Chemistry Reference
In-Depth Information
with traditional ESO made through a chemical process. Thus it provides
people with an alternative and more effective method for the synthesis
of ESO.
With the presence of the epoxide functional groups in the fatty acid
chains, ESO can undergo ring-opening polymerization (ROP) reactions. Liu
et al.
21
carried out a ROP of ESO in methylene chloride using BF
3
OEt
2
as a
catalyst. The product was a highly cross-linked polymer with T
g
ranging from
16 to
48 1C. It was noted that the reaction was conducted in a toxic
solvent, therefore the authors turned their attention to a green medium,
subcritical CO
2
(ref. 22). The ROP of ESO catalyzed by the same catalyst was
conducted under mild conditions such as room temperature and a sub-
critical CO
2
pressure of 65.5 bar. The formed product has a similar structure
and properties, with T
g
ranging from
11.9 to
24.1 1C. On the other hand,
because of the concerns regarding the toxicity of the BF
3
OEt
2
catalyst, the
application of these soy-based polymers in food and medicinal areas is
limited. Therefore, they conducted the ROP of ESO in ethyl acetate catalyzed
by the super acid HSbF
6
6H
2
O (ref. 23). The results indicated that ESO was
effectively polymerized by the catalyst and formed polymers with relatively
high cross-link densities and with T
g
values ranging from
13 to
21 1C.
All these soybean-oil-based polymers synthesized via ROP of ESO can be
functionalized to hydrogels by hydrolysis and the products can be used in
personal and health care areas.
21-23
In addition to the ROP reaction, ESO can be thermally cured by many
systems such as maleinated linseed oil
24
and pyridine catalysts
25
or UV
cured with different types of photo-initiators
26,27
for the synthesis of epoxy
resins. However, there are some limitations of these systems such as long
curing times, high curing temperatures or poor thermophysical properties.
Recently, in order to overcome some of these problems, Tan et al.
28
reported
that ESO can be thermally cured with a methylhexahydrophthalic anhydride
(MHHPA) curing agent in the presence of a 2-ethyl-methylimidazole (EMI)
catalyst. With the increase of EMI concentration, the rate of polyesterifica-
tion, degree of conversion and cross-link density of the thermally cured ESO
were higher and the T
g
and storage modulus of cured ESO also increased.
One possible explanation could be associated with the fact that a zwitterion
was formed from MHHPA and EMI during the pre-mixed reaction and can
initiate the polymerization of ESO with MHHPA. Another catalyst reported
by Tan et al.
29
that can be used for ESO curing with MHHPA is tetra-
ethylammonium bromide (TEAB). It is proposed that the ring opening of the
MHHPA curing agent by the TEAB catalyst involves an S
N
2 reaction. The
triethylamine formed as a result of the dequaternization reaction of the
TEAB catalyst, serves as a nucleophile and attacks the MHHPA curing agent
to yield zwitterions, which are generated during the pre-mixed reaction. The
zwitterions formed eventually react with the epoxy rings on the ESO back-
bone chains and subsequently generate alkoxide intermediates, which will
cleave another MHHPA curing agent to yield carboxylate anions and then
react with other ESO resins to yield reaction intermediate product and
