Stereospecific catalysts, structure polymerization

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. 

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. 

Unfortunately, no catalyst is found which gives an isotactic polymer in quantitative yield. This fact obstructs the determination of the structure of the real active species for isotactic polymerization and the collection of unequivocal information about the mechanism of stereoregulation. Formation of the highly isotactic polymer which is cleanly separable from the atactic polymer indicates the existence of a highly stereospecific catalyst species in the polymerizing system. In order to answer to these unsolved problems, it will be necessary to do more experiments by utilizing new ideas or by more ingenious experimental techniques. 

Consistent with the discussion on alkali metal alkyls, the least stereospecific catalysts for vinyl polymerizations should be those which are derived from the least electronegative metals having the weakest p or d bonding orbitals. On this basis, one expects increasing stereospecificity for making isotactic or cis-1,4 products in the order Ba < Sr < < Ca Mg Be, with some variations due to monomer structure. 

Staudinger, Hermann (1881-1965). Fundamental research on high-polymer structure, catalytic synthesis, polymerization mechanisms, resulting eventually in development of stereospecific catalysts by Ziegler and Natta (stereoregular polymers). Nobel Prize 1963. 

Synthetic polyisoprene, prepared by free-radical polymerization of isoprene monomer, is a copolymer of six structurally distinct kinds of isoprene chain units. Unlike natural rubber, which is a regularly repeating Class I structure (cis-1,4), such synthetic polyisoprene does not crystallize. On the other hand, by the use of the appropriate stereospecific catalyst, isoprene monomer can be converted to a regular Class I polymer with the same structure as natural rubber (. ... 

Many ot-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many such polymers were found to be stereospecific and crystalline. Polymerizations of a-olefins of the general structure of CH2 = CH — (CH2) — R, where x is 0-3 and R denotes CH3, CH-(CH3)2, C(CH3)3, or CsHs, can be catalyzed by vanadium trichloride/triethyl aluminum [80]. The conversions are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [81]. Addition of Lewis bases, such as ( 4119)20, (C4H9)3N, or ( 4119)3 , to the catalyst system further increases crystallinity [82]. 

Stereospecific n. Of polymerization catalysts, implying a specific or definite order of spatial arrangement of molecules in the polymer resulting from the catalyzed polymerization. This ordered regularity of molecules (tacticity), in contrast to the branched or random structure found in other plastics, permits close packing of the molecular segments and leads to high crystallinity, as in polypropylene. The adjective is sometimes applied imprecisely to polymers to mean tactic. 

Tactic PS. Isotactic (iPS) and syndiotactic (sPS) PSs can be obtained by the polymerization of styrene with stereospecific catalysts of the Ziegler-Natta-type. Aluminum-activated TiCls yields iPS while soluble Ti complexes [eg, ( j -CsHslTiCls] in combination with a partially hydrolyzed alkylaluminum [eg, methylalumoxane] yield sPS. The discovery of the sPS catalyst system was first reported in 1986 (33). As a result of the regular tactic structure, both iPS (phenyl groups cis) and sPS (phenyl groups alternating trans) are highly crystalline. Samples of iPS quenched from the melt are amorphous, but become... 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

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. 

As a result of polymerization of 1,3-butadiene and its derivatives one obtains synthetic rubbers (elastomers) whose properties depend on the structure of the formed products. The stereospecific Ziegler-Natta catalysts offer new opportunities for the synthesis of rubbers with a defined structure. It turned out that by choosing an appropriate catalyst one can be obtain the following polymer structures cis-1.4, trans-1.4, isotactic 1.2, and syndiotactic 1.2, all in a relatively pure form. The influence of the catalyst structure on the stereospecific polymerization of butadiene is shown in Table 8.11. 

The first metallocene catalyst (CP2MCI2.MAO) was completely nonstereospecific for the polymerization of propylene and the polymer was atactic. It was then shown that when the catalyst structure was modified by replacing the halogen atoms with phenyl groups (Cp2MPh2.MAO) the catalyst became stereospecific at a temperature of -30°C. ... 

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