A process for preparing defined molecular weight polymers, particularly polyquaternary ammonium compounds, is disclosed. As the molecular weights of these compounds can be absolutely defined, rather than defined merely in terms of 'number average molecular weight', these compounds are particularly useful as identification standard for other polymers which are made using conventional polymerization processes. The polyquaternary ammonium compounds described are further suitable for use as antimicrobial agents, particularly in ophthalmic compositions.