polystyrene A clear glasslike ma-
terial manufactured by free-radical
polymerization of phenylethene
(styrene) using benzoyl peroxide as
an initiator. It is used as both a ther-
mal and electrical insulator and for
packing and decorative purposes.
polysulphides See sulphides.
polytetraÛuoroethene (PTFE)A
thermosetting plastic with a high
softening point (327°C) prepared
by the polymerization of tetra-
Ûuoroethene under pressure (45–50
atmospheres). The reaction requires
an initiator, ammonium peroxosul-
phate. The polymer has a low coefÜ-
cient of friction and its ‘anti-stick’
properties are probably due to its he-
lical structure with theÛuorine
atoms on the surface of an inner ring
of carbon atoms. It is used for coating
cooking utensils and nonlubricated
bearings.
polytheneSee polyethene.
polythionate A salt of a poly-
thionic acid.
polythionic acidsOxo acids of sul-
phur with the general formula
HO.SO 2 .Sn.SO 2 .OH, where n = 0–4. See
also sulphuric acid.
polyurethane A polymer contain-
ing the urethane group –NH.CO.O–,
prepared by reacting di-isocyanates
with appropriate diols or triols. A
wide range of polyurethanes can be
made, and they are used in adhe-
sives, durable paints and varnishes,
plastics, and rubbers. Addition of
water to the polyurethane plastics
turns them into foams.
polyvinylacetate(PVA) A thermo-
plastic polymer used in adhesives
and coatings. It is made by polymer-
izing vinyl acetate (CH 2 :CHCOOCH 3 ).
polyvinyl alcoholA water-soluble
polymer made from polyvinylacetate
(PVA) by hydrolysis with sodium hy-
droxide. It is used for making adhe-
sives that are miscible with water, as
a size for textiles, and for making
syntheticÜbres.
polyvinyl chlorideSee poly-
chloroethene.polyyneAn unsaturated hydrocar-
bon that contains two or more triple
carbon–carbon bonds in its molecule.POPSee persistent organic pollu-
tant.population inversionSee laser.
porphyrinAny of a group of or-
ganic pigments characterized by the
possession of a cyclic group of four
linked nitrogen-containing rings (a
tetrapyrrole nucleus), the nitrogen
atoms of which are often coordinated
to metal ions. Porphyrins differ in
the nature of their side-chain groups.
They include the *chlorophylls,
which contain magnesium; and
*haem, which contains iron and
forms the *prosthetic group of
haemoglobin, myoglobin, and the cy-
tochromes. Porphyrins are also used
in host–guest chemistry. See
supramolecular chemistry.429 potash
p
NH NN NHPorphyrinpositronium See exotic atom.potashAny of a number of potas-
sium compounds, such as the carbon-
ate or the hydroxide.