V. I. Tsvetkova

A New Catalytic System (2-PhInd)2ZrMe2/Al(iso-Bu)3 for the Synthesis of Stereoblock Polypropylene

A new efficient two-component homogeneous system consisting of (2-PhInd)2ZrMe2 and Al(iso-Bu)3 cocatalyst was developed for the synthesis of elastomeric stereoblock polypropylene. The catalytic behavior of this system was studied in toluene and liquid propylene. It was shown that a change in the Al/Zr molar ratio enables one to vary the system activity, the kinetic characteristics of the polymerization process, and the properties of polypropylene over wide ranges. For example, an increase in the Al/Zr ratio from 50 to 300 mol/mol is accompanied by an increase in the activity by a factor of 4, an increase in the molecular weight from 80 000 to 220 000, and a decrease in isotactic pentad content from 39 to 16%. It was found that the polymerization rate is proportional to the concentration of Al(iso-Bu)3 and to the square of the monomer concentration. Some assumptions about the nature of active centers and about reactions leading to the formation of these centers were made. A model for the formation of sequences of isotactic and atactic blocks was proposed. This model takes into account the multiple inversion of the active center (fast dynamic equilibrium) from the iso-specific to the nonspecific state during chain growth. This inversion is due to the coordination of an additional Al(iso-Bu)3 molecule. This model satisfactorily describes the pentad composition of polypropylene samples as a function of the Al/Zr ratio.

Synthesis of propylene-ethylene copolymers in liquid propylene using ansa-metallocenes of the C 1 symmetry

The copolymerization of ethylene with propylene in the liquid propylene initiated by ansa-metallocenes of the C 1 symmetry, rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]zirconium dichloride and rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]hafnium dichloride, activated by methylaluminoxane has been studied. Triisobutylaluminum has been used as a cocatalyst. The propylene-ethylene copolymers thus prepared contain 5–60 mol % ethylene units. The reactivity ratios have been measured. In the case of the zirconocene-based catalyst, the molecular mass of the copolymers decreases with an increase in the content of ethylene units. The reverse situation is observed in the case of the hafnocene-based catalytic system. The copolymers are characterized by the low T g values (down to −45°C). Incorporation of a small amount of ethylene units (5 mol %) results in a rise in the elastomeric behavior of the polymers.

Bulk propylene polymerization in the presence of ansa-metallocenes of C2 and C1 symmetries: From a rigid thermoplastic to elastomeric stereoblock polypropylene

Bulk propylene polymerization in the presence of ansa-metallocenes with C2 and C1 symmetries has been studied. The catalytic activity, polymerization kinetics, and the molecular weight of polypropylene (PP) depend strongly on catalyst formation conditions. Mixtures of rac and meso isomers of metallocenes make it possible to rapidly produce a high-molecular-weight isotactic PP with high stereoregularity and mechanical characteristics and thus skip the stage of the isolation of pure rac isomer in the catalyst synthesis. The ability of triisobutylaluminum to serve as a cocatalyst is studied for ansa-metallocenes of C1 symmetry. In this case, the molecular weight of PP is higher, indicating that organoaluminum compounds participate in chain termination reactions. An increase in the reaction temperature results in an increase in the stereoregularity and crystallinity of PP. Polypropylene synthesized using ansa-metallocenes of C1 symmetry has good elastomeric properties.

Isospecific Bulk Polymerization of Propylene with the Use of ansa-Metallocenes as a Mixture of rac and meso Isomers

The effects of various pathways of the formation of catalytic systems based on the rac-Me2SiInd2ZrCl2 metallocene on the activity and properties of polypropylene prepared by the bulk polymerization of propylene were studied in detail. It was found that the conditions of formation of the catalytic system affect not only its activity and the character of kinetic curves but also the molecular weight of the synthesized polymer. Propylene polymerization was studied with the use of a number of bisindenyl derivatives of zirconium, which were mixtures of rac and meso forms that differ in the nature of substituents in the Si bridge or the indenyl ligand. The conclusion was drawn that, with the use of metallocenes as a mixture of rac and meso forms, high-molecular-weight isotactic polypropylene can be prepared with high stereoregularity at a very high rate; thereby, the stage of separation of a pure rac isomer can be excluded in the synthesis of the catalyst.

The Particularities of Isospecific Polypropylene Synthesis in Bulk with Ansa-metallocene Catalysts

hiral ansa-metallocenes with indenyl ligands are useful as catalysts for producig isotactic polypropylene (iso-PP). In this work the kinetics of propylene polymerization in bulk with ansa-zirconocenes C2 symmetry at 30-80’C with using of different cocatalysts, mainly the polymethylaluminoxane, have been investigated. The following aspects were in focus of our interest: the nature of the bridge, the presence of substituents in the bridge and on the indenyl ligands; conditions of the pre-activation stage and the polymerization process and the study of relations between these parameters and PP properties. Using the Me2Si(4-Ph-2-Et- Indenyl)2ZrCl2 (rac : meso = 1 : 2)/MAO the high molecular iso-PP was produced. The activity during iso-PP synthesis reaches 900 kgPP/gcat.h. Influence of hydrogen on activity and molecular weight characteristics also were studied. We compared properties of metallocene iso-PP with the same of PP produced with heterogeneous catalytic Systems on the base of high effective Ti/Mg catalysts.