Page 171 - 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 679

FIGURE 10 Black and red phosphorus.

R R Polysiloxaneshavemanycommercialandconsumerap-
O O O O
Si Si plications. Some of the uses of polysiloxane fluids, elas-
C H H C H tomers, and resins are summarized in Table II. Some of the
Pt Catalyst (2)
C H C H important physical properties of polysiloxanes that lead to
Hydrosilation
H H
H Si applications are their low glass transition temperature T g ,
O O
Si R low bulk viscosity, low thermal coefficient of viscosity,
O O
R high gas permeability, low surface energy, and low sur-
face tension.
CH 3
H ( O Si ) O
c
CH 3
CH 3
2. Polysilanes
O CH 3 CH 3
100 C
[(CH 3 ) 2 SiO] 3,4 CH 3 O Si O CH 3 acid or base H ( O Si ) O Si O ( Si O ) H
a b Polysilanes are chains, rings, and 3D network polymers of
CH 3 OH
O CH 3 CH 3
silicon that contain the Si Si bond in the polymer back-
CH 3
CH 3 bone. The tetravalency of silicon is usually completed with
Cyclic Oligomers Tetramethoxysilane O ( Si O ) H
d
H or organic groups. The basic structural unit of a polysi-
CH 3
Crosslinked Polymer lane is shown in Fig. 14.
There are several methods of polysilane synthesis:
(3)
reductive or Wurtz coupling [Eq. (4)], dehydrogena-
Resins differ from elastomers in the proportion of tive coupling [Eq. (5)], ring-opening polymerization of
crosslinks that are present. Highly crosslinked polysilox- cyclic silane oligomers [Eq. (6)], ring-opening of cyclic
anes are sometimes called T-resins. In general, T-resins organodisilanes [Eq. (7)], and polymerization of silylenes
are prepared by the hydrolysis of tri- and tetrafunctional [Eq. (8)]:
silanes.Theresultingresinshaveacomplexnetworkstruc-
R
ture and are insoluble in common organic solvents. Under
specially controlled synthetic conditions, lower MW ma- n R 2 SiHal 2 2n M ( Si ) n 2n MHal (4)
terials containing ladder and polyhedral oligomeric silox- Hal Cl, Br R
R H, Organic (M Na, Li)
anes called silsesquioxanes (Fig. 13) are obtained that are
soluble in organic solvents. The resins are used as coatings R
in microelectronic devices and binding agents in high- metallocene
n RSiH 3 ( Si ) n n H 2 (5)
performance refractory materials. catalyst
H

FIGURE 11 Allotropic forms of elemental sulfur. FIGURE 12 Coordinate (dative) bond.