oxygen is subjected to a pressure in
excess of 1500 atmospheres and a
temperature of 200°C. Low-density
polyethene (r.d. 0.92) has a formula
weight between 50 000 and 300 000,
softening at a temperature around
110 °C, while the high-density poly-
thene (r.d. 0.945–0.96) has a formula
weight up to 3 000 000, softening
around 130°C. The low-density poly-
mer is less crystalline, being more at-
actic. Polyethene is used as an
insulator; it is acid resistant and is
easily moulded and blown. See
phillips process; ziegler process.polyethyleneSee polyethene.polyhydric alcohol (polyol)An
*alcohol that has several hydroxyl
groups per molecule.
polymerA substance having large
molecules consisting of repeated
units (the monomers). See Feature
pp. 430–431.
A- Information about IUPAC nomenclature
polymerizationA chemical reac-
tion in which molecules join to-
gether to form a polymer. If the
reaction is an addition reaction, the
process is addition polymerization;
condensation reactions cause con-
densation polymerization, in which a
small molecule is eliminated during
the reaction. Polymers consisting of a
single monomer are homopolymers;
those formed from two different
monomers are copolymers.polymethanalA solid polymer of
methanal, formed by evaporation of
an aqueous solution of methanal.polymethylmethacrylateA clear
thermoplastic acrylic material made
by polymerizing methyl meth-
acrylate. The technical name is
poly(methyl 2-methylpropenoate). It
is used in such materials as Perspex.polymorphismThe existence of
chemical substances in two (dimor-
phism) or more physical forms. See
allotropy.
polyolSee polyhydric alcohol.polypeptideA *peptide compris-
ing ten or more amino acids.
Polypeptides that constitute proteins
usually contain 100–300 amino acids.
Shorter ones include certain antibiot-
ics, e.g. gramicidin, and some hor-
mones, e.g. ACTH, which has 39
amino acids. The properties of a
polypeptide are determined by the
type and sequence of its constituent
amino acids.polypropene (polypropylene) An
isotactic polymer existing in both
low and high formula-weight forms.
The lower-formula-weight polymer is
made by passing propene at moder-
ate pressure over a heated phos-
phoric acid catalyst spread on an
inert material at 200°C. The reaction
yields the trimer and tetramer. The
higher-formula-weight polymer is
produced by passing propene into an
inert solvent, heptane, which con-
tains a trialkyl aluminium and a tita-
nium compound. The product is a
mixture of isotactic and atactic
polypropene, the former being the
major constituent. Polypropene is
used as a thermoplastic moulding
material.
polypropylene See polypropene.polysaccharideAny of a group of
carbohydrates comprising long
chains of monosaccharide (simple-
sugar) molecules. Homopolysaccha-
ridesconsist of only one type of
monosaccharide; heteropolysaccha-
ridescontain two or more different
types. Polysaccharides may have mo-
lecular weights of up to several mil-
lion and are often highly branched.
Some important examples are starch,
glycogen, and cellulose.polyethylene 428p