Condensation polymers:

The previously known cation exchange resin was a condensation product of phenol and formaldehyde. A list became broader and more extensive. Another monovalent or polyvalent phenol such as resorcinol and naphthol instead of phenol and other aldehydes instead of formaldehyde could be used. Phenolic group could act as a fixed ionic group but the resins have an extremely low acid strength. Groups with higher acid strength could be introduced through various methods. The simplest course is sulphonation of phenol prior to polymerization.