The conversion of monomers during radical polymn., e.g., cyclopolymn. of diallyldimethylammonium chloride [7398-69-8] in water, is detd. continuously by detg. the change of d. of the soln. with an app. which detects changes in the frequency of elec. stimulated vibrations of the ends of a glass polymn. tube contg. the soln. The frequency of vibration is controlled by the soln. d. which changes with increasing conversion of monomer. The app. dets. d. changes of +-4 mg/cm3 for water. [on SciFinder (R)]