Radical Polymerization of Alkenes Chain-Growth Polymers

10 Radical Polymerization of Alkenes Chain-Growth Polymers 

Polymers are substances that consist of very large molecules called macromolecules that are made up of many repeating subunits. The molecular subunits that are used to synthesize polymers are called monomers, and the reactions by which monomers are joined together are called polymerizations. Many polymerizations can be initiated by radicals. 

Ethylene (ethene), for example, is the monomer that is used to synthesize the familiar polymer called polyethylene. 

Because polymers such as polyethylene are made by addition reactions, they are often called chain-growth polymers or addition polymers. Let us now examine in some detail how polyethylene is made. 

Ethylene polymerizes by a radical mechanism when it is heated at a pressure of 1000 atm with a small amount of an organic peroxide (called a diacyl peroxide). 

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RADICAL POLYMERIZATION OF ALKENES CHAIN-GROWTH POLYMERS... 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Many alkenes undergo chain-growth polymerization when treated with small amounts of suitable initiators. Table 26-1 shows some of the most common addition polymers, all made from substituted alkenes. The chain-growth mechanism involves addition of the reactive end of the growing chain across the double bond of the monomer. Depending on the monomer and the initiator used, the reactive intermediates may be free radicals, carbocations, or carbanions. Although these three types of chain-growth polymerizations are similar, we consider them individually. 

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