Page 172 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GKY/MBQ P2: GLQ Final Pages
Encyclopedia of Physical Science and Technology en012i-947 July 26, 2001 11:11

680 Polymers, Inorganic and Organometallic

FIGURE 14 Polysilane repeating unit.

yields of soluble polysilanes with MW as high as
FIGURE 13 Polyhedral silsesquioxanes. 17 500. Arylsilanes couple more rapidly than alkylsilanes.
Diorganosilanes (R 2 SiH 2 ) under the same reaction condi-
tions give only dimers, (R 2 Si) 2 .
R R R R Ring-opening polymerization of strained cyclosilanes
Si Si R R
R R nBuLi or leads to polysilanes. The reaction is anionically catalyzed
n Si K Si Si (6)
+
+
−
R Si Si R Si Si n with n-BuLi or R 3 Si K . This method is most effective if
R R R R R
R the rings are small (e.g., Si 4 ) and the substituents are not
excessively bulky (e.g., H, CH 3 ,C 6 H 5 ).
R R
Polysilanes are formed by ring-opening polymerization
Si
R
R R of strained organodisilanes. To illustrate, when a disila-
Si
nBuLi bicyclooctadiene [Eq. (7)] is treated with n-BuLi, polysi-
n ( Si ) n n C 6 H 5 C 6 H 5 (7)
lanes with MW as high as 50 000 are obtained. Biphenyl is
H
eliminated as a by-product. This synthetic method is par-
C 6 H 5
ticularly useful since addition of vinyl monomers such as
methyl methacrylate can lead to block copolymers [e.g.,
CH 3 CH 3
275 C poly(methyl methacrylate-co-polysilane)].
n CH 3 O Si Si OCH 3 n (CH 3 ) 2 Si(OCH 3 ) 2 n [CH 3 Si:]
Polysilanes can also be prepared from silylenes. Con-
sequently, thermolysis of 1,2-dimethoxytetramethylsilane
CH 3 CH 3
CH 3
generates the transient intermediate dimethylsilylene
( Si )
n that polymerizes at ambient temperature to poly(dimethy-
silane) [Eq. (8)].
CH 3
The physical properties of polysilanes depend on the
(8) groups attached to the silicon. Polymers with relatively
small and identical organic groups (e.g., R = CH 3 ,C 2 H 5 ,
Reductive coupling is a heterogeneous reaction in-
C 6 H 5 ) bonded to Si are highly crystalline and generally
volving well-dispersed alkali metals with difunctional di-
insoluble and infusible solids. The crystallinity decreases
halosilanes. If the product conversion is kept low, high-
and the solubility increases with the length of the alkyl
MW polymers are obtained. As the reaction proceeds, the
substitutent and the presence of two different substituents
cyclic oligomers (mainly Si 6 and Si 8 ) are also formed to
on Si. For example, [CH 3 SiR] n in which R = n-hexyl is
give an overall binodal distribution of chain and ring prod-
◦
an elastomer at room temperature with T g =−75 C. How-
ucts. If a trifunctional silane (e.g., RSiCl 3 ) is used, a net-
ever, if R = C 6 H 5 , resinous solids that are soluble in or-
work polysilane is formed.
ganic solvents and have reasonable melting points are
Dehydrogenative coupling of primary alkylsilanes
formed.
(RSiH 3 ) using titanocene, zirconocene, or organolan-
An interesting and potentially useful property of polysi-
thanum compounds (e.g., [C 5 (CH ) 5 ] 2 LaH) gives good
3
lanes is their sigma electron delocalization that results
from relatively loosely held σ-bonding electrons and rel-
atively low-lying σ antibonding orbitals associated with
∗
TABLE II Some Applications of Polysiloxanes
the Si Si backbone. These unusual molecular orbitals are
Insulators Surgery implants Coupling agents
responsible for strong electronic absorptions, electro- and
Dielectric materials Mold-forming agents Water repellants
photoconductivity, thermochromism, and photosensitiv-
Heat exchange fluids Antifoaming agents Lubricants
ity. Polysilanes have found applications as photoresists in
Seals and gaskets Masonry additives Hydraulic fluids
microlithography and free radical photoinitiators in poly-
Caulking agents Surfactants Ceramic composites
merization of unsaturated hydrocarbons. Soluble polysi-
Emulsifying agents Paper release coatings Pressure-sensitive
lanes can be spun into fibers that, when pyrolyzed, give sil-
adhesives
icon carbide (SiC) ceramic materials (see Section III.A.3).