N-carboxyethylation of chitosan by b-halopropionic acids in the presence of various proton and halogen ion acceptors was investigated. It has been observed that carboxyethylation of chitosan in aqueous medium is accompanied by the by-processes of hydrolysis and dehydrohalogenation of the b-halopropionic acids yielding b-hydroxypropionic acid, bis(2-carboxyethyl) ether, and acrylic acid. Degree of carboxyethyl substitution (DS) of chitosan and the relative rates of the by-processes varied significantly depending on the conditions used and nature of the proton or halogen ion acceptor. At carboxyethylation of chitosan with the alkaline b-bromopropionates, the DS increased in the order Csþ 5 Rbþ 5 Kþ
Naþ 5 Liþ. For alkaline earth salts BrCH2CH2COOM0.5 (M 5 Be2þ, Mg2 þ, Ca2 þ, Sr2 þ, Ba2 þ), the highest DS was obtained with strontium and barium salts, which could be subsequently removed from the reaction mixture by precipitation as sulfates. Among the organic bases applied (tetrabutylammonium hydroxide, triethylamine, trimethylamine, pyridine, 4-N,N-dimethylaminopyridine, 2,6-lutidine, and 1,5-diazabicyclo[4.3.0] non-5-ene), the highest DS was obtained using a moderately strong base triethylamine. For the halogen acceptors (Pb2þ, Agþ, Tlþ), the stoichiometrically highest DS was achieved in a system comprising iodopropionic acid plus Tlþ and a comparable conversion rate was obtained using also a combination of chloropropionic acid and Agþ. A novel alternative preparative approach—gel-state synthesis—was suggested that provides for the highest DS at the optimum reaction conditions.
Journal of Applied Polymer Science. 2008 108(1) 119-127.