A monomer is a repeating structure, or unit, within a type of large molecule known as a polymer. The word comes from the Greek mono meaning one and meros meaning part; it is one of many similar parts of a long chain that makes up the molecule. Monomers bond together to form polymers during a chemical reaction called polymerization, where the molecules link together by sharing electrons in what is called a covalent bond. They can also link with one another to form smaller structures: a dimer is made up of two monomers and a trimer, three, for example. Polymers may consist of many thousands of these units.
The structural properties of a polymer depend upon the arrangement of the monomers of which it is composed. This may affect its solubility in water, its melting point, its chemical reactivity or its durability. Two polymers can contain the same monomer molecules, but due to their arrangements, they can have different properties.
A key feature of a monomer unit is its ability to link to at least two other molecules. The number of molecules a unit is able to link up with is determined by the number of active sites where the covalent bonds can be formed. If it can only join with two other molecules, chain-like structures are formed. If it can link up with three or more other monomers, more advanced, three dimensional, cross-linked polymers can be built. The element carbon forms the basis of most polymers, as it is one of the few elements that can bond in four different directions with four other atoms.
The bonding process does not necessarily involve two monomer units simply joining together. In many cases, each unit loses one or two atoms, which form another product. For example, one unit may give up a hydrogen atom, and the other a hydroxyl, or hydrogen-oxygen, group in order to form a bond, producing water (H2O) as a by-product. This type of polymerization is known as a condensation reaction.
Types of Polymer
A polymer that consists entirely of one type of monomer unit is called a homopolymer. If there is more than one type of unit, this is known as a co-polymer. These can be grouped into different categories, depending on how the units are arranged:
- Alternating: two different units alternate with one another, for example, …ABABAB…
- Periodic: a given sequence of units is repeated, for example, …ABCABCABC…
- Block: two or more different homopolymers are bonded together, for example, …AAAABBBB…
- Statistical: the sequence of units has no fixed pattern, but certain combinations are more likely than others
- Random: the sequence has no discernible pattern
One of the most common natural monomers is glucose, a simple carbohydrate. It can join with other glucose molecules in various ways to form a number of different polymers. Cellulose, found in the cell walls of plants, consists of chains of glucose molecules up to 10,000 or more units long, giving it a fibrous structure. In starch, the glucose units form branched chains. The numerous branch ends form points at which enzymes can begin breaking the molecule down, making it more easily digestible than cellulose.
Other examples are amino acids, which can join together to form proteins, and nucleotides, which can polymerize along with certain carbohydrate compounds to form DNA and RNA, the molecules on which all known life is based. Isoprene, a hydrocarbon compound found in many plants, can polymerize into natural rubber. The elasticity of this substance is due to the fact that the units form coiled chains that can be stretched out, and which will contract back into a coiled state when released.
Many synthetic polymers have been produced, and they include everyday materials such as plastics and adhesives. Often, the monomers from which they are constructed are naturally occurring compounds, although they may often be produced synthetically. In most cases, these compounds are hydrocarbons — molecules containing only carbon and hydrogen.
One example is ethylene (C2H4, a simple hydrocarbon that is produced by plants, but which is manufactured on a large scale from petroleum. It can be polymerized to form polyethylene — sometimes called polythene — the most commonly used plastic. In essence, it is made by converting the double bond between ethylene’s two carbon atoms into a single bond, leaving each one able to form another single bond with a neighboring atom of carbon, and allowing long chains to form. Other examples are propylene and styrene, which are used to manufacture polypropylene and polystyrene, respectively.