Polyethylene, from Ziegler-Natta polymerization

We have seen that free-radical polymerization of ethylene leads to the formation of low-density polyethylene (LDPE). The reason for the low density of LDPE is the lack of crystallinity owing to branching in the polymer. Straight-chain polyethylene can be obtained from Ziegler-Natta polymerization, which involves the use of an organometallic catalytic system [1-5, 18-20]. 

The polydispersity with heterogeneous Ziegler-Natta polymerization may range from 5 to 30. High-density polyethylene (HDPE) produced by co-ordination polymerization has a degree of branching of about 1-5 per 1000 monomer units, compared with 25-50 for... 

In polyethylene, the tertiary carbon atom, which dominated the chemistry of the oxidative degradation of PP, is present only at branch points. This suggests that there may be a difference among LDPE, LLDPE and HDPE in terms of the expected rates of oxidation. This is complicated further by the presence of catalyst residues from the Ziegler-Natta polymerization of HDPE that may be potential free-radical initiators. The polymers also have differences in degree of crystallinity, but these should not impinge on the melt properties at other than low temperatures at which residual structure may prevail in the melt. Also of significance is residual unsaturation such as in-chain tra s-vinylene and vinylidene as well as terminal vinyl, which are defects in the idealized PE strucmre. 

Copolymers of this type with these moderate length branches (C, Cg, and so on) are called linear low-density polyethylene, or LLDPE. These are useful materials because they have many of the properties of LDPE made from radical reactions but are formed at the substantially milder conditions associated with Ziegler-Natta polymerizations. 

A major advance in polymer science occurred in the 1950s when Ziegler and Natta both established that aluminum alkyls could polymerize ethylene under high pressure. They discovered that addition of transition metal compounds such as TiCL or VCI5 accelerated the reaction, so that ethylene could be polymerized at atmospheric pressure and room temperature (Eq. 13.23). Propylene could also be polymerized by these systems. While these very simple, and readily available, olefins can be polymerized under radical conditions with difficulty, such reactions were far from optimal. The discovery of Ziegler-Natta polymerization launched a huge effort in polyethylene and polypropylene science that continues to this day, with tens of millions of pounds of each being produced every year. 

Chromium trioxide based catalysts supported on silica were developed by Phillips Petroleum at the same time as the original work of Ziegler and Natta. These catalysts polymerize non-polar olefins by mechanisms which are similar to those involved in Ziegler-Natta polymerization but do not give such good stereochemical control and are used principally for the preparation of linear polyethylene. More recently, supported catalysts of very high activity for the polymerization of ethylene have been prepared from chromates and also from chromacene. 

In PP and polyethylene, hydrotaldtes serve in immobilizing and neutralizing addic catalyst residues derived from the Ziegler-Natta polymerization process. 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

In this case, the aluminum alkyl is functioning as a cocatalyst, sometimes also called an "activator." Titanium alkyls, believed to be active centers for polymerization, are created through transfer of an alkyl from aluminum to titanium, known as "alkylation." Molar ratios of cocatalyst to transition metal (Al/Ti) are typically 30 for commercial polyethylene processes using Ziegler-Natta catalysts (lower ratios are used for polypropylene). The vast majority of aluminum alkyls sold into the polyethylene industry today is for use as cocatalysts. With TEAL, the most widely used cocatalyst, alkylation proceeds as in eq 4.8 ... 

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