Page 170 - 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

678 Polymers, Inorganic and Organometallic

TABLE I Metal Boride Polymeric Structures
Formula M B B unit Anion structure
MB Fe, Ni Zigzag chains B B B
B B B
M 11 B 8 Ru Branched chains B B B
B B B B

M 3 B 4 Cr, Ta Ladder B B B
B B B B
B B B B
B B B
MB 2 Mg, Ti, Zr, Gd Sheet
B B B
B B B B
B B B B B
B B B B B
B B B B B
B B B B
B B B B B
B B B

MB 4 La Network
(octahedron) a

M

a
M = metal; B at each vertex of octahedra.

ing on the degree of polymerization. A broad range of
CH 3
H 2 O 100 C poly(dimethylsiloxane)s are commercially available with
(CH 3 ) 2 SiCl 2 [(CH 3 ) 2 SiO] 3,4 acid or base HO ( Si O ) H
-HCI x MW from several thousand to several million and poly-
cyclic trimer
and tetramer CH 3 dispersities as low as 1.05. Fluids have viscosities from
linear hydroxy end functional ∼1 to more than 2.5 × 10 centistoke. Low-viscosity flu-
6
poly(dimethylsiloxane)
ids are used as heat transfer agents and dielectric me-
(1) dia. Intermediate-viscosity materials are used as brake
fluids, mold releases, and lubricants and in cosmetics.
High-viscosity fluids are used as shock absorbers, ad-
Depending on the conditions of the reaction, the pri-
ditives to paints, and impact modifiers for thermoplastic
mary products from hydrolysis are oligomers that are
resins.
mainly the cyclic trimer and tetramer, [(CH 3 ) 2 SiO] 3,4 .
Elastomeric polysiloxanes are prepared by crosslinking
Polymerization results from heating the cyclics in the
fluids or gums. Crosslinking may be effected by reactions
presence of an acid or base catalyst. Polysiloxanes pre-
involving functional groups on the polysiloxane chain.
pared in this manner are fluids, gums, or waxes, depend-
For example, reaction between polysiloxanes that contain
a few percent vinyl groups and H gives an elastomer
with CH 2 CH 2 crosslinks (hydrosilation reaction)
[Eq. (2)]. Crosslinks that contain ester, amide, urethane,
and other organic groups are also known. Elastomers
may also be prepared by including a small quantity of
tri- and/or tetrafunctional silanes in the polymerization
of cyclic oligomers [Eq. (3)]. In this case the crosslinking
FIGURE 9 White phosphorus, P 4 . unit is a siloxane bond.