Isomerization polymerization coordination

Isomerization polymerizations can be associated with coordination catalyst systems, ionic catalyst systems, and free radical systems. The cationic isomerization polymerization of 4-methyl-1-pentene is of interest because the product can be viewed as an alternating copolymer of ethylene and isobutylene. This structure cannot be obtained by conventional... 

Isomerism in the Metal-ammines.—Werner claimed for the coordination theory that in certain cases isomerism should occur, that isomerism being brought about by different causes. lie divided isomerism in the ammines into five groups, namely, structure isomerism, ionisation isomerism, hydrate isomerism, polymerism, and stereoisomerism. 

The relatively low angle slope of the Hammett plot indicates that the polarization induced by complexing the monomer with the electrophilic site involves only a partial charge. Weak polarization of monomer is supported by the fact that vinylcyclopropane does not isomerize during coordinated anionic polymerization but does so during conventional cationic polymerization (307). 

The same conclusions can be reached for the cis isomer. As already mentioned, isomerization of this isomer occurs in the polymerization medium. If we admit that only the trans isomer formed through isomerization polymerizes, we fall under the case examined above. If we admit also that the cis isomer can polymerize to cis-1,4 units, we must assume that also in this case the coordination occurs only through the vinyl group and not through the two double bonds, in the cis conformation. The cis isomer, in fact, cannot assume the cis conformation for steric reasons. On the other hand, coordination by the internal double bond, both for the cis and trans isomers, appears less probable (for steric reasons) than the coordination by the vinyl group. 

Ionization isomerism Hydration isomerism Coordination isomerism Linkage isomerism Polymerization isomerism... 

How does isomerization polymerization take place by coordinated anionic mechanism ... 

In fact, Werner played such a central and almost monopolistic role in coordination chemistry that his name is virtually synonymous with the field. Even today, almost 75 years after his death in 1919, coordination compounds, particularly metal-ammines, are still colloquially called Werner complexes. The coordination theory not only provided a logical explanation for known "molecular compounds, but also predicted series of unknown compounds, whose eventual discovery lent further weight to Werner s controversial ideas. He showed how ammonia could be replaced by water or other groups, and he demonstrated the existence of transition series between ammines, double salts, and hydrates. Werner recognized and named many types of inorganic isomerism such as coordination isomerism, polymerization isomerism, ionization isomerism, hydrate isomerism, salt isomerism, coordination position isomerism, and valence isomerism. He also postulated explanations for polynuclear complexes, hydrated metal ions, hydrolysis, and acids and bases. His view of the two types of chemical... 

Butadiene could be polymerized using free radical initiators or ionic or coordination catalysts. When butadiene is polymerized in emulsion using a free radical initiator such as cumene hydroperoxide, a random polymer is obtained with three isomeric configurations, the 1,4-addition configuration dominating ... 

We have designed PBUILD, a new CHEMLAB module, for easy construction of random copolymers. A library of monomers has been developed from which the chemists can select a particular sequence to generate a polymeric model. PBUILD takes care of all the atom numbering, three dimensional coordinates, and knows about stereochemistry (tacticity) as well as positional isomerism (head to tail versus head to head attachment). The result is a model of the selected polymer (or more likely a polymer fragment) in an all trans conformation, inserted into the CHEMLAB molecular workspace in literally a few minutes. 

The ability of complexes to catalyze several important types of reactions is of great importance, both economically and intellectually. For example, isomerization, hydrogenation, polymerization, and oxidation of olefins all can be carried out using coordination compounds as catalysts. Moreover, some of the reactions can be carried out at ambient temperature in aqueous solutions, as opposed to more severe conditions when the reactions are carried out in the gas phase. In many cases, the transient complex species during a catalytic process cannot be isolated and studied separately from the system in which they participate. Because of this, some of the details of the processes may not be known with certainty. 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... 

Furthermore, isomerization of the heptadentate XXXI, coordinated to Ni(II) via transmetallation (91), occurs again aimed at its adaptation to a octahedral coordination sphere around ions with d8 configuration. In the case of the more flexible pentadentate ligands, XXXII and its analogue with three methylene groups (L2) (92), formation of monomeric (with XXXII) and dimeric (with L2), but also of polymeric (with L2) Ni(II) complexes with an octahedral environment around the metal is possible (94). 

The coordinative polymerization with soluble transition metal systems is part of the growing field of homogeneous catalysis on transition metal centers (Oxo-Process, Wacker-Process, Isomerization, Cyclooligomerization of olefis, etc.). The mechanisms of these reactions have not yet been completely elucidated. Any new knowledge could perhaps contribute to the detection of common trends and parallels, and would thus facilitate prediction and development of new processes. 

The Raman spectrum1018 of coordinated zinc(II) in KSCN at 200 °C shows the presence of the tetrahedral [Zn(NCS)4]2 species, somewhat distorted by polymerization, and bands due to Zn—NCS—Zn bridges are also observed. Linkage isomerism is also present, as evidenced by bands due to both Zn—NCS and Zn—SCN bonding modes. 

In conventional organic nomenclature, a polymer is not considered to be an isomer of the repeating molecular unit, because the molecular formulas formally differ. This is a somewhat arbitrary distinction, however, because it is never really an isolated, single molecule of monomer that is compared with the polymer. In an aggregate of monomer molecules, intermolecular forces exist and the constitutional difference from an aggregate of polymer molecules is simply that some intermolecular forces have been converted into true chemical bonds. In any case, the term polymerization isomerism has had a long-standing use in coordination chemistry. It may refer... 

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