Ziegler polymerizations stereospecificity

Ziegler-Natta polymerization Stereospecific polymerization of olefines using a Ziegler catalyst. See titanium(IIl) chloride. 

A very important field of polymerization, stereospecific polymerization, was opened in 1955. In this year, Natta and his coworkers (1—3) polymerized a-olefins to crystalline isotactic poly-a-olefins with the Ziegler catalyst, and Pruitt and Baggett (4,5) polymerized dl-propylene oxide to crystalline polypropylene oxide, which was later identified as an isotactic polymer by Price and his coworkers (6,7). Since then, a large number of compounds including both unsaturated and cyclic compounds were polymerized stereospecifically and asymmetrically. Development of the stereospecific polymerization stimulated... 

The first stereoregular 1,4-polybutadiene was obtained by Morton and his associates by using an Alfin catalyst in 1947 (34) and had a predominantly trans structure. The structure of this polymer was determined by careful X-ray and IR-excimination (61). A few years later the first synthesis of 1,4-cis-polyisoprene was accomplished (62) by using a Li metal dispersion. In the same period, Ziegler-type catalysts were reported to polymerize stereospecifically to 1,4-trans- or 1,4-cis-polymers a number of conjugated dienes. More recently, new types of catalysts for the stereospecific polymerization of dienes were prepared starting with TT-allyl derivatives of transition metals. Some catalysts for the stereospecific 1,4-polymerization of butadiene are indicated in Scheme 12. 

In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. Natta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. 

In the mid-1950s, the Nobel Prize-winning work of K. Ziegler and G. Natta introduced anionic initiators which allowed the stereospecific polymerization of isoprene to yield high cis-1,4 stmcture (3,4). At almost the same time, another route to stereospecific polymer architecture by organometaHic compounds was aimounced (5). 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

The revolutionary development of stereospecific polymerization by the Ziegler-Natta catalysts also resulted ia the accomplishment ia the 1950s of a 100-year-old goal, the synthesis of i7j -l,4-polyisoprene (natural mbber). This actually led to the immediate termination of the U.S. Government Synthetic Rubber Program ia 1956 because the technical problem of dupHcating the molecular stmcture of natural mbber was thereby solved, and also because the mbber plantations of the Far East were again available. 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

Polymerization reactions of olefins and dienes cannot be treated here in detail. Knowledge of the early steps which occur on nickel, as in oligomerization reactions, help explain the course of polymerization reactions and particularly their stereospecific character, as in Ziegler-Natta polymerization. 

MPC [Mitsui Petrochemical] A continuous process for polymerizing propylene, based on the Ziegler-Natta process, but using a much more active catalyst so that de-ashing (catalyst removal) is not required. The catalyst contains magnesium in addition to titanium successive versions of it have been known as HY-HS (high yield, high stereospecifity), HY-HS II, and T-catalyst. Developed jointly by Mitsui Petrochemical Industries, Japan, and Montedison SpA, Italy, in 1975, and now licensed in 56 plants worldwide. 

The examination of models for stereospecific Ziegler-Natta polymerizations indicate the importance of the nonbonded interactions for the stereoselectivity mechanisms. 

Solution polymerization is bulk polymerization in which excess monomer serves as the solvent. Solution polymerization, used at approximately 13 plants, is a newer, less conventional process than emulsion polymerization for the commercial production of crumb mbber. Polymerization generally proceeds by ionic mechanisms. This system permits the use of stereospecific catalysts of the Ziegler-Natta or alkyl lithium types which make it possible to polymerize monomers into a cis structure characteristic that is very similar to that of natural rubber. This cis structure yields a rubbery product, as opposed to a trans stmcture which produces a rigid product similar to plastics. 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

Stereospecific Polymerization of Propylene with Ziegler-Natta-Catalysts in Organic Suspension... 

Stereospecific Polymerization of Butadiene with Ziegler-Natta-Catalysts Preparation of c/s-1,4-Polybutadiene... 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

In a number of classes of systems, the catalytic and other chemical effects of metal ions on reactions of organic and inorganic molecules are generally recognized the catalysis of nucleophilic reactions such as ester hydrolysis the reactions of alkenes and alkynes in the presence of metal carbonyls (8, 9, 69) stereospecific polymerization in the presence of Ziegler catalysts (20, 55, 56) the activation of such small molecules as H2 (37), 02 (13), H202 (13), and possibly N2 (58) and aromatic substitution reactions of metal-cyclopentadienyl compounds (59, 63). 

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