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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Repeat unit</span></span>
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<p>A <b>repeat unit</b> or <b>repeating unit</b> , or <b>mer</b>, is a part of a <a href="Polymer" title="Polymer">polymer</a> whose repetition would produce the complete polymer chain (except for the <a href="End_group" title="End group">end groups</a>) by linking the repeat units together successively along the chain, like the beads of a necklace.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>
<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
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<div class="quotebox-title" style=""><a href="International_Union_of_Pure_and_Applied_Chemistry" title="International Union of Pure and Applied Chemistry">IUPAC</a> definitions</div>
<blockquote class="quotebox-quote left-aligned" style="">
<p><b>constitutional unit</b>: An atom or group of atoms (with pendant atoms or groups, if any) comprising a part of the essential structure of a macromolecule, an oligomer molecule, a block or a chain.<sup id="cite_ref-Gold_Book_&quot;constitutional_unit&quot;_3-0" class="reference"><a href="#cite_note-Gold_Book_&quot;constitutional_unit&quot;-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><br>
<b>constitutional repeating unit</b>: The smallest constitutional unit the repetition of which constitutes a regular macromolecule, a regular oligomer molecule, a regular block or a regular chain.<sup id="cite_ref-Gold_Book_&quot;constitutional_repeating_unit&quot;_4-0" class="reference"><a href="#cite_note-Gold_Book_&quot;constitutional_repeating_unit&quot;-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>
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<p>A repeat unit is sometimes called a mer (or mer unit) in <a href="Polymer_chemistry" title="Polymer chemistry">polymer chemistry</a>. "Mer" originates from the <a href="Greek_Language" class="mw-redirect" title="Greek Language">Greek</a> word <i>meros</i>, which means "a part". The word polymer derives its meaning from this, which means "many mers". The mer is not the same thing as a <a href="Monomer" title="Monomer">monomer</a>—a mer is a repeating unit within a larger molecule, whereas a monomer is an actual molecule that exists independently, either prior to <a href="Polymerization" title="Polymerization">polymerization</a> or after decomposition.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>
<p>One of the simplest repeat units is that of the <a href="Addition_polymer" title="Addition polymer">addition polymer</a> <a href="Polyvinyl_chloride" title="Polyvinyl chloride">polyvinyl chloride</a>,
-[CH<sub>2</sub>-CHCl]<sub>n</sub>-, whose repeat unit is -[CH<sub>2</sub>-CHCl]-.
In this case the repeat unit has the same atoms as the monomer <a href="Vinyl_chloride" title="Vinyl chloride">vinyl chloride</a> CH<sub>2</sub>=CHCl. When the polymer is formed, the C=C <a href="Double_bond" title="Double bond">double bond</a> in the monomer is replaced by a C-C <a href="Single_bond" title="Single bond">single bond</a> in the polymer repeat unit, which links by two new bonds to adjoining repeat units.
</p><p>In <a href="Condensation_polymer" title="Condensation polymer">condensation polymers</a> (see examples below), the repeat unit contains fewer atoms than the monomer or monomers from which it is formed.
</p><p>The subscript "n" denotes the <a href="Degree_of_polymerisation" class="mw-redirect" title="Degree of polymerisation">degree of polymerisation</a>, that is, the number of units linked together. The <a href="Molecular_mass" title="Molecular mass">molecular mass</a> of the repeat unit, M<sub>R</sub>, is simply the sum of the <a href="Atomic_mass" title="Atomic mass">atomic masses</a> of the <a href="Atom" title="Atom">atoms</a> within the repeat unit. The molecular mass of the chain is just the product nM<sub>R</sub>. Other than <a href="Monodisperse" class="mw-redirect" title="Monodisperse">monodisperse</a> polymers, there is normally a <a href="Molar_mass_distribution" title="Molar mass distribution">molar mass distribution</a> caused by chains of different length.
</p><p>In <a href="Copolymer" title="Copolymer">copolymers</a> there are two or more types of repeat unit, which may be arranged in alternation, or at random, or in other more complex patterns.
</p>
<div class="mw-heading mw-heading2"><h2 id="Other_vinyl_polymers">Other vinyl polymers</h2></div>
<p><a href="Polyethylene" title="Polyethylene">Polyethylene</a> may be considered either as -[CH<sub>2</sub>-CH<sub>2</sub>-]<sub>n</sub>- with a repeat unit of -[CH<sub>2</sub>-CH<sub>2</sub>]-, or as [-CH<sub>2</sub>-]<sub>n</sub>-, with a repeat unit of -[CH<sub>2</sub>]-. Chemists tend to consider the repeat unit as -[CH<sub>2</sub>-CH<sub>2</sub>]- since this polymer is made from the monomer <a href="Ethylene" title="Ethylene">ethylene</a> (CH<sub>2</sub>=CH<sub>2</sub>).
</p><p>More complex repeat units can occur in <a href="Vinyl_polymer" title="Vinyl polymer">vinyl polymers</a> -[CH<sub>2</sub>-CHR]<sub>n</sub>-, if one hydrogen in the ethylene repeat unit is substituted by a larger fragment R. <a href="Polypropylene" title="Polypropylene">Polypropylene</a> -[CH<sub>2</sub>-CH(CH<sub>3</sub>)]<sub>n</sub>- has the repeat unit -[CH<sub>2</sub>-CH(CH<sub>3</sub>)]. <a href="Polystyrene" title="Polystyrene">Polystyrene</a> has a chain where the <a href="Substituent" title="Substituent">substituent</a> R is a <a href="Phenyl_group" title="Phenyl group">phenyl group</a> (C<sub>6</sub>H<sub>5</sub>), corresponding to a <a href="Benzene" title="Benzene">benzene</a> ring minus one hydrogen: -[CH<sub>2</sub>-CH(C<sub>6</sub>H<sub>5</sub>)]<sub>n</sub>-, so the repeat unit is -[CH<sub>2</sub>-CH(C<sub>6</sub>H<sub>5</sub>)]-.
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<div class="mw-heading mw-heading2"><h2 id="Condensation_polymers:_repeat_unit_and_structural_units">Condensation polymers: repeat unit and structural units</h2></div>

<p>In many <a href="Condensation_polymer" title="Condensation polymer">condensation polymers</a>, the repeat unit contains two <a href="Structural_unit" title="Structural unit">structural units</a> related to the <a href="Comonomer" title="Comonomer">comonomers</a> which have been polymerized. For example, in <a href="Polyethylene_terephthalate" title="Polyethylene terephthalate">polyethylene terephthalate</a> (PET or "polyester"), the repeat unit is -CO-C<sub>6</sub>H<sub>4</sub>-CO-O-CH<sub>2</sub>-CH<sub>2</sub>-O-. The polymer is formed by the <a href="Condensation_reaction" title="Condensation reaction">condensation reaction</a> of the two monomers <a href="Terephthalic_acid" title="Terephthalic acid">terephthalic acid</a> (HOOC-C<sub>6</sub>H<sub>4</sub>-COOH) and <a href="Ethylene_glycol" title="Ethylene glycol">ethylene glycol</a> (HO-CH<sub>2</sub>-CH<sub>2</sub>-OH), or their chemical <a href="Derivative_(chemistry)" title="Derivative (chemistry)">derivatives</a>. The condensation involves loss of water, as an H is lost from each HO- group in the glycol, and an OH from each HOOC- group in the acid. The two structural units in the polymer are then considered to be -CO-C<sub>6</sub>H<sub>4</sub>-CO- and -O-CH<sub>2</sub>-CH<sub>2</sub>-O-.
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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">Rudin A. "Elements of Polymer Science and Engineering" (Academic Press 1982) p.3 <style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a>&nbsp;<bdi>0-12-601680-1</bdi></span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.open.edu/openlearn/science-maths-technology/science/chemistry/introduction-polymers/content-section-2.2">"2.2 Chain repeat units"</a>. <i>Introduction to Polymers</i>. The Open University (GB)<span class="reference-accessdate">. Retrieved <span class="nowrap">31 July</span> 2019</span>.</cite></span>
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<li id="cite_note-Gold_Book_&quot;constitutional_unit&quot;-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-Gold_Book_&quot;constitutional_unit&quot;_3-0">^</a></b></span> <span class="reference-text"><cite id="Gold_Book_C01288" class="citation web cs1"><a rel="nofollow" class="external text" href="https://goldbook.iupac.org/terms/view/C01288">"constitutional unit"</a>. <i>Gold Book</i>. IUPAC. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1351%2Fgoldbook.C01288">10.1351/goldbook.C01288</a><span class="reference-accessdate">. Retrieved <span class="nowrap">1 April</span> 2024</span>.</cite></span>
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<li id="cite_note-Gold_Book_&quot;constitutional_repeating_unit&quot;-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-Gold_Book_&quot;constitutional_repeating_unit&quot;_4-0">^</a></b></span> <span class="reference-text"><cite id="Gold_Book_C01286" class="citation web cs1"><a rel="nofollow" class="external text" href="https://goldbook.iupac.org/terms/view/C01286">"constitutional repeating unit"</a>. <i>Gold Book</i>. IUPAC. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1351%2Fgoldbook.C01286">10.1351/goldbook.C01286</a><span class="reference-accessdate">. Retrieved <span class="nowrap">1 April</span> 2024</span>.</cite></span>
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<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text">Callister, William D. (2007). <i>Materials science and engineering&nbsp;: an introduction</i> (7th ed.) New York&nbsp;: John Wiley &amp; Sons. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a>&nbsp;<bdi>978-0-471-73696-7</bdi></span>
</li>
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