Addition Ziegler-Natta

Methods for identifying additional Ziegler-Natta co-catalysts in a catalyst system containing procatalysts magnesium and titanium are described by Campbell et al. (4). 

In addition Ziegler-Natta polymerization reactions have also shown some success when carried out in ionic liquids. The most common production methods for this form of polymerization involve the use of triethylaluminium catalysts at ca. 100°C and 100 atmospheres pressure. Advances have been developed through the use of organometallic transition metal catalysts, typically nickel or titanium. Given the solvent characteristics of ionic liquids it should be possible to effectively immobilize such catalysts in an ionic liquid solvent. Indeed, Carlin and Wilkes have reported the Ziegler-Natta polymerization of ethene in an ionic liquid solvent. In these reactions an acidic [Cj-mimJCl-AlClj ionic liquid solvent was used to support dichlorobis(Ti -cyclopentadienyl)titanium(IV) with an alkyl-chloroaluminium(III) co-catalyst. 

This definition is both too narrow and too wide, since 25egler-Natta polymerizations are not only started with compounds of metals from the I to III main periodic table groups but also by organometallic compounds of tin and lead, that is, from metals of the IV main group. On the other hand, not all combinations made in the sense of the classic definition are effective. In addition, Ziegler-Natta catalysts do not necessarily induce polyinsertion polymerizations. 

In 1958, Natta et al. published a study on Ziegler-Natta polymerizations of various vinyl monomers [362]. Natta and his co-workers showed that these polymerizations are very sensitive to steric hindrance at the double bound. In addition, Ziegler-Natta catalysts can generate a series of Lewis adds, which lead to complex side reactions. Therefore, only few attempts have been made to polymerize vinyl arenes by Ziegler-Natta catalysts. Heller and Miller obtained stereoregular polymers consisting of 1-vinylnaphthalene, 2-vinylnaphtha-lene and 4-vinylbiphenyl [363]. Polymerization by triethyl aluminum/titanium tetrachloride gave polymers in 75% to 90% conversion, which were characterized by IR and H-NMR spectroscopy and found to be at least 90% isotactic. Only 1-vinylnaphthalene produced a crystallizable polymer [362,363]. 

The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

Cyclic Polyolefins (GPO) and Gycloolefin Copolymers (GOG). Japanese and European companies are developing amorphous cycHc polyolefins as substrate materials for optical data storage (213—217). The materials are based on dicyclopentadiene and/or tetracyclododecene (10), where R = H, alkyl, or COOCH. Products are formed by Ziegler-Natta polymerization with addition of ethylene or propylene (11) or so-called metathesis polymerization and hydrogenation (12), (101,216). These products may stiU contain about 10% of the dicycHc stmcture (216). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Olig omerization and Polymerization. Siace an aHyl radical is stable, linear a-olefins are not readily polymerized by free-radical processes such as those employed ia the polymerization of styrene. However, ia the presence of Ziegler-Natta catalysts, these a-olefins can be smoothly converted to copolymers of various descriptions. Addition of higher olefins during polymerization of ethylene is commonly practiced to yield finished polymers with improved physical characteristics. 

Carbometalation, an important reaction of RTi(TV) compounds ia which RTi adds to a C=C or CM2 multiple bond and results ia a net R—H addition, is iavolved ia Ziegler-Natta polymerisation as follows ... 

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

Factors affecting laboratory polymerisation of the monomer have been discussed" and these indicate that a Ziegler-Natta catalyst system of violet TiCl3 and diethyl aluminium chloride should be used to react the monomer in a hydrocarbon diluent at atmospheric pressure and at 30-60°C. One of the aims is to get a relatively coarse slurry from which may be washed foreign material such as catalyst residues, using for example methyl alcohol. For commercial materials these washed polymers are then dried and compounded with an antioxidant and if required other additives such as pigments. 

Examine the sequence of structures corresponding to Ziegler-Natta polymerization of ethene, or more specifically, one addition step starting from a zirconocene-ethene complex where R=CH3. Plot energy (vertical axis) vs. frame number (horizontal axis). Sketch Lewis structures for the initial complex, the final adduct and the transition state. Indicate weak or partial bonding by using dotted lines. 

Alkenes undergo addition polymerization. When a Ziegler-Natta catalyst is used, the polymer is stereoregular and has a high density. 

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

In order to obtain good mixing of ethene with the catalyst, the original Ziegler-Natta processes used hexane as a solvent. Although the solvent is almost completely recovered, the use of a hazardous material such as hexane detracts from the greenness of the process. Since the catalyst is highly moisture sensitive it needs to be deactivated at the end of the process by addition of water or alcohol, and this produces a small waste... 

We have reviewed experiments on two classes of systems, namely small metal particles and atoms on oxide surfaces, and Ziegler-Natta model catalysts. We have shown that metal carbonyls prepared in situ by reaction of deposited metal atoms with CO from the gas phase are suitable probes for the environment of the adsorbed metal atoms and thus for the properties of the nucleation site. In addition, examples of the distinct chemical and physical properties of low coordinated metal atoms as compared to regular metal adsorption sites were demonstrated. For the Ziegler-Natta model catalysts it was demonstrated how combination of different surface science methods can help to gain insight into a variety of microscopic properties of surface sites involved in the polymerization reaction. 

Oxidative addition of the silyl species to nickel is followed by insertion of unsaturated substrates. Zero-valent nickel complexes, and complexes prepared by reducing nickel acetylacetonate with aluminum trialkyls or ethoxydialkyls, and in general Ziegler-Natta-type systems, are effective as catalysts (244, 260-262). Ni(CO)4 is specific for terminal attack of SiHCl3 on styrene (261). 

Several articles on these topics have appeared in earlier volumes. For the convenience of readers they are listed here. In addition, articles on Ziegler-Natta catalysis and on organolithium compounds in diene polymerization are planned for the next volume of this serial publication. 

---