Chemistry Reference
In-Depth Information
4.4.2 Pre-polymers
The thiol-ene reaction was successfully used to introduce other functional
groups in place of the double bonds. Most often, it is used to introduce
primary -OH groups, but also -COOH, -COOCH
3
, -NH
2
, or -SH groups. The
authors, reporting on these syntheses, do not report the functional group,
which has been introduced, to have a strong (negative) effect on the thiol-ene
reactivity. The only exception was the amine group, which decreased the
miscibility and reaction rate, but this problem was overcome by protecting it
as a hydrochloride salt during the reaction. Afterwards, the products were
utilized for the synthesis of various polymeric materials.
Most often, the double bonds were reacted with thiols containing -OH
groups
34,42-47
and the product was used either in polyurethane (PU) or in
polyester synthesis. The thio-alcohol was usually 2-mercaptoethanol (2ME),
but 1-thioglycerol was also utilized to introduce two hydroxyl groups per
double bond.
42
Reacting the double bonds with 3-mercaptopropionic acid
48,49
or 2-mer-
captoacetic acid methyl ester
43,50
permits the transformation to carboxylic
acids and esters.
Amino groups were introduced using cysteamine hydrochloride, and
consecutive washes with a solution of Na
2
CO
3
(ref. 51) or K
2
CO
3
(ref. 52).
When direct reaction with cysteamine was attempted, lower yields were
observed.
52
Thiol groups can be introduced several ways. One is direct addition of
excess H
2
S
10-13,53
or polyfunctional thiols
54
to double bonds. Caution should
be used when performing this method, as the formed thiol groups can also
react with double bonds and form sulfides and H
2
S is a highly toxic gas. An
indirect method can be used, utilizing the thiol-ene reaction with thioacetic
acid and consecutive transesterification (see Scheme 4.10).
55,56
Gonzalez-Paz et al.
44
obtained diols by reacting the double bonds of
undecylenic and oleic acids with 2ME. Two routes were successfully tested to
add a second hydroxyl group to the compound. One was esterifying the acid
group with allyl alcohol, whose olefinic double bond reacted afterwards with
the 2ME. The other route was esterifying the acid group with methanol and,
afterwards, reducing it with LiAlH
4
(see Scheme 4.11).
O
+ MeOH,
base cat
.
-MeOAc
radical
initiator
O
S
HS
+
HS
R
R'
R
R'
R
R'
or/and
or/and
O
SH
S
R
R'
R
R'
Scheme 4.10 Method for the conversion of double bonds into thiol groups.
56
