25.3. Organic Reactions http://www.ck12.org
repeating unit. There is another class of polymers called copolymers. This group contains two different repeating
units in its structure. Kevlar®, used in body armor, is one such copolymer:
FIGURE 25.34
Structure of Kevlar®.One of the repeating units is a substituted benzene ring with attached amine groups, while the other repeating
unit contains carbonyl groups connected to the benzene ring. In addition to the strong covalent bonds that link
these monomers together, the polymeric chains interact with one another via hydrogen bonds, further increasing the
strength of the material.
Synthesis of Polymers
We will now look at two of the reactions that are commonly used to create synthetic polymers. One way to form
polymers is through addition reactions. If each monomer contains an alkene or alkyne, various methods can be used
to connect each side of the multiple bond with one of the other monomers, resulting in a long chain.
One way to form an addition polymer is through a radical chain reaction. The steps in this process are as follows:
- A free radical initiator attacks the carbon-carbon double bond. The initiator can be something like hydrogen
peroxide, which can be split with light or heat to form two radical species, each of which contains a reactive,
unpaired electron (H-O-O-H→2 H-O). This free radical attacks a carbon-carbon double bond. The initiator
binds to one of the carbon atoms using its unpaired electron and one of the pi electrons from the double bond.
The other pi electron forms a new free radical on the other carbon atom. - The new free radical adds to another alkene, generating yet another unpaired electron and continuing the
process of chain growth. - Termination occurs whenever two free radicals meet. The two unpaired electrons form a covalent bond, ending
the chain reaction.
The other general category of polymer synthesis reactions are the condensation reactions. In these situations, a
reaction occurs between two different functional groups with the expulsion of water. For example, polyesters and
polyamides can be formed via the condensation reactions discussed earlier in this lesson.
Another condensation reaction can be seen in the synthesis of Kevlar®: