Editing Polymer (section)

==Synthesis==
{{Main|Polymerization}}

[[File:Polymerization classification ENmod.png|thumb|A classification of the polymerization reactions|alt=|440x440px]]
Polymerization is the process of combining many [[small molecule]]s known as monomers into a covalently bonded chain or network. During the polymerization process, some chemical groups may be lost from each monomer. This happens in the polymerization of [[polyethylene terephthalate|PET polyester]]. The monomers are [[terephthalic acid]] (HOOC{{--}}C<sub>6</sub>H<sub>4</sub>{{--}}COOH) and [[ethylene glycol]] (HO{{--}}CH<sub>2</sub>{{--}}CH<sub>2</sub>{{--}}OH) but the repeating unit is {{--}}OC{{--}}C<sub>6</sub>H<sub>4</sub>{{--}}COO{{--}}CH<sub>2</sub>{{--}}CH<sub>2</sub>{{--}}O{{--}}, which corresponds to the combination of the two monomers with the loss of two water molecules. The distinct piece of each monomer that is incorporated into the polymer is known as a [[repeat unit]] or monomer residue.

Synthetic methods are generally divided into two categories, [[step-growth polymerization]] and [[addition polymerization|chain polymerization]].<ref>{{cite book |last1= Sperling |first1= L. H. (Leslie Howard) |title= Introduction to physical polymer science |year= 2006 |publisher= Wiley |location= Hoboken, N.J. |isbn= 978-0-471-70606-9 |page= 10}}</ref> The essential difference between the two is that in chain polymerization, monomers are added to the chain one at a time only,<ref>Sperling, p. 11</ref> such as in [[polystyrene]], whereas in step-growth polymerization chains of monomers may combine with one another directly,<ref>Sperling, p. 15</ref> such as in [[polyester]]. Step-growth polymerization can be divided into [[polycondensation]], in which low-molar-mass by-product is formed in every reaction step, and [[polyaddition]].
[[File:Styrene radical chain polymerization.jpg|Example of chain polymerization: Radical polymerization of styrene, R. is initiating radical, P. is another polymer chain radical terminating the formed chain by radical recombination.|alt=|left|frame]]

Newer methods, such as [[plasma polymerization]] do not fit neatly into either category. Synthetic polymerization reactions may be carried out with or without a [[catalysis|catalyst]]. Laboratory synthesis of biopolymers, especially of [[peptide synthesis|proteins]], is an area of intensive research.

===Biological synthesis===
{{Main|Biopolymer}}

[[File:DNA animation.gif|thumb|right|Microstructure of part of a DNA [[double helix]] biopolymer]]

There are three main classes of biopolymers: [[polysaccharide]]s, [[polypeptide]]s, and [[polynucleotide]]s.
In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by [[DNA polymerase]]. The [[protein biosynthesis|synthesis of proteins]] involves multiple enzyme-mediated processes to [[transcription (genetics)|transcribe]] genetic information from the DNA to [[ribonucleic acid|RNA]] and subsequently [[translation (biology)|translate]] that information to synthesize the specified protein from [[amino acid]]s. The protein may be [[posttranslational modification|modified further]] following translation in order to provide appropriate structure and functioning. There are other biopolymers such as [[rubber]], [[suberin]], [[melanin]], and [[lignin]].

===Modification of natural polymers===
Naturally occurring polymers such as [[cotton]], [[starch]], and rubber were familiar materials for years before synthetic polymers such as [[polyethene]] and [[perspex]] appeared on the market. Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples include the reaction of [[nitric acid]] and [[cellulose]] to form [[nitrocellulose]] and the formation of [[vulcanized rubber]] by heating natural rubber in the presence of [[sulfur]]. Ways in which polymers can be modified include [[oxidation]], [[cross-link]]ing, and [[endcapping|end-capping]].

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Especially in the production of polymers the gas separation by membranes has acquired increasing importance in the [[petrochemical industry]] and is now a relatively well-established unit operation.
The process of polymer [[degassing]] is necessary to suit polymer for extrusion and pelletizing, increasing safety, environmental, and product quality aspects. Nitrogen is generally used for this purpose, resulting in a vent gas primarily composed of monomers and nitrogen.<ref name="Improvement Economics Program">{{cite book|url=http://www.slideshare.net/intratec/membranes-on-polyolefins-plants-vent-recovery |title= Membranes on Polyolefins Plants Vent Recovery, Improvement Economics Program|publisher=Intratec|isbn= 978-0615678917|url-status=dead |archive-url= https://web.archive.org/web/20130513040249/http://www.slideshare.net/intratec/membranes-on-polyolefins-plants-vent-recovery|archive-date= 2013-05-13|date= 2012-08-02}}</ref>
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