An essential concept which defines polymer structure, the repeat unit is the simplest structural entity of a polymer chain. So a polymer consists of several repeat units linked together successively along the chain, like the beads of a necklace. A repeat unit is sometimes called a mer or mer unit. "Mer" originates from the Greek word "meros," which means part. The word polymer derives its meaning from this, which means "many mers." A repeat unit (or mer), is not to be confused with the term monomer, which refers to the small molecule from which a polymer is synthesized[1].
One of the simplest repeat units is that of polyethylene:
-[CH2-CH2-]n-
while polypropylene has the repeat unit
-[CH2-CH(CH3)]n-
The subscript "n" denotes the degree of polymerisation, that is, the number of units linked together. The molecular mass of the repeat unit, MR, is simply the sum of the atomic masses of the atoms within the repeat unit. The molecular mass of the chain is just the product nMR. Other than monodisperse polymers, there is normally a molar mass distribution caused by chains of different length.
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Vinyl Polymers
More complex repeat units occur in vinyl polymers, where one of the hydrogen atoms is substituted by another different atom, or by a larger fragment. So PVC for example possesses a chain in which the hydrogen atom is substituted by a chlorine atom:
-[CH2-CHCl]n-
Polystyrene has a chain where the substituent is a phenyl group (C6H5), corresponding to a benzene ring minus one hydrogen:
-[CH2-CH(C6H5)]n-
See also
Sources
- ^ Callister p.492
References
- Structure of polymer chains
- Callister, William D. (2007). Materials science and engineering : an introduction (7th ed.) New York : John Wiley & Sons. ISBN 978-0-471-73696-7
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