Editing Polypropylene (section)

==Production==
Polypropylene is produced by the [[chain-growth polymerization]] of [[propene]]:

[[File:Polymerization of polypropylene - en.svg|class=skin-invert-image|frameless|467x467px]]

The industrial production processes can be grouped into gas phase polymerization, [[bulk polymerization]] and [[slurry]] polymerization. All state-of-the-art processes use either gas-phase or bulk reactor systems.<ref name="Ullmanns" />

*In gas-phase and slurry-reactors, the polymer is formed around heterogeneous catalyst particles. The gas-phase polymerization is carried out in a [[fluidized bed reactor]], [[propene]] is passed over a bed containing the [[Heterogeneous catalysis|heterogeneous (solid) catalyst]] and the formed polymer is separated as a fine powder and then converted into [[Pelletizing|pellets]]. Unreacted gas is recycled and fed back into the reactor.
*In bulk polymerization, liquid propene acts as a solvent to prevent the precipitation of the polymer. The polymerization proceeds at 60 to 80&nbsp;°C and 30–40&nbsp;atm are applied to keep the propene in the liquid state. For the bulk polymerization, typically [[Chemical reactor#CSTR (continuous stirred-tank reactor)|loop reactor]]s are applied. The bulk polymerization is limited to a maximum of 5% ethene as comonomer due to a limited solubility of the polymer in the liquid propene.
*In the slurry polymerization, typically C4–C6 alkanes ([[butane]], [[pentane]] or [[hexane]]) are utilized as [[Chemically inert|inert]] diluent to [[Suspension (chemistry)|suspend]] the growing polymer particles. Propene is introduced into the mixture as a gas.

===Catalysts===
The properties of PP are strongly affected by its [[tacticity]], the orientation of the [[methyl group]]s ({{chem|CH|3}}) relative to the methyl groups in neighboring monomer units. A [[Ziegler–Natta catalyst]] is able to restrict linking of monomer molecules to a specific orientation, either isotactic, when all methyl groups are positioned at the same side with respect to the backbone of the polymer chain, or syndiotactic, when the positions of the methyl groups alternate.

Commercially available isotactic polypropylene is made with two types of Ziegler-Natta catalysts. The first group of the catalysts encompasses solid (mostly supported) catalysts and certain types of soluble [[metallocene]] catalysts. Such isotactic macromolecules coil into a [[Helix|helical]] shape; these helices then line up next to one another to form the crystals that give commercial isotactic polypropylene many of its desirable properties.

Modern supported Ziegler-Natta catalysts developed for the polymerization of propylene and other [[1-alkene]]s to isotactic polymers usually use {{chem|TiCl|4}} as an active ingredient and {{chem|MgCl|2}} as a support.<ref>{{cite book|author=Kissin, Y. V. |title=Alkene Polymerization Reactions with Transition Metal Catalysts|url=https://books.google.com/books?id=JjhZiJEY5FsC&pg=PA207|year=2008|publisher=Elsevier|isbn=978-0-444-53215-2|pages=207–}}</ref><ref>{{cite book|author1=Hoff, Ray |author2=Mathers, Robert T. |name-list-style=amp |title=Handbook of Transition Metal Polymerization Catalysts|url=https://books.google.com/books?id=5_uqKL9VUmQC&pg=PA158|year= 2010|publisher=John Wiley & Sons|isbn=978-0-470-13798-7|pages=158–}}</ref><ref>{{cite book |last=Moore |first=E. P. |year=1996 |title=Polypropylene Handbook. Polymerization, Characterization, Properties, Processing, Applications |publisher=Hanser Publishers |location=New York |isbn=1569902089}}{{page needed|date=July 2018}}</ref> The catalysts also contain organic modifiers, either aromatic acid esters and diesters or ethers. These catalysts are activated with special co-catalysts containing an [[organoaluminium]] compound such as Al(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub> and the second type of a modifier. The catalysts are differentiated depending on the procedure used for fashioning catalyst particles from MgCl<sub>2</sub> and depending on the type of organic modifiers employed during catalyst preparation and use in polymerization reactions. Two most important technological characteristics of all the supported catalysts are high productivity and a high fraction of the crystalline isotactic polymer they produce at 70–80&nbsp;°C under standard polymerization conditions. Commercial synthesis of isotactic polypropylene is usually carried out either in the medium of liquid propylene or in gas-phase reactors.

[[Image:Syndiotactic polypropene.png|thumb|right|A ball-and-stick model of syndiotactic polypropylene]]

Commercial synthesis of syndiotactic polypropylene is carried out with the use of a special class of metallocene catalysts. They employ bridged bis-metallocene complexes of the type bridge-(Cp<sub>1</sub>)(Cp<sub>2</sub>)ZrCl<sub>2</sub> where the first Cp ligand is the [[cyclopentadienyl]] group, the second Cp ligand is the fluorenyl group, and the bridge between the two Cp ligands is -CH<sub>2</sub>-CH<sub>2</sub>-, >SiMe<sub>2</sub>, or >SiPh<sub>2</sub>.<ref>{{cite book |editor-last1=Benedikt |editor-first1=G. M. |editor-last2=Goodall |editor-first2=B. L. |year=1998 |title=Metallocene Catalyzed Polymers |publisher=ChemTech Publishing |location=Toronto |isbn=978-1-884207-59-4}}{{page needed|date=July 2018}}</ref> These complexes are converted to polymerization catalysts by activating them with a special organoaluminium co-catalyst, [[methylaluminoxane]] (MAO).<ref>{{cite book |editor-last1=Sinn |editor-first1=H. |editor-last2=Kaminsky |editor-first2=W. |editor-last3=Höker |editor-first3=H. |year=1995 |title=Alumoxanes, Macromol. Symp. 97 |publisher=Huttig & Wepf |location=Heidelberg}}{{page needed|date=July 2018}}</ref>

Atactic polypropylene is an amorphous rubbery material. It can be produced commercially either with a special type of supported Ziegler-Natta catalyst or with some metallocene catalysts.