Ethylene polymerization molecular weight control

Then the powder is purged with nitrogen and transported pneumatically to the finishing area for stabilization and pelletizing. It has been reported that ethylene conversion in the reactor is around 98%. This process relies on polymerization temperature for average molecular weight control, while the MWD is controlled by the type of catalyst employed and certain proprietary operational adjustments which alter MWD. 

For initiation and molecular weight control in free radical polymerization of ethylene, a combination of several substances can be applied. Typically used free radical initiators belong to the classes of di-alkyl peroxides, peroxy alkyl esters, peroxy-carbonates, or di-acyl peroxides. The choice of the initiator mainly depends on its half lifetime at application temperature. To generate a more or less constant radical concentration level over a wide range of temperatures (e.g., 150-300 °C), a combination of different initiators is commonly applied. A typical mixture consists of a low and high temperature decomposing peroxide dissolved in hydrocarbons. 

Most chromium-based catalysts are activated in the beginning of a polymerization reaction through exposure to ethylene at high temperature. The activation step can be accelerated with carbon monoxide. Phillips catalysts operate at 85—110°C (38,40), and exhibit very high activity, from 3 to 10 kg HDPE per g of catalyst (300—1000 kg HDPE/g Cr). Molecular weights and MWDs of the resins are controlled primarily by two factors, the reaction temperature and the composition and preparation procedure of the catalyst (38,39). Phillips catalysts produce HDPE with a MJM ratio of about 6—12 and MFR values of 90—120. 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

Further improvements in reaction rates and polymerization control have led to the commercial availabihty of poly(ethylene oxide) of varying molecular weights. 

The living radical polymerization process is also valid for the polymerization of water-soluble monomers. The polymerization of sodium styrenesulfonate in aqueous ethylene glycol (80%) in the presence of TEMPO using potassium per-sulfate/sodium bisulfite as the initiator at 125 °C gave a water-soluble polymer with well-controlled molecular weight and its distribution [207]. 

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. 

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