a structure-activity correlation

The polymerization of octamethylcyclotetrasiloxane (D4) is investigated using several five-, six- and seven-membered N-heterocyclic carbenes (NHCs). The catalysts are delivered in situ from thermally susceptible CO2 adducts. It is demonstrated that the polymerization can be triggered from a latent state by mild heating, using the highly nucleophilic 1,3,4,5-tetramethylimidazol-2-ylidene as organocatalyst. This way, high molecular weight PDMS is prepared (up to >400 000 g/mol, 1.6 < ÐM < 2.5) in yields >95%, using low catalyst loadings (0.2–0.1 mol %). Furthermore, the results suggest that a nucleophilic, zwitterionic mechanism is in operation, in preference to purely anionic polymerization

Polymeric monolithic materials: Syntheses, properties, functionalization and applications

The synthetic particularities for the synthesis of polymer-based monolithic materials are summarized. In this context, monoliths prepared via thermal-, UV- or electron-beam triggered free radical polymerization, controlled TEMPO-mediated radical polymerization, polyaddition, polycondensation as well as living ring-opening metathesis polymerization (ROMP) will be covered. Particular attention is devoted to the aspects of controlling pore sizes, pore volumes and pore size distributions as well as functionalization of these supports. Finally, selected, recent applications in separation science, (bio-) catalysis and chip technology will be summarized. © 2007 Elsevier Ltd. All rights reserved

Tandem ring‐opening–ring‐closing metathesis for functional metathesis catalysts

Use of a tandem ring-opening–ring-closing metathesis (RORCM) strategy for the synthesis of functional metathesis catalysts is reported. Ring opening of 7-substituted norbornenes and subsequent ring-closing metathesis forming a thermodynamically stable 6-membered ring lead to a very efficient synthesis of new catalysts from commercially available Grubbs’ catalysts. Hydroxy functionalized Grubbs’ first- as well as third-generation catalysts have been synthesized. Mechanistic studies have been performed to elucidate the order of attack of the olefinic bonds. This strategy was also used to synthesize the ruthenium methylidene complex

A novel modeling approach for sulfurized polyacrylonitrile (SPAN) electrodes in Li metal batteries

Li -sulfur batteries represent a promising class of next -generation batteries with high theoretical gravimetric capacity. Moreover, the absence of scarce elements such as nickel or cobalt makes them a sustainable alternative to Li -ion batteries. However, it is well known that the main problem of Li -sulfur batteries is the so-called polysulfide shuttle, which leads to self -discharge, capacity fading and low coulombic efficiency. In recent years, several mitigation strategies have been developed for Li -sulfur batteries to reduce the polysulfide shuttle effect. One promising approach is to covalently bond the sulfur to a polymer backbone. A well-known class of materials is sulfurized poly(acrylonitrile) (SPAN), for which long cycle life and high specific capacities have been reported. In this work, we present a novel continuum model for SPAN electrodes and demonstrate its application in Li -SPAN batteries. Within our simulation framework we include both red/ox reactions of covalently sulfur on PAN as well as transport and reactions of polysulfides in the solution. By combining simulations and experimental data we analyze the discharge mechanism and provide guidelines for electrode design

Li‐Ion Storage and Diffusivity in Sulfurized Polybutadiene Containing Covalently Bound Sulfur as a Polysulfide Shuttle‐Free Cathode Material for Li−S Batteries

In this work, a new polymer has been explored as a cathode host for lithium‐sulfur batteries (LSBs). Sulfurized polybutadiene materials were synthesized by a single‐step, scalable, and easily tailored heat treatment method. The optimized synthesis process allows for high sulfur loadings of up to 50 wt %. Thermogravimetric analysis‐mass spectrometry (TGA‐MS) and X‐ray photoelectron spectroscopy (XPS) studies confirm that the sulfur is covalently bound to the polymeric backbone, which overcomes the otherwise common capacity‐fading polysulfide shuttle effect of lithium‐sulfur (LSBs) batteries. The absence of free elemental sulfur in the synthesized active materials allows for a stable capacity of up to 1200 mAh g −1 at a rate of C/20. The porous polymer networks reduce the pulverization of the cathode during cycling, resulting in long‐term cycling stability of 1500 continuous galvanostatic charge/discharge (GCD) cycles. Capacity contribution studies depict that at a scan rate of 1 mV s −1 , the sulfurized polybutadiene cathode‐based cells have 65 % capacitive and 35 % diffusive contribution of the total charge stored. A comprehensive study on Li‐ion storage with capacity contribution and diffusion studies of polysulfide shuttle‐free sulfurized polybutadiene cathode material for LSBs is presented. Sulfurized polybutadiene with 50 wt % of covalently bound sulfur was synthesized by a one‐step heat treatment method and used as cathode material in Li−S batteries. The absence of free sulfur overcomes the polysulfide shuttle effect and porous polymer networks enable 1500 stable GCD cycles. Charge storage study depicts cells have 65 % capacitive and 35 % diffusive contribution at 1 mV s −1 . imag

Olefin Ring‐closing Metathesis under Spatial Confinement: Morphology−Transport Relationships

Spatial confinement effects on hindered transport in mesoporous silica particles are quantified using reconstructions of their morphology obtained by electron tomography as geometrical models in direct diffusion simulations for passive, finite‐size tracers. We monitor accessible porosity and effective diffusion coefficients resulting from steric and hydrodynamic interactions between tracers and pore space confinement as a function of λ=d$_{tracer}$/d$_{meso}$, the ratio of tracer to mean mesopore size. For λ=0, pointlike tracers reproduce the true diffusive tortuosities. For λ>0, derived hindrance factors quantify the extent to which diffusion through the materials is hindered compared with free diffusion in the bulk liquid. Morphology‐transport relationships are then discussed with respect to the immobilization, formation, and transport of key molecular species in the ring‐closing metathesis of an α,ω‐diene to macro(mono)cyclization product and oligomer, with a 2nd‐generation Hoveyda‐Grubbs type catalyst immobilized inside the mesopores of the particles

Toward sustainable fiber‐reinforced polymer composites

Fiber‐reinforced polymer composites (FRPCs) are versatile materials with applications in diverse fields such as transportation, construction, and electronics. With the composites market expected to reach 15.5 Mt by 2026, increasing the sustainability of FRPCs is imperative. The main factors driving the sustainability of FRPCs, namely end‐of‐life management and recyclability, the use of natural, bio‐based, and sustainable materials, as well as biodegradability and product simplification are presented and discussed

A spirocyclic parabanic acid masked N‐heterocyclic carbene as thermally latent pre‐catalyst for polyamide 6 synthesis and epoxide curing

1,3‐Dicyclcohexyl‐6,9‐dimethyl‐1,3,6,9‐tetraazaspiro[4.4]non‐7‐ene‐2,4‐dione, a spirocyclic parabanic acid derivative of N,N‐dimethylimidazole, is used as thermally latent, protected N‐heterocyclic carbene (NHC) in polymerizing anhydride‐cured epoxide resins, and azepan‐2‐one, respectively. The protected carbene is synthesized from 1,3‐dimethylimidazolium‐2‐carboxylate in the presence of two equivalents of cyclohexyl isocyanate. In the synthesis of epoxide resin thermosets, this class of latent NHC allows the production of fast and fully cured materials with high crosslinking content. Fast and complete conversion is found in the anionic ring opening polymerization (AROP) of azepan‐2‐one (ε‐caprolactam, CLA) with and without additional activators.Ministry of Science, Research and Arts of the Federal State of Baden‐WürttembergProjekt DEA

Catalytic living ring-opening metathesis polymerization

In living ring-opening metathesis polymerization (ROMP), a transition-metal–carbene complex polymerizes ring-strained olefins with very good control of the molecular weight of the resulting polymers. Because one molecule of the initiator is required for each polymer chain, however, this type of polymerization is expensive for widespread use. We have now designed a chain-transfer agent (CTA) capable of reducing the required amount of metal complex while still maintaining full control over the living polymerization process. This new method introduces a degenerative transfer process to ROMP. We demonstrate that substituted cyclohexene rings are good CTAs, and thereby preserve the ‘living’ character of the polymerization using catalytic quantities of the metal complex. The resulting polymers show characteristics of a living polymerization, namely narrow molecular-weight distribution, controlled molecular weights and block copolymer formation. This new technique provides access to well- defined polymers for industrial, biomedical and academic use at a fraction of the current costs and significantly reduced levels of residual ruthenium catalyst