Radical Additions to Alkenes Alkene Polymers

We had a brief introduction to radical reactions in Section 6.3 and said at that time that radicals can add to alkene double bonds, taking one electron from the double bond and leaving one behind to yield a new radical. Let s now look at the process in more detail, focusing on the industrial synthesis of alkene polymers. A polymer is simply a large—sometimes very large—molecule built up by repetitive bonding together of many smaller molecules, called monomers. 

Nature makes wide use of biological polymers. Cellulose, for instance, is a polymer built of repeating glucose monomer units proteins are polymers built of repeating amino acid monomers and nucleic acids are polymers built of repeating nucleotide monomers. 

Synthetic polymers, such as polyethylene, are chemically much simpler than hiopolymers, hut there is still a great diversity to their structures and properties, depending on the identity of the monomers and on the reaction conditions used for polymerization. The simplest synthetic polymers are those that result when an alkene is treated with a small amount of a radical as catalyst. Ethylene, for example, yields polyethylene, an enormous alkane that may have up to 200,000 monomer units incorporated into a gigantic hydrocarbon chain. Approximately 19 million tons per year of polyethylene are manufactured in the United States alone. 

Historically, ethylene polymerization was carried out at high pressure (1000-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as benzoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved half-arrow, or fishhook A, is used to show the movement of a single 

Propagation Polymerization occurs when the carbon radical formed in the initiation step adds to another ethylene molecule to yield another radical. Repetition of the process for hundreds or thousands of times builds the polymer chain. 

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