In
polymer chemistry
Polymer chemistry is a sub-discipline of chemistry that focuses on the structures of chemicals, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry are a ...
, a structural unit is a building block of a
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
chain. It is the result of a
monomer
In chemistry, a monomer ( ; '' mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
...
which has been
polymerized
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many for ...
into a long chain.
There may be more than one structural unit in the
repeat unit
In polymer chemistry, a repeat unit or repeating unit (or mer) is a part of a polymer whose repetition would produce the complete polymer chain (except for the end-groups) by linking the repeat units together successively along the chain, like the ...
. When different monomers are polymerized, a
copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
is formed. It is a routine way of developing new properties for new materials.
Example
Consider the example of
polyethylene terephthalate (PET or "polyester"). The monomers which could be used to create this polymer are
ethylene glycol
Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
and
terephthalic acid:
HO-CH
2-CH
2-OH
and
HOOC-C
6H
4-COOH
In the polymer, there are two structural units, which are
-O-CH
2-CH
2-O-
and
-CO-C
6H
4-CO-
The repeat unit is
-CH
2-CH
2-O-CO-C
6H
4-CO-O-
Functionality of structural units
The functionality of a monomeric structural unit is defined as the number of covalent bonds which it forms with other reactants.
Definition of functionality
in IUPAC Gold Book
The International Union of Pure and Applied Chemistry publishes many books which contain its complete list of definitions. The definitions are divided into seven "colour books": Gold, Green, Blue, Purple, Orange, White, and Red. There is also an e ...
A structural unit in a linear polymer chain segment forms two bonds and is therefore bifunctional, as for the PET structural units above.
Other values of functionality exist. Unless the macromolecule is cyclic, it will have monovalent structural units at each end of the polymer chain. In branched polymers, there are trifunctional units at each branch point. For example, in the synthesis of PET, a small fraction of the ethylene glycol can be replaced by glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
which has three alcohol groups. This trifunctional molecule inserts itself in the polymeric chain and bonds to three carboxylic acid groups forming a branch point.
Finally, the formation of cross-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ed polymers involves tetrafunctional structural units. For example, in the synthesis of cross-linked polystyrene, a small fraction of monomeric styrene
Styrene () is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid, although aged samples can appear yellowish. The compound evaporates easily and has a sweet smell, although high concen ...
(or vinylbenzene) is remplaced by 1,4-divinylbenzene
Divinylbenzene (DVB) consists of a benzene ring bonded to two vinyl groups. It is related to styrene (vinylbenzene) by the addition of a second vinyl group.CRC Handbook of Chemistry and Physics 65Th Ed. It is a colorless liquid manufactured by ...
(or ''para''-divinylbenzene). Each of the two vinyl groups is inserted into a polymeric chain, so that the tetravalent unit is inserted into both chains, linking them together.
See also
*Repeat unit
In polymer chemistry, a repeat unit or repeating unit (or mer) is a part of a polymer whose repetition would produce the complete polymer chain (except for the end-groups) by linking the repeat units together successively along the chain, like the ...
References
Polymer chemistry
{{Polymer-stub