Editing Polymer (section)

===Monomers and repeat units===
The identity of the repeat units (monomer residues, also known as "mers") comprising a polymer is its first and most important attribute. Polymer nomenclature is generally based upon the type of monomer residues comprising the polymer. A polymer which contains only a single type of [[repeat unit]] is known as a '''homopolymer''', while a polymer containing two or more types of repeat units is known as a '''[[copolymer]]'''.<ref>Rudin, p. 17</ref> A '''terpolymer''' is a copolymer which contains three types of repeat units.<ref>Cowie, p. 4</ref>

[[Polystyrene]] is composed only of [[styrene]]-based repeat units, and is classified as a homopolymer. [[Polyethylene terephthalate]], even though produced from two different [[monomer]]s ([[ethylene glycol]] and [[terephthalic acid]]), is usually regarded as a homopolymer because only one type of repeat unit is formed. [[Ethylene-vinyl acetate]] contains more than one variety of repeat unit and is a copolymer. Some biological polymers are composed of a variety of different but structurally related monomer residues; for example, [[polynucleotide]]s such as DNA are composed of four types of [[nucleotide]] subunits.

:{| class="wikitable" style="text-align:left; font-size:90%;" width="80%"
|-
| class="hintergrundfarbe6" align="center" colspan="4" |'''Homopolymers and copolymers (examples)'''
|- style="vertical-align:top" class="hintergrundfarbe2"
| [[File:Polystyrene skeletal.svg|80px|center]]
| [[File:Poly(dimethylsiloxan).svg|100px|center]]
| [[File:PET.svg|200px|center]]
| [[File:Styrol-Butadien-Kautschuk.svg|180px|center]]
|- style="vertical-align:top"
| Homopolymer [[polystyrene]]
| Homopolymer [[polydimethylsiloxane]], a [[silicone]]. The main chain is formed of silicon and oxygen atoms.
| The homopolymer [[polyethylene terephthalate]] has only one [[repeat unit]].
| Copolymer [[styrene-butadiene rubber]]: The repeat units based on [[styrene]] and [[1,3-Butadiene|1,3-butadiene]] form two repeating units, which can alternate in any order in the macromolecule, making the polymer thus a random copolymer.
|}

A polymer containing ionizable subunits (e.g., pendant [[carboxylic acid|carboxylic groups]]) is known as a [[polyelectrolyte]] or [[ionomer]], when the fraction of ionizable units is large or small respectively.