Kinetics of propylene polymerization initiated by rac-Me2Si(1-C5H2-2-Me-4-tBu)2Zr(NME2)2/MAO catalyst

✍ Scribed by Il Kim; Jia-Min Zhou; Mi-Sook Won

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 264 KB
• Volume: 37
• Category: Article
• ISSN: 0887-624X
• DOI: 10.1002/(sici)1099-0518(19990315)37:6<737::aid-pola9>3.0.co;2-#

✦ Synopsis

The kinetics of propylene polymerization initiated by ansa-metallocene diamide compound rac-Me 2 Si(CMB) 2 Zr(NMe 2 ) 2 (rac-1, CMB ϭ 1-C 5 H 2 -2-Me-4-t Bu)/ methylaluminoxane (MAO) catalyst were investigated. The formation of cationic active species has been studied by the sequential NMR-scale reactions of rac-1 with MAO. The rac-1 is first transformed to rac-Me 2 Si(CMB) 2 ZrMe 2 (rac-2) through the alkylation mainly by free AlMe 3 contained in MAO. The methylzirconium cations are then formed by the reaction of rac-2 and MAO. Small amount of MAO ([Al]/[Zr] ϭ 40) is enough to completely activate rac-1 to afford methylzirconium cations that can polymerize propylene. In the lab-scale polymerizations carried out at 30°C in toluene, the rate of polymerization (R p ) shows maximum at [Al]/[Zr] ϭ 6,250. The R p increases as the polymerization temperature (T p ) increases in the range of T p between 10 and 70°C and as the catalyst concentration increases in the range between 21.9 and 109.6 M. The activation energies evaluated by simple kinetic scheme are 4.7 kcal/mol during the acceleration period of polymerization and 12.2 kcal/mol for an overall reaction. The introduction of additional free AlMe 3 before activating rac-1 with MAO during polymerization deeply influences the polymerization behavior. The iPPs obtained at various conditions are characterized by high melting point (approximately 155°C), high stereoregularity (almost 100% [mmmm] pentad), low molecular weight (MW), and narrow molecular weight distribution (below 2.0). The fractionation results by various solvents show that iPPs produced at T p below 30°C are compositionally homogeneous, but those obtained at T p above 40°C are separated into many fractions.