Ziegler-Natta systems

Among other possible reactions, these free radicals can initiate ordinary free-radical polymerization. The Ziegler-Natta systems are thus seen to encompass several mechanisms for the initiation of polymerization. Neither ionic nor free-radical mechanisms account for stereoregularity, however, so we must look further for the mechanism whereby the Ziegler-Natta systems produce this interesting effect. 

Polybut-1-ene became available in the early 1960s as Vestolen BT produced by Chemische Werke Hiils in Germany. Today it is manufactured by Shell in the United States. It is produced by a Ziegler-Natta system and the commercial materials have very high molecular weights of 770 000 to 3 000 000, that is about ten times that of the normal low-density polyethylenes. 

It is useful to note here a fundamental distinction between cationic and anionic polymerizations (including Ziegler-Natta systems). In the latter, residual water merely inactivates an equivalent quantity of catalyst, whereas in the former water may be a cocatalyst to the metal halide catalyst in excess it may decrease the rate by forming catalytically inactive higher hydrates and in very many systems it, or its reaction product(s) with a metal halide, act as extremely efficient chain-breakers, thus reducing the molecular weight of the polymers (see sub-section 5.4). 

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

Poly acetylene was initially produced using Ziegler-Natta systems, which resulted in the production of what is now known as Shirakawa polyacetylene. These materials are not easily processable and are mainly fibrillar. Recently, other approaches have been taken. In the... 

The active center I, which first initiates and then propagates the chain, can be a free radical, an anion, a cation or a transition-metal based initiator (Ziegler-Natta systems or metathesis reactions). 

It is to be noted in this connection that alkyl radicals normally formed during reduction of the transition metal compound in Ziegler-Natta systems [scheme (7)] do not initiate the radical polymerisation of olefins, in contrast to that of polar monomers. Most of the modified Ziegler-Natta catalysts for polar monomer polymerisation are characterised by low activities and lack of stereospecificity, producing polymers with properties that are very similar to those of polymers obtained by more conventional procedures for radical polymerisation [28],... 

In standard Ziegler/Natta-systems traces of water are considered an impurity, even though water is known to have a beneficial effect in the stereospecific polymerization by Co-based catalyst systems [411]. So far, in Nd-catalyzed polymerizations the influence of water has received only little attention. For the system NdV/EASC/TIBA it was mentioned that water had no impact as long as the total water content in the polymerization mixture stayed below 5 ppm. In this study water was added as a byproduct of the organic solution of Nd versatate. The water content of different lots varied in the range 25-700 ppm [412]. Other authors also recommended a low water content in the organic solutions of Nd carboxylates (< 2000 ppm, preferably... 

Though NdX3-based catalyst systems are among the first Nd Ziegler/Natta systems, studies directed at the elucidation of the active species became more numerous since 1986. Essentially there are the research groups of Hsieh, Iovu and Monakov who made major contributions to this topic. 

The intensive search for the active cocatalyst responsible for this activation led in 1977 to the isolation of MAO, a component in which aluminum and oxygen atoms are positioned alternately and free valences are saturated by methyl groups (65, 66). When metallocenes, especially zirconocenes, are combined with MAO, the resulting catalyst can polymerize olefins 10-100 times faster than those used in the most active Ziegler-Natta systems (67). 

Assume as a model for a Ziegler-Natta system the diffusion of monomer to a site of catalytic activity—presumably one of a number of sites on a solid particle—where it is inserted into a growing polymer chain. For the bulk polymerization of a monomer such as 4-methylpentene-l where polymer is insoluble in monomer, the solid catalyst particle becomes the center of an expanding sphere of precipitated polymer chain (s) growing from the inside. On this molecular level, the rate of chain growth will be directly proportional to the monomer activity at the individual sites. At equilibrium the monomer activity at each site encapsulated in precipitated polymer will equal that of the surrounding bulk monomer, [Mo]. Under nonequilibrium conditions, where the rate of diffusion of monomer from the bulk monomer thru the precipitated polymer to the polymerization site becomes comparable to the rate of polymerization at that site, the localized activity will be lower, and the rate of polymerization will be correspondingly lower. 

Depressed rates have been observed in Ziegler-Natta systems with monomers other than 4-methylpentene-l. Bier (7) suggested that the slowly decreasing rates of propylene polymerization under polymer precipitating conditions with the catalyst system a-TiCb-Al HsbCl are caused by diffusion control. In another case Burnett and Tait (3) found depressed rates of styrene polymerization under polymer-precipitating conditions with the catalyst system a-TiCb-Al HsU. At styrene concentrations less than 3.5M in heptane (isotactic polystyrene precipitates in this region of monomer concentration) a plot of polymerization rate vs. styrene concentration falls below the extrapolated linear plot by a factor of 2. 

A plot of rate (taken from Figure 1) vs. weight of TiCh in Figure 2 is linear, indicating the usual situation in Ziegler-Natta systems that the number of active sites is proportional to the TiCh present. 

Figure 6.23 Control of Ziegler-Natta system involves a combination of a-TiCl3, TiCl4 and ethyl...

The nature of the cocatalyst in the Ziegler-Natta systems exerts a significant effect on the polymerization rate and isotacticity of the polymer product Interesting data were obtained by Keii et al. when the cocatalyst was changed during polymerization. The pronounced effect of the cocatalyst is the basis of the concept on the bimetallic composition of ACg according to which the latter are represented, for example, as binuclear complexes including titanium and aluminium. 

A. Cationic Alkyl Complexes in Ziegler-Natta Systems.370... 

By far the most important group 16 compounds with aluminum are those of oxygen. This is due not only to the fact that the A1 O bond is the thermodynamic driving force behind much of the chemistry of aluminum, but also due to the fact that A1 O compounds have found great utility in various industrial and catalytic processes. Indeed, one of the most important recent developments in this area may be found in a class of compounds knows as aluminoxanes. Aluminoxanes, methylaluminoxane (MAO) in particular, are very active cocatalysts in Ziegler-Natta systems. Two of the most common A1 O compounds are aluminum hydroxide, Al(OH)3, and aluminum oxide, AI2O3. 

Insertion reations of olefins into metal-carbon bonds are fundamental to catalytic oligomerization and polymerization (e.g., Ziegler-Natta systems). Furthermore, this reaction may provide a method for stereoselective formation of a carbon-carbon bond. 

Complexes, CpjM—CH3 [M = Yb(III) or Lu(III), Cp = j -C5(CH3)5], have been examined as olefin polymerization catalysts because of their similarity to paramagnetic Ti(III) intermediates in Ziegler-Natta systems. The similarities between rates of... 

Neodymium-based Ziegler-Natta systems play a major role in the industrial polymerization of 1,3-butadiene to poly-cw-1,4-butadiene [182-184]. Therefore, the catalytic activity of the neodymium complex 229 was investigated for the polymerization of 1,3-butadiene [181]. The observed catalytic... 

Progress in (heterogeneous) Ziegler-Natta catalysis has continued unabated over the last 50 years, while the last 20 years have seen the advent of homogeneous (metallocene and other single-site) catalysts. However, despite the enormous research effort and many advances made in the field of homogeneous catalysis, polyolefins manufacture is still dominated by Ziegler-Natta systems. 

Remarkably high activities for the polymerization of ethene similar to those of active Ziegler-Natta systems have been reported and the physical properties of the polyolefins produced can be tailored by the choice of the metal center and the substitution pattern of the hgand backbone [12-14]. Some of these catalytic systems are not only compatible with polar monomers, but also copolymerization of... 

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