Preparation of Polyethylene by a Free-Radical Mechanism

RGURE 5.1. IVibular reactor as described in a Monsanto Co. U.S. Patent 2,856,494. 

Polyethylene prepared in this way may have as many as 20 to 30 short branches per 10,000 caibon atoms in the chain and one or two long-chain branches per molecule, due to backbiting  

The reaction results in predominantly ethyl and butyl branches. The ratio of ethyl to butyl groups is roughly 2 1. Chain transferring to the tertiary hydrogens at the location of the short branches causes elimination reactions and formation of vinylidine groups. This mechanism also accounts for formation of low molecular weight species 

Commercial grades of low-density polyethylene vary widely in the number of short and long branches, average molecular weights, and molecular weight distributions Mw/Mn is between 20 and 50 for commercial low-density materials. The short branches control the degree of crystallinity, stiffness, and polymer density. They also influences the flow properties of the molten material. 

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Describe conditions and the procedure for commercial preparation of polyethylene by a free-radical mechanism, the role of oxygen, and the problems associated with oxygen. 

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