The Active Species in Homogeneous Ziegler‐Natta Catalysts for the Polymerization of Ethylene

Abstract

I CpzTiCI2 and CpzTiEtCl (Cp = cyclopentadienyl, Et = ethyl), in combination with aluminum alkyls, have been studied as model catalyst systems for the polymerization of ethylene in toluene. The activity of these systems resides in the presence of a complex molecule involving an alkylated Ti(w) and an alkylated A1 species, linked together by chlorine bridge bonds. The environment of the Ti is of octahedral symmetry with one vacant site available for the coordination of ethytene. The bridged Al-alkyf causes a destabilization of the Ti-alkyl bond ("trans efect"), thus rendering it active for polymerization. The reduction Ti(w) + T i f m ) , inevitable in these systems, involves the irreversible change from octahedral to tetrahedral symmetry with loss of the catalytic activity. Polymerizafion and reduction are supposed to occur at the same catalyst site. Magnetic methods (susceptibility and EPR) as well as the kinetics of polymerization were used in the investigation of these catalyst systems.

Kinetic and mechanistic aspects of metallocene polymerisation catalysts

“…While BreslowÕs mechanistic suggestions (Scheme 1) involved a zwitterionic species and adhered to a tetrahedral geometry for the titanium centre [6,7], alternative suggestions were made, such as octahedral intermediates suggested by Henrice-Olivé and Olivé [10]. An early proposal by Zefirova and Shilov [11] that the active species was likely to be cationic (Scheme 2) received little attention until the advent of well-defined polymerisation catalysts based on metallocenium salts in the mid-1980s [12][13][14][15].…”

Another example of carborane based trianionic ligand: Syntheses and catalytic activities of cyclohexylamino tailed ortho-carboranyl zirconium and titanium dicarbollides

“…Moreover, the carborane ligands are thermally stable and robust. The neutral icosahedral carborane cage can be heated to 600°C without decomposition, and the C 2 B 9 -cage ions can be recovered unchanged after several hours of heating in solution with aqueous acids and bases [11][12][13]. As these compounds are isoelectronic with their respective Cp analogues, one would expect similar, but polar group tolerable, catalytic behaviors in the polymerization of a-olefins.…”

Catalytic Olefin Polymerization

“…Pino et al [372] propose a bimetallic model where the two metal atoms may also be of the same nature and where one of the metal atoms forms a 1t bond with the olefin whereas the other is bound to the growing chain (cf, scheme g, Figure 24). Henrici-Olive [405] proposes a model of the catalytic centre containing two halogen bridges between two metal atoms (see scheme e, Figure 24). Ballard [21] hypothesizes the presence of a chlorine atom bridge between two metals atoms (see scheme d, Figure 24).…”