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Polymers, Inorganic and Organometallic 677

FIGURE 5 Graphite sheet polymer. In graphite (above), the C
FIGURE 7 Network polymer (diamond and silica). In diamond,
atoms are located at each vertex and the layers are staggered. In
each atom is carbon; in silica, silicon is surrounded tetrahedrally
boron nitride, the B and N atoms are located at alternate vertices
by four oxygen atoms.
and the layers are eclipsed.
more, the dative interaction tends to stabilize the polymer
stacked in a manner similar to graphite. In addition to flak- toward thermal degradation and aids in the regulation of
ing, black phosphorus is a good electrical conductor. Red structure, chain mobility, and interchain forces that en-
◦
phosphorus results from heating P 4 at ∼400 C in a sealed hance useful dynamic–mechanical properties.
vessel. One crystalline form of red phosphorus (Hittorf’s
phosphorus) has a linear tubular arrangement of eight P
atoms in a pentagonal wedge structure (Fig. 10B). III. INORGANIC POLYMERS OF
Like phosphorus, elemental sulfur is a polyatomic,
THE MAIN GROUP ELEMENTS
polymorphic substance. Oligomeric sulfur rings that con-
tain from 6 to 12 sulfur atoms have been identified
A. Silicon-Based Polymers
(Fig. 11). The orthorhombic form, α-S 8 , has the most
thermodynamically stable structure and exists as a yel- 1. Polysiloxanes (Silicones)
low crystalline solid consisting of eight-member puck-
Polysiloxanes are chains, rings, ladders, and 3D network
ered rings in a crown conformation stacked together in
polymers that contain siloxane (Si O Si) bonds in the
“crankcase” fashion. The chain polymer form of sulfur
backbone. The basic structural units of polysiloxanes are
called catena or plastic sulfur is obtained by quenching
molten S 8 in water. If stretched, catena sulfur has a helical
polymeric structure. Selenium forms polymers similar to R R R O
those of sulfur. R Si O O Si O O Si O O Si O
Heteroatom polymers contain a metal or semimetal in R R O O
addition to one or more other atoms (e.g., O, S, N, P, and C) Monofunctional unit Difunctional unit Trifunctional unit Tetrafunctional unit
(end group) (linear backbone) (crosslink) (crosslink and network)
in the polymer backbone. It is well-known that polymer
stability and inertness increases when the heteroatom is
Chain polysiloxanes are composed of difunctional
electron rich. Presumably, such atoms serve as electron
units. The framing groups (R) are either H or organic
sources toward low-lying vacant orbitals on the metal or
moieties and the end groups are usually OR or a mono-
semimetal (Fig. 12). The resulting coordinate (dative) in- functional siloxyl unit. Chain polysiloxanes such as poly-
teraction generally decreases the nucleophilicity of the (dimethylsiloxane)s are synthesized by the hydrolysis of
metal or semimetal thereby reducing the susceptibility of dichlorodimethysilane [Eq. (1)].
the polymer to attack by Lewis acids and bases. Further-

FIGURE 6 Poly(diphenylsiloxane) ladder polymer. FIGURE 8 Homoatom and heteroatom polymers.