Recognition that polymeric macromolecules make up many important natural materials was followed by the creation of synthetic analogs having a variety of properties. Indeed, applications of these materials as fibers, flexible films, adhesives, resistant paints and tough but light solids have transformed modern society. Some important examples of these substances are discussed in the following sections.There are two general types of polymerization reactions: addition polymerization and condensation polymerization. In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers. Ethylene molecules are joined together in long chains.
Strictly speaking, this chapter is on radical reactions, which include most addition polymerization processes. However, in order to cover the topic of polymers properly, we are including a brief section on condensation polymerization (usually based on substitution chemistry) here as well.
Many natural materials—such as proteins, cellulose and starch, and complex silicate minerals—are polymers. Artificial fibers, films, plastics, semisolid resins, and rubbers are also polymers. More than half the compounds produced by the chemical industry are synthetic polymers.
Chain-Reaction (Addition) Polymerization
The polymerization can be represented by the reaction of a few monomer units:
The bond lines extending at the ends in the formula of the product indicate that the structure extends for many units in each direction. Notice that all the atoms—two carbon atoms and four hydrogen atoms—of each monomer molecule are incorporated into the polymer structure. Because displays such as the one above are cumbersome, the polymerization is often abbreviated as follows:
nCH2=CH2 → [ CH2CH2 ] n
During the polymeriation of ethene, thousands of ethene molecules join together to make poly(ethene) – commonly called polythene. The reaction is done at high pressures in the presence of a trace of oxygen as an initiator.
Some common addition polymers are listed in the Table below . Note that all the monomers have carbon-to-carbon double bonds. Many polymers are mundane (e.g., plastic bags, food wrap, toys, and tableware), but there are also polymers that conduct electricity, have amazing adhesive properties, or are stronger than steel but much lighter in weight.
Monomer | Polymer | Polymer Name | Some Uses |
---|---|---|---|
CH2=CH2 | ~CH2CH2CH2CH2CH2CH2~ | polyethylene | plastic bags, bottles, toys, electrical insulation |
CH2=CHCH3 | polypropylene | carpeting, bottles, luggage, exercise clothing | |
CH2=CHCl | polyvinyl chloride | bags for intravenous solutions, pipes, tubing, floor coverings | |
CF2=CF2 | ~CF2CF2CF2CF2CF2CF2~ | polytetrafluoroethylene | nonstick coatings, electrical insulation |
Step 1: Chain Initiation
The oxygen reacts with some of the ethene to give an organic peroxide. Organic peroxides are very reactive molecules containing oxygen-oxygen single bonds which are quite weak and which break easily to give free radicals. You can short-cut the process by adding other organic peroxides directly to the ethene instead of using oxygen if you want to. The type of the free radicals that start the reaction off vary depending on their source. For simplicity we give them a general formula: Ra∙