Epoxide polymers active sites

The formation of epoxide polymers with a very high molecular weight by the discussed catalysts containing associated multinuclear species (— Zn-0— Zn O >) indicates that only a small fraction of the metal species in the catalyst is effective for the polymerisation. The broad molecular weight distribution of polymers yielded by these catalysts corresponds to the existence of various active sites [30]. 

Characteristic of these models of active sites is the appearance of the OZn O Zn(0)Et species. Considering the structural properties of the discussed catalysts as well as the polymer chain microstructure and the structure of the end groups of poly(propylene oxide) obtained with the PhOZnOCeHi ( -Bu)OZnEt catalyst, a concerted mechanism of epoxide ring-opening polymerisation with catalysts containing multinuclear species, including those with condensed zinc atoms, has been postulated [65,74] ... 

Following these results, Darensbourg et al. have continued the research and used other bifunctional Cr(salen) complexes as catalysts for polycarbonate synthesis. They observed that when a monofunctional Cr(salen) complex (5) was used to catalyze the reaction between epoxide and CO2, the product formed was cyclic carbonate. However, when a bifunctional Cr(salen) catalyst (6) was used, 79% selectivity towards the polycarbonate was obtained at 70 °C. The reason for this difference lies in the structure of the bifunctional catalyst, which provides steric hindrance in the epoxide ring-opening process to form the cyclic carbonate. Therefore, it can be inferred that spatial requirements in the active site of the metal catalyst determine the selectivity for the kinetic polymer product over the thermodynamically more stable cyclic carbonate product. 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

This unified volume explains the mechanistic basics of tactic polymerizations, beginning with an extensive survey of the most important classes of metallocene and post-metallocene catalysts used to make polypropylenes. It also focuses on tactic stereoblock and ethylene/propylene copolymers and catalyst active site models, followed by chapters discussing the structure of more stereochemically complex polymers and polymerizations that proceed via non-vinyl-addition mechanisms. Individual chapters thoroughly describe tactic polymerizations of a-olefins, styrene, dienes, acetylenes, lactides, epoxides, acrylates, and cyclic monomers, as well as cyclopolymerizations and ditactic structures, olefin/CO copolymers, and metathesis polyalkenamers. 

The epoxide is used as a monomer for polymer production. The byproduct ethylbenzene alcohol can be dehydrated to styrene, also a monomer for the production of polymers. If isobutane is used, iso-butylhydroperoxide replaces ethylbenzene-hydroperoxide as the oxidant. The byproduct tert-butanol can be converted with methanol to an ether that is an important additive in new environmental friendly gasolines. Complexes of Mo, V, or Ti are used in homogeneous epoxidation catalysis, while heterogeneous Ti02/Si02 catalysts can be used also. The active sites consist of a titanium ion with a fourfold coordination of oxygen in a tetrahedral geometry. Titanium acts essentially as a Lewis acid to activate the 0-0 bond in the hydroperoxide. 

The epoxidation of diene-based polymers is advantageous because it provides active sites along the polymer chains for further modification. Furthermore, introducing an epoxide functional group to unsaturated polymers can help improve abrasion resistance, adhesive strength, and heat stability. Epox-idized polymers can be prepared by polymerizing epoxidized monomers however, this method typically results in many side reactions. Numerous reports have focused on the epoxidation of diene-based polymers with... 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

Virtually all the reactions described for molecular level functionalization are, in principle, applicable to surface functionalization as well, and additional alternatives also become possible. In our program we have been able to do this using a wide variety of reagents to generate hydroxyl, arylamino, arylcarboxylate, sulfonic acid, epoxide, and allyl-units at the polymer surfaces, and to use these functional sites to control hydrophobicity or hydrophilicity or to link metals, enzymes, or other biologically active species to the surface regions (75-S9). Inherent in this work has been the... 

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