Breaking ester links
Polyesters contain ester links, , within their main chains. These links can be broken by hydrolysis, dividing a long polymer molecule into smaller fragments.
Hydrolysis restores alcohol-derived and carboxyl-derived groups at the point where an ester link is broken. The exact form of the carboxyl-containing product depends on whether the conditions are acidic or alkaline.
The hydrolysable functional group is the ester linkage:
Repeated cleavage of these links can eventually produce compounds corresponding to the monomers used to make the polyester.
Acid and alkaline hydrolysis
Polyesters can be hydrolysed using aqueous acid or aqueous alkali. The two conditions give different forms of the acid-derived product.
| Conditions | Products from an ester link |
|---|---|
| Aqueous acid | Carboxylic acid group + alcohol group |
| Aqueous alkali | Carboxylate salt group + alcohol group |
Complete hydrolysis of a polyester made from a dicarboxylic acid and a diol therefore gives fragments corresponding to those original monomers, although the acid-derived groups are present as carboxylate salts under alkaline conditions.
Exam Tip: Check the conditions before drawing the products. Under acidic conditions, restore a carboxylic acid and an alcohol; under alkaline conditions, show a carboxylate salt and an alcohol.