Comparison Between Polymorphic Behaviors Of Ziegler−Natta And Metallocene-Made Isotactic Polypropylene: The Role Of The Distribution Of Defects In The Polymer Chains

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...