light yellow. Some PCBs can exist as a vapor in air.
PCBs have no known smell or taste.
PCBs have been used as coolants and lubricants in
transformers, capacitors, and other electrical equip-
ment because they do not burn easily and are good
insulators. The manufacture of PCBs was stopped in
the United States in 1977 because of evidence that they
build up in the environment and can cause harmful
health effects. Cleanup of PCB-contaminated sites is an
ongoing controversy.
polycyclic A cyclic structure containing many closed
and fused rings of bonds, e.g., polycyclic AROMATIC
HYDROCARBONs (PAH), hydrocarbons with multiple
benzene rings such as naphthalene, the benzo pyrenes,
fluoranthene, and chrysene. PAHs are typical compo-
nents of asphalt, fuel, oils, and greases.
polydent A ligand that has two or more donor sites
that can be used simultaneously.
See alsoAMBIDENT.
polydentate SeeCHELATION; DONOR ATOM SYMBOL.
polyene A compound containing more than one dou-
ble bond per molecule.
polyester A group of polymers that consist basically
of repeated units of an ester and are used especially in
making fibers or plastics. A strong and durable human-
made fabric that was used in many types of clothing
made popular during the 1970s.
polyethylene A plastic made from ethylene. High-
density forms are used in the manufacturing of bottles
and other products that produce toxic fumes when
burned and thus are often recycled.
polyhedral symbol The polyhedral symbol indicates
the geometrical arrangements of the coordinating
atoms about the CENTRAL ATOM. It consists of one or
more capital italic letters derived from common geo-
metric terms (tetrahedron, square plane, octahedron,
etc.), which denote the idealized geometry of the LIG-
ANDs around the COORDINATIONcenter, and an Arabic
numeral that is the coordination number of the central
atom. The polyhedral symbol is used as an affix,
enclosed in parentheses, and separated from the name
by a hyphen. Examples are T-4, SP-4, TBPY-5, SPY-5,
OC-6, and CU-8.polymer A macromolecule of high relative molecular
mass composed of many similar or identical MONOMERs
linked together in chains. PLASTICs are polymers.polymerase chain reaction(PCR) A laboratory
technique used to rapidly amplify predetermined
regions of double-stranded DNA. Generally involves
the use of a heat-stable DNA polymerase.polymerization The creation of large molecules by
combining many small ones.polymorphous Materials that crystallize in more
than one crystalline arrangement.polypeptide A polymer chain of amino acids linked
by covalent peptide bonds. One or more polypeptides
form proteins. Each polypeptide has two terminal ends:
one called the amino terminal, or N-terminal, has a free
amino group; the other end is called the carboxyl ter-
minal, or C-terminal, with a free carboxyl group.polyprotic acid An acid that can give up more than
one proton per molecule in a reaction. Examples
include sulfuric acid (H 2 SO 4 ).polysaccharide A carbohydrate (polymer) made by
polymerizing over 1,000 monosaccharides; a complex
sugar.p orbital An electron with one unit of angular
momentum. Orbital letters are associated with the
angular-momentum quantum number, which is
assigned an integer value from 0 to 3 and where scor-p orbital 217