high-throughput synthesis, spherical support, olefin Polymerization, particle architecture, structural modulator ■ INTRODUCTION The heterogeneous Catalyst has played a pivotal role in chemical processes for more than a century. Driven by a growing concern in environmental sustainability and energy legislation, a desire to discover highly performant Catalysts for more selective and efficient chemical synthesis evolves as an important target in both of academy and industry. Superior performance in heterogeneous Catalysts often arises from synergistic contribution of multicomponents and hierarchical structures. For example, a multisite solid Catalyst, whose different active species are spatially distributed within a single (often hierarchical) support, collaboratively enables tandem reactions to be executed in one-pot, thus giving benefits not only in economic and environmental aspects but also allowing the use of reaction intermediates that may not be able to isolate. 1,2 The maximum product yield heavily relies on an optimal balance between catalytic sites and support architec-tures. To enable such a sophisticated Catalyst design, an understanding of the interplay among relevant chemical and structural factors in connection to the catalytic performance is essential. In contrast to molecular Catalysts, heterogeneous Catalysts are often composed by a multiple component placed over a multiple length scale of a solid support. Each factor plays different roles in catalysis and contributes to the performance in different extents. One example is a Pt/BaO/CeO 2 /Al 2 O 3 washcoated monolith Catalyst for NO x storage and reduction from lean-burn engines. 3−5 A monolithic structure is designed to maximize the air/fuel contact efficiency and to minimize the pressure drop in the converter, in which cordierite is generally chosen due to its high thermal stability. Few micron-sized Al 2 O 3 is washcoated into cordierite macropores to provide a surface area for Pt dispersion. Under the oxygen-rich condition, Pt oxidizes NO into NO x , which can be stored on the neighboring Ba sites. The copresence of CeO 2 improves the stability of metal dispersion, the SO x reduction and the selectivity toward N 2 by suppressing NH 3 formation. However, its capability to store oxygen also leads to the oxidation of reductants in a competitive way to NO x reduction. As such, many factors are involved in heterogeneous catalysis in a complicated manner and the clarification of its structure-performance relationship is multivariate in nature, in which a statistical analysis on a data set elaborating with detailed structural variables is an effective tool to project the true multidimensional nature of solid Catalysts. The heterogeneous Ziegler−Natta Catalyst is one of the most important industrial Catalysts. It is known that both of the multicomponent nature and hierarchical particle architectures manipulate the Polymerization performance in a synergistic way: At the active site scale, Ti species heterogeneously distributed on MgCl 2 surfaces are responsible for the activity and polydispersity of produced polymer. 6,7 Lewis base as an internal donor interacts with Ti species in a nonbonded manner to modulate its catalytic performance. 8−10 At the particle scale, the Catalyst morphology influences the final morphology of polymer through the replication phenomena. 11 The kinetic profile is influenced by Catalyst pore architectures, where olefinic monomer polymerizes at accessible pores and induces
Tobin J Marks - One of the best experts on this subject based on the ideXlab platform.
-
Sustainable high capacitance at high frequencies: Metallic aluminum-polypropylene nanocomposites
ACS Nano, 2013Co-Authors: Lisa A. Fredin, Michael T Lanagan, Mark A. Ratner, Zhong Li, Tobin J MarksAbstract:The high-frequency dielectric response of 0-3 polypropylene nanocomposites prepared with the activated metallocene Polymerization Catalyst [rac-ethylenebisindenyl]zirconium dichlororide absorbed on the native Al(2)O(3) surfaces of metallic aluminum nanoparticles is characterized. The nanocomposites produced are randomly dispersed in the polyolefin matrix with no visible defects that might degrade film dielectric properties. Electrical measurements show that as the volume fraction of Al nanoparticles is increased, the effective permittivity of the nanocomposites increases, with epsilon(r) values reaching ~10 at relatively low frequency (1 MHz). Because of the high permittivity and conductivity contrast between the metal nanoparticles and the polypropylene matrix, Maxwell-Wagner-Sillars theory can be applied to model the loss at high frequencies and provide insight into how the nanocomposite high frequency response scales with Al volume fraction. At higher Al nanoparticle volume fractions, mixing theories predict greater densities of nanoparticle aggregates, consistent with the experimentally observed shift of the dielectric relaxation to lower frequencies. Although these nanocomposites undergo the predicted initial dielectric relaxation with increasing frequency, the metallic nanoparticle complex permittivity imbues the higher Al volume fraction materials with relatively high, sustainable permittivities, 6, at frequencies as high as 7 GHz.
-
Substantial recoverable energy storage in percolative metallic aluminum-polypropylene nanocomposites
Advanced Functional Materials, 2013Co-Authors: Lisa A. Fredin, Michael T Lanagan, Mark A. Ratner, Zhong Li, Tobin J MarksAbstract:Chemisorption of the activated metallocene Polymerization Catalyst derived from [rac-ethylenebisindenyl]zirconium dichlororide (EBIZrCl2) on the native Al2O3 surfaces of metallic aluminum nanoparticles, followed by exposure to propylene, affords 0–3 metal-isotactic polypropylene nanocomposites. The microstructures of these nanocomposites are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. Electrical measurements show that increasing the concentration of the filler nanoparticles increases the effective permittivity of the nanocomposites to ϵr values as high as 15.4. Because of the high contrast in the complex permittivities and conductivities between the metallic aluminum nanoparticles and the polymeric polypropylene matrix, these composites obey the percolation law for two-phase composites, reaching maximum permittivities just before the percolation threshold volume fraction, vf ≈ 0.16. This unique method of in situ Polymerization from the surface of metallic Al particles produces a new class of materials that perform as superior pulse-power capacitors, with low leakage current densities of ≈10−7–10−9 A/cm2 at an applied field of 105 V/cm, low dielectric loss in the 100 Hz–1 MHz frequency range, and recoverable energy storage as high as 14.4 J/cm3.
Robert H. Grubbs - One of the best experts on this subject based on the ideXlab platform.
-
pulsed addition ring opening metathesis Polymerization Catalyst economical syntheses of homopolymers and block copolymers
Journal of the American Chemical Society, 2009Co-Authors: John B Matson, Scott C Virgil, Robert H. GrubbsAbstract:Poly(tert-butyl ester norbornene imide) homopolymers and poly(tert-butyl ester norbornene imide-b-N-methyloxanorbornene imide) copolymers were prepared by pulsed-addition ring-opening metathesis Polymerization (PA-ROMP). PA-ROMP is a unique Polymerization method that employs a symmetrical cis-olefin chain transfer agent (CTA) to simultaneously cap a living polymer chain and regenerate the ROMP initiator with high fidelity. Unlike traditional ROMP with chain transfer, the CTA reacts only with the living chain end, resulting in narrowly dispersed products. The regenerated initiator can then initiate Polymerization of a subsequent batch of monomer, allowing for multiple polymer chains with controlled molecular weight and low polydispersity to be generated from one metal initiator. Using the fast-initiating ruthenium metathesis Catalyst (H_2IMes)(Cl)_2(pyr)_2RuCHPh and cis-4-octene as a CTA, the capabilities of PA-ROMP were investigated with a Symyx robotic system, which allowed for increased control and precision of injection volumes. The results from a detailed study of the time required to carry out the end-capping/initiator-regeneration step were used to design several experiments in which PA-ROMP was performed from one to ten cycles. After determination of the rate of Catalyst death, a single, low polydispersity polymer was prepared by adjusting the amount of monomer injected in each cycle, maintaining a constant monomer/Catalyst ratio. Additionally, PA-ROMP was used to prepare nearly perfect block copolymers by quickly injecting a second monomer at a specific time interval after the first monomer injection, such that chain transfer had not yet occurred. Polymers were characterized by gel permeation chromatography with multiangle laser light scattering.
-
ring expansion metathesis Polymerization Catalyst dependent Polymerization profiles
Journal of the American Chemical Society, 2009Co-Authors: Andrew J Boydston, Julia A Kornfield, Irina A Gorodetskaya, Hans Wolfgang Spiess, Robert H. GrubbsAbstract: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 ...
-
Insights into the Deactivation of Neutral Nickel Ethylene Polymerization Catalysts in the Presence of Functionalized Olefins
Organometallics, 2004Co-Authors: Andrew W. Waltman, Todd R. Younkin, Robert H. GrubbsAbstract:A study on the products of reaction between a neutral nickel(II) olefin Polymerization Catalyst and methyl acrylate is presented. A deactivation mechanism involving hydrogen transfer from substrate to Catalyst is suggested.