Chemistry Reference
In-Depth Information
Another method of forming polydialkyldisiloxanes is by reacting difunctional oligomers with
cyclic organisilicons:
Si
O
O
Si
n
Si
Si
+
(CH
3
)
2
Cl
2
Cl
O
Cl
Si
O
4n
O i
Si
Si
Si
Si
2H
2
O
HO
O
Cl
O
OH
Cl
4n
4n
Molecular weights of polydimethyl siloxanes can reach 700,000 or higher. Within the range of
molecular weights between 4,000 and 25,000, the materials are fluids of various viscosities. Most
common commercial liquid polydimethyl siloxanes are prepared from dimethyl dichloro siloxane.
Many elastomers are also based on dimethylsiloxane. Special polymers are prepared with other
substituents.
7.14.2 Silicone Elastomers
The elastomers from high molecular weight silicone polymers must be cross-linked to obtain rubber-
like properties. One way this is accomplished is through hydrogen abstraction by free-radicals that are
generated by decomposition of added peroxides. 2,4-Dichlorobenzoyl peroxide is often used for this
purpose. It is decomposed between 110 and 150
C. The reaction can be shown as follows:
Cl
Cl
+
O
Si
Si
Si
Si
2
+
Cl
2
Cl
O
O
O
O
COOH
O
Si
Si
Si
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
O
CH
2
+
Si
Si
Si
Si
O
O
O
O
CH
2
Si
Si
Si
Si
O
O
O
O
