Active centres, Ziegler-Natta catalyst

Kinetic models referred to as adsorption models have been proposed, especially for olefin polymerisation with highly active supported Ziegler-Natta catalysts, e.g. MgCl2/ethyl benzoate/TiCU AIR3. These models include reversible processes of adsorption of the monomer (olefin coordination at the transition metal) and adsorption of the activator (complexation via briding bonds formation). There are a variety of kinetic models of this type, most of them considering the actual monomer and activator concentrations at the catalyst surface, m and a respectively, described by Langmuir-Hinshelwood isotherms. It is to be emphasised that M and a must not be the same as the respective bulk concentrations [M] and [A] in solution. Therefore, fractions of surface centres complexed by the monomer and the activator, but not bulk concentrations in solution, are assumed to represent the actual monomer and activator concentrations respectively. This means that the polymerisation rate equation based on the simple polymerisation model should take into account the... 

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

Curves presented in Figure 3.13 testify to the large specificity of supported Ziegler-Natta catalysts regarding the kind of monomer some centres that polymerise ethylene do not polymerise propylene (or higher a-olefins, which may also be differentiated by particular catalyst centres, depending on the structure of the oc-olefin, e.g. branched as in 3-methyl-1-butene or not branched). Therefore, no hints about the monomer reactivity can be obtained by simple comparison of polymerisation rates without simultaneous estimation of the concentration of active sites [241]. 

Termination of the olefin polymerisation with heterogeneous Ziegler-Natta catalysts by the addition of carbon monoxide to the system is often used in the laboratory to determine the active centres of the catalyst. 

A number of models of the active centres in Ziegler Natta catalysts have been postulated. The diversity of these models arises from the multitude of products found to be formed or believed to be formed in the reaction of the catalyst precursor with the activator [e.g. schemes (4) to (8) and (12)]. The proposed active centres fall into either of two general categories those containing monometallic species with the central transition metal atom (e.g. Ti), and those containing bimetallic species with the central transition metal atom linked via bridges with the metal atom originating from the activator (e.g. Al). 

Figure 9 Simplified model for the formation of active centres of Ziegler-Natta catalysts by adsorption ofTiCl4 on solid MgCl2, reduction ofTi(IV) to Ti(III) chloride-alkyl exchange with aluminium alkyls and polyethylene (PE) formation by coordination and insertion of ethylene molecules.

The main parameters of diene polymerisation with lanthanide-based catalytic systems are similar to those of polymerisation with ion-coordinated catalysts on the basis of d-metals. This can be seen from the following facts polymerisation of dienes has an anionic coordinated character [18] at polymerisation temperatures from 20 to 25 C, the reaction is of first order with respect to the monomer and catalyst (this property is independent of the natures of catalyst and hydrocarbon solvent, the only exception to this rule being the system considered in work [18]) for most of catalysts studied [18, 21, 26, 28, 41] and, the apparent activation energy of the reaction of polymerisation of dienes is of the order of 33.5 kj/mol [20, 41]. For lanthanide catalysts, the concentration of active centres is somewhat higher than for conventional Ziegler-Natta catalysts, e.g., for neodimium-based catalysts their content varies from 6-10% [12, 41, 42, 50] to 15-20% [54-57]. 

Modern theoretical concepts of the polymerisation of dienes on Ziegler-Natta catalysts are based on the principle of their inherent polycentrism [49]. Substantial changes in the MW characteristics of polyisoprene and polybutadiene, due to the formation of a microheterogeneous titanium catalyst in the turbulent mode, make it reasonable to analyse the distribution of macromolecular growth centres on their kinetic activity. 

The correlation between MW and polymer microstructure is also mentioned by the authors of [77] for fractions of dienes synthesised on complex Ziegler-Natta catalysts. The content of cis-1,4-links drops in fractions with a decrease of their MW. A substantial decrease of ds-l,4-links content was observed in the area of MW < 2 X lO (from 90 to 70%). The authors assume that the heterogeneity of a catalyst is the basis for the formation of different types of AC with different growth reaction rates and anti-syn-isomtnsanon. More active centres tend to form macromolecules with an increased content of the ds-l,4-links and a higher MW. 

High-temperature SEC finds wide application in polymerization studies, as the molecular mass distribution is an artefact of the various reactions involved in polymerization, initiation, termination, and transfer. It is diagnostic of living systems and random polymerization reactions, such as condensation and radical initiated polymerizations, for which the distributions are Poisson and normal respectively. In the polymerization of ethene and propene by Ziegler-Natta catalysts, the determination of the concentration of active centres as a function of conversion defines catalyst type. Similar studies have been made in the study of chain scission by thermal degradation or by irradiation, in defining the number of molecules produced from the inverse of the number average molecular mass and random chain scission eventually leads to a normal molecular mass distribution, with polydispersities close to 2.0. This has, of course, been widely used to produce narrow from broad molecular mass distribution samples prior to fractionation. 

This feature is due to the difference in reactivity ratios between comonomers which depend on the type of catalytic active site. The lower isospecific centres of traditional Ziegler-Natta catalysts are much more active toward ethylene than toward propylene or butene-1. Moreover, these less isotactic sites being the more reactive toward hydrogen, the atactic fractions will have lower mean molecular weight than isotactic ones. 

Studies on the Polymerization of Propylene Using Highly Active Magnesium Chloride Supported Ziegler-Natta Catalysts Effects of Alkyl Concentration on the Polymerization Rate and on the Active Centre Concentration... 

The coordination and organometallic chemistry of zirconium and hafnium are surveyed for the year 1991.107,108 There is useful material in a review of homogeneous Group 4 metallocene Ziegler-Natta catalysts, a review of the coordination chemistry of cyclopentadienyl titanium carboxylate and related complexes O and a review of bis(cycIopentadienyl)zirconium(IV) or hafnium(IV) complexes with Si-, Ge-, Sn-, N-, P-, As-, Sb-, 0-, S-, Se-, Te- or transition metal-centred anionic ligands. Cationic zirconocene or hafiiocene complexes serve as Lewis acids with unique reactivities, they are active for C-F bond activation, coordinative activation of ether linkages, carbonyl activation and C-O bond cleavage. New synthetic methods based on the... 

It is known [14,15] that, on the surface of the heterogeneous Ziegler-Natta catalysts promoting the isotactic polymerisation of propene, active centres are also present which may give rise to the formation of significant amounts of V-rich sequences (syndiotactic or syndiotactoid ). As a matter of fact, syndiotactic polypropene was isolated for the first time as an impurity from samples of isotactic polypropene prepared in the presence of catalyst systems such as, for example, a- or y -TiClj in combination with A1(C2H5)2F or LiC4H9 [16]. 

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