Crystallization of the α and γ Forms of Isotactic Polypropylene as a Tool To Test the Degree of Segregation of Defects in the Polymer Chains

The polymorphic behavior of samples of isotactic polypropylene prepared with unbridged oscillating metallocene catalysts is analyzed and compared with the behavior of samples prepared with a chiral isospecific metallocene catalyst having similar amounts of defects of stereoregularity. In the samples prepared with the unbridged catalysts, the amount of γ form, which develops upon melt-crystallization procedures, turns out to be always lower than that in the samples prepared with the chiral isospecific catalyst. This suggests that, in the samples obtained with the unbridged catalysts, the distribution of defects of stereoregularity is not random, the defects being segregated in more stereoirregular regions of the chains, and confirms that these samples are characterized by a stereoblock structure, with long isotactic sequences alternating to atactic or more stereoirregular sequences. An empirical method to establish the degree of segregation of defects along polypropylene chains is suggested.