Ziegler polymerizations molecular weight distributions

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. 

Borstar A catalytic process for polymerizing ethylene. Use of two reactors, a loop reactor and a gas-phase reactor, allows better control of molecular weight distribution. The loop reactor operates under super-critical conditions to avoid bubble formation. Either Ziegler-Natta or metallocene catalysts can be used. The first commercial unit was installed in Porvoo, Finland, in 1995. 

Zambelli, A. and Tosi, C. Stereochemistry of Propylene Polymerization. Vol. 15, pp. 31-60. Zucchini, U. and Cecchin, G. Control of Molecular-Weight Distribution in Polyolefins Synthesized with Ziegler-Natta Catalytic Systems. Vol. 51, pp. 101-154. 

Heterogeneous Ziegler-Natta catalysts composed of titanium trichloride and alkylaluminum have been used to prepare block copolymers of ethylene with a-olefins 44-46), even though there is no known example of such a catalyst meeting the requirement for a living polymerization. The produced block copolymers have broad molecular weight distributions (Mw/Mn = 4 20) and are present in small concentrations... 

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. 

Borstar A catalytic process for polymerizing ethylene or propylene, subdivided into Borstar PE and Borstar PP. Use of two reactors — a loop reactor and a gas-phase reactor — allows better control of molecular weight distribution. The loop reactor operates under supercritical conditions to avoid bubble formation. Either Ziegler-Natta or metallocene catalysts can be used. The latest version, Borstar PE 2G, uses a single, multizone gas-phase reactor to make polymers that have bimodal molecular weight distributions. Developed by Borealis A/S. The first commercial unit, for polyethylene, was installed in Porvoo, Finland, in 1995. The first polypropylene plant was operated by Borealis in Schwechat, Austria, in 2000. In 2005, Borstar s total capacity for PE and PP was 1.3 million tons. 

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. 

Additionally, if the initiation reaction is more rapid an the chain propagation, a very narrow molecular weight distribution, MJM = 1 (Poisson distribution), is obtained. Typically living character is shown by the anionic polymerization of butadiene and isoprene with the lithium alkyls [77, 78], but it has been found also in butadiene polymerization with allylneodymium compounds [49] and Ziegler-Natta catalysts containing titanium iodide [77]. On the other hand, the chain growth can be terminated by a chain transfer reaction with the monomer via /0-hydride elimination, as has already been mentioned above for the allylcobalt complex-catalyzed 1,2-polymerization of butadiene. 

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