Preparation and characterization of model catalysts: from ultrahigh vacuum to in situ conditions at the atomic dimension

MgCl2 supported Ziegler-Natta catalysts for the polymerization of olefins have had recently a spectacular success in simplyfling polymerization processes and improving polymer quality; a rich literature is available reporting various data and interpretations of the mechanisms involved in this type of catalysis.The authors’ aim was to review thoroughly and critically the scientific and patent literature. The characteristics of the components and their interaction are particularly borne in mind in view of the catalysts’ activity and selectivity. Other items concern structure and concentration of the active centers, polymerization kinetics and the role of the catalyst in determining polymer morphology.

The catalytic system Ti-complex/MgCl2

Toughening mechanisms in a nylon 6,6/polyphenylene oxide (PA/PPO) alloy are studied. This alloy consists of well dispersed PPO particles (containing an elastomeric phase) in a PA matrix. Both crazing and shear yielding mechanisms are found to be sequentially operative in this alloy. When a crack propagates in the material, a crazed zone forms ahead of the crack tip. This crazed zone then transforms into a shear yielded zone as the crack propagates through it. The crazes inside the original crazed zone are closed or distorted by the shear yielding process. In the transformed region, the shear yielded material is oriented at different angles as the crack propagates. These findings are consistent with the notion that hydrostatic tension has to be dissipated in order for the remaining deviatoric stress to reach the critical value for yielding. It is possible to partially attribute the increase in toughness to both crack blunting and zone shielding mechanisms.

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Toughening mechanisms in a multi-phase alloy of nylon 6,6/polyphenylene oxide

G. Guidetti

Papers

The activation of MgCl2-supported ziegler-natta catalysts: A structural investigation

The activation of MgCl2-supported ziegler-natta catalysts—II: Correlation between activity and structural disorder

A crystallographic study of disorder in titanium trichloride, a component of Ziegler-Natta catalysts for stereospecific polymerization