Condensation polymers:
An earliest known anion exchange resins were prepared from aromatic amines such m- phenylenediamine through condensation with formaldehyde.
The aldehyde reacts with amino groups. In the process, the secondary and tertiary amino groups are formed. Therefore, the resins are polyfunctional. Aliphatic polyamines which are not as weakly basic can also be condensed with aldehydes.