Chemistry Reference
In-Depth Information
The magnesium chloride forms magnesium hydroxide and acts as a nucleating agent. Bis(2-
chloroethyl) formal is then added over 2 h with stirring and external cooling of the exothermic
reaction to about 90
C. After addition, this temperature and stirring are usually maintained for
additional 2 h to complete the process. The product contains a distribution of polysulfide groups.
The polysulfide anions can interchange continuously:
+
S
+
S
S
R
S
S
S
R
S
S
R
S
S
R
S
S
S
S
R
R
S
S
This allows building up the molecular weights of the polymers by additions of excess polysulfide.
Also, low molecular weight fractions can be washed out. By such manipulations, molecular weights
of 500,000 are readily achieved. It is interesting that in this particular step-growth polymerization, in
order to obtain high molecular weights, strict stoichiometry is not only not required, but one of the
components is deliberately added in excess.
Several different grades of poly(alkyl sulfide)s are available commercially. One form, hydroxyl-
terminated, is formed by coagulating the formed polymer from the aqueous dispersion with sulfuric
acid. The terminal halogens hydrolyze in the process to hydroxyl groups. For easier processing, these
elastomers are usually reacted with disulfide like benzothiazyl disulfide. This reduces the molecular
weight through chain cleavage:
N
N
+
R
S
S
R
S
S
S
S
N
2
R
S
S
S
The elastomers are then chain-extended again with metal oxides, like zinc oxide, that couple the
terminal hydroxy groups. The same thing can also be done by using diisocyanates.
Another group consists of thiol-terminated, low molecular weight polymers. They form from
heating alkyl sulfides in aqueous dispersion of sodium bisulphite and sodium sulfite for about an hour
at 80
C. This results in mercaptide and thiothiol terminal groups:
+
S
S
R S S R S S
R
SH
S
S
R S
SH
+
S
R
S
S
R
The sulfite ion prevents the reversal of the equilibrium by splitting of the sulfur from the thiothiol:
SH
+
SO
3
S
S
R
S
S
S
R
SH
+
S
2
O
3
These polymers cross-link by oxidative coupling of the mercaptide groups:
+
+
HS
+
SH
[O]
S
S
H
2
O
and by reactions with metal peroxides like lead peroxide. They also react with epoxy resins.
