The reactivity ratios for the aqueous free-radical copolymerization of diallydimethylammonium chloride and N-vinylformamide were found to be 0.13 and 1.92, respectively, from a Fineman-Ross analysis of a series of batch polymerizations. Because batch polymerization could not give a uniform product in a high yield with two monomers of such different reactivities, a semibatch procedure was developed in which the more reactive N-vinylformamide was added in 10 steps over the course of the copolymerization. The poly(diallyldimethylammonium chloride-co-N-vinylformamide) copolymers were hydrolyzed to give poly(diallyldimethylammonium chloride-co-vinylamine). The utility of the vinylamine/diallydimethylammonium chloride copolymers was demonstrated by the preparation and characterization of three derivatives: (1) a copolymer with coupled dansyl groups for fluorescence detection, (2) a copolymer with coupled dabsyl groups for ultraviolet-visible detection; and (3) an ultra-high-molecular-weight (1.6 x 10(6) Da) poly(diallyldimethylammonium chloride) by chain extension (coupling) with glycerol cliglycidyl ether. (c) 2007 Wiley Periodicals, Inc.