e.g. polyethene. Sometimes brackets
are used in polymer names to in-
dicate the repeated unit, e.g.
poly(ethene).
polyacrylamideA polymer
formed from 2-propenamide
(CH 2 :CHCONH 2 ). Polyacrylamide gels
are made using a cross-linking agent
to form three-dimensional matrices.
They are used in gel electrophoresis
(see page).
polyacrylamide gel electro-
phoresis See page.
polyamideA type of condensation
polymer produced by the interaction
of an amino group of one molecule
and a carboxylic acid group of an-
other molecule to give a protein-like
structure. The polyamide chains are
linked together by hydrogen bond-
ing.
polyatomic molecule A molecule
formed from several atoms (e.g. pyri-
dine, C 5 H 5 N, or dinitrogen tetroxide,
N 2 O 4 ), as distinguished from di-
atomic and monatomic molecules.
polybasic acid An acid with more
than one replaceable hydrogen atom.
Examples include the dibasic sul-
phuric acid (H 2 SO 4 ) and tribasic phos-
phoric(V) (orthophosphoric) acid
(H 3 PO 4 ). Replacement of all the hy-
drogens by metal atoms forms nor-
mal salts. If not all the hydrogens are
replaced, *acid salts are formed.
polychlorinated biphenyl (PCB)
Any of a number of derivatives of
biphenyl (C 6 H 5 C 6 H 5 ) in which some
of the hydrogen atoms on the ben-
zene rings have been replaced by
chlorine atoms. Polychlorinated
biphenyls are used in the manufac-
ture of certain polymers as electrical
insulators. They are highly toxic and
are suspected to be carcinogenic;
their increasing use has caused con-
cern because they have been shown
to accumulate in the food chain.polychloroethene (PVC; polyvinyl
chloride)A tough white solid ma-
terial, which softens with the appli-
cation of a plasticizer, manufactured
from chloroethene by heating in an
inert solvent using benzoyl peroxide
as an initiator, or by the free-radical
mechanism initiated by heating
chloroethene in water with potas-
sium persulphate or hydrogen perox-
ide. The polymer is used in a variety
of ways, being easy to colour and re-
sistant toÜre, chemicals, and
weather.
polycyclicDenoting a compound
that has two or more rings in its mol-
ecules. Polycyclic compounds may
contain single rings (as in phenylben-
zene, C 6 H 5 .C 6 H 5 ) or fused rings (as in
naphthalene, C 10 H 8 ).polydioxoboric(III) acidSee boric
acid.polyeneAn unsaturated hydrocar-
bon that contains two or more dou-
ble carbon–carbon bonds in its
molecule.polyesterA condensation polymer
formed by the interaction of polyhy-
dric alcohols and polybasic acids.
Linear polyesters are saturated ther-
moplastics and linked by dipole–
dipole attraction as the carbonyl
groups are polarized. They are exten-
sively used asÜbres (e.g. Terylene).
Unsaturated polyesters readily
copolymerize to give thermosetting
products. They are used in the manu-
facture of glass-Übre products. See also
alkyd resin.polyethene(polyethylene; poly-
thene)AÛexible waxy translucent
polyalkene thermoplastic made in a
variety of ways producing a polymer
of varying characteristics. In the ICI
process, ethene containing a trace of427 polyethene
p