A monomer is a small molecule capable of bonding with other molecules to form larger molecules known as polymers. The combination of two monomers is known as dimer; of three monomers, a trimer; of four monomers, a tetramer; and so on. The term oligomer is used for the combination of a small number of monomers, and polymer for a large number--often a very large number--of monomers. Larger monomer combinations are often given numerical names, such as 18-mers (eighteen). Monomers are the starting materials for a wide range of materials. Numerous kinds are available from both natural and synthetic sources. The name monomer comes from the Greek words mono, meaning one, and meros, meaning part.
While polymers have been used by humans for centuries, it was not until the early twentieth century that their chemistry was understood. The groundwork for modern understanding of polymer science was laid by Nobel laureate Hermann Staudinger (1881-1965), a German chemist and molecular biologist, in the 1920s. He theorized that polymers were composed of simpler molecules, or monomer units, that were linked together in a chainlike fashion. In 1928, German chemists Kurt H. Meyer (1882-1952) and Herman F. Mark (1895-1992) used x-ray techniques to confirm Staudinger's models. By the 1930s, the monomer/polymer model was accepted by most polymer scientists.
The first type of polymers that were used by humans were naturally occurring. The primary categories for natural polymers include polysaccharides, proteins, and nucleic acids. These materials are made up of specific monomers including saccharides, amino acids, and nucleotides, respectively.
Amino acids are the basic monomer building blocks for proteins. They contain an amino group (-NH2), a carboxylic acid group (-COOH), and a side chain that differs from one amino acid to the next. Twenty common amino acids, when bonded together, comprise the protein structures. The simplest amino acid is glycine, which contains a hydrogen atom as its side group. Other examples of amino acids are alanine, leucine, cysteine, arginine, and tryptophan.
Simple carbohydrates, or monosaccharides, are monomers that bond together to form polysaccharides. Monosaccharides are composed of a number of linked carbon atoms and a functional group. They are classified by these two characteristics. Aldoses have an aldehyde functional group while ketoses contain a ketone. The most important monosaccharides are hexoses such as glucose and galactose. They react to form polymers such as starch, glycogen, chitin, and cellulose.
Another type of natural monomer is the nucleotides. These compounds create the polymers ribonucleic and deoxyribonucleic acids, which store the genetic information common to all living organisms. Nucleotides consist of three distinct sections: the cyclic five-carbon monosaccharide ribose, a phosphate diester (that links the riboses by connection to the hydroxy groups of the riboses), and a heterocyclic base. The bases (thus termed because they contain basic nitrogen atoms) are purines or pyrimidines, which provide the code which is translated into proteins in living systems. Nucleotides are linked together by the phosphate groups to form polynucleotide polymers.
Synthetic polymers are produced through two types of polymerization reactions. Condensation reactions depend on a repeated reaction between functional groups on the monomers. Addition polymerization reactions use free radical monomers to produce polymers. The chemistry of these monomers is slightly different.
Monomers involved in condensation reactions typically contain two reactive groups within the molecule. For example, polyesters are created by the reaction of a dihydric alcohol, such as ethylene glycol, which contains two hydroxyl (-OH) groups and a diacid, like adipic acid, containing two carboxyl groups (-COOH). During the reaction, these groups can react with either end of the monomer. This reaction continues until there are no reactive groups left. The result is a long chain, high molecular weight polymer. The production of polysulfide polymers is a result of a condensation reaction between dialkalide monomers with sulfide monomers. Nylon is produced from diamine and diacid monomers. These molecules have reactive amine (-NH) groups that bond with the carboxyl groups. Polyurethanes are created from dihydric alcohols and diisocyanate monomers. Other condensation monomers include epichlorohydrin, phenol, and formaldehyde.
Addition polymerization reactions are characterized by monomers that contain double bonds. When polymerization is initiated, the double bond is broken and a free radical is formed. This free radical attacks the double bonds on other monomers creating a longer chain molecule and an additional free radical. The reaction continues until a termination step is reached. Important monomers that undergo these reactions include styrene, methylmethacrylate, vinyl acetate, vinyl chloride, and other acrylates.