John A. Ewen

Symmetry rules and reaction mechanisms of Ziegler–Natta catalysts

Abstract Symmetry rules obeyed by metallocenes having well-defined, rigid structures are in accord with reaction mechanisms proposed in the 1960s and 1970s for Ziegler–Natta catalysts with unknown structures.

Catalyst system using aluminum alkyl with ion-pair metallocene catalysts

This invention is for a catalyst system for polymerization of olefins using an ionic metallocene catalyst with aluminum alkyl. The metallocene catalyst is an ion pair formed from a neutral metallocene compound and an ionizing compound. The invention can be used in any method of producing ionic metallocene catalysts. Use of aluminum alkyl with an ionic metallocene catalyst eliminates the need for using methylaluminoxane (MAO). Catalysts produced by the method of this invention have high activity. The invention reduces catalyst poisons which cause low activity, no activity or uncontrolled polymerizations. Polymerization using this catalyst system are reproducible and controllable.

Group 4 Cs Symmetric Catalysts and 1-Olefin Polymerization

Abstract Polymerization of propene and 1-butene promoted by group 4 Cs symmetric metallocene precursors has been tested in different reaction conditions. The stereochemical structure of the polymer is affected by the radius of the transition metal, the substituent of the olefin and in some cases the reaction temperature and the monomer concentration. The results are tentatively rationalized considering the possible catalytic cycles that may produce either syndiotactic or isotactic polymers. A sample of syndiotactic polypropylene melting at 170°C is also reported.

Expanding the Scope of Metallocene Catalysis: Beyond Indenyl and Fluorenyl Derivatives

Group 4 metallocenes with bridged and substituted Ind and Cp/Flu ligands have produced high-molecular-weight isotactic (I-PP), syndiotactic (S-PP), atactic (A-PP) and hemi-isotactic (HIT-PP) (Scheme 1) polypropylenes.i

Isospecific Pseudo-Helical Zirconocenium Catalysts

High molecular weight, isotactic polypropylene is obtained with MAO and Me2Si[l-Cp-3-t-Bu-l’-Flu]ZrCl2 (1-MAO) and with Me2C[1-Cp-3-t-Bu-1 ‘-Flu]ZrCl2 (2-MAO) catalysts (MAO = methylaluminoxane; Flu = fluorenyl; Cp = cyclopentadienyl.) Atactic polymers are not produced since meso isomers of the pseudo-helicenes are impossible. The… mmmrrmmm… polymer microstructure is that for the enantiomorphic site-control stereoregulation model. 1 is more stereospecific than 2. The relative stereospecificities of 1 and 2 conform to the trend with other pairs of isotactic specific analogs that are Me2C and Me2Si bridged.