Chemistry Reference
In-Depth Information
phase and the broader the temperature range of the mesophase. Thus,
together with the properties, the extent of reaction has to be carefully
characterized.
Liquid crystalline polysiloxanes are usually synthesized by hydrosilyla-
tion reactions between polysiloxanes containing reactive Si
−
H groups and
vinyl compounds with mesogenic units. Gray (1989) and Percec and Pugh
(1989) have discussed the problems and side reactions in hydrosilylation
reactions, including incomplection, catalyzed migration of double bonds
and production of isomeric products, the danger of cross-linking involving
side groups, and product contamination by residual alkenes. The synthesis
is further complicated by the possible coexistence of the
α
-addition and the
β
-addition as shown by Figure 3.7.
Even though the addition of silicon hydrides to vinyl compounds often
takes the form of
β
-addition, exceptions do exist. For example, the reaction
of methyldichlorosilane and styrene catalyzed by chloroplatinic acid or Pt/C
yielded 53%
β
-addition and 33%
α
-addition product (Ryan and Speier,
1959). For polysiloxanes, Huang and Wu (1992) studied the synthesis of
polymer
using NMR analysis. The result showed that at 50
◦
C using
chloroplatinic acid as catalyst, the product was actually a copolymer with
isomeric structures, 58% of which were from
β
-addition (
3
.
40
3
.
40
), the other
42% were from the
α
-addition
3
.
41
.
CH
3
O
Si
+
CH
2
=
CH
CH
2
O
COO
OCH
3
H
CH
3
O
S
i
O
COO
CH
2
OCH
3
3
3
.
40
CH
3
O
S i
CH
CH
CH
2
O
COO
OCH
3
3
3
.
41
Different catalyst and reaction conditions may result in different molec-
ular structures with various percentages of
α
-and
β
-additions. Thus, even
if the reaction went to a 100% completion, the product would still be a
