One of the characteristic reactions of carboxylic acids is esterification, a process that involves the condensation of a carboxylic acid with an alcohol. This transformation is depicted in general terms in Equation 1. The word condensation refers to the fact that the formation of the ester is accompanied by the formation of a molecule of water. In order to drive equilibrium 1 to the right the water is distilled and the vapors are condensed in a receiving flask. A more general definition of a condensation reaction is one in which the formation of the desired product is accompanied by the formation of another small molecule such as H2O, HCl, or NH3.
Reaction 1proceeds by a mechanism known as nucleophilic acyl substitution. As Equation 2 indicates, the process may be extended to the formation of polymers by the simple expedient of reacting a dicarboxylic acid with a diol.
Polymers that are formed by this pathway are commonly referred to as condensation polymers. In this topic we will examine the formation of two main classes of condensation polymers, polyesters, and polyamides.
One of the most ubiquitous polymers in our society today is poly(ethylene terephthalate), PET. It is used to make fibers for clothing as well as containers for carbonated beverages. When used to make clothing it's known as dacron. Figure 1 shows the repeat unit of this material.
Since acyl chlorides undergo facile nucleophilic acyl substitution reactions, many polyesters are prepared by treatment of a diacyl chloride with a diol. Figure 2 shows the repeat unit of the polycarbonate ester Lexan™ which is formed by the reaction of the diacid chloride of carbonic acid (H2CO3) and a diphenol.
Exercise 3 Draw the structures of the diacid chloride and the diphenol from which Lexan™ is made.
Exercise 4 The diphenol referred to above is prepared by the acid-catalysed reaction of phenol with acetone. What type of reaction is this? Write an equation for it.
Isocyanates, R-N=C=O, react with alcohols to form compounds known as urethanes. Equation 3 describes the reaction in general terms.
While this process is not technically a nucleophilic acyl substitution reaction, it is treated as such because isocyanates are derivatives of carbonic acid.
Extension of the chemistry described in Equation 3 to a diisocyanate and a diol provides a route to polyurethanes as shown in Equation 5.
If a small amount of water is added to the diisocyanate/diol mix, some of the diisocyanate undergoes a reaction analogous to that shown in Equation 4 and the CO2 that is liberated becomes entrapped in the polymer. This is the basis of polyurethane foams that are use as insulating materials.
Spandex™, a stretchable fabric made by DuPont, is an interesting example of a "mixed" polyurethane. As outlined in Figure 3, it is prepared in a 2-stage process, the first stage involving the condensation of two moles of methylene diisocyanate with one mole of poly(butylene oxide). This forms an intermediate known as a prepolymer. Treatment of this material with ethylenediamine yields the final product.
In the same way that they react with diols to produce polyesters, diacyl chlorides condense with diamines form polyamides. Commercially these compounds are more commonly called nylon. Equation 6 describes the process in general terms, while Figure 3 shows the repeat units of two common forms of nylon.
Nylon 6,6 is used to make fibers that are used to make carpets and fabrics. Kevlar is also spun into fibers which are woven into fabrics that are used in bullet-proof vests, as well as the cloth that is used to make high-end canoes, kayaks, and sailboats. It is also found in fishing poles and tennis racquets.
Natural fibers such as hair, silk, and wool are polyamides, too. We will discuss these materials more in the topic Biopolymers.