Polymerization
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/* See also */ Glurch
[[Image:Styrene polymerization.png|thumb|right|200px|An example of '''alkene polymerization''', in which each [[Styrene]] monomer unit's double bond reforms as a single bond with another styrene monomer and forms [[polystyrene]].]]
In [[polymer chemistry]], '''polymerization''' is a process of reacting [[monomer]] [[molecule]]s together in a [[chemical reaction]] to form three-dimensional networks or [[polymer]] chains <ref>''Introduction to Polymers'' 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5</ref> <ref>International Union of Pure and Applied Chemistry, ''et al.'' (2000) "''IUPAC Gold Book''" Retrieved on [[11 May]] [[2007]] from "IUPAC Gold Book" on [http://goldbook.iupac.org/ http://goldbook.iupac.org/]</ref> <ref>Clayden, J., Greeves, N. ''et al.'' (2000). "''Organic chemistry''" Oxford</ref>. There are many forms of polymerization and different systems exist to categorize them.
The main categories are
* [[Addition polymerization]]
* [[Condensation polymerization]]
==Introduction==
{| align="right" class="infobox" style="text-align:center;"
|
<div style="font-size:14px; font-weight:bold;">Single-monomer formed polymers</div>
:<math>A + A + A... \rightarrow AAA ...</math>
<div style="font-size:14px; font-weight:bold;">Co-polymers</div>
:<math>A + B + A... \rightarrow ABA ...</math>
|}
In chemical compounds, polymerization occurs via a variety of reaction mechanisms which vary in complexity due to [[functional group]]s present in reacting compounds<ref name="clayden_organic">Clayden, J., Greeves, N. ''et al.'' (2000), p1450-1466</ref> and their inherent [[steric effects]] explained by [[VSEPR Theory]]. In more straightforward polymerization, [[alkenes]], which are a relatively stable due to <math>\sigma</math> [[chemical bonding|bonding]] between carbon atoms form polymers through relatively simple radical reactions; conversely, more complex reactions such as those that involve substitution at the carbonyl atom require more complex synthesis due to the way in which reacting molecules polymerize.<ref name="clayden_organic" />
As alkenes can be formed in somewhat straightforward reaction mechanisms, they form useful compounds such as [[polyethylene]] and [[polyvinyl chloride]] (PVC) when undergoing radical reactions,<ref name="clayden_organic" /> which are produced in high tonnages each year<ref name="clayden_organic" /> due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. Polymers such as PVC are generally referred to as "'''singular'''" polymers as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one molecule are referred to as "'''co-polymers'''".
Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (>-80<sup>o</sup>C) to form [[trimer]]s;<ref name="clayden_organic" /> molecules consisting of 3 monomer units which can cyclize to form ring cyclic structures, or undergo further reactions to form [[tetramer]]s,<ref name="clayden_organic" /> or 4 monomer-unit compounds. Further compounds either being referred to as [[oligomer]]s<ref name="clayden_organic" /> in smaller molecules. Generally, because formaldehyde is an exceptionally reactive electrophile it allows [[nucleophile|nucleophillic]] addition of hemiacetal intermediates, which are generally short lived and relatively unstable "mid stage" compounds which react with other molecules present to form more stable polymeric compounds.
Polymerization that is not sufficiently moderated and proceeds at an undesirably fast rate can be very hazardous. This phenomenon is known as [[Hazardous polymerization]] and can cause fires and explosions.
==Chain-growth==
{{Main article|Chain-growth polymerization}}
Chain-growth polymerization or addition polymerization involves the linking together of molecules incorporating double or triple [[chemical bond]]s. These unsaturated ''monomers'' (the identical molecules which make up the polymers) have extra internal bonds which are able to break and link up with other monomers to form the repeating chain. Addition polymerization is involved in the manufacture of polymers such as [[polyethylene]], [[polypropylene]] and [[polyvinyl chloride]] (PVC). A special case of addition polymerization leads to [[living polymerization]].
In the polymerization of [[ethylene]], its pi bond is broken and these two electrons rearrange to create a new propagating center like the one that attacked it. The form this propagating center takes depends on the specific type of addition mechanism. There are several mechanisms through which this can be initiated. The [[free radical]] mechanism was one of the first methods to be used. Free radicals are very reactive atoms or molecules which have unpaired electrons. Taking the polymerization of ethylene as an example, the free radical mechanism can be divided in to three stages: [[chain initiation]], [[chain propagation]] and [[chain termination]].
[[Image:Ethylene polymerization.png|thumb|right|Polymerization of [[ethylene]]]]
Free radical addition polymerization of ethylene must take place at high temperatures and pressures, approximately 300°C and 2000 At. While most other free radical polymerizations do not require such extreme temperatures and pressures, they do tend to lack control. One effect of this lack of control is a high degree of branching. Also, as termination occurs randomly, when two chains collide, it is impossible to control the length of individual chains.
A newer method of polymerization similar to free radical, but allowing more control involves the [[Ziegler-Natta catalyst]] especially with respect to [[branching (chemistry)|polymer branching]].
Other forms of addition polymerization include [[cationic addition polymerization]] and [[anionic addition polymerization]]. While not used to a large extent in industry yet due to stringent reaction conditions such as lack of water and oxygen, these methods provide ways to polymerize some monomers that cannot be polymerized by free radical methods such as [[polypropylene]]. Cationic and anionic mechanisms are also more ideally suited for [[living polymerization]]s, although free radical living polymerizations have also been developed.
==Step-growth==
{{Main article|Step-growth polymerization}}
Step growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers. Most step growth polymers are also classified as condensation polymers, but not all step growth polymers (like [[polyurethane]]s formed from [[isocyanate]] and alcohol bifunctional monomers) release condensates. Step growth polymers increase in molecular weight at a very slow rate at lower conversions and only reach moderately high molecular weights at very high conversion (''i.e.'' >95%).
To alleviate inconsistencies in these naming methods, adjusted definitions for condensation and addition polymers have been developed. A condensation polymer is defined as a polymer that involves [[elimination reaction|elimination]] of small molecules during its synthesis, or contains functional groups as part of its [[backbone chain]], or its [[structural unit|repeat unit]] does not contain all the atoms present in the hypothetical monomer to which it can be degraded.
==See also==
* [[Plasma polymerization]]
* [[Ziegler-Natta catalyst]]
* [[Metallocene]]
* [[Glurch]]
==References==
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