Crystallization Behavior and Properties of Propylene/4-Methyl-1-pentene Copolymers from a Metallocene Catalyst

Copolymers of isotactic polypropylene (iPP) with 4-methyl-1-pentene (iPP4MP) were prepared with a highly isoselective homogeneous organometallic catalyst in a range of 4-methyl-1-pentene (4MP) concentrations between about 1.7 and 14 mol %. Crystallization from the melt at different crystallizations temperatures have been performed to study the effect of 4MP comonomeric units excluded from the crystals on the crystallization of α and γforms. All samples crystallize in mixtures of α and γforms, and for each sample, the fraction of γform increases with increasing crystallization temperature to achieve a maximum value fγ(max), which depends on the 4MP concentration. Compared to the homopolymer, the maximum fractional amount of γform fγ(max) rapidly increases with increasing 4MP content achieving the highest value of 92% at low 4MP concentration of 2.2 mol %, and decreases with a further increase of 4MP concentration. These data are compared with analogous data of the maximum amount of γform that develops in copolymers of iPP with ethylene and butene. This allows comparing the different effects of rejection of defects from the crystals, which produces interruption of the regular propene sequences and shortening of the length of the crystallizable sequences, the inclusion of defects into crystals of α and γforms, and the effect of the crystallization kinetics. Since 4MP comonomeric units are excluded from the crystals, the behavior of iPP4MP copolymers provides the sole interruption effect, which is highly efficient and produces the highest amount of γform of 92% at low 4MP concentration of nearly 2 mol %. The observed decrease of fγ(max) at higher 4MP concentrations is due to the too slow crystallization rate of the γform at these 4MP contents that induces the crystallization of the kinetically favored α form. In fact, crystals of γforms that develop in these copolymers are highly defective and show melting temperatures lower than those of the α form and, therefore, experience low undercooling at high crystallization temperatures. These results demonstrate that in metallocene iPP copolymers containing a significant amount of constitutional defects, the crystallization of the γform is favored because of the short regular propene sequences, whereas the crystallization of the α form is always kinetically favored