Propylene, stereoregular polymerization

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

Pino and Miilhaupt considered four diastereomeric jt complexes formed as a result of the four possible modes of complexation of propylene.328 They suggested that diastereomeric and rotameric equilibria between these jt complexes and/or activation energy for the insertion reaction control the large regioselectivity and enan-tioface discrimination necessary for stereoregular polymerization. 

Catalysts based on complexes 150 and 151 have also been tested for ethylene-propylene co-polymerizations but poor activity was reported. The NMR analysis of the co-polymers indicated random insertion of the monomers, with no stereoregularity in the propylene homosequences.1189... 

When the insertion of propylene molecules in the growing chain is such that all methyl branches are on the same hand, the regularity of the chain allows it to crystallize this is the isotactic polypropylene (iPP), synthesized by Natta [8, 9] and the most commercial type obtained by the Ziegler-Natta stereoregular polymerization process. When the monomer insertion is consistently in the opposite hand to previous monomer insertion, the polymer obtained is syndiotactic polypropylene (sPP), which achieves lower crystallinity and today has less commercial interest. The random stereo incorporation of monomer units results in an amorphous resin, atactic polypropylene (aPP). 

For stereoregular insertion there are two modes to consider—cis insertion and trans insertion. For both isotactic and syndiotactic production, the cis mechanism has been determined to be in operation. This was established by polymerizing with cis-, and trans-l-deuteriopropylene or related monomers. The expected stereochemistry was demonstrated when deuteriopropylene was polymerized. The cis monomers produce erythro monomer imits whereas the trans monomer yields the threo units when cis- and tra/is-l-d-propylene is polymerized. In some cases the nomenclature appearing in the literature can be confusing and contradictory, but all indicate cis insertion. To be specific, as defined below, stereochemical structures from cis and trans addition to the double bond of cis-(l-di) and trans-(l-di)-propylene to isotactic polypropylene are as follows (229) ... 

Esteb, J. J. Chien, J. C. W. Rausch, M. D. Novel Ci symmetric zirconocenes containing substituted indenyl moieties for the stereoregular polymerization of propylene. J. Organomet. Chem. 2003, 688, 153-160. 

Coordination or stereoregular polymerization is carried out in the presenee of speeial eatalyst-eoeatalyst systems, called Ziegler-Natta eatalysts. The eatalyst system normally eonsists of halides of transition elements of groups IV to VIII and alkyls or aryls of elements of groups I to IV For example, a mixture of TiCl3 and AIEtj eonstitutes the Ziegler-Natta eatalyst system for the polymerization of propylene. 

Figure 5.10 (a) Setup for stereoregular polymerization of propylene using TiCl3—AlEth3 catalyst in n-heptane. (b) Schematic representation of effect of stirring on polymerization for two speeds of stirring Ni and N2-... 

In the buildup period of decay-type stereoregular polymerization, we find that the rate when propylene is introduced after TiCl3 and AlEt3 are allowed to equilibrate is different from the rate when AlEt3 is added after the gas is introduced. It is assumed that the following equihbrium exists in the former case ... 

Eastman Chemical has utilized a unique, high temperature solution process for propylene polymerization. Polymerization temperatures are maintained above 150°C to prevent precipitation of the isotactic polypropylene product in the hydrocarbon solvent. At these temperatures, the high rate of polymerization decreases rapidly, requiring low residence times (127). Stereoregularity is also adversely affected by high temperatures. Consequentiy, the... 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

Natta A process for polymerizing propylene and other higher olefins, catalyzed by crystalline titanium trichloride and an alkyl aluminum compound such as triethyl aluminum. The polymer can exhibit various types of stereoregularity, depending on the catalyst and the conditions. Invented in 1954 by G. Natta at the Istituto de Chimica Industrial del Politecnico di Milano, Italy, and commercialized in 1957. Now used widely, worldwide. See also Ziegler, Ziegler-Natta. 

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