Search

The propene/ethene-[1-C-13] Copolymerization method has been applied to measure precisely the regioselectivity in propene polymerization of the "simple" MgCl2/TiCl4 catalyst (activated with Al((i)-Bu)(3)) which can be viewed as the ancestor of all present-day MgCl2-supported industrial catalysts containing Lewis bases as selective modifiers. It has been found that the average content of 2,1-regiomistakes in the polypropylene produced is fairly high (ca. 0.7 mol %), although-not unexpectedly, due to the multisite catalyst nature-these are distributed rather nonuniformly throughout the polymer. The difficulty in detecting the 2,1-units, despite their relatively high concentration, in the C-13 NMR spectra of homopolymer samples at natural C-13 abundance has been traced to their occurrence in a variety of stereochemical environments, which leads to splitting and broadening of the corresponding resonances. The fraction of "dormant" polypropylene chains with a 2,1-last-inserted unit was estimated to be in the 20-50% range. QM calculations of regioselectivity on classical Corradini-type models of epitactic octahedral catalytic Ti species turned out to be in basic agreement with the experiment.

Fundamental implications of occasional secondary (2,1-) insertions in propene polymerization with different MgCl2-supported Ziegler-Natta catalysts are discussed, with reference to the properties of the polymers obtained. The relatively narrow molecular weight distribution of polypropylene prepared using catalysts of type MgCl2/TiCl4/diether-AlR3 is ascribed to the fact that the active species in this system are relatively uniform, in the sense that significant 2,1-insertion takes place at both highly isospecific and weakly isospecific active species. In contrast, the isospecific species in the catalyst system MgCl2/TiCl4/diisobutyl phthalate-AlEt3-alkoxysilane undergo less 2,1-insertion and are therefore less responsive to chain transfer with hydrogen. The presence of (some) highly regiospecific active sites in such catalysts will therefore lead to the formation of a high molecular weight polymer fraction and, overall, a broad molecular weight distribution. The presence of high molecular weight chains leads to relatively slow molecular relaxation and a more rapid onset of crystallization of the polymer from the melt, as evidenced by rheological studies of polypropylenes prepared using different catalysts and having different molecular weight distributions.

The propene/ethene-[1-13C] copolymerization method has been applied to measure precisely the regioselectivity of two modern MgCl2-supported Ziegler-Natta catalyst systems for the industrial production of isotactic polypropylene, namely MgCl2/di(isobutyl)-o-phthalate/TiCl4-cyclohexylmethyldimethoxysilane/AlR3 (1) and MgCl2/2,2-di(isobutyl)-1,3-dimethoxypropane/TiCl4-AlR3 (2). System 2 turned out to be slightly less regioselective than system 1 (on average, 0.26% 2,1 misinsertions instead of 0.18%), but the distribution of the regiodefects throughout the polymer was found to be more uniform, most likely as a result of a lower differentiation of the active species. This provides a simple explanation for the different response of the two catalysts to molecular hydrogen as a chain transfer agent. In system 1, indeed, part of the active species are almost completely regioselective, which means that H2 finds very few preferential "cutting points" in the form of growing chains with a sterically hindered 2,1 last-inserted unit; therefore, a higher H2 partial pressure is neededcompared with system 2in order to achieve the same decrease of average polymer molecular mass.

Studies on the effects of hydrogen, monomer concentration and temperature in propene polymerization using high-activity MgCl2-supported catalysts have revealed that the effects of the catalyst on polymer yield, isotacticity and molecular weight are interrelated and dependent on both regio- and stereoselectivity. The high hydrogen sensitivity of catalysts containing a diether donor is mainly due to chain transfer following the occasional regioirregular (2,1-) monomer insertion. The probability of regio- and stereoirregular insertion decreases with monomer concentration, indicating the presence of nonsymmetric active centres. The stereoregularity of the isotactic polymer also increases with polymerization temperature, indicating a greater relative increase in productivity for highly isospecific as opposed to moderately isospecific centres.