Editing Copolymer (section)

===Alternating copolymers===
 {{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''alternating copolymer''': A [https://doi.org/10.1351/goldbook.C01335 copolymer] consisting of [https://doi.org/10.1351/goldbook.M03667 macromolecule] comprising two species of [https://doi.org/10.1351/goldbook.M04018 monomeric unit] in alternating [https://doi.org/10.1351/goldbook.ST06775 sequence]. (See Gold Book entry for note.)
<ref name='Gold Book "alternating copolymer"'>{{cite web |title=alternating copolymer |url=https://goldbook.iupac.org/terms/view/A00250 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book A00250 |doi=10.1351/goldbook.A00250}}</ref>
}}
An alternating copolymer has regular alternating A and B units, and is often described by the formula: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)<sub>n</sub>-. The molar ratio of each monomer in the polymer is normally close to one, which happens when the reactivity ratios r<sub>1</sub> and r<sub>2</sub> are close to zero, as can be seen from the Mayo–Lewis equation. For example, in the free-radical copolymerization of [[styrene maleic anhydride]] copolymer, r<sub>1</sub> = 0.097 and r<sub>2</sub> = 0.001,<ref name=Fried/> so that most chains ending in styrene add a maleic anhydride unit, and almost all chains ending in maleic anhydride add a styrene unit. This leads to a predominantly alternating structure.

A step-growth copolymer -(-A-A-B-B-)<sub>n</sub>- formed by the [[condensation reaction|condensation]] of two [[bifunctional]] monomers A–A and B–B is in principle a perfectly alternating copolymer of these two monomers, but is usually considered as a [[Polymer#Monomers and repeat units|homopolymer]] of the dimeric repeat unit A-A-B-B.<ref name=Cowie>{{cite book |last=Cowie |first=J.M.G. |date=1991 |edition=2nd |title=Polymers: Chemistry and Physics of Modern Materials |publisher=Blackie (USA: Chapman and Hall) |pages=[https://archive.org/details/polymerschemistr0000cowi/page/104 104–106] |isbn=978-0-216-92980-7 |url=https://archive.org/details/polymerschemistr0000cowi/page/104 }}</ref> An example is [[nylon 66]] with repeat unit -OC-( CH<sub>2</sub>)<sub>4</sub>-CO-NH-(CH<sub>2</sub>)<sub>6</sub>-NH-, formed from a [[dicarboxylic acid]] monomer and a [[diamine]] monomer.