Chemistry Reference
In-Depth Information
case of the grafting in a homogeneous medium in which the cellulose dissolves
molecularly, it is known that all the cellulose chains participate in grafting reaction
[
41
,
46
]. By performing the grafting reaction using soluble cellulose derivatives in a
homogeneous medium, the extent and frequency of grafting and the molecular
weight of graft chains and homopolymer can be controlled better, the formation of
by product homopolymer is minimized, the homogeneity of graft product or
uniform distribution of graft chains can be ensured, etc. [
35
,
47
]. In addition, in
homogeneous grafting, the molecular weight of graft chains is similar to that of
homopolymer occurring simultaneously with the graft copolymer [
48
]. The grafting
reactions in homogeneous medium can be performed by using a soluble cellulose
derivative (mostly water-soluble cellulose derivative) such as hydroxyethyl cellu-
lose (HEC), carboxymethyl cellulose sodium salt (CMCNa) [
49
,
50
],
carboxymethyl cellulose (CMC) [
51
], methyl cellulose (MC) [
52
,
53
], cellulose
acetate [
54
], and ethyl cellulose [
42
,
46
] or dissolving the cellulose in a suitable
solvent pair such as
N
,
N
-dimethylacetamide/LiCl system [
43
], dimethyl
sulfoxide-paraformaldehyde (DMSO-PF) system [
39
-
41
,
55
] and using
carboxymethyl cellulose by xanthate method [
56
]. In another group of homoge-
neous grafting works, the reaction is performed in two steps. In the first step, a
derivative of cellulose is prepared, and then the product of the first step is reacted
with the monomer in the second step. For example, Bianchi et al. [
7
,
43
] prepared a
cellulose derivative, such as cellulose acrylate or cellulose methacrylate, by
reacting cellulose dissolved in DMAc-LiCl with acryloyl chloride and
methacryloyl chloride (MACl), respectively, and then they grafted the acrylonitrile
(AN) or methyl methacrylate (MMA) onto acrylate or methacrylate derivative of
cellulose in homogeneous medium, respectively. In another work, Wang et al. [
57
]
grafted poly(caprolactone monoacrylate) (PCLA) with isocyanate end groups
(NCO) (NCO-PCLA) onto cellulose diacetate (CDA) in homogeneous medium of
acetone. Lin et al. [
58
] grafted acrylic acid (AA) onto cellulose dissolved in an ionic
liquid (IL) of 1-
N
-butyl-3-methylimidazolium chloride (BMIMCl) by microwave
irradiation in a short irradiation duration such as 3 min. ILs which are organic salts
are good solvents [
59
] and good reaction media for cellulose. They are environ-
mental friendly alternatives to volatile organic compounds (VOC) due to their
nonvolatility, nonflammability, thermal stability, chemically inertness, and recy-
clability [
60
]. Due to their high ionic conductivities and polarizability properties,
they absorb microwave irradiation, and as a consequence they provide high heating
rates and shorten the reaction duration [
58
]. Zhu et al. [
61
] grafted poly(
p
-
dioxanone) (PDO) onto ethyl cellulose (EC) by ring-opening polymerization with
a tin-2-ethylhexanoate (Sn(Oct)
2
) as catalyst in bulk at 120
C. In the grafting of
PDO onto EC in homogeneous medium by Sn(Oct)
2
catalyst, it was not necessary to
add any solvent in reaction mixture [
61
] due to well solubility of EC in PDO. In the
grafting methods summarized above, the grafting is performed mostly by creation
of free-radical sites on the cellulose backbone either by chemical means or by
irradiation. The growth of side chains from vinyl monomers is initiated from these
radical sites on the cellulose backbone. However, these techniques have some
drawbacks as stated above. More advanced, controlled/living polymerization
