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-halopropi...
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.