Ziegler type polymerization, molecular weight distribution

Molecular Weight Distribution. In industry, the MWD of PE resins is often represented by the value of the melt flow ratio (MER) as defined in Table 2. The MER value of PE is primarilly a function of catalyst type. Phillips catalysts produce PE resins with a broad MWD and their MER usually exceeds 100 Ziegler catalysts provide resins with a MWD of a medium width (MFR = 25-50) and metallocene catalysts produce PE resins with a narrow MWD (MFR = 15-25). IfPE resins with especially broad molecular weight distributions are needed, they can be produced either by using special mixed catalysts or in a series of coimected polymerization reactors operating under different reaction conditions. 

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. 

Keywords polymerization kinetics, polymerization reactors, mathematical modelling, molecular weight distribution (MWD), chemical composition distribution (CCD), Ziegler-Natta catalysts, metallocenes, microstructure, isotacticity distribution, mass transfer resistances, heat transfer resistances, effects of multiple site types. 

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. 

The study of polymerization in a tubular reactor involves simultaneous consideration of the velocity profile, heat release and conduction, as well as the kinetics of the polymerization. This work was undertaken to determine the effect of this type of reactor upon the molecular weight distribution of the polymer produced. The system studied was the copolymerization of an elastomer from ethylene, propylene, and 1,4-hexadlene In solution using Ziegler catalysis. Laminar flow polymerization similar to previously described work involving polymerization of sytrene in tubular reactors (1, 2, was analyzed, but the numerical... 

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