Thermosensitive copolymers with pyridyl and/or pyridinium groups were prepared by copolymerization of 4-vinylpyridine (4VP) and N-isopropylacrylamide (NIPAAm) followed by the reaction of 4VP-NIPAAm copolymers with benzyl chloride. The copolymer 4VP-NIPAAm was soluble in dilute HCl below 32 degrees C, but the quaternized 4VP-NIPAAm copolymer was soluble even in aqueous solutions of pH 6.5 below 28 degrees C. The thermosensitivity of both 4VP-NIPAAm and the quaternized 4VP-NIPAAm copolymer was observed. The quaternized 4VP-NIPAAm copolymer could adsorb food dyes dissolved in water and the copolymer-dye complexes became water-insoluble. The quaternized 4VP-NIPAAm copolymer also exhibited high antibacterial activity against bacteria such as Escherichia coli. (c) 2007 Elsevier Ltd. All rights reserved.