Editing Polystyrene (section)

===Tacticity===

In polystyrene, [[tacticity]] describes the extent to which the phenyl group is uniformly aligned (arranged at one side) in the polymer chain. Tacticity has a strong effect on the properties of the plastic. Standard polystyrene is atactic. The [[diastereomer]] where all of the phenyl groups are on the same side is called ''isotactic'' polystyrene, which is not produced commercially.{{Citation needed|date=January 2021}}

[[File:Polystyrene tacticity en.svg|class=skin-invert]]

====Atactic polystyrene====
The only commercially important form of polystyrene is ''atactic'', in which the phenyl groups are [[randomness|randomly]] distributed on both sides of the polymer chain. This random positioning prevents the chains from aligning with sufficient regularity to achieve any [[crystallinity]]. The plastic has a glass transition temperature ''T''<sub>g</sub> of ≈90&nbsp;°C.  Polymerization is initiated with [[free radical]]s.<ref name=Ullmann>Maul, J.; Frushour, B. G.; Kontoff, J. R.; Eichenauer, H.; Ott, K.-H. and Schade, C. (2007) "Polystyrene and Styrene Copolymers" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim, {{doi|10.1002/14356007.a21_615.pub2}}</ref>

==== Syndiotactic polystyrene ====
[[Ziegler–Natta catalyst|Ziegler–Natta polymerization]] can produce an ordered ''syndiotactic'' polystyrene with the phenyl groups positioned on alternating sides of the hydrocarbon backbone. This form is highly crystalline with a ''T''<sub>m</sub> (melting point) of {{convert|270|C|F|abbr=on}}.  Syndiotactic polystyrene resin is currently produced under the trade name XAREC by Idemitsu corporation, who use a metallocene catalyst for the polymerisation reaction.<ref>{{cite web|title = XAREC Syndiotactic Polystyrene – Petrochemicals – Idemitsu Kosan Global|url = http://www.idemitsu.com/products/petrochemicals/engineering/polystyrene.html|website = www.idemitsu.com|access-date = 2016-01-01|archive-date = 31 March 2019|archive-url = https://web.archive.org/web/20190331143055/http://www.idemitsu.com/products/petrochemicals/engineering/polystyrene.html|url-status = dead}}</ref>