Page 173 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology en012i-947 July 26, 2001 11:11

Polymers, Inorganic and Organometallic 681

Polygermanes, (R 2 Ge) n , and polystannanes, (R Sn) n ,
2
are synthesized in a comparable manner to polysilanes
and have similar physical properties. Like polysilanes,
(R 2 Ge) n and (R 2 Sn) n with R = hexyl or octyl are strongly
thermochromic. The former exhibits a red shift (20 nm)
and the latter exhibits a blue shift (8 nm) relative to anal-
ogous polysilanes. These polymers also have enhanced
σ-conductivity.

FIGURE 16 (A) Polysilazane repeating unit. (B) Cyclotrisilazane.
3. Polycarbosilanes (C) Cyclotetrasilazane.
Polycarbosilanes (Fig. 15) are polymers that contain sil-
4. Polysilazanes
icon bonded to carbon in the backbone. They are gener-
ally prepared by thermolysis of polysilanes [Eq. (9)], cat- Polysilazanes are another important class of polymer
alytic ring-opening of cyclic carbosilanes [Eq. (10)], or (Fig. 16). They are chain, ring, and network polymers
catalytic redistribution of a mixture of methylchlorodisi- that contain silicon bonded to nitrogen. There are sev-
lanes [Eq. (11)]. In these reactions the methylene group eral methods of polysilazane synthesis. These include re-

is inserted into the Si Si bond. The reaction mechanisms action of halosilanes (e.g., RR SiX 2 ) with ammonia or
are complex and sometimes lead to crosslinking through amines [Eq. (13)], ring-opening polymerization of cy-
carbon. The MW of uncrosslinked polycarbosilanes pre- closilazane [Eq. (14)], catalytic dehyrocoupling polymer-
pared by these methods is relatively low (<8000). Poly- ization [Eq. (15)], and deamination and condensation of
carbosilanes are ceramic precursors that, upon pyrolysis bis- and tris-alkyaminosilanes [Eq. (16)]. The reaction
◦
at 1300 C, give silicon carbide [Eq. (12)]: mechanisms are complex and the products depend on the
nature of the compounds used in the synthesis:
CH 3 H
R H
450 °C
n Si Si CH 2 (9)
n n Si N 2n NH 4 Cl (13)
n RR'SiCl 2 3n NH 3
n
CH 3 CH 3 R'
linear or cyclic
CH 3 H
Ru 3 (CO) 12
H [ (CH 3 ) 2 SiNH ] 4 Si N
135 °C x (14)
Cl Cyclic Tetramer H 2
Si CH 2 CH 3
H 2 Cl LiAIH 4 n Chains and
Cl Si C
H 2 PtCl 6 H Condensed Rings
Si CH 2
n n OR H 3 C H H
C Si Cl CH 3
Cl NaOR H H
H 2 H 3 C N Si H 3 C N Si
Cl Si CH 2 n [ (CH 3 ) 2 SiNH ] 4 KH
Si N Si N
OR
Cyclic Tetramer
(10) N Si N Si
Si N CH 3 Si N CH 3

n-Bu 4 P Cl Crosslinked Oligomeric
[(CH 3 ) x Cl 3−x Si] 2 (11) H 3 C N Si CH 3
mixture with Polycarbosilanes H
x = 1, 2
Si N
H 3 C
H HN Si
1300 °C
Si CH 2 β SiC (12) Si N
n N 2 H 3 C CH 3
CH 3
Sheet Polymer
Silicon carbides prepared in this manner have extremely (15)
2
high tensile strengths (>350 kg/mm ) and are among the CH 3 CH 3
strongest substances known. H 3 C N N ]
520 °C + n
n RSi(NHCH 3 ) 3 n CH 3 NH 2
[ Si Si
CH 3
N N
H 3 C Si CH 3
HN CH 3
CH 3
FIGURE 15 Polycarbosilane repeating unit. (16)