FREE ESSAY ON POLYMERS

POLYMERS

Polymer, substance consisting of large molecules that are made of many small, repeating
units called monomers, or mers. The number of repeating units in one large molecule is
called the degree of polymerization. Materials with a very high degree of polymerization
are called high polymers. Polymers consisting of only one kind of repeating unit are
called homopolymers. Copolymers are formed from several different repeating units.
Most of the organic substances found in living matter, such as protein, wood, chitin,
rubber, and resins, are polymers. Many synthetic materials, such as plastics, fibers,
adhesives, glass, and porcelain, are also to a large extent polymeric substances.
Structure of Polymers 
Polymers can be subdivided into three, or possibly four, structural groups. The molecules
in linear polymers consist of long chains of monomers joined by bonds that are rigid to a
certain degree-the monomers cannot rotate freely with respect to each other. Typical
examples are polyethylene, polyvinyl alcohol, and polyvinyl chloride (PVC).
Branched polymers have side chains that are attached to the chain molecule itself.
Branching can be caused by impurities or by the presence of monomers that have several
reactive groups. Chain polymers composed of monomers with side groups that are part of
the monomers, such as polystyrene or polypropylene, are not considered branched
polymers.
In cross-linked polymers, two or more chains are joined together by side chains. With a
small degree of cross-linking, a loose network is obtained that is essentially two
dimensional. High degrees of cross-linking result in a tight three-dimensional structure.
Cross-linking is usually caused by chemical reactions. An example of a two-dimensional
cross-linked structure is vulcanized rubber, in which cross-links are formed by sulfur
atoms. Thermosetting plastics are examples of highly cross-linked polymers; their
structure is so rigid that when heated they decompose or burn rather than melt.
Synthesis 
Two general methods exist for forming large molecules from small monomers: addition
polymerization and condensation polymerization. In the chemical process called addition
polymerization, monomers join together without the loss of atoms from the molecules. Some
examples of addition polymers are polyethylene, polypropylene, polystyrene, polyvinyl
acetate, and polytetrafluoroethylene (Teflon).
In condensation polymerization, monomers join together with the simultaneous elimination
of atoms or groups of atoms. Typical condensation polymers are polyamides, polyesters,
and certain polyurethanes.
In 1983 a new method of addition polymerization called group transfer polymerization was
announced. An activating group within the molecule initiating the process transfers to
the end of the growing polymer chain as individual monomers insert themselves in the
group. The method has been used for acrylic plastics; it should prove applicable to other
plastics as well.
Bibliography
Polymer, Microsoft? Encarta? 98 Encyclopedia. ? 1993-1997 Microsoft Corporation. All
rights reserved.