Biomedical Engineering Reference
In-Depth Information
1.5 Chemical Properties
1.5.1 O-Acylation and N-Acylation
Chitin and chitosan may react with various derivatives of organic acids such as anhydride
and acyl chloride to obtain aliphatic or aromatic acryls of different molecular weights.
Such a reaction is most studied among all the chitosan relevant reactions.
Residues of the chitosan include hydroxyls and aminos; hence acylation may occur at
hydroxyls to form ester or at aminos to form amide. A hydrogen atom on a nitrogen atom
of an acetamino of chitin is still active despite the fact that the acetamino is changed to
amido. This hydrogen atom can react if conditions permit. Hydroxyls on residues of chitin
and chitosan include C6 hydroxyls and C3 hydroxyls, which are secondary hydroxyls. C6
hydroxyl as primary hydroxyl can freely spin in space due to small steric hindrance
whereas C3 cannot due to large steric hindrance. For this reason, C6 is more active in
reaction than C3. On the other hand, aminos on the residues are more active than the pri-
mary hydroxyls. However, activity is not the only factor influencing acylation; reaction site
is also determined by reaction solvent, structure of acylating agent, catalyst, and reaction
temperature. Moreover, acylation hardly forms a single product, which means that
N-acylation and O-acylation occur at the same time or C6 acylation and C3 acylation occur
at the same time. If C3, C6, and C2-NH are all acylated, the product is fully acylated chito-
san, which is actually fully acylated chitin.
Strong intramolecular and intermolecular hydrogen bonds of chitin make the structure
especially tight, and thus acylation is difficult. Usually, the acylating agent is anhydride
and the reaction medium is the corresponding acid. Catalyst is a must and the reactant
must be cooled during reacting. Commonly used catalysts include hydrogen chloride,
methanesulfonic acid, perchloric acid, and so on. Main chain degradation is slight if the
catalyst is methanesulfonic acid or perchloric acid. Chitosan contains several aminos,
making acylation easier than chitin, and therefore a catalyst is not necessary. The reaction
medium is methanol or ethanol.
The preparation method of fully acrylated chitin comprises the following steps: dispers-
ing 1 g of chitosan powder that is dried sufficiently in 150 mL of methanol, adding excess
acetic anhydride (2-3 mol larger than glucosamino of chitosan), stirring the mixture at
room temperature for 16 h, filtering it, washing the filter cake with methanol twice, steep-
ing the filter cake in 50 mL of 0.5 mol/L ethanol-KOH solution for 16 h, filtering the mix-
ture, washing the filter cake with methanol sufficiently, dehydrating the mixture with
ether, and drying the product naturally in air. The yield is almost stable.
This preparation method is of significance because it is difficult to form fully acylated
chitin by using chitin, and chitin may degrade, causing molecular weight decline. These
problems can be solved by using chitosan as the raw material. Fully acylated chitin fiber or
film has excellent strength but is complex to make. Instead, acylation of chitosan will form
fully acylated chitin of good performance and with lower cost.
The above method is suitable for making other acylates by using different carboxylic
anhydrides. Fatty acid anhydride with more than six carbon atoms should be heated in
reflux for over 16 h instead of reacting at room temperature.
As amino is more active than C6 hydroxyl, which makes acylation primarily occur at
amino, the O-acylated chitosan is hard to form. Hence, aminos of chitosan are protected in
advance (e.g., by benzaldehyde) before acylation and unprotected after acylation, and the
acylation condition is moderate. Grant et al. [64,65] formed fully acylated chitosan by
 
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