Kinetics of propylene polymerization by hafnocene diamide catalyst

Abstract

The kinetics of propylene polymerization initiated by ansa-metallocene diamide compound rac-(EBI)Hf(NMe 2 ) 2 (EBI = C 2 H 4 -(indenyl) 2 , rac-1) were investigated. The rac-1 compound could be directly utilized for catalyst formations without converting to a dihalide or dialkyl complex in the presence or absence of methylaluminoxane (MAO). The MAO-free system rac-1/AlR 3 /[Ph 3 C][B(C 6 F 5 ) 4 ] (2) is much more effective than the rac-1/MAO catalyst. The activity of the rac-1/Al(iBu) 3 /2 system is much higher than that of the rac-(EBI)HfCl 2 /MAO or rac-(EBI)ZrCl 2 /MAO catalyst, and almost same as that of the rac-(EBI)Zr(NMe 2 ) 2 /Al(iBu) 3 /2 catalyst under similar conditions. The alkylation of rac-1 to rac-(EBI)HfR 2 by using AlR 3 needs more time than the corresponding zirconocene analogue. The activity increases by a factor of 7 by increasing the aging time from 1 min to more than 4 h. The activity of the rac-1/AlR 3 /2 catalyst is very sensitive to the type and concentration of AlR 3 , and decreases in the order: Al(iBu) 2 H > Al(iBu) 3 > AlEt 3 . > AlMe 3 . The catalyst keeps high activity in a narrow range of the [Al]/[Hf] ratio. In addition, the activity is influenced by the concentration of 2, and as a result, the maximum activity is observed when 2/rac-1 = 0.7. The activity of the rac-1/AlR 3 /2 catalyst is also sensitive to the polymerization temperature. The activation energies for the initiation and overall reactions are calculated as 7.61 and 7.14 kcal/mol, respectively. The properties of polymer such as isotacticity (as [mmmm]), molecular weight (MW), molecular weight distribution (MWD), melting temperature (T m ), and crystallinity are similar level with those obtained with the rac-(EBI)HfCl 2 /MAO system. The MW and isotacticity of the polymer produced by MAO-free system decreases monotonically as T p increases, and MWD becomes narrow from 2.90 to 2.10 when T p increases from 30 to 90°C because of the compositional homogeneity of the polymer produced at high T p , which is demonstrated by fractionation of the polymer. Both MW and [mmmm] values of polymers decrease as aging time and anion concentration increase.