- Repeat unit
An essential concept which defines
polymerstructure, the repeat unit is the simplest structural unit of a polymer chain. So a polymer consists of repeat units linked together, like the beads of a necklace. One of the simplest repeat units is that of polyethylene:
- [CH2-CH2-] n-
polypropylenehas 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 monodispersepolymers, there is normally a molar mass distributioncaused by chains of different length.
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 PVCfor example possesses a chain in which the hydrogen atom is substituted by a chlorineatom:
- [CH2-CHCl] n-
Polystyrenehas a chain where the substituent is a phenyl group(C6H5), corresponding to a benzene ring minus one hydrogen:
- [CH2-CH(C6H5)] n-
* [http://openlearn.open.ac.uk/mod/resource/view.php?id=196629 Structure of polymer chains]
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