Ziegler type polymerization, molecular

The mechanism of Ziegler-type polymerization has not only to explain rate expressions found by kinetic measurements, but also the structure of the polymer. The structure and the molecular-weight distribution of the polymers are a record of what happened during the polymerization reaction. What is to be explained may be summarized by discussing propylene as a monomer. 

Ultrahigh-molecular-weight polyethylene (UHMWPE) is defined by ATM as polyethylene with molecular weight over three million (weight average). The resin is made by a special Ziegler-type polymerization. 

Supported Ziegler-type neodymium surface species (54, see below) have been prepared by mixing molecular components composed of [Nd(naph)3] (derived from naphthenonic acids) and alkyl aluminium reagents such as Al2Et3Cl3, Al( Bu)3 and/or Al( Bu)2H at 50-60°C with silica (source QiLu Petrochemicals Co., China) [158-160]. Although the immobihzed neodymium species are iU-defined, the materials display interesting properties in butadiene polymerization (Section 12.4.1.2). 

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. 

This model, of the Rideal type, is of general applicability and includes and explains the most relevant phenomena involved in Ziegler-Natta polymerization. Mathematical formulation of the proposed reaction scheme led to the following equations for polymerization rate Rp and number average molecular weight of polymer Mn, in the absence of hydrogen ... 

QPC data were measured with a Model 200 of the Waters Co. the result is shown in Fig. 3 Although the polymerization is carried out by Ziegler type catalyst, the molecular distribution of the 1, 2-PBD is rather sharp. 

By treatment of these materials with titanium tetrachloride valuable supported catalysts for the propene Ziegler-Natta type polymerization were obtained. These catalysts were tested by slurry polymerization using triethylaluminium as cocatalyst and showed an interesting activity compared with that exhibited by a commercial catalysts. The polymer products were also characterized by measuring the molecular weight distribution by gel permeation chromatography technique. 

Commercial ethylene-propylene rubbers (EPR or EPM ) generally contain about 35 mole % propylene although rubbery properties are shown by copolymers with a propylene content ranging from 30—60 mole %. At the present time, these materials are prepared exclusively by Ziegler-type processes. Generally, true solution processes are preferred in which a soluble catalyst system is used and the polymer remains in solution rather than form a slurry. A common soluble catalyst system is based on vanadium oxychloride/aluminium trihexyl. Catalysts of this type favour the formation of amorphous atactic polymers and lead to narrower molecular weight distributions than solid catalysts. Typically, polymerization is carried out at about 40°C in a solvent such as chlorobenzene or pentane and the polymer is isolated by precipitation with an alcohol. 

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