Stereoblock polypropylene from a metallocene catalyst with a Hapto-flexible naphthyl-indenyl ligand

Stereoblock polypropylenes have been obtained performing the polymerization of propene at different temperatures in the presence of [1-methyl-1-naphthylethyl-2-inden-1-yl]zirconium(IV) trichloride and methylaluminoxane. The stereoblock microstructure probably arises from to fact that the zirconium complex exists in equilibrium among structures having the naphthyl group coordinated to the metal, corresponding to chiral and/or pseudo-achiral forms, and a structure having the naphthyl group not coordinated, corresponding to a semimetallocene form. The chiral form produces regular isotactic sequences, whereas the pseudo-achiral and the semimetallocene forms produce atactic sequences, giving rise to isotactic-atactic stereoblocks. The length of the isotactic block depends on the polymerization temperature. A structural characterization and an analysis of the thermal behavior of these stereoblock polypropylenes have been performed. The samples crystallize from the melt in mixtures of α and γ forms of isotactic polypropylene. In the samples polymerized at low temperatures (17 and 30°C), the amount of γ form is lower than that observed in samples prepared at higher temperatures (50°C). This indicates that the regular isotactic sequences are longer in samples prepared at low temperatures, according with the interconversion mechanism of the catalyst among isospecific and aspecific forms. In these samples the amount of γ form is, in any case, lower than that observed in the literature for samples having higher stereoregularity and a random distribution of stereodefects, confirming the stereoblock microstructure of the samples prepared with the flexible catalyst.