| Abstract
| - A comparative analysis of the polymorphic behavior of samples of isotactic polypropylene(iPP) prepared with heterogeneous Ziegler−Natta catalysts and with a single-center homogeneousmetallocene catalyst is presented. Different samples of Ziegler−Natta iPP, prepared with MgCl2-supportedcatalysts modified by adding different Lewis bases, have been fractionated by extraction with boilingsolvents. The irregular fraction, insoluble in diethyl ether and soluble in hexane, crystallizes from themelt almost totally in the γ form. The more stereoregular fractions crystallize instead basically in the αform. This confirms that, even in the case of Ziegler−Natta iPP samples, the γ form may develop bymelt-crystallization at atmospheric pressure in fractions containing a high concentration of defects. Therelative amount of γ form crystallized from the melt is, however, much lower that that observed in samplesof metallocene-made iPP containing comparable amount of defects. Since the γ form crystallizes in chainshaving short regular isotactic sequences, these data indicate that in Ziegler−Natta iPP samples the regularisotactic sequences are longer than those present in chains of metallocene-made iPP having a similaroverall concentration of defects. The different polymorphic behavior of metallocene and Ziegler−NattaiPP samples is related to the different distribution of defects in the polymeric chains, generated by thedifferent kinds of catalytic systems. While in the metallocene-made iPP the distribution of defects alongthe chains is random, in Ziegler−Natta iPP samples the majority of the defects are segregated in a smallfraction of poorly crystallizable macromolecules or in more irregular portions of the chain, so that muchlonger fully isotactic sequences can be produced, leading to the crystallization of the α form, even for arelatively high overall concentration of defects. These results confirm the idea that the structural analysisof iPP, in particular the crystallization of the γ form, may give information about the microstructure ofthe polymer chains. The measure of the maximum amount of γ form crystallized from the melt may beused as an indirect method to evaluate the average length of isotactic sequences. This analysis allowsconcluding that some fractions of Ziegler−Natta iPP are characterized by chains with a stereoblockmicrostructure, consisting of regular isotactic sequences linked to more irregular sequences. The lattercontain the major part of stereodefects mainly consisting in isolated rr triads, r diads, and longer ...rrrr...syndiotatic sequences. The hypothesis of a stereoblock microstructure for some of these less stereoirregularfractions is also consistent with the high degree of crystallinity observed in the samples crystallized fromsolution or from the melt, despite the high concentration of defects.
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