Editing Polymerization (section)

{{Short description|Chemical reaction to form polymer chains}}
{{TopicTOC-Polymer}}
{{Use American English|date=November 2020}}<!-- and IUPAC spelling (aluminium, sulfur, caesium) -->
{{Use dmy dates|date=November 2020}}

In [[polymer chemistry]], '''polymerization''' ([[American English]]), or '''polymerisation''' ([[British English]]), is a process of reacting [[monomer|monomer molecule]]s together in a [[chemical reaction]] to form  [[polymer]] chains or three-dimensional networks.<ref>{{cite book |doi=10.1351/goldbook.P04740 |doi-access=free |chapter=Polymerization |title=The IUPAC Compendium of Chemical Terminology |year=2014}}</ref><ref>{{Cite book |last=Young |first=Robert J. |url= |title=Introduction to polymers |publisher=Chapman and Hall |year=1981 |isbn=0-412-22170-5 |location=London |oclc=8086791}}</ref><ref name="clayden_organic">{{Cite book |last1=Clayden |first1=Jonathan |url= |title=Organic chemistry |last2=Greeves |first2=Nick |last3=Warren |first3=Stuart |publisher=Oxford University Press |year=2001 |isbn=0-19-850347-4 |location=Oxford |pages=1450–1466 |oclc=43338068}}</ref> There are many forms of polymerization<ref>{{Cite book |last=Manas |first=Chanda |title=Introduction to Polymer Science and Chemistry:A Problem-Solving Approach |publisher=CRC Press |year=2023 |isbn=978-1-4665-5385-9 |edition=2nd |publication-date=2013}}</ref> and different systems exist to categorize them.

[[File:IUPAC definition for polymerization.png|thumb|right|550px|link=https://doi.org/10.1351/goldbook.P04740|IUPAC definition for polymerization]]

In [[chemical compound]]s, polymerization can occur via a variety of reaction mechanisms that vary in complexity due to the [[functional group]]s present in the reactants<ref name="clayden_organic" /> and their inherent [[steric effects]]. In more straightforward polymerizations, [[alkenes]] form polymers through relatively simple [[free-radical reaction|radical reactions]]; in contrast, reactions involving substitution at a [[carbonyl group]] require more complex synthesis due to the way in which reactants polymerize.<ref name="clayden_organic"/>

[[File:Polystyrene formation.PNG|thumb|right|400px|An example of '''alkene polymerization''', in which each [[styrene]] monomer's double bond reforms as a single bond plus a bond to another styrene monomer. The product is [[polystyrene]].]]

As alkenes can polymerize in somewhat straightforward radical reactions, they form useful compounds such as [[polyethylene]] and [[polyvinyl chloride]] (PVC),<ref name="clayden_organic"/> which are produced in high tonnages each year<ref name="clayden_organic"/> due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. In general, polymers such as PVC are referred to as "'''homopolymers'''", as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one monomer unit are referred to as [[copolymer]]s (or co-polymers).<ref>{{Cite book |last=Cowie |first=J. M. G. |url= |title=Polymers : chemistry and physics of modern materials |publisher=CRC Press |others=V. Arrighi |year=2008 |isbn=978-0-8493-9813-1 |edition=3rd |location=Boca Raton |pages=4 |oclc=82473191}}</ref>

{| align="right" class="infobox" style="text-align:center;"
	
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<div style="font-size:14px; font-weight:bold;">Homopolymers</div>
	
:<math>A + A + A+... \rightarrow AAAA ...</math>
	

<div style="font-size:14px; font-weight:bold;">Copolymers</div>
	
:<math>A + B + A+B... \rightarrow ABAB ...</math>
:<math>A + B + A+B... \rightarrow AABB ...</math>	
:<math>A + B + A+B... \rightarrow BBAB ...</math>
|}

Other monomer units, such as [[formaldehyde]] hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (ca. −80&nbsp;°C) to form [[trimer (chemistry)|trimer]]s;<ref name="clayden_organic"/> molecules consisting of 3 monomer units, which can [[cyclization|cyclize]] to form ring cyclic structures, or undergo further reactions to form [[tetramer]]s,<ref name="clayden_organic"/> or 4 monomer-unit compounds. Such small polymers are referred to as [[oligomer]]s.<ref name="clayden_organic"/> Generally, because formaldehyde is an exceptionally reactive electrophile it allows [[nucleophile|nucleophilic]] addition of [[hemiacetal]] intermediates, which are in general short-lived and relatively unstable "mid-stage" compounds that react with other non-polar molecules present to form more stable polymeric compounds.

Polymerization that is not sufficiently moderated and proceeds at a fast rate can be very dangerous. This phenomenon is known as [[autoacceleration]], and can cause fires and explosions.