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{{short description|Chain polymerization involving cyclic monomers}}
{{Quote box
|title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
|quote = A [[polymerization]] in which a [[Cyclic compound|cyclic]] [[monomer]] yields a monomeric unit which is [[Open-chain compound|acyclic]] or contains fewer cycles than the monomer.
Note:
If
Modified from the earlier definition.<ref name="Goldbook">{{GoldBookRef|title=Ring-opening polymerization|file=R05396|accessdate=Mar 10, 2014}}</ref><ref name=PAC1996>{{cite journal
|url= http://iupac.org/publications/pac/68/12/2287/
|doi = 10.1351/pac199668122287
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|last1= Jenkins |first1= A. D. |last2= Kratochvíl |first2= P. |last3= Stepto |first3= R. F. T. |last4= Suter |first4= U. W.
|journal= Pure and Applied Chemistry |volume=68 |year=1996 |pages=2287–2311
|issue= 12|doi-access= free
|source = [http://www.iupac.org/publications/pac/80/10/2163/ Penczek S.; Moad, G. ''Pure Appl. Chem.'', '''2008''', 80(10), 2163-2193]
|align = right
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[[File:General scheme ionic prop.png|thumb|600px|General scheme ionic propagation. Propagating center can be radical, cationic or anionic.]]
In [[polymer chemistry]], '''ring-opening polymerization''' ('''ROP''') is a form of [[chain-growth polymerization]], in which the terminus of a [[polymer]] chain attacks [[cyclic compound|cyclic monomers]] to form a longer polymer (see figure). The reactive center can be [[Radical (chemistry)|radical]], [[anion]]ic or [[cation]]ic. Some cyclic monomers such as [[norbornene]] or [[cyclooctadiene]] can be [[polymerization|polymerized]] to high [[molecular mass|molecular weight]] polymers by using metal [[Catalysis|catalysts]]. ROP is a versatile method for the synthesis of [[biopolymer]]s.▼
▲In [[polymer chemistry]], '''ring-opening polymerization''' ('''ROP''') is a form of [[chain-growth polymerization]]
Ring-opening of cyclic monomers is often driven by the relief of [[ring strain|bond-angle strain]]. Thus, as is the case for other types of polymerization, the [[enthalpy]] change in ring-opening is negative.<ref name=Young>{{cite book|last=Young|first=Robert J.|title=Introduction to Polymers|year=2011|publisher=CRC Press|location=Boca Raton|isbn=978-0-8493-3929-5}}</ref>▼
▲Ring-opening of cyclic monomers is often driven by the relief of [[ring strain|bond-angle strain]]. Thus, as is the case for other types of polymerization, the [[enthalpy]] change in ring-opening is negative.<ref name=Young>{{cite book|last=Young|first=Robert J.|title=Introduction to Polymers|year=2011|publisher=CRC Press|location=Boca Raton|isbn=978-0-8493-3929-5}}</ref> Many rings undergo ROP.<ref>{{cite journal |doi=10.1007/s00726-006-0432-9}}</ref>
==Monomers==
Many [[cyclic compound|
</ref> and [[amino acid N-carboxyanhydride|amino acid ''N''-carboxyanhydride]]s.<ref>{{cite journal|author=Kricheldorf, H. R. |year=2006 |title=Polypeptides and 100 Years of Chemistry of α-Amino Acid ''N''-Carboxyanhydrides|journal=Angewandte Chemie International Edition |volume=45|issue=35|pages=5752–5784|doi= 10.1002/anie.200600693|pmid=16948174 }}</ref><ref>{{cite journal|title=Synthesis of Well-Defined Polypeptide-Based Materials via the Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydrides|author=Nikos Hadjichristidis |author2=Hermis Iatrou |author3=Marinos Pitsikalis |author4=Georgios Sakellariou |journal=Chemical Reviews|year=2009|volume=109|issue=11|pages= 5528–5578|doi=10.1021/cr900049t|pmid=19691359}}</ref> Many strained [[cycloalkene]]s, e.g [[norbornene]], are suitable monomers via [[ring-opening metathesis polymerization]]. Even highly strained [[cycloalkane]] rings, such as [[cyclopropane]]<ref>{{cite journal |title= The Polymerization of Cyclopropane |first1= R. J. |last1= Scott |first2= H. E. |last2= Gunning |journal= J. Phys. Chem. |year= 1952 |volume= 56 |issue= 1 |pages= 156–160 |doi= 10.1021/j150493a031 }}</ref> and [[cyclobutane]]<ref>{{cite journal |title= Ring-Opening Polymerization of the Cyclobutane Adduct of Methyl Tricyanoethylenecarboxylate and Ethyl Vinyl Ether |first1= Tsutomu |last1= Yokozawa |first2= Ei-ichi |last2= Tsuruta |journal= Macromolecules |year= 1996 |volume= 29 |issue= 25 |pages= 8053–8056 |doi= 10.1021/ma9608535 }}</ref> derivatives, can undergo ROP.
==History==
Ring-opening polymerization has been used since the beginning of the 1900s to produce [[polymer]]s. Synthesis of [[polypeptides]] which has the oldest history of ROP, dates back to the work in 1906 by Leuchs.<ref>{{cite journal|title=Glycine-carbonic acid|last=Leuchs|first=H.|journal=Berichte der
An industrial application is the production of [[nylon-6]] from [[caprolactam]].
==Mechanisms==
Ring-opening polymerization can proceed via [[Radical (chemistry)|radical]], anionic, or cationic polymerization as described below.<ref name=nuyken>{{cite journal|last=Nuyken|first=Oskar|author2=Stephen D. Pask |title=Ring-Opening Polymerization—An Introductory Review|journal=Polymers|date=25 April 2013|volume=5|issue=2|pages=361–403|doi=10.3390/polym5020361|doi-access=free}}</ref> Additionally, radical ROP is useful in producing polymers with [[functional group]]s incorporated in the backbone chain that cannot otherwise be synthesized via conventional [[chain-growth polymerization]] of [[Vinyl group|vinyl]] monomers. For instance, radical ROP can produce polymers with [[ethers]], [[esters]], [[amide]]s, and [[carbonates]] as functional groups along the main chain.<ref name=nuyken /><ref name=dubois>{{cite book|last=Dubois|first=Philippe|title=Handbook of ring-opening polymerization|year=2008|publisher=Wiley-VCH|location=Weinheim|isbn=978-3-527-31953-4|edition=1. Aufl.}}</ref>
===Anionic ring-opening polymerization (AROP)===
{{main article|Anionic polymerization}}
[[File:Wiki566665.tif|thumb|400px|center|The general mechanism for anionic ring-opening polymerization. Polarized functional group is represented by X-Y, where the atom X (usually a carbon atom) becomes electron deficient due to the highly electron-withdrawing nature of Y (usually an oxygen, nitrogen, sulfur, etc.). The nucleophile will attack atom X, thus releasing Y
Anionic ring-opening polymerizations (AROP)
A typical example of anionic ROP is that of [[caprolactone|ε-caprolactone]], initiated by an [[alkoxide]].<ref name=dubois />
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Cationic initiators and intermediates characterize cationic ring-opening polymerization (CROP). Examples of [[cyclic compound|cyclic monomers]] that polymerize through this mechanism include [[lactone]]s, [[lactam]]s, [[amine]]s, and [[ether]]s.<ref name="cowie cation">{{cite book|last=Cowie|first=John McKenzie Grant|title=Polymers: Chemistry and Physics of Modern Materials|year=2008|publisher=CRC Press|location=Boca Raton, Florida|isbn=978-0-8493-9813-1|pages=105–107}}</ref> CROP proceeds through an [[SN1 reaction|S<sub>N</sub>1]] or [[SN2 reaction|S<sub>N</sub>2]] propagation, chain-growth process.<ref name=nuyken /> The mechanism is affected by the stability of the resulting [[ion|cationic]] species. For example, if the atom bearing the positive charge is stabilized by [[activating group|electron-donating groups]], polymerization will proceed by the S<sub>N</sub>1 mechanism.<ref name=dubois /> The cationic species is a [[heteroatom]] and the chain grows by the addition of cyclic monomers thereby opening the ring system.
[[
The monomers can be activated by [[Brønsted–Lowry acid–base theory|Bronsted acids]], [[carbenium ion]]s, [[Onium compound|onium ions]], and metal cations.<ref name=nuyken />
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===Ring-opening metathesis polymerization===
{{main article|Ring-opening metathesis polymerization}}
[[Ring-opening metathesis polymerisation
The mechanism for ROMP follows similar pathways as [[olefin metathesis]]. The initiation process involves the coordination of the cycloalkene monomer to the [[Transition metal carbene complex|metal alkylidene complex]], followed by a [2+2] type [[cycloaddition]] to form the metallacyclobutane intermediate that cycloreverts to form a new alkylidene species.<ref name=sutthasupa>{{cite journal|last=Sutthasupa|first=Sutthira|author2=Shiotsuki, Masashi |author3=Sanda, Fumio |title=Recent advances in ring-opening metathesis polymerization, and application to synthesis of functional materials|journal=Polymer Journal|date=13 October 2010|volume=42|issue=12|pages=905–915|doi=10.1038/pj.2010.94|doi-access=free}}</ref><ref name=hartwig>{{cite book|last=Hartwig|first=John F.|
[[File:Romp mechanism.png|thumb|center|850px|General scheme of the mechanism for ROMP.]] Commercially relevant [[Saturated and unsaturated compounds|unsaturated]] polymers synthesized by ROMP include
==Thermodynamics==
The formal thermodynamic criterion of a given monomer polymerizability is related to a sign of the [[free enthalpy]] ([[Gibbs free energy]]) of polymerization:
where:
:{{mvar|x}} and {{mvar|y}} indicate monomer and polymer states, respectively ({{mvar|x}} and/or {{mvar|y}} = l (liquid), g ([[gaseous]]), c ([[amorphous solid]]), c' ([[crystalline solid]]), s ([[Solution (chemistry)|solution]]));
The free [[enthalpy]] of polymerization (ΔG<sub>p</sub>) may be expressed as a sum of standard [[enthalpy]] of polymerization (ΔG<sub>p</sub>°) and a term related to instantaneous monomer molecules and growing [[macromolecules]] concentrations: ▼
:{{math|Δ''H<sub>p</sub>''(''xy'')}} is the [[enthalpy]] of polymerization (SI unit: joule per kelvin);
:<math>\Delta G_p = \Delta G^\circ_p + RT\ln\frac{[...-(m)_{i+1} m^\ast]}{[M][...-(m)_i m^\ast]}</math>▼
:{{math|Δ''S{{sub|p}}''(''xy'')}} is the [[entropy]] of polymerization (SI unit: joule);
:{{mvar|T}} is the [[absolute temperature]] (SI unit: kelvin).
Following [[Flory–Huggins solution theory]] that the reactivity of an active center, located at a [[macromolecule]] of a sufficiently long macromolecular chain, does not depend on its [[degree of polymerization]] (DPi), and taking in to account that ΔG<sub>p</sub>° = ΔH<sub>p</sub>° - TΔS<sub>p</sub>° (where ΔH<sub>p</sub>° and ΔS<sub>p</sub>° indicate a standard polymerization [[enthalpy]] and [[entropy]], respectively), we obtain: ▼
▲The free
:<math>\Delta G_p = \Delta H^\circ_p - T(\Delta S^\circ_p + R\ln[M])</math>▼
▲
At [[Chemical equilibrium|equilibrium]] (ΔG<sub>p</sub> = 0), when polymerization is complete the monomer concentration ([M]<sub>eq</sub>) assumes a value determined by standard polymerization parameters (ΔH<sub>p</sub>° and ΔS<sub>p</sub>°) and polymerization temperature:▼
where:
:<math>[M]_{eq} = e^{\frac{\Delta H^\circ_p}{RT} - \frac{\Delta S^\circ_p}{R}}</math>▼
:{{mvar|R}} is the [[gas constant]];
:<math>\ln\frac{DP_n}{DP_n - 1}[M]_{eq} = \frac{\Delta H^\circ_p}{RT} - \frac{\Delta S^\circ_p}{R}</math>▼
:{{math|M}} is the monomer;
:<math>[M]_{eq} = \frac{DP_n - 1}{DP_n} e^{\frac{\Delta H^\circ_p}{RT} - \frac{\Delta S^\circ_p}{R}}</math>▼
:{{math|(m)<sub>''i''</sub>}} is the monomer in an initial state;
Polymerzation is possible only when [M]<sub>0</sub> > [M]<sub>eq</sub>. Eventually, at or above the so-called [[ceiling temperature]] (T<sub>c</sub>), at which [M]<sub>eq</sub> = [M]<sub>0</sub>, formation of the high polymer does not occur. ▼
:{{math|m<sup>*</sup>}} is the active monomer.
:<math>T_c = \frac{\Delta H^\circ_p}{\Delta S^\circ_p + R\ln[M]_0} ; (\Delta H^\circ_p<0, \Delta S^\circ_p<0)</math>▼
▲Following [[Flory–Huggins solution theory]] that the reactivity of an active center, located at a [[macromolecule]] of a sufficiently long macromolecular chain, does not depend on its [[degree of polymerization]] (
:<math>T_f = \frac{\Delta H^\circ_p}{\Delta S^\circ_p + R\ln[M]_0} ; (\Delta H^\circ_p>0, \Delta S^\circ_p>0)</math>▼
▲:<math>\Delta G_p = \Delta H^\circ_p - T(\Delta S^\circ_p + R\ln[M])</math>
For example, [[tetrahydrofuran]] (THF) cannot be polymerized above T<sub>c</sub> = 84 °C, nor cyclo-octasulfur (S<sub>8</sub>) below T<sub>f</sub> = 159 °C.<ref>{{cite journal|last=Tobolsky|first=A. V.|title=Equilibrium polymerization in the presence of an ionic initiator|journal=Journal of Polymer Science|date=July 1957|volume=25|issue=109|pages=220–221|doi=10.1002/pol.1957.1202510909|bibcode=1957JPoSc..25..220T}}</ref><ref>{{cite journal|last=Tobolsky|first=A. V.|title=Equilibrium polymerization in the presence of an ionic initiator|journal=Journal of Polymer Science|date=August 1958|volume=31|issue=122|pages=126|doi=10.1002/pol.1958.1203112214|bibcode=1958JPoSc..31..126T|doi-access=free}}</ref><ref>{{cite journal|last=Tobolsky|first=Arthur V.|author2=Eisenberg, Adi |title=Equilibrium Polymerization of Sulfur|journal=Journal of the American Chemical Society|date=May 1959|volume=81|issue=4|pages=780–782|doi=10.1021/ja01513a004}}</ref><ref>{{cite journal|last=Tobolsky|first=A. V.|author2=Eisenberg, A. |title=A General Treatment of Equilibrium Polymerization|journal=Journal of the American Chemical Society|date=January 1960|volume=82|issue=2|pages=289–293|doi=10.1021/ja01487a009}}</ref> However, for many monomers, T<sub>c</sub> and T<sub>f</sub>, for polymerization in the bulk, are well above or below the operable polymerization temperatures, respectively.▼
▲At [[Chemical equilibrium|equilibrium]] (
The polymerization of a majority of monomers is accompanied by an [[entropy]] decrease, due mostly to the loss in the translational degrees of freedom. In this situation, polymerization is thermodynamically allowed only when the enthalpic contribution into ΔG<sub>p</sub> prevails (thus, when ΔH<sub>p</sub>° < 0 and ΔS<sub>p</sub>° < 0, the inequality |ΔH<sub>p</sub>| > -TΔS<sub>p</sub> is required). Therefore, the higher the ring strain, the lower the resulting monomer concentration at [[Chemical equilibrium|equilibrium]].▼
<math chem display=block>\begin{align}
▲
▲
▲
\end{align}</math>
▲
<math chem display=block>\begin{align}
▲
▲
\end{align}</math>
▲For example, [[tetrahydrofuran]] (THF) cannot be polymerized above {{mvar|T<sub>c</sub>
▲The polymerization of a majority of monomers is accompanied by an [[entropy]] decrease, due mostly to the loss in the translational degrees of freedom. In this situation, polymerization is thermodynamically allowed only when the enthalpic contribution into
==Additional reading==
*{{
*{{cite journal|
*{{cite book |title= Handbook of Ring‐Opening Polymerization |editor1-first= Philippe |editor1-last= Dubois |editor2-first= Olivier |editor2-last= Coulembier |editor3-first= Jean-Marie |editor3-last= Raquez |publisher= Wiley |year= 2009 |isbn= 9783527628407 |doi= 10.1002/9783527628407 }}<!-- see especially chapter 13 "Polymerization of Cycloalkanes" lead-ref for expanding our article -->
== References ==
<references />
[[Category:Polymerization reactions]]
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