Reversible-deactivation radical polymerization: Difference between revisions
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|title = IUPAC definition for<br/>Reversible-deactivation radical polymerization |
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|quote = [[Chain (growth) polymerization]], propagated by radicals<br/> that are deactivated reversibly, bringing them into<br/>active/dormant [[Equilibrium (chemistry|equilibria]] of which there might be more than one.<ref name=Jenkins2010> |
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{{cite journal |
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| url = http://www.iupac.org/publications/pac/82/2/0483/ |
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| title = Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization |
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| author = Aubrey Jenkins |
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| coauthors = Richard G. Jones, Graeme Moad |
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|journal = [[Pure and Applied Chemistry]] |volume=82 |year=2010 |pages=483-491 |
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}}</ref><br/>See also [[reversible-deactivation polymerization]] RDP. |
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}} |
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'''Reversible deactivation radical polymerizations''' are members of the class of [[reversible deactivation polymerization]]s which exhibit much of the character of [[living polymerization]]s but cannot be categorized as such as they are not with¬out chain transfer or chain termination reactions.<ref name=Szwarz1956>{{Cite journal |
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| author = Szwarz, M. |
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| year = 1956 |
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| journal = Nature |
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| volume = 178 |
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| issue = 1 |
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| pages = 1168-1169 |
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<ref name=Szwarz2000>{{Cite journal |
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| author = Szwarz, M. |
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| year = 2000 |
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| journal = J. Polym. Sci. A |
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| volume = 38 |
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| issue = 10 |
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| pages = 1710 |
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==History and character== |
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Conventional radical polymerization – sometimes misleadingly called ‘free’ radical polymerization – is one of the most widely used polymerization processes since it can be applied |
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*to a great variety of monomers |
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*it can be carried out in the presence of certain functional groups |
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*the technique is rather simple and easy to control |
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*the reaction conditions can vary from bulk over solution, emulsion, miniemulsion to suspension |
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*it is relatively inexpensive compared with competitive techniques |
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The [[steady-state]] concentration of the growing polymer chains is 10<sup>-7</sup> M by order of magnitude, and the average life time of an individual polymer radical before termination is about 5-10 s. A drawback of the conventional radical polymerization is the limited control of chain architecture, molecular weight distribution, and composition. In the late 20th century it was observed that when certain components were added to systems polymerizing by a chain mechanism they are able to react reversibly with the (radical) chain carriers, putting them temporarily into a ‘dormant’ state. |
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This had the effect of prolonging the lifetime of the growing polymer chains (see above) to values comparable with the duration of the experiment. At any instant most of the radicals are in the inactive (dormant) state, however, they are not irreversibly terminated (‘dead’). Only a small fraction of them are active (growing), yet with a fast rate of interconversion of active and dormant forms, faster than the growth rate, the same probability of growth is ensured for all chains, i.e., on average, all chains are growing at the same rate. Consequently, rather than a most probable distribution, the molecular masses (degrees of polymerization) assume a much narrower [[Poisson distribution]], and a lower [[dispersity]] prevails. |
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IUPAC also recognizes the alternative name, ‘controlled reversible-deactivation radical polymerization’ as acceptable, "provided the controlled context is specified, which in this instance comprises molecular mass and molecular mass distribution." These types of radical polymerizations are not necessarily ‘living’ polymerizations, since chain termination reactions are not precluded".<ref name=Jenkins2010>{{cite journal |url = http://www.iupac.org/publications/pac/82/2/0483/ |
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}}</ref><ref name=Szwarz1956>{{cite journal | journal = Nature |
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}}</ref><ref name=Szwarz2000>{{cite journal | journal = J. Polym. Sci. A |
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}}</ref> |
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==Controlled radical polymerization== |
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The adjective ‘controlled’ indicates that a certain kinetic feature of a polymerization or structural aspect of the polymer molecules formed is controlled (or both). The expression ‘controlled polymerization’ is sometimes used to describe a [[Radical polymerization|radical]] or [[ionic polymerization]] in which reversible-deactivation of the [[chain carrier]]s is an essential component of the mechanism and interrupts the propagation that secures control of one or more kinetic features of the [[polymerization]] or one or more structural aspects of the [[macromolecule]]s formed, or both. The expression ‘controlled radical polymerization’ is sometimes used to describe a radical polymerization that is conducted in the presence of agents that lead to e.g. atom-transfer radical polymerization (ATRP), nitroxide-(aminoxyl) mediated polymerization (NMP), or reversible-addition-fragmentation chain transfer (RAFT) polymerization. All these and further controlled polymerizations are included in the class of reversible-deactivation radical polymerizations. Whenever the adjective ‘controlled’ is used in this context the particular kinetic or the structural features that are controlled have to be specified. |
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==Examples== |
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===Atom-transfer radical polymerization (ATRP)=== |
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The initiator of the polymerization is usually an organohalogenid and the dormant state is achieved in a metal complex of a transition metal (‘radical buffer’). This method is very versatile but requires unconventional initiator systems that are sometimes poorly compatible with the polymerization media. |
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===Aminoxyl-mediated polymerization (NMP)=== |
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Given certain conditions a homolytic splitting of the C-O bond in alkoxylamines can occur and a stable 2-centre 3 electron N-O radical can be formed that is able to initiate a polymerization reaction. The preconditions for an alkoxylamine suitable to initiate a polymerization are bulky, sterically obstructive substituents on the secondary amine, and the substituent on the oxygen should be able to form a stable radical, e.g. benzyl. |
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[[File:ReversibleDeactivationReaction.png|thumb|center|500px|Example of a reversible deactivation reaction]] |
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===Reversible addition-fragmentation chain transfer (RAFT)=== |
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{{Main|Reversible addition−fragmentation chain-transfer polymerization}} |
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RAFT is one of the most versatile and convenient techniques in this context. The most common RAFT-processes are carried out in the presence of thiocarbonylthio compounds that act as radical buffers. |
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While in ATRP and NMP reversible deactivation of propagating radical-radical reactions takes place and the dormant structures are a halo-compound in ATRP and the alkoxyamine in NMP, both being a sink for radicals and source at the same time and described by the corresponding equilibria. RAFT on the contrary, is controlled by chain-transfer reactions that are in a deactivation-activation equilibrium. Since no radicals are generated or destroyed an external source of radicals is necessary for initiation and maintenance of the propagation reaction. |
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;Initiation step of a RAFT polymerization |
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[[File:RAFTinitiation.png|left|250px|Initation step]] {{clear-left}} |
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;Reversible chain transfer |
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[[File:RAFT ReversibleChainTransfer.png|left|500px|Reversible chain transfer]] {{clear-left}} |
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;Reinitiation step |
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[[File:RAFTreinitiation.png|left|250px|Reinitiation step]] {{clear-left}} |
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;Chain equilibration step |
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[[File:RAFT ChainEquilibration.png|left|450px|Chain equilibration step]] {{clear-left}} |
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;Termination step |
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[[File:RAFTtermination.png|left|220px|Termination step]] {{clear-left}} |
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===Other examples=== |
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*Cobalt (catalytic) mediated radical polymerization |
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*Iodine-transfer polymerization (ITRP) |
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*Radical catalysed transfer polymerization (RCTP) |
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*[[Ring-opening metathesis polymerization]] (ROMP) |
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*Stable-radical-mediated polymerization (SFRP), NMP is an example. |
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*Selenium centred radical mediated polymerization |
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*Telluride-mediated polymerization (TERP) |
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== References == |
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{{Reflist|2}} |
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Revision as of 03:19, 5 July 2013
Reversible-deactivation radical polymerization
Chain (growth) polymerization, propagated by radicals
that are deactivated reversibly, bringing them into
active/dormant equilibria of which there might be more than one.[1]
See also reversible-deactivation polymerization RDP.
Reversible deactivation radical polymerizations are members of the class of reversible deactivation polymerizations which exhibit much of the character of living polymerizations but cannot be categorized as such as they are not with¬out chain transfer or chain termination reactions.[2] [3]
History and character
Conventional radical polymerization – sometimes misleadingly called ‘free’ radical polymerization – is one of the most widely used polymerization processes since it can be applied
- to a great variety of monomers
- it can be carried out in the presence of certain functional groups
- the technique is rather simple and easy to control
- the reaction conditions can vary from bulk over solution, emulsion, miniemulsion to suspension
- it is relatively inexpensive compared with competitive techniques
The steady-state concentration of the growing polymer chains is 10-7 M by order of magnitude, and the average life time of an individual polymer radical before termination is about 5-10 s. A drawback of the conventional radical polymerization is the limited control of chain architecture, molecular weight distribution, and composition. In the late 20th century it was observed that when certain components were added to systems polymerizing by a chain mechanism they are able to react reversibly with the (radical) chain carriers, putting them temporarily into a ‘dormant’ state.
This had the effect of prolonging the lifetime of the growing polymer chains (see above) to values comparable with the duration of the experiment. At any instant most of the radicals are in the inactive (dormant) state, however, they are not irreversibly terminated (‘dead’). Only a small fraction of them are active (growing), yet with a fast rate of interconversion of active and dormant forms, faster than the growth rate, the same probability of growth is ensured for all chains, i.e., on average, all chains are growing at the same rate. Consequently, rather than a most probable distribution, the molecular masses (degrees of polymerization) assume a much narrower Poisson distribution, and a lower dispersity prevails.
IUPAC also recognizes the alternative name, ‘controlled reversible-deactivation radical polymerization’ as acceptable, "provided the controlled context is specified, which in this instance comprises molecular mass and molecular mass distribution." These types of radical polymerizations are not necessarily ‘living’ polymerizations, since chain termination reactions are not precluded".[1][2][3]
Controlled radical polymerization
The adjective ‘controlled’ indicates that a certain kinetic feature of a polymerization or structural aspect of the polymer molecules formed is controlled (or both). The expression ‘controlled polymerization’ is sometimes used to describe a radical or ionic polymerization in which reversible-deactivation of the chain carriers is an essential component of the mechanism and interrupts the propagation that secures control of one or more kinetic features of the polymerization or one or more structural aspects of the macromolecules formed, or both. The expression ‘controlled radical polymerization’ is sometimes used to describe a radical polymerization that is conducted in the presence of agents that lead to e.g. atom-transfer radical polymerization (ATRP), nitroxide-(aminoxyl) mediated polymerization (NMP), or reversible-addition-fragmentation chain transfer (RAFT) polymerization. All these and further controlled polymerizations are included in the class of reversible-deactivation radical polymerizations. Whenever the adjective ‘controlled’ is used in this context the particular kinetic or the structural features that are controlled have to be specified.
Examples
Atom-transfer radical polymerization (ATRP)
The initiator of the polymerization is usually an organohalogenid and the dormant state is achieved in a metal complex of a transition metal (‘radical buffer’). This method is very versatile but requires unconventional initiator systems that are sometimes poorly compatible with the polymerization media.
Aminoxyl-mediated polymerization (NMP)
Given certain conditions a homolytic splitting of the C-O bond in alkoxylamines can occur and a stable 2-centre 3 electron N-O radical can be formed that is able to initiate a polymerization reaction. The preconditions for an alkoxylamine suitable to initiate a polymerization are bulky, sterically obstructive substituents on the secondary amine, and the substituent on the oxygen should be able to form a stable radical, e.g. benzyl.
Reversible addition-fragmentation chain transfer (RAFT)
RAFT is one of the most versatile and convenient techniques in this context. The most common RAFT-processes are carried out in the presence of thiocarbonylthio compounds that act as radical buffers. While in ATRP and NMP reversible deactivation of propagating radical-radical reactions takes place and the dormant structures are a halo-compound in ATRP and the alkoxyamine in NMP, both being a sink for radicals and source at the same time and described by the corresponding equilibria. RAFT on the contrary, is controlled by chain-transfer reactions that are in a deactivation-activation equilibrium. Since no radicals are generated or destroyed an external source of radicals is necessary for initiation and maintenance of the propagation reaction.
- Initiation step of a RAFT polymerization
- Reversible chain transfer
- Reinitiation step
- Chain equilibration step
- Termination step
Other examples
- Cobalt (catalytic) mediated radical polymerization
- Iodine-transfer polymerization (ITRP)
- Radical catalysed transfer polymerization (RCTP)
- Ring-opening metathesis polymerization (ROMP)
- Stable-radical-mediated polymerization (SFRP), NMP is an example.
- Stibine mediated radical polymerization
- Selenium centred radical mediated polymerization
- Telluride-mediated polymerization (TERP)
References
- ^ a b
Aubrey Jenkins (2010). "Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization". Pure and Applied Chemistry. 82: 483–491.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) Cite error: The named reference "Jenkins2010" was defined multiple times with different content (see the help page). - ^ a b Szwarz, M. (1956). Nature. 178 (1): 1168–1169.
{{cite journal}}: Cite has empty unknown parameter:|month=(help); Missing or empty|title=(help) Cite error: The named reference "Szwarz1956" was defined multiple times with different content (see the help page). - ^ a b Szwarz, M. (2000). J. Polym. Sci. A. 38 (10): 1710.
{{cite journal}}: Cite has empty unknown parameter:|month=(help); Missing or empty|title=(help) Cite error: The named reference "Szwarz2000" was defined multiple times with different content (see the help page).