Monomer

A monomer is a molecule or compound, usually containing carbon, with a relatively low molecular weight and simple structure; monomers form the fundamental building blocks of polymers, synthetic resins, and elastomers. Thus, vinylidene chloride is the monomer from which polyvinylidene chloride is made, and styrene is the monomer from which polystyrene resins are produced.

One of the simplest monomers, ethylene, consists of two carbon atoms (represented by C) linked by a double bond, with two hydrogen atoms (represented by H) connected to each carbon.

CH₂=CH₂

In ethylene the carbon-hydrogen bonds are single covalent bonds, while the carbon-carbon bond is a double covalent bond. In the presence of heat, light, and an appropriate catalyst the ethylene molecule can be excited into a reactive state in which the carbon-carbon double bond is dissociated.

– CH₂ – CH_{2 –}

If two excited molecules come into contact, it is possible to link the ethylene molecules with single covalent bonds between molecules. When this process repeats itself, on the order of thousands of times, a chain is produced with a carbon backbone and a formula H-(CH₂–CH₂)_n–H

The building unit or monomer in this case is ethylene. After n units of ethylene have been linked, the reaction may be terminated by the addition of a hydrogen to each end of the polyethylene chain. Such polymerizations are referred to as addition polymerization, in contrast to condensation polymerization in which molecules with diverse end groups react to give chains composed of units of varying size and chemical complexity. Nylon 66 is the condensation product of two monomers: adipic acid and hexamethylenediamine.

There are a number of other polymers formed by addition polymerization from compounds much like ethylene

CH₂=CRH

where R may be a halide, a benzene ring, etc. These are called vinyl compounds. Table 1 lists some of the frequently encountered vinyl monomers.

The dienes constitute another class of addition polymers. Dienes monomers have the following generic structure:

CH₂=CR–CH=CH ₂

Table 2 lists some of the common diene monomers.

The vinylidenes constitute yet another class of polymers. Vinylidenes have monomers with the following generic structure

CX₂=CR₂

Table 3 lists some of the common vinylidene monomers.

When monomers of different kinds are united by addition polymerization, the product is known as a copolymer. Table 4 identifies some copolymers.

The Polymer Handbook (J. Brandrup and E.H. Immergut, eds., 3rd. ed., Wiley Interscience, New York, 1990) lists hundreds of other monomers in commercial use. The number of polymers that can be produced synthetically from monomers would seem to be limited only by the imagination and ingenuity of the chemist.

See also Polymer.