Phase transfer catalyzed reactions on polymers

Linear high-molecular weight polyepichlorohydrin elastomer (PECH) was reacted with potassium Superoxide in benzene and benzene-dimethylsulfoxide solutions in the presence of 18-crown-6 as a phase-transfer catalyst. Elemental analysis, spectroscopic studies, and viscosity measurements showed that Superoxide ion attacks both the chloromethyl group leading to the formation of various oxygen-containing groups, and main chain, causing its rapid degradation. A reaction mechanism has been proposed which appears to be dependent on the solvent.Part 4: A.S. Gód, Polymer Bull. 5, 591 (1981)     

Phase-transfer catalyzed synthesis of poly(etherketone)s

Summary A novel synthetic method for the preparation of high molecular weight aromatic poly- (etherketone)s is described. N-Alkyl(4-N,N-dialkylamino)pyridinium chlorides 1, 2 and 3 were used as phase-transfer catalysts in polycondensation reactions of 1,4-bis(4-chlorobenzoyl)benzene (4) with aromatic diphenols via nucleophilic displacement. The polycondensations were significantly promoted and, moreover, the results achieved in terms of molecular weight and reaction time were comparable to polymerizations conducted with 1,4-bis(4-fluorobenzoyl)benzene (5), which is a considerably more reactive monomer.     

Phase-transfer catalysis in the chemical modification of polymers: 1.

Phase-transfer catalysts have been successfully employed for the reaction of amines and alcohols with chloromethylated polymers and copolymers. Influence of solvent, catalyst and of reaction time has been studied. The nature of the amine as well as that of the polymeric substrate has been varied. This simple process of chemical modification has led to 100% conversions of chloromethyl groups in some cases.     

Synthesis of condensation polymers catalyzed by palladium-graphite

Palladium-graphite (Pd---Gr) was applied to polycondensations as a heterogeneous palladium catalyst. Catalytic activities of Pd---Gr for the Heck reaction and a carbonyl insertion reaction were investigated using the model reactions. Polycinnamamide was synthesized through the Heck reaction of N, N′-(3,4′-oxydiphenylene)bis(acrylamide) and bis(4-iodophenyl) ether catalyzed by Pd---Gr. The polymerization proceeded efficiently in the presence of tributylamine, but required a long reaction time compared with the case of homogeneous catalysts. The resulting polymer was almost white in color, which means that it was less contaminated by palladium metal. The removal and recycling of Pd---Gr were much easier than the case of homogeneous catalysts. Aramid was also synthesized through the carbonyl insertion reaction of m-diiodobenzene and bis(4-aminophenyl) ether catalyzed by Pd---Gr. The rate of carbon monoxide consumption for the polymerization catalyzed by Pd---Gr was almost equal to that catalyzed by the homogeneous palladium catalyst. In both cases, the structures of the resulting polymers were identical to those prepared by homogeneous palladium catalysts.     

Ruthenium catalyzed chemical modification of unsaturated polymers

Summary Chemical modification of unsaturated polysiloxanes with pendant or terminal vinyl groups and polycarbosilanes with pendant Si-vinyl groups has been achieved by a novel ruthenium catalyzed reaction. Dihydridocarbonyltris(triphenylphosphine)-ruthenium catalyzes the addition of the ortho C–H bond of 2-methylacetophenone across the C–C double bond of pendant or terminal vinyl groups of polymers such as copoly(dimethylsiloxane/vinylmethylsiloxane), polydimethylsiloxane which is terminated with vinyldimethylsiloxy groups and copoly(vinylmethylsilylene/1,4-phenylene).     

Transesterification reactions in molten polymers

The transesterification reaction of molten ethylene and vinyl alcohol copolymers (EVA), in presence of paraffinic alcohols and basic catalysts, leads to high conversion of the ester groups to secondary alcohol in both discontinuous and continuous processing equipment. Various kinds of alcohols and two different catalysts were used. Sodium methoxide is a powerful catalyst for the equilibrated transesterification reaction, but we also observed side reactions, such as cross-linking with low-molecular-weight alcohols and hydrolysis of the catalyst followed by partial saponification of the EVA. Kinetic studies were performed in the presence of dibutyltin dilaurate, an efficient catalyst without any side reactions. The solubility of the main alcohol reagents was verified by diffusion measurements. The general reaction scheme and the related kinetics, corresponding to a homogeneous system, lead to a fair evaluation of the rate constants.     

Competitive reactions catalyzed by NiMo-containing zeolites

The rates of benzene hydrogenation and of n-heptane hydroconversion were measured simultaneously over a series of bifunctional catalysts. The catalysts were Y zeolites containing a Ni or Mo sulfide. At 280 °C, and for the more acidic catalysts, both rates appeared linearly correlated, suggesting that the hydrogenation function was limiting.     

钨锗酸的催化脱水反应

报道了钨锗酸对醇脱水成醚或成烯的催化反应。     

Balancing wear and traction with lithium catalyzed polymers

This paper is a report of methods of balancing wear and traction in passenger tire treads with lithium catalyzed polymers. The results demonstrate the versatility of the alkyllithium system for preparing a wide variety of polymers and copolymers with differing wear and traction properties in passenger tire treads. In addition, the use of this system for preparing polymers with improved processing and green strength is briefly discussed.     

Azo polymers — syntheses and reactions

Summary Polycarbonates containing different amounts of thermolabile azo functions have been synthesized. The resulting polymeric initiators have been used to start the polymerization of methyl methacrylate. The kinetic results are explained on the basis of a high degree of termination reactions by primary radicals.     

Azo polymers — syntheses and reactions

Summary Azo group containing polycarbonates were used as initiators for the polymerization of methyl methacrylate. The grafting reaction was characterized by the terms: grafting success U_g, grafting efficiency and degree of grafting . The effect of the monomer and initiator concentration on these terms is discussed in detail. The number of grafted sidearms is determined by complete hydrolysis of the polycarbonate backbone.     

Rhodium catalyzed chelation-assisted C-H bond functionalization reactions

Over the last several decades, researchers have achieved remarkable progress in the field of organometallic chemistry. The development of metal-catalyzed cross-coupling reactions represents a paradigm shift in chemical synthesis, and today synthetic chemists can readily access carbon-carbon and carbon-heteroatom bonds from a vast array of starting compounds. Although we cannot understate the importance of these methods, the required prefunctionalization to carry out these reactions adds cost and reduces the availability of the starting reagents. The use of C-H bond activation in lieu of prefunctionalization has presented a tantalizing alternative to classical cross-coupling reactions. Researchers have met the challenges of selectivity and reactivity associated with the development of C-H bond functionalization reactions with an explosion of creative advances in substrate and catalyst design. Literature reports on selectivity based on steric effects, acidity, and electronic and directing group effects are now numerous. Our group has developed an array of C-H bond functionalization reactions that take advantage of a chelating directing group, and this Account surveys our progress in this area. The use of chelation control in C-H bond functionalization offers several advantages with respect to substrate scope and application to total synthesis. The predictability and decreased dependence on the inherent stereoelectronics of the substrate generally result in selective and high yielding transformations with broad applicability. The nature of the chelating moiety can be chosen to serve as a functional handle in subsequent elaborations. Our work began with the use of Rh(I) catalysts in intramolecular aromatic C-H annulations, which we further developed to include enantioselective transformations. The application of this chemistry to the simple olefinic C-H bonds found in α,β-unsaturated imines allowed access to highly substituted olefins, pyridines, and piperidines. We observed complementary reactivity with Rh(III) catalysts and developed an oxidative coupling with unactivated alkenes. Further studies on the Rh(III) catalysts led us to develop methods for the coupling of C-H bonds to polarized π bonds such as those in imines and isocyanates. In several cases the methods that we have developed for chelation-controlled C-H bond functionalization have been applied to the total synthesis of complex molecules such as natural products, highlighting the utility of these methods in organic synthesis.     

Metal catalyzed photocrosslinking of polymers containing pendant propargyl groups

Polymers containing the pendant propargyl groups can be effectively crosslinked upon UV irradiation in the presence of a small amount of W(CO)₆. The alkyne-tungsten carbonyl complex was formed initially as evident by IR spectroscopy, which was involved in the subsequent crosslinking reaction. Analyses by UV, IR, and ¹³C NMR CP/MAS spectroscopic methods confirmed the polymer crosslinking as a result of the conversion of the acetylenic units to the polyene and diacetylene moieties in the crosslinked system.     

Photochemical reactions of polymers bearing chalcone residues

Photosensitive polymers based on pendent chalcone moieties were prepared directly by radical polymerization of vinyl-chalcone monomers. The monomers were synthesized by the esterification reactions of the hydroxyl groups of 4- and 4′-hydroxychalcones with methacryloyl chloride. The monomers (4- and 4′-(methacryloyloxy)chalcones) were polymerized to give the corresponding homo-and co-polymers containing different ratios of p-nitrophenyl methacrylate as photosensitizer group. Investigation of the photosensitivities of the chalcone polymers was carried out through study of the photochemical reactions on exposure to UV light and measuring the changes in UV spectra before and after irradiation. The results obtained from the disappearance rates of the C=C bonds indicate that the photoreactivities of the polymers are largely affected by the position of the attachment of the chalcone to the polymer backbone and by the concentration of the photosensitizer group.     

Kinetically induced unstable behaviour in heterogeneously catalyzed reactions

Evidence has appeared in the literature that ignition-extinction phenomena in heterogeneously catalyzed reactions can be induced by pure kinetic effects. This paper presents an elegant and simple analytical reasoning based upon elementary properties of polynomials to derive explicit necessary and sufficient conditions for this behaviour for various reaction mechanisms, including Langmuir as well as non-Langmuir-type of adsorption. It is inspired on geometric arguments on the steady state equation, involving tangent adsorption and surface reaction rate curves as a function of surface coverage, which represent the limiting cases of rate multiplicity. The approach is particularly attractive for Langmuir-type adsorption as the adsorption rate equations are linear in this case. The technique is also applicable with the same ease in most situations of nonlinear adsorption characteristics and is to be preferred over a number of more intricate alternatives, because of its simplicity and its ability of defining explicit maximal bounds on the fundamental parameters.     

Quantum chemical studies of zeolite proton catalyzed reactions

Theoretical chemistry applied to zeolite acid catalysis is becoming an important tool in the understanding of the adsorption and interaction of guest molecules with the zeolitic lattice. Especially the understanding of the mechanisms by which zeolite catalyzed chemical reactions proceed becomes possible. It is shown here that the old interpretation of carbonium and carbenium ions as intermediates for zeolite catalyzed reactions has to be replaced by a new approach in terms of positively charged transition states that are strongly stabilized by the zeolitic lattice. The large deprotonation energy of the acidic zeolite is overcome by stabilization of the intermediate or transition state positive charge by the negative charge left in the lattice. The zeolitic sites responsible for the adsorption and/or reaction of guest molecules are the Brønsted-acid and Lewis-base sites. We also show that different transition states are responsible for different kinds of reactions, such as cracking, dehydrogenation, etc.