Editing Polystyrene (section)

== Structure and production ==
[[File:Poliestireno.JPG|thumb|right|Polystyrene is [[fire|flammable]], and releases large amounts of [[soot|black smoke]] upon burning.]]
[[File:201904 Man carrying polystyrene boxes in Guiyang, China.jpg|thumb|Expanded polystyrene is lightweight. This is a man in [[Guiyang]], China carrying a lot of expanded polystyrene packaging.]]
In [[chemistry|chemical]] terms, polystyrene is a long chain hydrocarbon wherein alternating carbon centers are attached to [[phenyl group]]s (a derivative of [[benzene]]). Polystyrene's chemical formula is {{chem|(C|8|H|8|)|n}}; it contains the [[chemical elements]] [[carbon]] and [[hydrogen]].{{Citation needed|date=May 2024}}

The material's properties are determined by short-range [[Van der Waals force|van der Waals]] attractions between polymer chains. Since the molecules consist of thousands of atoms, the cumulative attractive force between the molecules is large.  When heated (or deformed at a rapid rate, due to a combination of viscoelastic and thermal insulation properties), the chains can take on a higher degree of confirmation and slide past each other. This [[intermolecular]] weakness (versus the high ''[[Intramolecular force|intramolecular]]'' strength due to the hydrocarbon backbone) confers flexibility and elasticity.  The ability of the system to be readily deformed above its glass transition temperature allows polystyrene (and thermoplastic polymers in general) to be readily softened and molded upon heating. Extruded polystyrene is about as strong as an unalloyed [[aluminium]] but much more flexible and much less dense (1.05 g/cm<sup>3</sup> for polystyrene vs. 2.70 g/cm<sup>3</sup> for aluminium).<ref>{{Cite patent|number=US9738739B2|title=Method of fixing radioactive tritiated water in a stable tritiated polystyrene product|gdate=2017-08-22|invent1=Digenis|invent2=Digenis|inventor1-first=George A.|inventor2-first=Alexander G.|url=https://patents.google.com/patent/US9738739/en}}</ref>

=== Production ===
Polystyrene is an [[addition polymer]] that results when styrene [[monomer]]s [[polymerization|polymerize]] (interconnect). In the polymerization, the carbon-carbon [[pi bond|π bond]] of the [[vinyl group]] is broken and a new carbon-carbon [[sigma bond|σ bond]] is formed, attaching to the carbon of another styrene monomer to the chain. Since only one kind of monomer is used in its preparation, it is a homopolymer. The newly formed σ bond is stronger than the π bond that was broken, thus it is difficult to depolymerize polystyrene. About a few thousand monomers typically comprise a chain of polystyrene, giving a [[Molar mass distribution#Number average molar mass|molar mass]] of 100,000–400,000&nbsp;g/mol.{{Citation needed|date=January 2021}}

[[File:Polystyrene formation.PNG|class=skin-invert|center]]

Each carbon of the backbone has [[tetrahedral geometry]], and those carbons that have a [[phenyl group]] (benzene ring) attached are [[chirality (chemistry)|stereogenic]]. If the backbone were to be laid as a flat elongated zig-zag chain, each phenyl group would be tilted forward or backward compared to the plane of the chain.{{Citation needed|date=January 2021}}

The relative [[stereochemical]] relationship of consecutive phenyl groups determines the [[tacticity]], which affects various physical properties of the material.<ref>{{Cite web |date=2013-10-02 |title=The Phenyl Group |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Arenes/Properties_of_Arenes/The_Phenyl_Group |access-date=2022-06-18 |website=Chemistry LibreTexts |language=en}}</ref>

===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>