Monomers are repeating units that bond together to form polymers through polymerization. The arrangement of monomers affects the properties of the polymer. Monomers can bond with two or more molecules, forming chain or crosslinked polymers. Natural monomers include glucose, amino acids, and nucleotides, while synthetic polymers include plastics and adhesives made from hydrocarbons like ethylene.
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, in which molecules bond by sharing electrons in what is called a covalent bond. They can also bond with each other to form smaller structures: a dimer is made up of two monomers and a trimer, 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, melting point, chemical reactivity or shelf life. Two polymers can contain the same monomer molecules, but due to their arrangement they can have different properties.
Bonding
A key feature of a monomer unit is its ability to bind to at least two other molecules. The number of molecules a unit is able to bond with is determined by the number of active sites where covalent bonds can form. If it can only join with two other molecules, chain structures are formed. If it can bond with three or more other monomers, more advanced, three-dimensional crosslinked polymers can be built. The element carbon forms the basis of most polymers, as it is one of the few elements capable of bonding in four different directions with four other atoms.
The bonding process does not necessarily involve two monomer units simply bonding together. In many cases, each unit loses one or two atoms, which form another product. For example, one unit may donate a hydrogen atom and the other a hydroxyl group, or hydrogen-oxygen, to form a bond, producing water (H2O) as a byproduct. 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. These can be grouped into several categories, depending on how the units are arranged:
Alternate: Two different units alternate with each other, for example …ABABAB…
Periodic: a given sequence of units repeats itself, for example …ABCABCABC…
Block: two or more different homopolymers are linked together, for example …AAAABBBB…
Statistic: The sequence of units has no fixed pattern, but some combinations are more likely than others
Random: The sequence has no recognizable pattern
Natural monomers
One of the most common natural monomers is glucose, a simple carbohydrate. It can join with other glucose molecules in a variety of ways to form a number of different polymers. Cellulose, found in the cell walls of plants, is made up of chains of glucose molecules up to 10,000 or more units long, giving it a fibrous structure. In starch, glucose units form branched chains. The numerous ends of the branches form points where 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 upon 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 spiral chains which can be stretched and which will contract back into a spiral state when released.
Artificial polymers
Many synthetic polymers have been produced and include everyday materials such as plastics and adhesives. Often, the monomers from which they are constructed are natural 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 (C2H4, a simple hydrocarbon produced by plants, but produced on a large scale from petroleum. It can be polymerized to form polyethylene, sometimes called polythene, the most commonly used plastic. Basically, it is made by converting the double bond between the two carbon atoms of ethylene in a single bond, leaving each able to form another single bond with a neighboring carbon atom, and allowing long chains to form. Other examples are propylene and styrene, which are used to produce polypropylene and polystyrene respectively.
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