A novel pH- and salt-responsive carboxybetaine monomer, 4-(N,N-diallyl-N-methylammonio)butanoate (2), was prepared and cyclocopolymerized with the cationic monomer N,N-diallyl-N,N-dimethylammonium chloride (3) in 0.5 M NaCl aqueous solution (pH = 7.0) using 2-hydroxy-1-[4(hydroxyethoxy)phenyll-2-methyl-1-propanone (Irgacure 2959) as the free-radical photoinitiator. The molar feed ratio of 2:3 was varied from 100:0 to 0:100 with the total monomer concentration held constant at 2.5 M. Cyclopolymerization to five-membered ring structures common to diallylammonium salts was confirmed by C-13 NMR spectroscopy. Reactivity ratio studies indicate that 2 and 3 copolymerize in a nearly ideal fashion (r(1) = 0.86, r(2) = 0.99). Weight-average molecular weights and second virial coefficients vary from (6.0 to 12.8) x 10(4) g mol(-1) and (1.62 to 5.36) x 10(-4) mL Mol g(-2), respectively. Dilute solution viscosity behavior depends on copolymer composition, ionic strength, and pH. Copolymers with a large excess charge exhibit typical polyelectrolyte behavior, while those with balanced charge exhibit "anti-polyelectrolyte" behavior reported for zwitterionic (co)polymers. The polymers studied here are closely analogous cyclocopolymers we prevoiusly reported containing sulfobetaine mer units.(25) Unlike solutions of sulfobetaine-containing polymers, soltuions of poly(2-co-3) remain soluble at very low ionic strength even up to 100% incorporation of 2. Additionally, while the sulfobetaine systems are insensitive to pH, solutions of poly(2-co-3) exhibit a pH-dependent viscosity. All the copolymers have an apparent pK(a) of similar to3.6. Copolymers with 2 incorporations of 28 mol % and above have viscosity responsiveness between pH values of 2.5 and 4.5. The maximum pH response was achieved for solutions of poly(2-co-3) containing 37 and 56 mol % of the carboxybetaine monomer 2.