Chemistry Reference
In-Depth Information
Fig. 5.11
Preparation procedure of hydroxypropyl gum
5.3.6 Hydrophobic Derivatives
The natural gums are mainly hydrophilic and soluble in aqueous medium, but it
cannot meet the requirements for the application in organic solvent system. Other
polysaccharide (i.e., starch, cellulose, chitosan) may form hydrophobic derivatives by
introducing short chain hydrocarbon substituents through etherification reaction and
show solubility in organic solvents. Similarly, the reactive groups of gum may be
etherized by RO-X, phO-X (X
ΒΌ
Cl or Br) to produce the derivatives with hydro-
philic and hydrophobic characters. The hydrophobic modification of gums can form
hydrophilic-hydrophobic gums with the characteristics similar to surfactants and can
form a micelle in solution. For instance, the modification of gum under alkaline
catalyst may form hydroxypropyl gum [
105
], which could trigger and accelerate the
sol
gel transition of tetraethoxy-silicone in water and induce rapid formation of
homogeneous gel matrix without the addition of any organic solvents or catalysts,
and can modulate the gel strength of the silica matrix by the amount of hydroxypropyl
gum (Fig.
5.11
)[
106
]. The substitution and the average length of the hydroxypropyl
substituents usually affect the properties of products [
107
].
5.4 Synthesis of Gum-g-Copolymers
5.4.1 Gum-g-Copolymers via Conventional Radical Grafting
Methods
The conventional grafting method of gum was involved with the direct grafting
reaction and the ring-opening reaction of macromolecular chains of gum. The direct
grafting reaction usually occurred on the -OH groups of gum by a radical polymer-
ization reaction process, and the used initiators are usually thermal initiator or redox
initiator. In this process, the radicals were generated from the decomposition of
thermal initiator (i.e., ammonium persulfate, sodium persulfate, potassium
persulfate, azodiisobutyronitrile) or the oxide-redox action of redox initiation
pairs (i.e., H
2
O
2
/Fe
2+
,K
2
S
2
O
4
/Fe
2+
,K
2
S
2
O
4
/ascorbic acid, benzoyl peroxide/
dimethylaniline). The formed anionic radicals may strip down the H atom of
-OH groups and initiate the macromolecular chains of gums to generate macro-
radicals, and these radical reactive sites may initiate the vinyl groups of monomers
to process the chain propagation.
