Synthesis, Characterization, and Aqueous Solution Behavior of Electrolyte- and pH-Responsive Carboxybetaine-Containing Cyclocopolymers

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)phenyl]-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 13 C 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) × 10 4 g mol -1 and (1.62 to 5.36) × 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 p K a of ∼3.6. 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 .