Radical Additions to Alkenes: Chain-Growth Polymers

form a polymer. Many substituted ethylenes, called vinyl monomers, also undergo polymerization to yield polymers with substituent groups regularly spaced on alternating carbon atoms along the chain. Propylene, for example, yields polypropylene, and styrene yields polystyrene.

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH₂·) or a secondary radical (R₂CH·). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed.

Table 8.1 shows some commercially important alkene polymers, their uses, and the monomers from which they are made.

Table 8.1 Some Alkene Polymers and Their Uses

Monomer	Formula	Trade or common name of polymer	Uses
Ethylene	H2C═CH2H2C═CH2	Polyethylene	Packaging, bottles
Propene (propylene)	H2C═CHCH3H2C═CHCH3	Polypropylene	Moldings, rope, carpets
Chloroethylene (vinyl chloride)	H2C═CHClH2C═CHCl	Poly(vinyl chloride)	Insulation, films, pipes
Styrene	H2C═CHC6H5H2C═CHC6H5	Polystyrene	Foam, moldings
Tetrafluoroethylene	F2C═CF2F2C═CF2	Teflon	Gaskets, nonstick coatings
Acrylonitrile	H2C═CHCNH2C═CHCN	Orlon, Acrilan	Fibers
Methyl methacrylate		Plexiglas, Lucite	Paint, sheets, moldings
Vinyl acetate	H2C═CHOCOCH3H2C═CHOCOCH3	Poly(vinyl acetate)	Paint, adhesives, foams

Worked Example 8.4

Predicting the Structure of a Polymer

Show the structure of poly(vinyl chloride), a polymer made from H2C═CHClH2C═CHCl, by drawing several repeating units.

Strategy

Mentally break the carbon–carbon double bond in the monomer unit, and form single bonds by connecting numerous units together.

Solution

The general structure of poly(vinyl chloride) is