From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 22

CH 2 OH CH 2 OH

H H H

H

O O

C+ O

H C OH O

O

OH

OH

*

* *

*

H

*

H

OH

O

N NH

CH 2
OH

n

n

H H HO

+n

CH

OH

Figure 1.14
Reaction of chitosan and formaldehyde hydrate.

The Schiff base of formaldehyde and chitosan can be reduced by NaBH 3 CN to form
N -methyl chitosan [74]. The method comprises the following steps [75]: suspending 20 g
of chitosan powder with 2 L of water, adding 20 mL of glacial acetic acid to the suspen-
sion, stirring to dissolve glacial acetic acid, adding formaldehyde of the calculated
amount in two batches, stirring for 30 min, adjusting the pH to 4.5 by using NaOH solu-
tion, dissolving 5 g of NaBH 4 in 50 mL of water, adding the solution to the reactant in 1 h
during stirring, reacting for 1 h until the pH is 5.5, adjusting the pH to 10 by using alkali
until N -methyl chitosan deposits, filtering, washing deposits with water to neutral, and
extracting them with ethanol and ether via a Soxhlet extractor to remove the remaining
formaldehyde and inorganic compounds.
A similar method can be used to prepare N -dimethyl chitosan: adding 50 mL of glacial
acetic acid to 2 L of water, adding 50 g of 100-200 mesh chitosan to the solution, stirring the
mixture to dissolve solid and form a solution of pH 3.2, adding 500 mL of 35% formalde-
hyde solution, stirring strongly to immediately form N -methylene chitosan gel, keeping it
still for 12 h, adding 13 g of NaBH 4 in 8 h, reacting at 15-20°C until the pH is 4.0, which
shows that the reaction is complete, adjusting the pH to 9.0 by using NaOH solution, wash-
ing the gel with water, and using acetone for depositing, filtering, and extracting deposits
by using ether in a Soxhlet extractor to obtain a white powder that is insoluble in acetic
acid or hydrochloric acid.
A small amount of formaldehyde cross-links chitosan, increasing the viscosity and even
changing the solution into a gel. The specific steps are as follows: swelling chitosan by 95%
ethanol for 2 h, adding excess salicylaldehyde to the mixture, stirring in reflux for 8 h,
filtering to harvest the yellow product, extracting it by using ethanol in a Soxhlet extractor
for 24 h to get salicylaldehyde chitosan Schiff base, and reducing it by NaBH 4 in methanol

CH 2 OH

O

O

OH

*

*

N

n

H 2 C CH 2

OH OH

Figure 1.15
N -dihydroxylmethyl derivative of chitosan.