Ziegler Natta electron structure

Modification of an initiator system to increase activity has often come at the expense of stereoselectivity. The great utility of the Ziegler-Natta initiator system is the ability to change one or another of the components, or to add additional components (usually electron donors) to achieve very high stereoselectivity with high activity. The choice of the initiator components evolved in an empirical manner because of a less-than-complete understanding of the detailed structure of these initiators and the mechanism of their stereoselectivity. 

While this review discloses the kinetic and stereochemical features of soluble Ziegler-Natta catalysts, we have little information on the structure of the active center. The steric environments of active centers must be very important in determining the monomer reactivity, regiospecificity and stereospecificity of soluble catalyst. The influence of ligands such as the aluminum components on the rates of chain propagation and chain-terminating steps should be correlated to the electronic structure of... 

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. 

In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous catalysts based on biscyclopentadienyl derivatives of group 4 transition metals, which contain cationic metallocene species of formally d° 14-electronic structure, hardly promote the polymerisation of conjugated dienes, since the diene can act as a donor of four electrons rather than of two electrons as in monoolefin polymerisation (let us recall that the polymerisation of conjugated dienes is catalysed by half-sandwich metallocene-based catalysts). However, it has been reported [162] that statistical copolymers of ethylene and butadiene were obtained with the Cp2ZrCl2— [Al(Me)0]x catalyst. 

Metallocenes (Fig. 2) are sandwich structures, typically incorporating a transition metal such as titanium, zirconium, or hafnium in the center. The metal atom is linked to two aromatic rings with five carbon atoms and to two other groups—often chlorine or alkyl. The rings play a key role in the polymerization activity (23-27). Electrons associated with the rings influence the metal, modifying its propensity to attack carbon-carbon double bonds of the olefins. The activities of these metallocenes combined by aluminum alkyls, however, are too low to be of commercial interest. Activation with methylaluminoxane, however, causes them to become 10-100 times more active than Ziegler-Natta catalysts. 

The electronic configuration of titanium is [Ar] 3d24s2, which means that Ti(IV) compounds are d° species with free coordination sites 1-27,28). H-NMR and 13C-NMR data are known and have been occasionally discussed in terms of bond polarity 19), but such interpretations are obviously of limited value. The electronic structure of methyltitanium trichloride 17 and other reagents have been considered qualitatively 52) and quantitatively S3 56> using molecular orbital procedures. It is problematical to compare these calculations in a quantitative way with those that have been carried out for methyllithium 57> since different methods, basis sets and assumptions are involved, but the extreme polar nature of the C—Li bond does not appear to apply to the C—Ti analog. Several MO calculations of the w-interaction between ethylene and methyltitanium trichloride 17 (models for Ziegler-Natta polymerization) clearly emphasize the role of vacant coordination sites at titanium 58). 

Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

At the present time, the most likely concept of the mechanism of a heterogeneous polymerization catalyzed by a Ziegler-Natta catalyst involves a complex in which the organometallic component and the transition metal component—i.e., the A1 and Ti atoms—are joined by electron-deficient bonds. Natta, Corradini, and Bassi (13) have reported such a structure for the active catalyst prepared from bis (cyclopentadienyl) titanium dichloride and aluminum triethyl. Natta and Pasquon (14), Patat and Sinn (18), and Furukawa and Tsuruta (2) have proposed mechanisms for the stereospecific polymerization of a-olefins in terms of such electron-deficient complexes. 

To a large extent, current interest in solid-state polymerization of monoacetylenes derives from the observation of interesting electrical, magnetic, and optical phenomena in polyacetylene, (CH)j (45), a pEutially crystalline material unstable to ambient conditions typically synthesized by Ziegler-Natta techniques. The fundamental study of (CH), and its electron-transferred ( doped ) forms has been retarded by the lack of fully ordered materials. Ftilly ordered polyacetylenes are also of interest because it is conceivable that their crystal structures could allow significant interchain interactions, a situation precluded in most PDA by side chains. 

Polyacetylene (PA), the simplest linear conjugated polymer, has been actively studied for two main reasons. First, the discovery of the direct synthesis method of PA films on the surface of a Ziegler-Natta catalyst solution [1]. Second, the discovery of a large increase in electronic conductivity, due to a synthetic metal by doping with small quantities of electron-attracting species such as iodine, AsFs, etc., or with an electron donor such as sodium. However, because of its high reactivity and poor solubility, it is difficult to obtain the experimental structural data of PA. 

T his work concerns the study of the polymerization of cyclopropane, substituted cyclopropanes, and conjugated cyclopropanes in the presence of cationic and Ziegler-Natta polymerization. The unsaturation of cyclopropane has been described by several workers in the same way as unsaturated compounds. The unsaturation of cyclopropane compounds, which is the basis for the polymerization of these structures, can be explained by the electronic repartition on the three carbon atoms of the ring. Determination of the dipolar moment of chlorocyclopropane has shown that the carbonium ion resulting from the attack of the ring by a carbo cation is stabilized in a homoallylic structure. 

For the higher activity Ziegler-Natta catalysts (Table II) based on reaction products of specific magnesium, titanium, and aluminum compounds, the similarity in size, coordination preference, electronic structure, and electronegativity of Ti(IV), Mg(II), and Al(III) ions is reflected in structural parameters and chemical properties (38) (Table III). The similarity in size between Mg(II) and Ti(IV) probably permits an easy substitution between ions in a catalyst framework. 

Figure 3.2 Structures of various electron donors used in the preparation of different generations of Ziegler-Natta catalysts.

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