Titanium complexes polymeric hydride

The syndiotacticity of sPS resnlts from the homogeneons coordinative polymerization process (1,42). Styrene monomer complexes at a vacant coordination site on the transition metal, typically titanium, and inserts into a titanium carbon or hydride bond (Fig. 5). In the case of the growing polsrmer chain, the insertion occurs via cis-addition with secondary insertion so that the titaniiun is attached to the carbon bearing the phenyl substituent (43). Chain transfer occurs typically via -hydride elimination, forming a titanium hydride, or via reaction with an... 

As in the previous section, hydrogen increases the concentration of the active sites. Moreover, we think that the polymerization is preceded by the formation of a titanium hydride complex. If this is correct, hydrogen also increases the overall catalytic activity. The effects of hydrogen pressure on catalytic activity and molecular weight of SPS are summarized in Table 17.7. 

Additionally, if the initiation reaction is more rapid an the chain propagation, a very narrow molecular weight distribution, MJM = 1 (Poisson distribution), is obtained. Typically living character is shown by the anionic polymerization of butadiene and isoprene with the lithium alkyls [77, 78], but it has been found also in butadiene polymerization with allylneodymium compounds [49] and Ziegler-Natta catalysts containing titanium iodide [77]. On the other hand, the chain growth can be terminated by a chain transfer reaction with the monomer via /0-hydride elimination, as has already been mentioned above for the allylcobalt complex-catalyzed 1,2-polymerization of butadiene. 

For the polymerization to proceed at a reasonable rate, the use of a transesterification catalyst is needed. Compounds which are usually used as a catalyst for the preparation of polyesters through transesterification can be used here. These include lithium, sodium, zinc, magnesium, calcium, titanium, maganese, cobalt, tin, antimony, etc. in the form of a hydride, hydroxide, oxide, halide, alcoholate, or phenolate or in the form of salts of organic or mineral acids, complex salts, or mixed salts.(10) In this study, tetrabutyl titanate (TBT) in the amount of 1000 ppm was used normally. 

Ziegler catalyst. A type of stereospecific catalyst, usually a chemical complex derived from a transition metal halide and a metal hydride or a metal alkyl. The transition metal may be any of those ingroups IV to VIII of the periodic table the hydride or alkyl metals are those of groups I, II, and III. Typically, titanium chloride is added to aluminum alkyl in a hydrocarbon solvent to form a dispersion or precipitate of the catalyst complex. These catalysts usually operate at atmospheric pressure and are used to convert ethylene to linear polyethylene and also in stereospecific polymerization of propylene to crystalline polypropylene (Ziegler process). 

The A1 coordinates with the halogen on the titanium and activates it for insertion by removing electron density as in the polymerization reactions described in 5.8.2.3.5.ii.a. The titanium hydride reacts more easily with ethylene than with the higher 1-alkene present and forms the ethyltitanium complex. This complex then selectively inserts the... 

Hydrides and alkyls of the titanium triad metals are polymerization catalysts, so it is understandable that there are few stable r-allyl intermediates. The only example we can locate is the reaction between the zirconium hydride XIV and butadiene, which yields the homoallyl complex XV in place of the expected t-allyl complex ... 

It was found that titanacyclobutanes could be prepared by removing the aluminum from 3 with a strong Lewis base in the presence of an olefin, as shown in Eq. 9. These metallacycles were stable with respect to rearrangement to an olefin via a 3 hydride process, and showed a tendency to generate intermediate alkylidene complexes in situ. They ultimately proved to be useful as catalysts for the ring opening metathesis polymerization of a variety of strained olefins. [47,48] It should be noted that the masked titanium methylene complex 3 (Eq. 

The in situ formed basic cyclopentadienyl complex of titanium, ethylene-bis( f-tetrahydroindenyl)-titanium hydride (EBTHI)TiH) containing a planar element of chirality, is active for the reduction of a C=N double bond of both N-aryl-imines (Schiff bases) and unsubstituted imines (108,109). (EBTHl)TiX2 derivatives are used as precursors of the hydride analogue. The chiral zirconium analogues of the above-mentioned complexes provided a breakthrough in asymmetric polymerization of terminal oleflns (110). 

The microstructures are influenced primarily by the nature of the alkylaluminum compound. With triethylaluminum the portion of trans-, 4 double bonds reaches a relatively high level of 10%, while tris(2-methylpropyl)aluminum and bis(2-methylpropyl) aluminum hydride yield cis-, A contents as high as 99% [190]. Similarly, high cis-1,4 portions are obtained in the polymerization of 1,3-butadiene with j -allyluranium complexes. The osmometric measured mole mass ranges from 50 to 150 000, the molecular mass distribution between 3 and 7. The extremely high temperature-induced crystallization rate of uranium polybutadiene in comparison with titanium or cobalt polybutadiene corresponds to a greater tendency toward expansion-induced erystallization. A technical application, however, is in conflict with the costly removal of weakly radioactive catalyst residues from the products [132],... 

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