Linear Polymeric Catalysts

PEGs are one of the most used soluble polymeric supports in organic synthesis. They are well soluble in water and many organic solvents, such as alcohols, dimethyl formamide, toluene, dichloromethane, and acetone, but are insoluble in 

Specifically, the reversal was found to occur with a molecular weight change of only 800. As the PEG chains did not affect the inherent chirality of the ligand, they proposed that the enantioreversal might occur as a result of coordination models. Further equilibrium measurements revealed that the predominant species in Ti/PEG-tartrate ester mixtures was a distinct 2 1 Ti-ligand complex, as opposed to the 2 2 Ti-ligand complex of traditional Sharp less asymmetric epoxidation. 

In addition, this Ru catalyst could be easily recycled by solvent precipitation, and the recovered catalysts showed no loss of enantioselectivity upon reuse. 

Zheng and coworkers [35] disclosed a new approach to soluble linear polymeric salen ligands with main-chain chirality for the Mn-catalyzed asymmetric epoxidation of olefins. The chiral polymer ligands (16) were prepared by the 

The above work is reminiscent of the elegant works of main-chain organometal-lic polymers [39] and self-supported catalysts [6], and the latter has proven to be a novel and versatile immobilization method. It is noticeable that, however, these catalysts are often insoluble for the whole catalytic process, and thus will not 

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At attempt has been made to use gels as phase transfer catalysts [190]. A considerable difference was observed between the effects of cross-linked and linear polymeric catalysts. 

Chiral Linear Polymeric Catalysts 1115 Catalyst recycling... 

Alkylation of aluminum with ethyleae yields products that fiad appHcatioa as iaitiators and starter compounds ia the productioa of a-olefias and linear primary alcohols, as polymerization catalysts, and ia the syathesis of some monomers like 1,4-hexadieae. Triethyl aluminum [97-93-8] A1(C2H3)2, is the most important of the ethylene-derived aluminum alkyls. 

Strong interest in late transition metal olefin polymerization catalysts resulted in the development of new five-coordinate Fe and Co systems (69) that afford highly linear, crystalline, high-density polyethylene.587-589 A new class of single-component, neutral Ni catalysts based on salicylaldimine ligands (70) was reported to be active in the polymerization of ethylene 590,591... 

Chemicals. Polymerization Catalysts are used in the production of polymers, such as linear and low-density polyethylene (LLDPE). An example of these catalysts are Ziegler Natta catalysts, which are combinations of titanium halides with aluminium and magnesium alkyls. 

In aqueous solution the catalytic esterolyses with polymeric catalysts is accelerated by hydrophobic bonding . The catalytic effect of synthetic polymers on hydrolysis has been reviewed by Kunitake and Okahata (57). Linear polyvinylimid-azole (Scheme 5) increases the rate of hydrolyses of 3-nitro-4-acyloxybenzoic acid by a factor of 103 with respect to monomeric imidazole 58-62). The primary reaction is the acylation of polyvinylimidazole. The effect is dependent on chain length of the acid and the solvent. The hydrophobic interaction causes an autocatalytic course of reaction rate. The acylated polyvinylimidazole is more hydrophobic than the starting polymer. At a 75% conversion the rate is five times higher than the initial rate. 

Because of their Lewis acidity, the MX5 species, usually the fluorides or the chlorides, can act as catalysts in cyclotrimerizing or linearly polymerizing acetylenes (although in the latter case metal alkylidene intermediates may be important) and in Friedel-Crafts and related alkylation reactions. They also polymerize THF and can catalyze the hydrolysis of acetonitrile.21... 

Continuous homogeneous catalysis is achieved by membrane filtration, which separates the polymeric catalyst from low molecular weight solvent and products. Hydrogenation of 1-pentene with the soluble pofymer-attached Wilkinson catalyst affords n-pentane in quantitative yield A variety of other catalysts have been attached to functionalized polystyrenes Besides linear polystyrenes, poly(ethylene glycol)s, polyvinylpyrrolidinones and poly(vinyl chloride)s have been used for the liquid-phase catalysis. Instead of membrane filtration for separating the polymer-bound catalyst, selective precipitation has been found to be very effective. In all... 

One of the major fields of isocyanate catalysis is pol3mierization. The formation of cyclic dimers and trimers from aryl isocyanates was established over 100 years ago by Hofmann (8). This early work has been reviewed by Saunders (1). Dimers are formed by aryl isocyanates at room temperature in the presence of certain amines or phosphines. Trimeriza-tion occurs in the presence of bases such as potassium acetate. Linear polymerization has been recently reported by Shashoua and co-workers at lower temperatures in polar solvents with an anionic catalyst such as metallic sodium (9, 10). 

Shashoua and co-workers (9, 10) described cyclic trimer formation with a sodium catalyst under certain conditions which were not ideal for linear polymerization. 

In 1998, Grubbs and co-workers [83, 84] reported on a new type of neutral Ni(II) complexes with salicylaldimin ligands (32). With these catalysts low-branched polyethylenes were obtained with a narrow molecular weight distribution. The copolymerization of ethylene and norbomene is possible. Fe and Co catalysts were used for the linear polymerization of ethylene by Gibson [85] and Brookhart [86] independently (33). Activities of 10 TONs were reported. The polyethylenes obtained are highly crystalline with a broad molecular weight distribution. 

Several oxovanadium(IV) complexes (141) with ON-donor ligands were prepared from [YOCl2(thf)2] (thf = tetrahydrofuran) and /// /////-substituted phenols to study the electronic effects of para substituents in order to develop better [VCl2(OR)2]-type olefin polymerization catalysts.676 The hyperfine coupling constants, the HOMO-LUMO transitions, and the oxidation potentials were all found to be linearly related to the Hammett a constant of the substituent on the monoanionic aryloxy ring.676... 

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