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[[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]].]] | |||
{{Quote box | |||
|title = IUPAC definition | |||
|quote = The process of converting a [[monomer]] or a mixture of monomers into a [[polymer]]. <ref>{{cite journal|title=Glossary of basic terms in polymer science (IUPAC Recommendations 1996)|journal=[[Pure and Applied Chemistry]]|year=1996|volume=68|issue=12|pages=2287–2311|doi=10.1351/pac199668122287|url=http://pac.iupac.org/publications/pac/pdf/1996/pdf/6812x2287.pdf|last1=Jenkins|first1=A. D.|last2=Kratochvíl|first2=P.|last3=Stepto|first3=R. F. T.|last4=Suter|first4=U. W.}} See definition 3.1, p. 2305.</ref> | |||
}} | |||
In [[polymer chemistry]], '''polymerization''' is a process of reacting [[monomer]] [[molecule]]s together in a [[chemical reaction]] to form [[polymer]] chains or three-dimensional networks.<ref>Young, R. J. (1987) ''Introduction to Polymers'', Chapman & Hall ISBN 0-412-22170-5</ref><ref>[http://goldbook.iupac.org/P04740.html International Union of Pure and Applied Chemistry, ''et al.'' (2000) ''IUPAC Gold Book'', '''Polymerization''']</ref><ref name="clayden_organic">Clayden, J., Greeves, N. and Warren, S. (2000). [http://books.google.com/books?id=kQgu2j_ber0C&printsec=frontcover ''Organic chemistry''], Oxford University Press ISBN 0198503466 pp. 1450–1466</ref> There are many forms of polymerization and different systems exist to categorize them. | |||
==Introduction== | |||
{| align="right" class="infobox" style="text-align:center;" | |||
| | |||
<div style="font-size:14px; font-weight:bold;">Homopolymers</div> | |||
:<math>A + A + A... \rightarrow AAA ...</math> | |||
<div style="font-size:14px; font-weight:bold;">Copolymers</div> | |||
:<math>A + B + A... \rightarrow ABA ...</math> | |||
|} | |||
In chemical compounds, polymerization occurs via a variety of reaction mechanisms that vary in complexity due to [[functional group]]s present in reacting compounds<ref name="clayden_organic"/> and their inherent [[steric effects]]. In more straightforward polymerization, [[alkenes]], which are relatively stable due to <math>\sigma</math> [[chemical bonding|bonding]] between carbon atoms, form polymers through relatively simple radical reactions; in contrast, more complex reactions such as those that involve substitution at the carbonyl group 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. In general, polymers such as PVC are referred to as "'''homopolymers'''," 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 [[copolymer]]s (or co-polymers).<ref>Cowie, J.M.G. (1991) ''Polymers: Chemistry and Physics of Modern Materials'', Chapman and Hall, p. 4 ISBN 0849398134</ref> | |||
Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (ca. −80 °C) to form [[trimer (chemistry)|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 in general short-lived and relatively unstable "mid-stage" compounds that react with other molecules present to form more stable polymeric compounds. | |||
Polymerization that is not sufficiently moderated and proceeds at a fast rate can be very hazardous. This phenomenon is known as [[hazardous polymerization]] and can cause fires and explosions. | |||
==Step-growth== | |||
{{Main|Step-growth polymerization}} | |||
Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers, usually containing [[heteroatoms]] such as nitrogen or oxygen. Most step-growth polymers are also classified as [[condensation polymer]]s, but not all step-growth polymers (like [[polyurethane]]s formed from [[isocyanate]] and alcohol bifunctional monomers) release condensates; in this case, we talk about [[addition polymer]]s. Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only 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 loss of small molecules during its synthesis, or contains heteroatoms 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. | |||
==Chain-growth== | |||
{{Main|Chain-growth polymerization}} | |||
Chain-growth polymerization (or addition polymerization) involves the linking together of molecules incorporating double or triple carbon-carbon [[chemical bond|bonds]]. These unsaturated ''monomers'' (the identical molecules that make up the polymers) have extra internal bonds that are able to break and link up with other monomers to form a repeating chain, whose backbone typically contains only carbon atoms. Chain-growth polymerization is involved in the manufacture of polymers such as [[polyethylene]], [[polypropylene]], and [[polyvinyl chloride]] (PVC). A special case of chain-growth polymerization leads to [[living polymerization]]. | |||
In the [[radical polymerization]] of [[ethylene]], its [[pi bond|π bond]] is broken, and the two electrons rearrange to create a new [[Reactive center|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 is one of the first methods to be used. Free radicals are very reactive atoms or molecules that have unpaired electrons. Taking the polymerization of ethylene as an example, the free radical mechanism can be divided into 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 atm. 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 chain growth 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. | |||
[[Ester]]s of [[acrylic acid]] contain a carbon-carbon double bond which is [[conjugated system|conjugated]] to an ester group. This allows the possibility of both types of polymerization mechanism. An acrylic ester by itself can undergo chain-growth polymerization to form a [[homopolymer]] with a carbon-carbon backbone, such as [[poly(methyl methacrylate)]]. Also, however, certain acrylic esters can react with [[diamine]] monomers by [[nucleophilic conjugate addition]] of amine groups to acrylic C=C bonds. In this case the polymerization proceeds by step-growth and the products are poly(beta-amino ester) [[copolymer]]s, with backbones containing nitrogen (as amine) and oxygen (as ester) as well as carbon.<ref>{{cite journal|doi=10.1021/ja0015388 |title= Degradable Poly(β-amino esters): Synthesis, Characterization, and Self-Assembly with Plasmid DNA|year=2000|last1=Lynn|first1=David M.|last2=Langer|first2=Robert|journal=Journal of the American Chemical Society|volume=122|issue=44|pages=10761}}</ref> | |||
==See also== | |||
{{colbegin|3}} | |||
* [[Addition polymer]] | |||
* [[Condensation polymer]] | |||
* [[Cross-link]] | |||
* [[Metallocene]] | |||
* [[Plasma polymerization]] | |||
* [[Polymer characterization]] | |||
* [[Polymer physics]] | |||
* [[Reversible addition−fragmentation chain transfer polymerization]] | |||
* [[Ring-opening polymerization]] | |||
* [[Sequence-controlled polymer]]s | |||
* [[Sol-gel]] | |||
* [[Ziegler-Natta catalyst]] | |||
{{colend}} | |||
==References== | |||
{{reflist}} | |||
[[Category:Polymerization reactions| ]] | |||
Revision as of 15:55, 16 August 2013
Fitter (Standard ) Stephan Sampley from Nicolet, enjoys reading, top 20 property developers in singapore developers in singapore and aromatherapy. Have been recently gonna León Cathedral. In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks.[1][2][3] There are many forms of polymerization and different systems exist to categorize them.
Introduction
|
Homopolymers
Copolymers
|
In chemical compounds, polymerization occurs via a variety of reaction mechanisms that vary in complexity due to functional groups present in reacting compounds[3] and their inherent steric effects. In more straightforward polymerization, alkenes, which are relatively stable due to bonding between carbon atoms, form polymers through relatively simple radical reactions; in contrast, more complex reactions such as those that involve substitution at the carbonyl group require more complex synthesis due to the way in which reacting molecules polymerize.[3]
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,[3] which are produced in high tonnages each year[3] due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. In general, polymers such as PVC are referred to as "homopolymers," 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 copolymers (or co-polymers).[4]
Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (ca. −80 °C) to form trimers;[3] molecules consisting of 3 monomer units, which can cyclize to form ring cyclic structures, or undergo further reactions to form tetramers,[3] or 4 monomer-unit compounds. Further compounds either being referred to as oligomers[3] in smaller molecules. Generally, because formaldehyde is an exceptionally reactive electrophile it allows nucleophillic addition of hemiacetal intermediates, which are in general short-lived and relatively unstable "mid-stage" compounds that react with other molecules present to form more stable polymeric compounds.
Polymerization that is not sufficiently moderated and proceeds at a fast rate can be very hazardous. This phenomenon is known as hazardous polymerization and can cause fires and explosions.
Step-growth
Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers, usually containing heteroatoms such as nitrogen or oxygen. Most step-growth polymers are also classified as condensation polymers, but not all step-growth polymers (like polyurethanes formed from isocyanate and alcohol bifunctional monomers) release condensates; in this case, we talk about addition polymers. Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only 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 loss of small molecules during its synthesis, or contains heteroatoms as part of its backbone chain, or its repeat unit does not contain all the atoms present in the hypothetical monomer to which it can be degraded.
Chain-growth
Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Chain-growth polymerization (or addition polymerization) involves the linking together of molecules incorporating double or triple carbon-carbon bonds. These unsaturated monomers (the identical molecules that make up the polymers) have extra internal bonds that are able to break and link up with other monomers to form a repeating chain, whose backbone typically contains only carbon atoms. Chain-growth polymerization is involved in the manufacture of polymers such as polyethylene, polypropylene, and polyvinyl chloride (PVC). A special case of chain-growth polymerization leads to living polymerization.
In the radical polymerization of ethylene, its π bond is broken, and the 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 is one of the first methods to be used. Free radicals are very reactive atoms or molecules that have unpaired electrons. Taking the polymerization of ethylene as an example, the free radical mechanism can be divided into three stages: chain initiation, chain propagation, and chain termination.
Free radical addition polymerization of ethylene must take place at high temperatures and pressures, approximately 300 °C and 2000 atm. 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 polymer branching.
Other forms of chain growth 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 polymerizations, although free radical living polymerizations have also been developed.
Esters of acrylic acid contain a carbon-carbon double bond which is conjugated to an ester group. This allows the possibility of both types of polymerization mechanism. An acrylic ester by itself can undergo chain-growth polymerization to form a homopolymer with a carbon-carbon backbone, such as poly(methyl methacrylate). Also, however, certain acrylic esters can react with diamine monomers by nucleophilic conjugate addition of amine groups to acrylic C=C bonds. In this case the polymerization proceeds by step-growth and the products are poly(beta-amino ester) copolymers, with backbones containing nitrogen (as amine) and oxygen (as ester) as well as carbon.[5]
See also
- Addition polymer
- Condensation polymer
- Cross-link
- Metallocene
- Plasma polymerization
- Polymer characterization
- Polymer physics
- Reversible addition−fragmentation chain transfer polymerization
- Ring-opening polymerization
- Sequence-controlled polymers
- Sol-gel
- Ziegler-Natta catalyst
References
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- ↑ Young, R. J. (1987) Introduction to Polymers, Chapman & Hall ISBN 0-412-22170-5
- ↑ International Union of Pure and Applied Chemistry, et al. (2000) IUPAC Gold Book, Polymerization
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Clayden, J., Greeves, N. and Warren, S. (2000). Organic chemistry, Oxford University Press ISBN 0198503466 pp. 1450–1466
- ↑ Cowie, J.M.G. (1991) Polymers: Chemistry and Physics of Modern Materials, Chapman and Hall, p. 4 ISBN 0849398134
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