Influence of Chain Microstructure on the Crystallization Kinetics of Metallocene-Made Isotactic Polypropylene Claudio De Rosa,* ,† Finizia Auriemma, † and Luigi Resconi ‡ Dipartimento di Chimica, Universita ` di Napoli “Federico II”, Complesso Monte S.Angelo, Via Cintia, 80126 Napoli, Italy, and Basell Polyolefins, Centro Ricerche G. Natta, P.le G. Donegani 12, I-44100 Ferrara, Italy Received May 27, 2005; Revised Manuscript Received September 19, 2005 ABSTRACT: Highly regioregular isotactic polypropylene samples containing only rr defects of stereo- regularity in a wide range of concentration have been obtained with a series of regiospecific metallocene catalysts. A study of the relationships between kinetics of melt-crystallization and the microstructure of polypropylene chains is reported. The equilibrium melting temperatures for isotactic polypropylenes of different stereoregularity have been estimated. The kinetic study has shown that the crystallization rate increases with increasing the concentration of rr defects and decreasing the molecular weight. During isothermal crystallizations from the melt, crystals of R form develop at the beginning of the crystallization, whereas the formation of crystals of γ form is observed only at longer crystallization times. According to the literature, the crystals of γ form are nucleated over the preformed crystals of R form. Introduction With the development of metallocene catalytic sys- tems for the isospecific polymerization of olefins, new isotactic polypropylenes, showing a wide variety of microstructures, have become available. 1 Depending on the specific metallocene catalyst, as well as on the polymerization conditions (i.e., monomer concentration, temperature, nature of cocatalyst, etc.), isotactic polypro- pylene (i-PP) samples containing different amounts and combinations of defects of stereoregularity (primary insertions with the wrong enantioface, or stereodefects) and regioregularity (meso and racemo secondary inser- tions, or regiodefects) can be obtained. 1 It has been shown that the microstructure strongly influences the crystallization properties of i-PP and its polymorhic behavior. 2-17 In particular, the crystallization of R and γ forms depends on the stereoregularity and, in general, on the concentration of stereo- and regio- defects 2-4,7,9-12,15-17 and on the distribution of defects along the polymer chains. 10,12-14 We have recently studied the crystallization behavior and the physical properties of metallocene-made i-PP samples containing only one kind of stereo-irregularity (isolated rr triads), in a wide range of concentration. 16,17 This study has allowed clarifying the single effect of the presence of rr defects on the polymorphic behavior and mechanical properties of i-PP. 16,17 This stereodefect induces crystallization of γ form and of disordered modifications intermediate between R and γ forms. A linear relationship between the amount of γ form that crystallizes from the melt and the average length of isotactic sequences has been found. 16 Depending on the concentration of rr defects, polypropylenes with melting temperatures variable from 160 to 80 °C and mechanical properties intermediate between those of stiff-plastic and elastomeric materials have been produced. 16,17 All these studies have indicated that the crystalliza- tion of γ form seems to be thermodynamically favored in samples containing an appreciable amount of stereo- defects. As a consequence, a high amount of γ form develops in the slow crystallizations at high tempera- tures. However, it has also been demonstrated that even for samples with high concentration of defects, high concentration of the R form may be obtained when the crystallization is very fast, for instance by quenching the melt to room temperature, 11 indicating that for these samples the R form is kinetically favored. Therefore, the crystallization of R and γ forms of i-PP in metallocene- made samples containing a random distribution of defects is a result of two competing kinetic and ther- modynamic effects. 3,4,11 In this paper the kinetics of crystallization from the melt of i-PP samples containing only rr stereodefects and free from regiodefects are analyzed. The effect of the presence of rr defects on the overall crystallization rate is studied, independent of the number and the amount of crystalline forms that develop during iso- thermal crystallizations. Experimental Section i-PP samples have been prepared with the highly regiospe- cific C2- and C1-symmetric zirconocene catalysts shown in Chart 1, 18-22 activated with methylalumoxane (MAO). The C2- symmetric complexes 1, rac-H2C(3-tert-butylindenyl)2ZrCl2, and 2, rac-isopropylidene[bis(3-trimethylsilylindenyl)]ZrCl2, have been prepared as described in refs 19 and 18, respec- tively. The C 1-symmetric ansa-zirconocenes 3-5 are based on the substituted indenyl-dimethylsilyl[bis(2-methylthieno)cy- clopentadienyl] ligand framework and have been prepared as described in refs 20-22. All the analyzed i-PP samples are listed in Table 1. The samples show similar molecular weight and different concen- trations of rr defects. The more isotactic samples iPP1-iPP3 have been prepared with the C 2-symmetric catalyst 1, 18,19 whereas samples iPP4-iPP7 have been prepared with the C1- symmetric ansa-zirconocenes 3-5, 20-22 which are highly active in propylene polymerization and produce highly regioregular, high molecular weight i-PPs characterized by different ste- reoregularity, depending on the indenyl substituents. 16,17,22 Sample R3 has been prepared with catalyst 2/MAO at 60 °C † Universita ` di Napoli “Federico II”. ‡ Basell Polyolefins. * To whom correspondence should be addressed: Tel ++39 081 674346; Fax ++39 081 674090; e-mail claudio.derosa@unina.it, derosa@chemistry.unina.it. 10080 Macromolecules 2005, 38, 10080-10088 10.1021/ma0510845 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/27/2005