Ziegler-Natta polymerization single-site catalysts

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

Medium Density High Density Polyethylene MDPE (or MDHDPE) is produced by copolymerization of ethylene with a-olefins using Ziegler-Natta, supported chromium or single site catalysts. MDPE cannot be produced by free radical polymerization. MDPE has a linear structure similar to LLDPE, but comonomer content is lower. Density is typically 0.93-0.94 g/cm. MDPE is used in geomembrane and pipe applications. 

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

There are substantial differences between the mechanisms of polymerization with single site catalysts and the closely related Ziegler-Natta catalysts (37-42). Most notably, the active centers of single site catalysts are believed to be cationic. Currently, cocatalysts are used in all commercial processes using single site catalysts, but this may change in the not-too-distant future (see p. 76). 

There is no single mechanism for Ziegler-Natta polymerization because of the variety of catalyst and co-catalyst systems as well as the different phases in which the reaction may take place. The process of stereoregular polymerization can be understood tfom the mechanism of initiation and propagation as the monomer is inserted at the polymerization site on the catalyst surface. The detailed mechanism for achieving stereospecificity is an active area of research (Corradini and Busico, 1989, Tait and Watkins, 1989), but some general principles may be learned tfom the simple Ziegler-Natta catalysts (Allcock and Lampe, 1981). 

Single-site catalysts offer significant advantages over heterogeneous catalysts for the polymerization of functional olefins. One of the inherent liabilities of heterogeneous Ziegler—Natta catalysts is that they... 

The polymerization of olefins has long been one of the most industrially important applications of organometaiiic complexes.The use of transition metals as catalysts ranges from early metal (e.g., Ti, Zr) Ziegler-Natta type polymerizations to late-metal single site catalysts as well as atom transfer radical polymerizations (ATRPs)... 

Isoprene polymerization Particles size effect Single site catalysts Ziegler-Natta catalyst... 

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