Active centers in propylene polymerization catalysts of the fourth generation

• Isospecific centers in propylene polymerization catalysts are nonuniform.
• Populations of active centers differ in kinetic parameters and in stereospecificity.
• Kinetic differences result in differences in the average polymerization degree.
• Differences in stereospecificity affect the content of meso-pentad and Tm.
• Modifiers RnSi(OCH3)4−n are direct participants of polymerization centers.

Isospecific centers in propylene polymerization catalysts of the 4th generation [combinations of solid catalysts of the TiCl4/aromatic diester/MgCl2 composition and binary cocatalysts containing Al(C2H5)3 and methoxysilanes RnSi(OCH3)4−n] contain several populations of active centers, which differ in kinetic parameters and in the level of stereospecificity control. The differences in the kinetic parameters between the active centers translate into large differences in the average polymerization degree of the polymer material the centers produce, from 500–800 to 8000–17,000. Differences in the stereospecificity level between the active centers result in large differences in melting points of the polymer components. Methoxysilanes RnSi(OCH3)4−n directly affect each type of the polymerization center; both the kinetic parameters of the centers and their level of stereocontrol are influenced by the nature of the alkyl groups R.

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