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Structural unit
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<h2 id="example">Example</h2> <p>Consider the example of <a href="/facts/Polyethylene_terephthalate/Sc5UhmCB">polyethylene terephthalate</a> (PET or "polyester"). The monomers which could be used to create this polymer are <a href="/facts/Ethylene_glycol/stFz6CVe">ethylene glycol</a> and <a href="/facts/Terephthalic_acid/ClRNc6m0">terephthalic acid</a>: </p><p>HO-CH2-CH2-OH and HOOC-C6H4-COOH </p><p>In the polymer, there are two structural units, which are -O-CH2-CH2-O- and -CO-C6H4-CO- </p><p>The repeat unit is </p><p>-CH2-CH2-O-CO-C6H4-CO-O- </p> <h2 id="functionality-of-structural-units">Functionality of structural units</h2> <p>The functionality of a monomeric structural unit is defined as the number of covalent bonds which it forms with other reactants.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> A structural unit in a linear polymer chain segment forms two bonds and is therefore bifunctional, as for the PET structural units above. </p><p>Other values of functionality exist. Unless the macromolecule is cyclic, it will have monovalent structural units at each end of the polymer chain. In <a href="/facts/Branching_(polymer_chemistry)/KYF9YA1j">branched polymers</a>, 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 <a href="/facts/Glycerol/sRQ6NBQQ">glycerol</a> 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. </p><p>Finally, the formation of <a href="/facts/Cross-link/zeQ9hEAA">cross-linked</a> polymers involves tetrafunctional structural units. For example, in the synthesis of cross-linked <a href="/facts/Polystyrene/dfIVntty">polystyrene</a>, a small fraction of monomeric <a href="/facts/Styrene/B8uXm78L">styrene</a> (or vinylbenzene) is replaced by 1,4-<a href="/facts/Divinylbenzene/5bueSnj4">divinylbenzene</a> (or <i>para</i>-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. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Repeat_unit/WIxA68X4">Repeat unit</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Definition of functionality in IUPAC Gold Book <a href="http://goldbook.iupac.org/FT07505.html" target="_blank">http://goldbook.iupac.org/FT07505.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> </ol>