Comparison between Polymorphic Behaviors of Ziegler−Natta and Metallocene-Made Isotactic Polypropylene:  The Role of the Distribution of Defects in the Polymer Chains

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 homogeneous metallocene catalyst is presented. Different samples of Ziegler−Natta iPP, prepared with MgCl2-supported catalysts modified by adding different Lewis bases, have been fractionated by extraction with boiling solvents. The irregular fraction, insoluble in diethyl ether and soluble in hexane, crystallizes from the melt almost totally in the γ form. The more stereoregular fractions crystallize instead basically in the α form. This confirms that, even in the case of Ziegler−Natta iPP samples, the γ form may develop by melt-crystallization at atmospheric pressure in fractions containing a high concentration of defects. The relative amount of γ form crystallized from the melt is, however, much lower that that observed in samples of metallocene-made iPP containing comparable amount of defects. Since the γ form crystallizes in chains having short regular isotactic sequences, these data indicate that in Ziegler−Natta iPP samples the regular isotactic sequences are longer than those present in chains of metallocene-made iPP having a similar overall concentration of defects. The different polymorphic behavior of metallocene and Ziegler−Natta iPP samples is related to the different distribution of defects in the polymeric chains, generated by the different kinds of catalytic systems. While in the metallocene-made iPP the distribution of defects along the chains is random, in Ziegler−Natta iPP samples the majority of the defects are segregated in a small fraction of poorly crystallizable macromolecules or in more irregular portions of the chain, so that much longer fully isotactic sequences can be produced, leading to the crystallization of the α form, even for a relatively high overall concentration of defects. These results confirm the idea that the structural analysis of iPP, in particular the crystallization of the γ form, may give information about the microstructure of the polymer chains. The measure of the maximum amount of γ form crystallized from the melt may be used as an indirect method to evaluate the average length of isotactic sequences. This analysis allows concluding that some fractions of Ziegler−Natta iPP are characterized by chains with a stereoblock microstructure, consisting of regular isotactic sequences linked to more irregular sequences. The latter contain 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 stereoirregular fractions is also consistent with the high degree of crystallinity observed in the samples crystallized from solution or from the melt, despite the high concentration of defects.

Thermal Behavior, Structure and Morphology of Propene/Higher 1-Olefin Copolymers

“…These results are well in line with the outcome of the extensive investigations reported in the literature [29][30][31][32][33][34][35][36][37][38][39][40][41][42]49,50] on different polypropenes containing random regio, stereo and constitutional defects. The fractional content of gpolymorph strongly depends on the total concentration of defects and on crystallization temperature.…”

“…[21][22][23][24] The availability of structurally well defined isotactic polypropenes has renewed the interest on the formation of the g-polymorph, which displays unique crystallographic structure [25,26] and is typical of polypropenes with short isotactic sequences. [11,27,28] The recent extensive investigations performed on propene homopolymers with various contents of regio and stereo defects and on random copolymers with 1-olefins by Alamo et al [29][30][31][32][33][34] and De Rosa et al [35][36][37][38][39][40][41][42] have shown that random constitutional defects essentially behave as any other type of irregular unit in the development of the g-polymorph. The amount of g-form was found to be strictly correlated with the length of isotactic sequences which, for randomly placement of mistakes, is inversely related to their total content.…”

Aging Effects on the Phase Composition and Chain Mobility of Isotactic Poly(propylene)

“…The appearance of the different crystalline forms depends to a large extend on the chain microstructure, [20,21] processing conditions [22,23] and presence of specific nucleating agents. [24,25] However, under C. Hedesiu et al conditions as usually applied during industrial processing the a-form is always predominant.…”

Isoselectivity Distribution of Isospecific Centers in Supported Titanium‐Based Ziegler‐Natta Catalysts

“…[12][13][14][15] The results of these analyses are mainly dependent on stereoregularity of poly(propylene) and are almost independent of molecular weight. [16,17] This technique reveals even minor differences in stereoregularities of polymers prepared with similar catalysts.…”