Ziegler Natta olefin interaction

The detail of the structure of the polymerisation centre present in suppported Ziegler-Natta catalysts for a-olefin polymerisation has been the subject of much research effort (e.g./-/2) The catalyst consists of a solid catalyst MgC /TiC /electron donor and a co-catalyst, an aluminium alkyl complexed with an electron donor. Proposed mechanisms for the polymerisation involve a titanium species attached to magnesium chloride with the olefin coordinated to titanium. The detail of the site at which the titanium species is attached is an important area of study in understanding the mechanism of catalysis and several recent papers 10-12) have investigated the surface structure of magnesium chloride and the attachment of TiCl4, in particular the interaction of titanium species with the 100 and 110 planes of a and (3- magnesium chloride. 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

The ability of a /i-olefin to copolymerise with ethylene in the presence of Ziegler-Natta catalysts arises from minimisation of steric interactions at the catalytic active site by ethylene units the steric hindrance, which prevents homopropagation of the /1-olefin, is overcome when the /<-olefin monomer is... 

Functionalised a-olefins capable of undergoing insertion polymerisation with Ziegler-Natta catalysts are, in principle, monomers in which the heteroatom (X) does not electronically interact with the double bond to be polymerised in such monomers, the heteroatom is separated from the double bond CH2=CH-(CH2)x X [326,384,518,522-528], Monomers with the heteroatom directly bound to the double bond, i.e. those of the CH2=CH-X type, may also undergo polymerization, but when the heteroatom is silicon or tin (X= Si, Sn) [522-526], Representative examples of the insertion polymerisation of functionalised a-olefins and their copolymerisation with ethylene and a-olefins in the presence of heterogeneous Ziegler-Natta catalysts are shown in Table 3.7 [2,241,326,384,518,522-528],... 

Ziegler-Naita caialysts consist of a combination of alkyls or hydrides of Group I-III metals with salts of the Group IV-VHI metals. The most generally efficient catalyst combinations are those in which an aluminum alkyl derivative is interacted with titanium, vanadium, chromium or zirconium salts. The most important application of these catalysts is in the polymerization of olefins and conjugated dienes. Not every catalyst combination is equally effective in such polymerizations. As a general rule, Ziegler-Natta combinations that will polymerize 1-olefins will also polymerize ethylene, but the reverse is not true. 

Conventionally, HAS are blended with PO during processing. 2-(Diethy-lamino)-4,6-bis[butyl(l,2,2,6,6-pentamethyl-4-piperidyl) amino]-l,3,5-triazine may be fed with an olefin directly into the low pressure polymerization process catalyzed with a modified MgCl2 supported Ziegler-Natta catalyst [142]. The catalytic activity was not impaired [143], Tetramethylpiperidine was reported to be a useful component in MgC -supported Ziegler-Natta catalysts as well. Very high stereospecificity of the synthesised PO was achieved. A complex of HAS with the alkyl aluminium activator was envisaged without interaction with the catalytically active alkyl titanium compound [144],... 

The utility of the Tebbe type complex in carbonyl olefination is discussed in Chapter 4. The bridged complex may be regarded as a special type of a carbene complex where the Cp2Ti=CH2 unit is masked by interaction with the AlMe2Cl entity. Formation of the Tebbe s complex suggests the occurrence of a-hydrogen elimination in the preparation of the Ziegler-Natta and Kaminsky type olefin polymerization catalysts from titanium chlorides and methylaluminum compounds. 

When 1,2-disubstituted olefins are polymerized with Ziegler-Natta catalysts, the ditacticity of the products depends on the mode of addition. It also depends on the structure of the monomer, whether it is cis or trans. A threodiisotactic structure results from a syn addition of a trans monomer. A syn addition of a cis monomer results in the formation of an erythrodiisotactic polymer. For instance, cis and /ra/i -l- Z-propylenes give erythro and threo diisotactic polymers, respectively. To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bond in the monomer units rotate after the addition of the monomer to the polymeric chain. 

The approach and insertion of an olefin molecule may or may not pass through a local minimum or coordination complex (first in brackets in eq. 16) recent theoretical work (128) indicates that the well, if it indeed exists, is very shallow. The insertion of the new molecule into the growing chain is represented in equation 13 as a structure intermediate between reactants and products. The mechanism for this apparently concerted reaction does not involve the participation of metal-based electrons, and can be considered to be a Lewis acid-assisted anionic attack of the zirconium alkyl (ie, the polymer chain) upon one end of a carbon-carbon double bond. The concept of this reaction pre-dates metallocene study, and is merely a variant of the Cossee-Arlman mechanism (129) routinely invoked in Ziegler-Natta polymerization. Computational studies indicate (130) that an a-agostic interaction (131) provides much needed stabilization during the process of insertion. 

EP copolymers are obtained through polymerization promoted by Ziegler-Natta-type catalysts. A Ziegler-Natta catalyst polymerizes olefins by inserting them into a metal-carbon bond. As a consequence, the exact structure of the catalytic site, determined by the interaction between the organometallic complex and the growing polymer chain, has a dramatic influence on the insertion reaction and hence on copolymer characteristics. 

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