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 polythene (r.d. 0.945–0.96) has a formula
weight up to 3 000 000, softening
around 130°C. The low-density polymer is less crystalline, being more atactic. Polyethene is used as an
insulator; it is acid resistant and is
easily moulded and blown. See
phillips process; ziegler process.
polyethylene See polyethene.
polyhydric alcohol (polyol) An
*alcohol that has several hydroxyl
groups per molecule.
polymer A substance having large
molecules consisting of repeated
units (the monomers). See Feature
pp. 430–431.
A
• Information about IUPAC nomenclature
polymerization A chemical reaction in which molecules join together to form a polymer. If the
reaction is an addition reaction, the
process is addition polymerization;
condensation reactions cause condensation 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.
polymethanal A solid polymer of
methanal, formed by evaporation of
an aqueous solution of methanal.
polymethylmethacrylate A clear
thermoplastic acrylic material made
by polymerizing methyl methacrylate. The technical name is
poly(methyl 2-methylpropenoate). It
is used in such materials as Perspex.
polymorphism The existence of
chemical substances in two (dimorphism) or more physical forms. See
allotropy.
polyol See polyhydric alcohol.
polypeptide A *peptide comprising ten or more amino acids.
Polypeptides that constitute proteins
usually contain 100–300 amino acids.
Shorter ones include certain antibiotics, e.g. gramicidin, and some hormones, 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 moderate pressure over a heated phosphoric 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 contains a trialkyl aluminium and a titanium 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.
polysaccharide Any of a group of
carbohydrates comprising long
chains of monosaccharide (simplesugar) molecules. Homopolysaccharides consist of only one type of
monosaccharide; heteropolysaccharides contain two or more different
types. Polysaccharides may have molecular weights of up to several million and are often highly branched.
Some important examples are starch,
glycogen, and cellulose.
polyethylene
428
p
www.AzShimi.ir www.AzShimi.com
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 polythene (r.d. 0.945–0.96) has a formula
weight up to 3 000 000, softening
around 130°C. The low-density polymer is less crystalline, being more atactic. Polyethene is used as an
insulator; it is acid resistant and is
easily moulded and blown. See
phillips process; ziegler process.
polyethylene See polyethene.
polyhydric alcohol (polyol) An
*alcohol that has several hydroxyl
groups per molecule.
polymer A substance having large
molecules consisting of repeated
units (the monomers). See Feature
pp. 430–431.
A
• Information about IUPAC nomenclature
polymerization A chemical reaction in which molecules join together to form a polymer. If the
reaction is an addition reaction, the
process is addition polymerization;
condensation reactions cause condensation 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.
polymethanal A solid polymer of
methanal, formed by evaporation of
an aqueous solution of methanal.
polymethylmethacrylate A clear
thermoplastic acrylic material made
by polymerizing methyl methacrylate. The technical name is
poly(methyl 2-methylpropenoate). It
is used in such materials as Perspex.
polymorphism The existence of
chemical substances in two (dimorphism) or more physical forms. See
allotropy.
polyol See polyhydric alcohol.
polypeptide A *peptide comprising ten or more amino acids.
Polypeptides that constitute proteins
usually contain 100–300 amino acids.
Shorter ones include certain antibiotics, e.g. gramicidin, and some hormones, 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 moderate pressure over a heated phosphoric 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 contains a trialkyl aluminium and a titanium 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.
polysaccharide Any of a group of
carbohydrates comprising long
chains of monosaccharide (simplesugar) molecules. Homopolysaccharides consist of only one type of
monosaccharide; heteropolysaccharides contain two or more different
types. Polysaccharides may have molecular weights of up to several million and are often highly branched.
Some important examples are starch,
glycogen, and cellulose.
polyethylene
428
p
www.AzShimi.ir www.AzShimi.com