Chemistry Reference
In-Depth Information
Material
Parts
Water
150
Chloroprene
100
Rosin
4 (stabilizer)
NaOH
0.8 (stabilizer)
K
2
S
2
O
8
Sulfur
Methylene-bis-(Na-naphthalenesulfonic acid)
0.7
When the polymerization reaches 90% conversion, the reaction mixture is cooled to 20
C and
tetraethylthiuram disulfide is added. This is done to prevent the pendant unsaturation in the
polychloroprene backbones from cross-linking or forming branches. An unmodified polymer is
difficult to process even at a 70% conversion. To overcome this, a sulfur-tetraethylthiuram modifi-
cation is carried out.
When the product is treated with the thiuram, an exchange reaction takes place to yield a stable,
thiuram-modified polymer of reduced molecular weight. It is believed that the reaction takes place
through cleavage of the sulfur links, formed during polymerization in the presence of sulfur, and
formation of free radicals [
128
]
Cl
Cl
S
S
S
S
m
n
Cl
Cl
S
S
S
S
m
n
S
N(C
2
H
5
)
2
S
+
S
N(C
2
H
5
)
2
S
Cl
S
S
N(C
2
H
5
)
2
+
N(C
2
H
5
)
2
S
S
S
n
S
After the reaction with tetraethylthiuram disulfide is completed, the latex is acidified with acetic
acid, short of coagulation. The rubber is then recovered at a low temperature (about
15
C) in the
form of sheets by deposition of the latex on cooled rotating drums [
127
,
128
].
The polymer, formed by this technique, consists of about 85% of
1,4
units, 1.5% of 1,2 units, and 1.0% of 3,4 units. The polymer is essentially linear with a molecular
weight equal to approximately 100,000. The sulfur-modified polychloroprenes are sold under a trade
name of Neoprene-G. An unmodified version prepared with mercaptan chain transferring agents
(Neoprene W) is a polymer with a molecular weight of about 200,000 [
128
,
130
].
Table
6.9
lists the structures of polychloroprenes that form by free-radical polymerization at different
temperatures. Chloroprene polymerizes by cationic polymerization with the aid of Lewis acids
in chlorinated solvents. When aluminum chloride is used in a mixture of ethyl chloride-methylene
chloride solvent mixture at
trans-
1,4 units, 10% of
cis-
80
C, the polymer has 50% 1,4 units [
131
,
132
]. If it is polymerized
with boron trifluoride, the product consists of a 50-70% 1,4-adducts. A very high
1,4 poly(2-
chloro-1,3-butadiene) forms by X-ray radiation polymerization of large crystals of chloroprene at
trans-
130
180
C. It is 97.8%
to
trans
-1,4 [
130
]. Presumably the mechanism of polymerization is free radical.
