Chain transfer reaction by trialkylaluminum (AIR₃) in the stereospecific polymerization of propylene with metallocene - AIR₃/Ph₃CB(C₆F₅)₄

Stereospecific polymerization of propylene was carried out with rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)₂ZrCl₂) (1), rac-dimethylsilylenebis(indenyl)zirconiurn dichloride (rac-Me₂Si(Ind)₂ZrCl₂) (2) and isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (i-Pr(Cp)(Flu)ZrCl₂) (3) combined with trialkyl-aluminum (AIR₃: R = C₂H₅, i-C₄H₉)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph₃CB(C₆F₅)₄) (4). In isospecific polymerization with 1 and 2, the molecular weight of polypropylenes decreased with increase in the molar ratio of AlEt₃ (Et = C₂H₅)/Zr, whereas, an effect of AliBu₃ (iBu = i-C₄H₉) concentration on molecular weight was not observed. The microstructures of resulting polypropylenes were studied by ¹³C n.m.r. and an increase in the molar ratio of ethyl end groups (derived from chain transfer to AlEt₃) to n-propyl end groups (derived from β-hydrogen transfer) was observed with increase in the molar ratio of AlEt₃/Zr (1 and 2). The chain transfer reactions by both AlEt₃ and AliBu₃ were also detected in syndiospecific polymerization with 3. The molar ratio of alkyl (R) end groups (derived from chain transfer to AIR₃) to n-propyl end groups was higher in the polypropylene obtained with AlEt₃ than that obtained with AliBu₃. The relative constants k_trA/k_p (k_trA = rate constant of chain transfer to AIR₃, k_p = rate constant of propagation) were determined by kinetic study.