Chemistry Reference
In-Depth Information
This reaction is used commercially to prepare an aromatic polymercaptan from
p
-bis
(mecaptomethyl)benzene:
+
HS
SH
n
n
O
S
S
n
The polymer melts at 150
C and can be spun into fibers [
109
].
Another polymercaptan, poly(phenylene sulfide), can be prepared by several routes. This polymer
forms from sodium
-bromothiophenol at 250-305
C[
109
]. It has good thermal resistance and melts
p
at 287
C.
Br
SNa
+
S
NaBr
n
It also forms by a reaction of dichlorobenzene with sodium sulfide [
110
]. To date, the exact
mechanism of this polymerization, which is carried out commercially in
N
-methylpyrrolidone
solution, has not been fully established. Recent evidence indicates that in this solvent, an ionic,
step growth S
N
Ar mechanism predominates [
110
]:
Cl
S
Na
solvent
+
Cl
Cl
NaS
Na
Cl
NaS
Cl
Cl
Cl
Na
2
S
Cl
S
Cl
S
n
85
C.
An important characteristic of poly(phenylene sulfide) is its ability to undergo changes upon
heating [
110
]. This change is complex and not completely understood. It appears to involve varying
degrees of oxidation, cross-linking, and chain scission. When heated from 315 to 415
C, the polymer
melts, thickens, gels, and eventually solidifies to a dark infusible solid. This curing phenomenon
makes the polymer useful in many applications that range from coatings (powder or slurry) to
molding (by injection, compression, or sintering). Poly(phenylene sulfide) also becomes highly
conductive electrically when a dopant is added. Conducting polymers are discussed in Chap.
8
.
Elastomers
The polymer possesses a broad, high melting point and a glass transition temperature of
are still another group of sulfur-containing polymers.
They can be represented by a general structural formula of:
, based on
poly
(
alkylene sulfide
)
s,
½
R
S
x
n
where
x ¼
2
4
The most widely used methods of preparation are based on reactions of sodium polysulfides with
alkyl dichlorides [
110
]:
n
Cl
R
Cl
þ
Na
2
S
x
! ½
R
S
x
n
þ
2
n
NaCl
These reactions are usually carried out in dispersions. An aqueous sodium polysulfide containing a
surfactant, like alkyl aryl sulfonate, sodium hydroxide, and magnesium chloride, is heated to 80
C.
