Ziegler systems

Studies with transition metal alkyls discussed have simplified out understanding of the Ziegler system in the following way ... 

A number of catalysts are known to effect homogeneous hydrogenation of aromatic hydrocarbons, e.g., some oxidized rhodium complexes (/, p. 238), some rhodium 7r-complexes with phenyl carboxylates (/, p. 283), some Ziegler systems (/, p. 363), and Co2(CO)8 (/, p. 173). However, the catalysts in the first three systems are not well characterized, and the carbonyl systems require fairly severe hydroformylation conditions, although they are reasonably selective, possibly via radical pathways (Section II, C). 

With Ziegler systems containing Ni, terminal acetylenes gave coupled diadducts as in Eq. (45) ... 

The zirconocene/MAO catalysts are about 10-100 times more active for ethene polymerization than the conventional Ziegler systems. Using bis(cyclopen-tadienyl) zirconium dichloride Cp2ZrCl2 and MAO up to 40000000 g, poly-ethene/g Zr-h are obtained (Table 2) [54]. 

It has been reported that vinylferrocene is polymerized by using a radical, cationic, anionic, or Ziegler system initiator [29 — 34]. In particular, higher molecular weight products can be obtained using radical-initiated bulk polymerization [31]. Indeed, both bulk copolymerization and solution copolymerization (in benzene) of 18 with vinylferrocene by using a radical initiator (AIBN) afforded the chiral polymers 19a —e (Scheme 3-13), which were purified by reprecipitation of the benzene solution with methanol. The ratios of the two comonomers were varied in copolymerization. The composition data of the copolymers obtained revealed nearly the same reactivity between 18 and vinylferrocene, which suggests that 19a—e are random copolymers. 

Ziegler systems from halides of the transition metal in its highest valence state frequently exhibit rates which pass through maxima, in some cases sharply (Fig. 5), and then fall as the ratio of transition metal to metal alkyl is decreased. It is unlikely, however, that this results from equilibria giving rise to eqns. (6) and (7) and the explanation is more probably that the polymerization rate reflects the activity of the catalyst sites produced by the particular stoichiometry. Table 1 shows the dependence of catalyst composition on molar ratio of reagents. 

In ethylene—propene copolymerization the former monomer is greatly favoured and a value for r, of 72 was found. Hydrogen is particularly active as a chain transfer agent for this catalyst, a value of fetr,H,/ tr,M 3.8 X 10 being quoted, some ten times greater than that for a conventional Ziegler system [133b]. The active species in both these systems was ascribed to a low valence Cr complex. 

The Ziegler system ML -A1(C2H5)3 (M=Ni, Co or Fe n=2, 3 L=acac or Cl) has been used mostly in the addition of trialkyl and triphenylsilanes to conjugated dienes and trienes (especially Ni(acac)2-A1(C2H5)3) as well as in hydrosilylation of olefins with functional groups (the three-component system with additional PPh3) [2, 5]. 

Thus, until the last decade, three families of catalysts have been reported to catalyze the addition, or vinyl-type homopolymerization of norbornene resulting in poly(2,3-bicyclo[2.2.1]hept-2-ene). These three catalyst types are the classical TiC -based Ziegler systems (type 1), the zirconocene/aluminoxane systems (type 2) and certain electrophilic palladium(II) complexes (type 3). 

A third catalyst system was discovered in the last half of 1950 by Alex Zletz of Standard Oil of Indiana [11,12], It consisted of reduced molybdenum on alumina. The first patents were filed on April 28,1951. Polymer with a density of about 0.96 g mL-1 was reported. This discovery took a direction different from those mentioned above. Unsure of its importance, the company hired an outside consultant to evaluate their new linear PE. He gave a discouraging assessment. This assessment apparently caused a delay in plans for commercialization until after the early Phillips and Ziegler systems were well on the way toward commercial success... 

In contrast to other catalysts, a Ziegler system containing nickel and triethylaluminum gives coupling products in the hydrosilylation of 1-alkynes64 (equation 17). 

The above reactions illustrate the fact that the compounds used in Ziegler s polymerization systems are very reactive and often unstable, so that situations easily arise in which the catalyst undergoes a further reaction to give an inactive product. Another example of this occurs with the soluble catalyst system (13) vanadium di(isopropyl salicylate)-AlEt2Cl where the rate also declines rapidly, as shown in Fig. 8b. This type of behavior is characteristic of many Ziegler systems and is sometimes attributed to the encapsulation of the catalyst by the polymer. 

Today HDPE is still made almost entirely through chromium or Ziegler systems. The two systems produce different t5 es of polymer, which is useful for different applications. The Phillips catalyst generally produces broader molecular weight distributions than that of typical Ziegler catalysts. 

Polymerizations initiated by organometallic compounds—Ziegler systems—will be covered in section 1.15.1 which deals with ethylene and propylene polymerizations. 

Polybutadiene (BR rubber) and the random styrene/butadiene copolymer (SBR rubber) are the most widely used polymers. Their principal use is in tyres, which are typically blends of natural/synthetic rubber. BR rubber has good resilience, abrasion resistance and low heat build-up. SBR contains 10-25% styrene which is added chiefly to reduce cost but also to improve wearing and blending characteristics compared with BR alone. BR and SBR are polymerized by a free-radical mechanism as a water emulsion at 50-60 °C (hot rubber) or 0°C (cold rubber). Typical compositions are 70% trans-1,4, 15% cis-1,4 and 15% 1,2. Ziegler systems used in solution polymerization yield an SBR which has higher MW, narrower MWD and higher cis-1,4-content than the emulsion free-radical type. 

Metallocenes, in combination with the conventional aluminum alkyl cocatalysts used in Ziegler systems, are indeed capable of polymerising ethene, but only at a very low activity. Only with the discovery and application of methylaluminoxane (MAO) it was possible to enhance the activity, surprisingly, by a factor of 10000 [113]. Therefore, MAO plays a crucial part in the catalysis with metallocenes. 

An example of the physical impregnation method is provided by U.S. Patent 4,302,565, which was discussed above. The dissolution of a magnesium compound and a titanium compound in a suitable solvent, usually an electron donor solvent such as tetrahydrofuran (THF), can lead to the formation of a new species (catalyst precursor) that provides a highly-active Ziegler system when activated. Removing the solvent at elevated temperatures in the presence of a support material such as silica will precipitate the precursor into the pores of the silica. In this method it is necessary that... 

As pointed out by Kaminsky and Arndt [24], this Cp ZrCl /MAO catalyst is about 10-100 times more active in ethylene polymerization than the conventional Ziegler systems, as an activity of 4 x lO g polyethylene/g Zr/hour are obtained, which translates to about 46,000 polymer molecules/ hour or one ethylene insertion in 3 x 10 seconds. 

In the case of cobalt, modified Ziegler systems have limited activity toward ethylene dimerization and oligomerization [141]. Cobalt(II) or cobalt(IIl) and a reducing organometallic compound system have been specially proposed for this reaction. Tris(acetylacetonato)Co(llI) and triethylaluminum convert ethylene at 30°C into n-butenes with a selectivity of 99.5% [142]. The product consists of a mixture of 95% 2-butenes and 5% 1-butenes. The molar ratio of AIR3/C0 must be between 2 and 5 beyond that, the activity decreases. On the other hand, the addition of triphenylphosphine decreases the rate of the reaction. 

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