MAO-free polymerization of propylene byrac- Me2Si(1-C5H2-2-Me-4-tBu)2Zr(NMe2)2 compound

Ansa-zirconocene diamide complex rac-Me 2 Si(CMB) 2 Zr(NMe 2 ) (rac-1, CMB = 1-C 5 H 2 -2-Me-4- t Bu) reacts with AlR 3 (R = Me, Et, i-Bu) and then with [CPh 3 ] + [B(C 6 F 5 ) 4 ] (2) in toluene in order to in situ generate cationic alkylzirconium species. In the sequential NMR-scale reactions of rac-1 with various amount of AlMe 3 and 2, rac-1 transforms first to rac-Me2Si(CMB) 2 Zr(Me)(NMe 2 (rac-3) and rac-Me 2 Si(CMB) 2 ZrMe 2 (rac-4) by the reaction with AlMe 3 , and then to [rac-Me 2 Si-(CMB) 2 ZrMe] + (5 + ) cation by the reaction of the resulting mixtures with 2. The activities of propylene polymerizations by rac-1/Al(i-Bu) 3 /2 system are dependent on the type and concentration of AlR 3 , resulting in the order of activity: rac-1/Al(i-Bu) 3 /2 > rac-1/AlEt 3 /2 > rac-1/MAO » rac-1/AlMe 3 /2 system. The bulkier isobutyl substituents make inactive catalytic species sterically unfavorable and give rise to more separated ion pairs so that the monomers can easily access to the active sites. The dependence of the maximum rate (R p,max ) on polymerization temperature (T p ) obtained by rac-1/Al(i-Bu) 3 /2 system follows Arrhenius relation, and the overall activation energy corresponds to 0.34 kcal/mol. The molecular weight (MW) of the resulting isotactic polypropylene (iPP) is not sensitive to Al(i-Bu) 3 concentration. The analysis of regio-chemical errors of iPP shows that the chain transfer to Al(i-Bu) 3 is a minor chain termination. The 1,3-addition of propylene monomer is the main source of regiochemical sequence and the [mr] sequence is negligible, as a result the meso pentad ([mmmm]) values of iPPs are very high ([mmmm > 94%). These results can explain the fact that rac-1/Al(i-Bu) 3 /2 system keeps high activity over a wide range of [Al(i-Bu) 3 ]/[Zr] ratio between 32 and 3,260.