One of the most common natural monomers is glucose, a simple carbohydrate.
A monomer is a structure or repeating 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 join together to form polymers during a chemical reaction called polymerization, where molecules bond by sharing electrons in what is called a covalent bond. They can also link together to form smaller structures: a dimer is made up of two monomers and a trimer is made up of three, for example. Polymers can consist of many thousands of these units.
The structural properties of a polymer depend on the arrangement of the monomers of which it is composed. This can affect its water solubility, its melting point, its chemical reactivity, or its durability. Two polymers can contain the same monomer molecules, but because of their arrangement, they can have different properties.
A key feature of a monomeric unit is its ability to bind at least two other molecules. The number of molecules with which a unit is able to bond is determined by the number of active sites where covalent bonds can be formed. If it can only join two other molecules, chain-like structures will form. If it can connect 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 linking process does not necessarily involve two monomer units simply joined together. In many cases, each unit loses one or two atoms, which form another product. For example, one unit may give 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 copolymer. They can be grouped into different categories depending on how the units are organized:
Alternating: two different units alternate, e.g.… ABABAB… Periodical: a given sequence of units is repeated, e.g.… ABCABCABC… Block: two or more different homopolymers are bonded together, e.g. … AAAABBBB … Statistics: 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 other glucose molecules in a variety of ways to form many different polymers. Cellulose, found in plant cell walls, consists of chains of glucose molecules up to 10,000 or more units in length, giving it a fibrous structure. In starch, glucose units form branched chains. The numerous ends of the branches form points at which enzymes can begin to break down the molecule, making it more easily digestible than cellulose.
Other examples are amino acids, which can join together to form proteins, and nucleotides, which can polymerize together 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 and that contract again to a coiled state when released.
Many synthetic polymers have been produced and include everyday materials such as plastics and adhesives. Often, the monomers from which they are constructed are naturally occurring compounds, although they can often be produced synthetically. In most cases, these compounds are hydrocarbons – molecules containing only carbon and hydrogen.
An example is ethylene (C 2 H 4 , a simple hydrocarbon that is produced by plants but 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 done by converting the double bond between the two carbon atoms of ethylene into a single bond, leaving each capable of forming another single bond with a neighboring carbon atom and allowing the formation of long chains. Other examples are propylene and styrene, which are used in the manufacture of polypropylene and polystyrene, respectively.