Effects of Regiochemical and Stereochemical Errors on the Course of Isotactic Propene Polyinsertion Promoted by Homogeneous Ziegler-Natta Catalysts

Abstract

The effects of regiochemical and stereochemical errors on the kinetic course of isotactic propene polyinsertion promoted by two typical homogeneous metallocene catalysts are analyzed in detail. It is shown, in particular, that occasional regioirregular 2,1 insertions not only slow down chain propagation to a substantial extent-as already reported in a preliminary communication-but also practically inhibit chain transfer to the monomer and to the aluminum alkyl cocatalyst. Active centers bearing a growing chain with a 2,1 last-inserted propene unit are therefore trapped in a "dormant" state, in which the only feasible alternative to the formation of a sterically demanding head-to-head enchainment, at least at sufficiently high reaction temperatures, is 2,1 to 1,3 isomerization. Experimental data suggesting that chain propagation is hindered after a stereoirregular monomer insertion are also discussed.

Addition of Lewis bases and acids. Effect on α-olefins polymerization with soluble metallocenes, 2 Propylene

“…[18,20] The low selectivity of this catalysts would be due to a low ratio k ins (insertion constant)/k epim (epimerization constant). Busico et al found a value of 70 l/mol for EtInd 2 ZrCl 2 / MAO and 8 l/mol for Et(IndH 4 ) 2 ZrCl 2 .…”

Ethylene polymerization reactions with Ziegler-Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

“…It is formed after the regioirregular 2,1-insertion of a propylene molecule into a Zr-C bond in metallocene-catalyzed propylene polymerization reactions. 36 This center also has the methyl group in the ␤-position to the metal atom, Zr-CH(CH 3 )-CH 2 -polymer, and can involve coordination between its H atom and the vacancy at the Zr atom.…”

Development of a refined poly(propylene) growth model for silica supported metallocene catalyst systems

“…Comparison of the activitiy curves between an 1-octene and a propylene prepolymerized catalyst system 1 and the untreated system lower molecular weights of the produced polymers due to chain transfer reactions. Additionally, the activity is enhanced dramatically because of the multiplication of the total number of active centers [8][9][10] . This activation effect was used to get a more comprehensive insight into the early polymerization stages.…”