Melt-Crystallizations of alpha and gamma Forms of Isotactic Polypropylene in Propene-Butene Copolymers

Random isotactic propene-butene copolymers (iPPC4) of different stereoregularity have been synthesized with three different homogeneous single center metallocene catalysts having different stereoselectivity. All samples crystallize from the polymerization solution in mixtures of alpha and gamma forms, and the relative amount of gamma form increases with increasing concentrations of butene and of rr stereodefects. All samples crystallize from the melt in mixtures of alpha and gamma forms and the fraction of gamma form increases with decreasing cooling rate. At high cooling rates, the crystallization of the alpha form is always favored, even for samples that contain high total concentration of defects that should crystallize in the gamma form. The results demonstrate that in iPPs containing significant concentrations of defects, such as stereodefects and comonomeric units, the gamma form is the thermodynamically stable form of iPP and crystallizes in selective conditions of very slow crystallization, whereas the alpha form is the kinetically favored form and crystallizes in conditions of fast crystallization.