Stereospecific catalyst systems

The reverse reaction of oxidation of the metal or reduction of the moiety is also known to occur in stereospecific catalyst systems. It has been long known that the polymerization of olefine materials can be accomplished in non-alkyl systems. Diem, Tucker and Gibbs (43) have shown that the lithium metal polymerization of isoprene proceeds with the initial reduction by the electron seeking lithium of the nucleophilic diene to produce the corresponding alkyl lithium. Fukui, Schimidzu, Yagi, Fukumoto, Kagiya and Yuosa (127) have studied the polymeriza-... 

Table IV lists the more important combinations for the preparation of highly active and highly stereospecific catalyst systems. As Weissermel et at. (27) pointed out, catalyst efficiencies of higher than 40,000 g of polymer per gram of titanium and isotacticities greater than 97% for propylene polymerization can be obtained.

Propylene is readily polymerized in bulk that is, in the liquid monomer itself. Arco, El Paso, Phillips, and Shell are practitioners of bulk processing in stirred or loop reactor systems. In either case, liquid propylene (and ethylene, if random copolymer is desired) is continuously metered to the polymerization reactor along with a high-activity/high-stereospecificity catalyst system. Polymerization temperatures are normally in the range of 45-80°C with pressures sufficient to maintain propylene in the liquid phase... 

Catalysts. Iodine and its compounds ate very active catalysts for many reactions (133). The principal use is in the production of synthetic mbber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-83-4], are employed for producing stereospecific polymers, such as polybutadiene mbber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymeri2a tion (66) (see RUBBER CHEMICALS). Hydrogen iodide is used as a catalyst in the manufacture of acetic acid from methanol (66). A 99% yield as acetic acid has been reported. In the heat stabiH2ation of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

Montedison and Mitsui Petrochemical iatroduced MgCl2-supported high yield catalysts ia 1975 (7). These third-generation catalyst systems reduced the level of corrosive catalyst residues to the extent that neutralization or removal from the polymer was not required. Stereospecificity, however, was iasufficient to eliminate the requirement for removal of the atactic polymer fraction. These catalysts are used ia the Montedison high yield slurry process (Fig. 9), which demonstrates the process simplification achieved when the sections for polymer de-ashing and separation and purification of the hydrocarbon diluent and alcohol are eliminated (121). These catalysts have also been used ia retrofitted RexaH (El Paso) Hquid monomer processes, eliminating the de-ashing sections of the plant (Fig. 10) (129). 

Soluble metathesis catalysts yield trans products in reactions with // / v-2-pentene, but generally are not very stereospecific with c/.v-2-pen-tene. In the latter case, the initially formed butenes and hexenes are typically about 60 and 50% cis, respectively. Basset noted (19) that widely diverse catalyst systems behaved similarily, and so it was suggested that the ligand composition about the transition metal was not a significant factor in the steric course of these reactions. Subsequently, various schemes to portray the stereochemistry have been proposed which deal only with interactions involving alkyl substituents on the reacting olefin or on the presumed metallocyclobutane intermediate. 

Occasionally, however, stereospecific results are encountered in the literature which clearly implicate ligands about the transition metal in steric control. For example, when a typical catalyst system based on WCle was modified by the addition of triphenylphosphine, Dall Asta found (77) that the reaction of c/s-2-pentene led very selectively to the formation of tr n.s-olefinic products. On the other hand, Katz demonstrated (75) that when (CO)5W=C(Ph)2 was used, e/.v-2-pentene afforded butenes and hexenes having about 95% cis structure, and notably that this specificity persisted even for reactions carried to near-equilibrium. 

A main feature of the new homogeneous catalysts is that they can be "single site", that is they can include all identical catalytic sites. This can be a great advantage with respect to the heterogeneous catalytic systems, for which several sites with different characteristics are present. Several aspects relative to the catalytic behavior ofthese single site stereospecific catalysts have been described in some recent reviews [9-14],... 

Solution polymerization is bulk polymerization in which excess monomer serves as the solvent. Solution polymerization, used at approximately 13 plants, is a newer, less conventional process than emulsion polymerization for the commercial production of crumb mbber. Polymerization generally proceeds by ionic mechanisms. This system permits the use of stereospecific catalysts of the Ziegler-Natta or alkyl lithium types which make it possible to polymerize monomers into a cis structure characteristic that is very similar to that of natural rubber. This cis structure yields a rubbery product, as opposed to a trans stmcture which produces a rigid product similar to plastics. 

Alternatively, Soga and co-workers described how a series of (CsH4R)TiCl3 compounds in association with MAO could mediate 1,4-at-stereospecific living BD polymerization (e.g., R = Bu, —25°C, 1,4-m-selectivity = 93.5%, = 126000, PDI = 1.04). It was also revealed that an increase in the steric bulk of the substituent R results in an increase in both catalytic activity and 1,4-m-selectivity. Additionally, Shiono and co-workers found that stereospecific quasi-living BD polymerization could be catalyzed by a rather simple catalyst system composed of C0CI2 and MAO (0°C, l,4- j-selectivity = 98.2%, M = 361000, PDI = 1.3). ... 

Polypropylene (PP) is a stereospecific polymer prepared by polymerization using an organometallic catalyst system. Commercial polypropylenes have up to 95% isotactic content, which means that pendant methyl groups are almost all on the same side of the chain. 

Polypropylene (PP) is a stereospecific polymer prepared by polymerization using organo-metallic catalyst system. Commercial PPs have up to 95% isotactic content, which means that pendant methyl groups are almost all on the same side of the chain. When PP is exposed to ionizing radiation, free radicals are formed that cause chemical changes. Because PP is highly crystalline, these radicals are relatively immobile and consequently may not be available for reaction for long periods of time.96... 

Highly stereospecific catalysts for the polymerization of these monomers were found quite naturally along two lines of search starting from the triethylaluminum-water and triethylaluminum-alcohol catalyst systems, which were known to be stereospecific polymerization catalysts for these monomers when we started the experiments on this subject. Development and interrelation of these catalysts in our research are shown in Scheme 1 (8). 

Furukawa and his coworkers also observed that, in the polymerization of acetaldehyde by the in situ triethylaluminum-alcohol (1 1) catalyst system, the bulkiness of the alkoxyl group bonded to an aluminum atom had an influence on the stereospecificity of poly-... 

The work on the AlR3-aeid amide catalyst system has its origin in the studies on the AlR3-ketone catalyst system (35). In the course of studies on the latter catalyst, we speculated that the dialkylaluminum monoenolate [X] might be superior to dialkylaluminum monoalcoholate [XI] as a stereospecific polymerization catalyst (35). Although the speculation on the active species of the AlR3-ketone catalyst system was disproved later by our more detailed studies, fortunately we could find out about the AlR3-acid amide catalyst system (35). 

The AlR3-acid amide (1 1) catalyst system thus proved to be an excellent catalyst for the stereospedfic polymerization of acetaldehyde and to be far superior to the AlR3-ketone (1 1) catalyst system. Both linear and cyclic acid amides gave a similar degree of stereospecificity with good reproducibility (35). We considered that the former catalyst system would be more suitable for our purpose, because this type of... 

In order to find a highly stereospedfic, homogeneous catalyst for the polymerization of propylene oxide, we selected the organometallic compound-primary amine catalyst system which exhibited excellent stereospecificity in the polymerization of acetaldehyde. 

Unfortunately, no catalyst is found which gives an isotactic polymer in quantitative yield. This fact obstructs the determination of the structure of the real active species for isotactic polymerization and the collection of unequivocal information about the mechanism of stereoregulation. Formation of the highly isotactic polymer which is cleanly separable from the atactic polymer indicates the existence of a highly stereospecific catalyst species in the polymerizing system. In order to answer to these unsolved problems, it will be necessary to do more experiments by utilizing new ideas or by more ingenious experimental techniques. 

Tani,H., Yasuda,H., Araki,T. Stereospecific polymerization of acetaldehyde with catalyst systems AlEt3-ketone-H20 and AlEt3-amide-(H20). J. Polymer Sci. B 2,933 (1964). 

Campbell (22) refers that copolymers of 4-methyl- 1-pentene (4MP) with M-pentene-1 and with w-hexene-1, synthesized with the aid of stereospecific catalysts, were crystalline for M-hexene contents up to 25%, probably because of a partial cocrystallization. Analogous conclusions were attained by Hewett and Weir (23) for the same copolymer systems studied by Campbell on the basis of dynamic measurements correlated with DTA observations. 

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