Chemistry Reference
In-Depth Information
The same materials can also be formed by the Grignard reaction:
+
+
RMgCl
SiCl
4
RSiCl
3
MgCl
2
+
+
RMgCl
R
2
SiCl
2
R
3
SiCl
MgCl
2
+
+
(CH
3
)
3
Si
MgCl
2
(CH
3
)
3
SiCl
MgCl
Alkyl silanes can also be prepared by additions of trichlorosilanes to ethylene or acetylene:
+
SiCl
3
HSiCl
3
+
HSiCl
3
SiCl
3
Trichlorosilane reacts with aromatic compounds in the presence of boron trichloride:
BCl
3
HSiCl
3
+
2
SiCl
2
2
The siloxane linkages can result from hydrolysis of the halides. The products of hydrolyses,
silanols, are unstable and condense:
R
R
R
R
H
2
O
Cl
Si
Cl
HO
Si
OH
+
Si
O
Si
O
H
2
O
R
R
R
R
The above shown reaction is one possible route to siloxane polymers. As a general method,
however, this approach is not very satisfactory, because ring formations accompany the reactions.
Some rings that form from hydrolyses of trichlorosilanes are structurally complex [
161
]. They may
even possess three-dimensional structures [
162
,
163
]. High molecular weight polymers, however,
form readily by ring opening polymerizations. Such polymerizations can be applied to the simple
rings that form from dihalides or complex ones from trihalides. Ring opening polymerizations,
carried out on purified (by distillation) cyclic intermediates, are catalyzed by either acids or bases
[
164
], leading to linear siloxane polymers:
Si
H
2
SO
4
O
O
Si
O
n
Si
Si
O
Acid-catalyzed polymerizations yield lower molecular weight polymers that are mostly oils. The
molecular weights of these oils can be controlled by additions of hexamethyldisiloxane during the
polymerization reactions. When catalyzed by bases, high molecular weight elastic polymers form.
O
Si
Si
O
+
Si
Na
4n
2
NaOH
NaO
O
n
O
Si
Si
O
