Propylene polymerization stereospecific

Rausch MD, Chien JCW, Thomas EJ (2000) Substituent effects on the stereospecificity of propylene polymerization by novel asymmetric bridged zirconocenes. A mechanistic discussion. Macromolecules 33 1546-1552... 

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... 

Table IV lists the more important combinations for the preparation of highly active and highly stereospecific catalyst systems. As Weissermel et at. (27) pointed out, catalyst efficiencies of higher than 40,000 g of polymer per gram of titanium and isotacticities greater than 97% for propylene polymerization can be obtained.

For stereospecific propylene polymerization, no enantiomer separation is necessary (229). The enantiomerically pure catalyst has been used in the enantioselective hydrooligomerization of a-olefins (230). 

The above Cp2MQ2 species are immediate precursors for highly active Ziegler-Natta catalysts for die preparation of isotactic polypropylene with methylalumoxane as a cocatalyst die racemic mixtures can be directly employed in stereospecific propylene polymerizations (see Section IV,D,2) (225,227,228) the enantiomers are employed in asymmetric hydrooligomerizations ofa-olefins... 

Titanium-containing catalysts, generally called Ziegler-Natta catalysts, in honour of Karl Ziegler, who discovered them in 1953, and Giulio Natta, who initiated and developed their use for stereospecific propylene polymerization, and... 

TiCl4/MgCl2 catalysts may be used for the polyethylene synthesis, but not for the preparation of isotactic polypropylene. In fact, although these catalysts are highly reactive for the propylene polymerization, they are not stereospecific enough to give a commercially interesting product. 

Other catalysts, highly active in ethylene polymerization, have been obtained by co-milling MgCl2 with Ti compounds other than chlorides37). Even though these catalysts are active for the propylene polymerization, they are stereospecifically poor and have mainly been used to determine kinetic parameters at short polymerization times 38). 

Actually, studies on the propylene polymerization at atmospheric pressure carried out in our laboratories 101 > have demonstrated that R0 and the deactivation rate depend, in a complex manner, on both the organoaluminum and external donor concentrations (see Sect. 6.1.2 and 6.1.3). The kinetic curves obtained cannot be reduced to a single model for the deactivation of active centers according to a simple 1 st and 2nd order law, but rather they seem to follow a more complicated behavior. This is not surprising if one considers that the decay of polymerization rate is probably the effect of an evolution, in time, of a plurality of different catalytic species having different stability, reactivity and stereospecificity (see Sect. 6.3). 

It is known that in propylene polymerization, both with conventional and supported Ziegler-Natta catalysts, at least two types of active centers can be distinguished. Such species can be associated with the so-called isotactic and atactic polymeric fractions, which have different configurations and may be separated by simple extraction with boiling heptane. Based on the 13C NMR analysis of the microstructure of the atactic and isotactic fractions, Inoue 1451 has recently proposed a two site model. At one site the stereospecific polymerization proceeds according to the Bernouillian model, and at the other it proceeds according to the enantiomorphic site model. However, it is understood that a two site model is an oversimplification. As a matter of fact, the crude polypropylene can usually be separated into several fractions having different tacticity 51>. 

Table 4. Representative kp data for propylene polymerization referred to centers of differing stereospecificities... 

D-Limonene and ot-pinene have been used as renewable solvents and chain transfer agents in metallocene-methylaluminoxane (MAO) catalysed polymerization of ot-olefins. Chain transfer from the catalyst to the solvent reduces the achieved in limonene compared with toluene and also reduces the overall catalyst activity. This was confirmed, as in the ROMP studies, by performing identical reactions in hydrogenated limonene. However, an increase in stereospecificity was seen when D-limonene was used as the solvent. This is measured as the mole fraction of [mmmm] pentads seen in NMR spectra of the polymer. 100% isotactic polypropylene would give a value of 1.0. On performing the same propylene polymerization reactions in toluene and then in limonene, the mole fraction of [mmmm] pentads increased from 0.86 to 0.94, indicating that using a chiral solvent influences the outcome of stereospecific polymerizations. Unfortunately, when a-pinene was used, some poly(a-pinene) was found to form and this contaminates the main polymer product. 

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