A2 Chemistry 9701: Polyamide Repeat Unit, Hydrolysis and Biodegradability

4-Aminobutanoic acid, $\text{H}_2\text{N}(\text{CH}_2)_3\text{COOH}$ , can undergo condensation polymerisation on its own to form a polyamide.

(a) Deduce the repeat unit of this polyamide, showing the amide link clearly, and state the number of water molecules eliminated per repeat unit. [3]

(b) This polyamide is hydrolysed by heating under reflux with an excess of dilute hydrochloric acid. Name the type of bond broken in the polymer backbone, and give the structural formula of the organic product formed per repeat unit under these acidic conditions. [3]

(c) Poly(alkenes) such as poly(ethene) are not biodegradable, whereas this polyamide can be broken down by hydrolysis in the environment. Explain, in terms of the polarity of the bonds involved, why the poly(alkene) backbone resists this kind of attack but the amide linkages in the polyamide do not. [2]

Part (a): Repeat unit of a self-condensing amino acid

4-Aminobutanoic acid carries one $-\text{NH}_2$ group and one $-\text{COOH}$ group. Each molecule condenses with the next by forming a single amide bond between its $-\text{NH}_2$ and the neighbouring molecule’s $-\text{COOH}$ , releasing one water molecule:

$-\text{NH}_2 + \text{HOOC}- \;\longrightarrow\; -\text{NH-CO}- + \text{H}_2\text{O}$

Because only one $-\text{NH}_2$ and one $-\text{COOH}$ are contributed per monomer, only one amide bond (and hence one water molecule) forms per repeat unit added to the chain. The repeat unit is:

$\left(-\text{NH}(\text{CH}_2)_3\text{CO}-\right)_n$

Part (b): Acidic hydrolysis of the polyamide

Hydrolysis breaks the amide bond ( $-\text{CO-NH}-$ ) in the polymer backbone: a water molecule adds across the bond, regenerating a $-\text{COOH}$ group and an $-\text{NH}_2$ group at the point of cleavage.

Because the hydrolysis is carried out with an excess of dilute hydrochloric acid, the newly-formed amine group, a base, is immediately protonated by the acid present. So, per repeat unit, the organic product recovered is the protonated form of the amino acid:

$^+\text{H}_3\text{N}(\text{CH}_2)_3\text{COOH}$

(isolated as its chloride salt, since $\text{HCl}$ is in excess).

Part (c): Why one polymer biodegrades and the other does not

A poly(alkene) backbone consists entirely of carbon–carbon single bonds. Because carbon atoms have very similar electronegativity, these bonds are essentially non-polar, so water molecules (which attack as nucleophiles, via their slightly negative oxygen) have no partially-positive carbon to attack. The backbone is resistant to hydrolysis, and the polymer is non-biodegradable.

In the polyamide, however, the carbon of each $\text{C=O}$ group in the amide link is bonded to the strongly electronegative oxygen, giving that carbon a significant partial positive charge ( $\delta+$ ). This makes it an effective target for nucleophilic attack by water, allowing the amide bond to be hydrolysed and the polymer chain to be broken down, which is why polyamides (and polyesters) are biodegradable while poly(alkenes) are not.

Final answers

(a) Repeat unit $\left(-\text{NH}(\text{CH}_2)_3\text{CO}-\right)_n$ ; 1 water molecule eliminated per repeat unit.
(b) Amide bond broken; product per repeat unit is $^+\text{H}_3\text{N}(\text{CH}_2)_3\text{COOH}$ (as its chloride salt).
(c) The poly(alkene)‘s non-polar $\text{C-C}$ backbone resists nucleophilic attack by water; the polyamide’s polarised carbonyl carbon does not, so it can be hydrolysed.