Page 181 - 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

Polymers, Inorganic and Organometallic 689

FIGURE 31 Synthesis of polythiazyl.

FIGURE 30 Boron–oxygen polymers. D. Other Main Group Element Polymers
1. Heteroatom–Sulfur Polymers
which is prepared by warming liquid borazine under vac-
Catenated sulfur polymers were discussed earlier. The low
uum, gives α-BN [Eq. (26)]:
cost and ready supply of sulfur has led to the development
of a number of heteroatom polymers involving sulfur with
H H H H
other main group elements (e.g., N, C, O, P, B, etc.). Sev-
N B N B eral examples with their applications are given in Table IV.
70 C 1200 C
n H B N H B N -BN A particularly interesting polymer of sulfur and nitro-
vacuum
N B N B gen known as polythiazyl or poly(sulfur nitride) is ob-
H H H H n tained by the sequence of reactions illustrated in Fig. 31.
Thepolymeriscrystallineandfibrouslikeasbestos.Ithasa
(26) gold-luster color when viewed perpendicular to the chain
axis and behaves in many ways like a metal: high mal-
The advantage of preceramic polymers is that they are leability, reflectivity, and electrical conductivity at room
soluble in polar solvents, readily purified, and produce temperature. Moreover, it exhibits anisotropic supercon-
ceramic materials in high purity and high yield. ductivity at 0.26 K. The conductivity has been explained
Like silicon, boron forms a large number of anionic in term of the polymer structure (Fig. 32) and bonding in
oligomers and polymers with oxygen (borates). Some which long S N chains assume a cis–trans–planar con-
of these anions have been well characterized. For ex- formation with π-bond delocalization along the chains
ample, anhydrous metal metaborates have the general and from S S, S N, and N N orbital overlap between
formula M x (BO 2 ) y . These polymeric salts are stable at adjacent chains. Polythiazyl absorbs bromine to produce
atmospheric pressure and contain the cyclotrimeric B 3 O 3− (SNBr x ) n (x = 0.25–0.40), a black, fibrous polymer with
6
anion or the infinite linear (BO 2 ) n− anion where each B increased conductivity. Despite their unusual properties,
n
atom is trivalent (Fig. 30). At high pressures, however, polythiazyls have not been commercialized because of
they undergo reorganization to tetravalent boron with a their oxidative and thermal instability in air and their ten-
zinc-blend superstructure. dency to detonate with heat and pressure.

TABLE IV Sulfur-Containing Polymers and Applications
Polymer General formula Applications
Poly(thioethers) [ R S ] n Engineering plastics, elastomers, photoresists,
optical filters, heat-resistant adhesives
Poly(thiazoles) Conducting polymers, heat-dissipating coatings
N N
R
S S n
Poly(sulfamides) [ N S ] n Coatings for imaging paper
O R
Poly(thiophenes) S Antistatic coatings and films, biosensors,
[ n ] storage batteries
S
Poly(sulfones) O Engineering plastics
[ R S ] n
O