= Solution
\b[An aromatic–aliphatic <polyamide> and methanol are formed.] Let $\mathrm{Ar}=p\text{-}\mathrm{C_6H_4-C_6H_4}\text{-}p$. The <polymer repeat unit> is
$$
\boxed{[\mathrm{-NH(CH_2)_3NH-C(O)-Ar-C(O)-}]_n.}
$$
Each <amide> linkage comes from <nucleophilic acyl substitution>: an <amine> attacks an <ester> <carbonyl>, the <tetrahedral intermediate> expels <methoxide>, and <proton transfer> produces <methanol>. There are approximately $2n$ <molecules> of <methanol> for $n$ complete repeat units in a long chain; the precise finite-chain count depends on its end groups. Removal of <methanol> drives this <step-growth polymerization>.
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