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{{short description|Chain polymerization involving cyclic monomers}}
{{Quote box
|title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
|title = IUPAC definition for '''ring-opening polymerization'''
|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 the monomer is [[Polycyclic compound|polycyclic]], the opening of a single ring is sufficient to classify the [[Chemical reaction|reaction]] as ring-opening polymerization.
 
Modified from the earlier definition.<ref name="Goldbook">{{GoldBookRef|title=Ring-opening polymerization|urlfile=http://goldbook.iupac.org/R05396.html|accessdate=Mar 10, 2014}}</ref><ref name=PAC1996>{{cite journal
.<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}}</ref>
|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 terminal end of a [[polymer]] chain acts as a [[reactive center]] where further [[cyclic compound|cyclic monomers]] can react by opening its ring system and form a longer polymer chain (see figure). The propagating 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 continues to be the most versatile method of synthesis of major groups of [[biopolymer]]s, particularly when they are required in quantity.
 
In [[polymer chemistry]], '''ring-opening polymerization''' ('''ROP''') is a form of [[chain-growth polymerization]] in which the [[End group|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.
The driving force for the ring-opening of cyclic monomers is via the relief of [[ring strain|bond-angle strain]] or [[steric effects|steric repulsions]] between atoms at the center of a ring. 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>
[[cyclic compound|Cyclic monomers]] that are polymerized using ROP encompass a variety of structures, such as:
 
*[[alkanes]], [[alkenes]],
==Monomers==
*compounds containing [[heteroatoms]] in the ring:
Many [[cyclic compound|cyclic monomers]] are amenable to ROP.<ref>{{cite journal |doi=10.3390/polym5020361|doi-access=free |title=Ring-Opening Polymerization—An Introductory Review |date=2013 |last1=Nuyken |first1=Oskar |last2=Pask |first2=Stephen |journal=Polymers |volume=5 |issue=2 |pages=361–403 }}</ref> These include [[epoxide]]s,<ref name=Sarazin>{{cite journal|title=Discrete Cationic Complexes for Ring-Opening Polymerization Catalysis of Cyclic Esters and Epoxides|author=Yann Sarazin |author2=Jean-François Carpentier |journal=Chemical Reviews|year=2015|volume=115|issue=9|pages=3564–3614|doi=10.1021/acs.chemrev.5b00033|pmid=25897976}}</ref><ref name=Longo>{{cite journal|title=Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure–Property Relationships|first1=Julie M.|last1=Longo|first2=Maria J.|last2= Sanford|first3=Geoffrey W.|last3=Coates|journal=Chemical Reviews|year=2016|volume=116|issue=24|pages=15167–15197|doi=10.1021/acs.chemrev.6b00553|pmid=27936619}}</ref> cyclic trisiloxanes,{{cn|date=December 2023}} some lactones<ref name=Sarazin/><ref name=Jerome>{{Cite journal|last1=JEROME|first1=C|last2=LECOMTE|first2=P|date=2008-06-10|title=Recent advances in the synthesis of aliphatic polyesters by ring-opening polymerization☆|journal=Advanced Drug Delivery Reviews|volume=60|issue=9|pages=1056–1076|doi=10.1016/j.addr.2008.02.008|pmid=18403043|hdl=2268/3723|issn=0169-409X|url=http://orbi.ulg.ac.be/handle/2268/3723|hdl-access=free}}</ref> and [[lactide]]s,<ref name=Jerome/> cyclic [[anhydride]]s,<ref name=Longo/> [[cyclic carbonate]]s,<ref>{{cite journal|last=Matsumura|first=Shuichi|author2=Tsukada, Keisuke |author3=Toshima, Kazunobu |title=Enzyme-Catalyzed Ring-Opening Polymerization of 1,3-Dioxan-2-one to Poly(trimethylene carbonate)|journal=Macromolecules|date=May 1997|volume=30|issue=10|pages=3122–3124|doi=10.1021/ma961862g|bibcode=1997MaMol..30.3122M}}
**[[oxygen]]: [[ethers]], [[acetals]], [[esters]] ([[lactones]], [[lactides]], and [[carbonates]]), and [[anhydrides]],
</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.
**[[sulfur]]: polysulfur, [[sulfides]] and [[polysulfides]],
**[[nitrogen]]: [[amine]]s, [[amide]]s (lactames), [[imides]], N-carboxyanhydrides and 1,3-oxaza derivatives,
**[[phosphorus]]: [[phosphates]], [[phosphonates]], [[phosphites]], [[phosphines]] and phosphazenes,
**[[silicon]]: [[siloxanes]], silathers, carbosilanes and [[silanes]].
 
==History==
'''Ring-opening polymerization''' ('''ROP''') has been used since the beginning of the 1900s in order to synthesizeproduce [[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=Ber.Berichte Dtsch.der Chem.Deutschen Chemischen Gesellschaft|year=1906|volume=39|page=857|doi=10.1002/cber.190603901133|url=https://zenodo.org/record/1426172}}</ref> Many years later came the method ofSubsequently, the ROP of anhydro [[sugars]], providingprovided [[polysaccharides]], including synthetic [[dextran]], [[xanthan gum]], [[welan gum]], [[gellan gum]], diutan gum, and [[pullulan]]. Mechanisms and thermodynamics of ring-opening polymerization was furtherwere established in the 1950s.<ref>{{cite journal|last=Dainton|first=F. S.|author2=Devlin, T. R. E. |author3=Small, P. A. |title=The thermodynamics of polymerization of cyclic compounds by ring opening|journal=Transactions of the Faraday Society|year=1955|volume=51|pagespage=1710|doi=10.1039/TF9555101710}}</ref><ref>{{cite journal|last=Conix|first=André|author2=Smets, G. |title=Ring opening in lactam polymers|journal=Journal of Polymer Science|date=January 1955|volume=15|issue=79|pages=221–229|doi=10.1002/pol.1955.120157918|bibcode=1955JPoSc..15..221C}}</ref> The first high-molecular weight polymers (M<sub>n</sub> up to 10<sup>5</sup>) with a [[repeat unit|repeating unit]] were prepared by ROP as early as in 1976.<ref>{{cite journal|last1= Kałuz̀ynski|first1=Krzysztof|last2=Libiszowski|first2=Jan|last3=Penczek|first3=Stanisław|title=Poly(2-hydro-2-oxo-1,3,2-dioxaphosphorinane). Preparation and NMR spectra|journal=Die Makromolekulare Chemie|volume=178|issue=10|year=1977|pages=2943–2947|issn=0025-116X|doi=10.1002/macp.1977.021781017}}</ref><ref>{{cite journal|last=Libiszowski|first=Jan|author2=Kałużynski, Krzysztof |author3=Penczek, Stanisław |title=Polymerization of cyclic esters of phosphoric acid. VI. Poly(alkyl ethylene phosphates). Polymerization of 2-alkoxy-2-oxo-1,3,2-dioxaphospholans and structure of polymers|journal=Journal of Polymer Science: Polymer Chemistry Edition|date=June 1978|volume=16|issue=6|pages=1275–1283|doi=10.1002/pol.1978.170160610|bibcode=1978JPoSA..16.1275L}}</ref>
<ref>{{cite journal|last=Libiszowski|first=Jan|author2=Kałużynski, Krzysztof |author3=Penczek, Stanisław |title=Polymerization of cyclic esters of phosphoric acid. VI. Poly(alkyl ethylene phosphates). Polymerization of 2-alkoxy-2-oxo-1,3,2-dioxaphospholans and structure of polymers|journal=Journal of Polymer Science: Polymer Chemistry Edition|date=June 1978|volume=16|issue=6|pages=1275–1283|doi=10.1002/pol.1978.170160610}}</ref>
 
An industrial application is the production of [[nylon-6]] from [[caprolactam]].
Nowadays, ROP plays an important role in industry such as production of [[nylon-6]]. ROP can introduce [[functional group]]s such as [[ether]], [[ester]], [[amide]], and [[carbonate]] into the polymer main chain, which cannot be achieved by [[vinyl polymer]]ization affording polymers only with C-C main chain. Polymers obtained by ROP can be also prepared by [[polycondensation]] in most cases, but following [[controlled radical polymerization]] is possible in ROP, which is difficult in polycondensation.
Recently, development of novel [[monomer]]s and [[Catalysis|catalysts]] has enabled polymer chemists to control [[molecular mass|molecular weights]], structure, and configuration of the polymers precisely.<ref>{{cite book|last=Luck|first=edited by Rajender K. Sadhir, Russell M.|title=[[Expanding Monomers]]: Synthesis, Characterization, and Applications|year=1992|publisher=CRC Press|location=Boca Raton, Florida|isbn=9780849351563}}</ref> Cyclic carbonates undergo both [[cationic polymerization]] and [[anionic polymerization]] to afford the corresponding [[polycarbonates]], which are expected as biocompatible and biodegradable polymers.<ref>{{cite journal|last=Matsumura|first=Shuichi|author2=Tsukada, Keisuke |author3=Toshima, Kazunobu |title=Enzyme-Catalyzed Ring-Opening Polymerization of 1,3-Dioxan-2-one to Poly(trimethylene carbonate)|journal=Macromolecules|date=May 1997|volume=30|issue=10|pages=3122–3124|doi=10.1021/ma961862g}}</ref>
Recently, ultra high molecular weight bisphenol A polycarbonate (> 2,000 kDa) has been synthesized by ROP of a large-membered bisphenol A-based [[cyclic carbonate]].<ref>{{cite journal|last=Sugiyama|first=J|author2=R. Nagahata |author3=M. Goyal |author4=M. Asai |author5=M. Ueda |author6=K. Takeuchi |journal=ACS Polymer Preprints|year=1998|volume=40|series=1|page=90}}</ref> The resulted polymer can be used as engineering plastics due to its thermal stability and high impact resistance.
When the reactive center of the propagating chain is a [[carbocation]], the polymerization is called '''cationic ring-opening polymerization'''.
 
==Mechanisms of ROP==
Ring-opening polymerization can proceed via [[Radical (chemistry)|radical]], anionic, or cationic polymerization as described below.<ref inname=nuyken>{{cite morejournal|last=Nuyken|first=Oskar|author2=Stephen detailsD. 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 canis involveuseful metalin producing polymers with [[Catalysis|catalystsfunctional group]]s tooincorporated andin isthe bestbackbone exemplifiedchain bythat thecannot otherwise be synthesized via conventional [[chain-growth polymerization]] of [[AlkeneVinyl group|olefinsvinyl]] whilemonomers. maintainingFor instance, radical ROP can produce polymers with [[Saturationethers]], (chemistry)|unsaturation[[esters]], in[[amide]]s, theand resulting[[carbonates]] polymer.as functional Thisgroups mechanismalong isthe knownmain aschain.<ref [[Ring-openingname=nuyken metathesis/><ref polymerisationname=dubois>{{cite book|last=Dubois|first=Philippe|title=Handbook of ring-opening metathesis polymerization|year=2008|publisher=Wiley-VCH|location=Weinheim|isbn=978-3-527-31953-4|edition=1. (ROMP)]]Aufl.}}</ref>
 
===RadicalAnionic ring-opening polymerization (AROP)===
{{main article|Radical polymerization}}
 
With radical ring-opening polymerization, it is possible to produce polymers of the same or lower density than the monomers. This is important for applications that require constant volume after polymerization, such as [[Dental restoration|tooth fillings]], coatings, and the molding of electrical and electronic components.<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|pages=361–403|doi=10.3390/polym5020361}}</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]] 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><ref name=mori>{{cite journal|last=Mori|first=Hideharu|author2=Shigeki Masuda |author3=Takeshi Endo |title=Ring-Opening RAFT Polymerization Based on Aromatization as Driving Force: Synthesis of Well-Defined Polymers Containing Anthracene Units in the Main Chain|journal=Macromolecules|date=July 20, 2006|volume=39|pages=5976–5978|doi=10.1021/ma0612879}}</ref>
 
[[Radical polymerization|Free radical polymerization]] techniques have been recently developed to control radical ROPs, thereby controlling the [[Molecular mass|molecular weight]] of the synthesized polymer chains. [[Reversible addition−fragmentation chain-transfer polymerization|Reversible Addition Fragmentation Transfer (RAFT)]] has been applied to radical ROP of a cyclopropane monomer.<ref name=dubois /> For instance, the RAFT polymerization of the [[cyclic compound|cyclic monomer]] to synthesize polymers with [[anthracene]] along the backbone chain has been demonstrated.<ref name=mori />
 
Examples of monomers that undergo radical ROP include vinyl substituted cyclic monomers, [[methylene group|methylene]] substituted cyclic monomers, bicyclobutanes, spiro monomers (which undergo double ring-opening). Degradable [[polyester]] can be synthesized via radical ring-opening homo- and [[copolymer]]ization.<ref name=nuyken />
 
A recent hot topic among scientists has been the study of radical ROP to undergo copolymerization for the production of [[copolymers]] with ketoester linkages in the main chain. The goal is to synthesize a final copolymer that is both [[Hydrolysis|hydrolyzable]] and [[Photodegradation|photodegradable]].<ref name=dubois />
 
====Mechanism====
In free radical ROP, the [[cyclic compound|cyclic]] structure will undergo [[Homolysis (chemistry)|homolytic dissociation]] rather than undergoing [[Heterolysis (chemistry)|heterolytic dissociation]] (as is the case for any ionic ROP). There are two typical mechanistic schemes in radical ROP.
 
'''Scheme 1:''' The terminal vinyl group accepts a [[Radical (chemistry)|radical]]. The radical will be transformed into a [[carbon]] radical stabilized by [[functional group]]s (i.e. [[halogen]], [[aromaticity|aromatic]], or [[ester]] groups). This will lead to the generation of an internal [[Alkene|olefin]].
 
[[File:Grace figure revised.tif|thumb|800px|center|Radical ring-opening polymerization of vinyl cyclopropane]]
 
'''Scheme 2:''' In this case, the [[methylene group|exo-methylene group]] is the radical [[electron acceptor|acceptor]]. The ring-opening reaction will form an ester bond, and the radical produced is stabilized by a [[phenyl group]].<ref name=dubois />
 
[[File:Radical ROP of ketene acetal..png|thumb|800px|center|Radical ROP of ketene acetal.]]
 
===Anionic ring-opening polymerization===
{{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-<sup>−</sup>. The newly formed nucleophile will then attack the atom X in another monomer molecule, and the sequence would repeat until the polymer is formed.<ref name=dubois />]]
Anionic ring-opening polymerizations (AROP) are ring-opening polymerizations that involve [[nucleophile|nucleophilic reagents]] as initiators. Monomers with a three-member ring structure - such as [[epoxideepoxides]], [[aziridineaziridines]], and [[episulfideepisulfides]] - are able to undergo anionic ROP due to the ring-distortion, despite having a less [[electrophile|electrophilic]] functional group (e.g. [[ether]], [[amine]], and [[thioether]]). These [[cyclic compound|cyclic monomers]] are important for many practical applications. The [[polar effect|polarized]] [[functional group]] in cyclic monomers is characterized by one [[atom]] (usually a carbon) that is [[Electron deficiency|electron-deficient]] due to an adjacent atom that is highly [[deactivating groups|electron-withdrawing]] (e.g. [[oxygen]], [[nitrogen]], [[sulfur]] etc.) Ring-opening will be triggered by the nucleophilic attack of the initiator to the carbon, forming a new species that will act as a [[nucleophile]]. The sequence will repeat until the polymer is formed.<ref name=dubois />
 
A typical example of anionic ROP is that of [[caprolactone|ε-caprolactone]], initiated by an [[alkoxide]] functional group.<ref name=dubois />
[[File:Wiki65656.tif|thumb|600px|center|The anionic ring-opening polymerization of ε - caprolactone, initiated by alkoxide function]]
 
====Initiation====
Common [[nucleophile|nucleophilic]] reagents used for the initiation of AROP usually will include [[organometallics|organometals]] (e.g. [[Organolithium reagent|alkyl lithium]], alkyl magnesium bromide, alkyl aluminum, etc.), metal [[amide]]s, [[alkoxides]], [[phosphines]], [[amine]]s, [[alcohols]] and water. The monomers that undergo AROP will contain [[polar bond|polarized bonds]] ([[ester]] [[carbonate]], [[amide]], [[Carbamate|urethane]], and [[phosphate]]), which respectively leads to the production of the corresponding [[polyester]], [[polycarbonate]], [[polyamide]], [[polyurethane]] and [[polyphosphate]].<ref name=dubois />
 
Monomer rings that are unsymmetrically substituted will open with [[nucleophile|nucleophilic]] attack on the least substituted carbon atom.<ref name=nuyken />
 
====Propagation====
The general mechanism of propagation for anionic ROP relies on the [[nucleophile|nucleophilic]] attack of a propagating chain end to a monomer.
 
Another possible mechanism for propagation is the [[nucleophile|nucleophilic]] attack of an activated monomer to the growing chain end. [[caprolactam|ε-caprolactam]] and N-carboxyanhydride undergo this kind of mechanism.<ref name=dubois />
 
====Transfer and termination====
Termination in AROP can be described as [[chain transfer]] reactions to monomer that is available. The [[Active center (polymer science)|active centers]] of AROP monomers are [[nucleophile|nucleophilic]] and also act as [[base (chemistry)|bases]] to abstract [[proton]]s from the monomer, initiating new chains. Thus, AROP often results in low [[molecular mass|molecular-weight]] polymers. A possible method to increase the molecular mass of the polymer products is by adding [[crown ether]]s as [[Chelation|complexing agents]] for [[Counterion|counter-ions]] in the polymerization system. This causes the free-ions to preferentially add to monomer rather than abstract protons.<ref name=nuyken />
 
===Cationic ring-opening polymerization===
{{main article|Cationic polymerization}}
 
Cationic initiators and intermediates characterize cationic ring-opening polymerization (CROP) is characterized by having a cationic initiator and intermediate. 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 predominance of one mechanism overis theaffected other depends onby 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 ana [[heteroatom]] and the chain grows by the addition of cyclic monomers thereby opening the ring system.
[[File:PTMEG synthesis.svg|450px|center|thumb|Synthesis of [[Spandex]].<ref name="kirk">{{cite encyclopedia |year=1996 |title =Polyethers, Tetrahydrofuran and Oxetane Polymers |first1= Gerfried|last1= Pruckmayr|first2= P.|last2= Dreyfuss|first3= M. P.|last3= Dreyfuss |encyclopedia=Kirk‑Othmer Encyclopedia of Chemical Technology |publisher=John Wiley & Sons }}</ref>]]
[[File:Cationic ROP..png|thumb|center|900px|S<sub>N</sub>1 and S<sub>N</sub>2 mechanisms of CROP.]]
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 />
 
CROP can be a [[living polymerization]] and can be terminated by nucleophilic reagents such as [[Alkoxy group|phenoxy anions]], [[phosphine]]s, or [[Polyelectrolyte|polyanions]].<ref name=nuyken /> When the amount of monomers becomes depleted, termination can occur intra or intermolecularly. The active end can "backbite" the chain, forming a [[macrocycle]]. [[Alkyl]] chain transfer is also possible, where the active end is quenched by transferring an alkyl chain to another polymer.
Not all cyclic monomers containing an heteroatom undergo CROP. Ring size influences whether the cyclic monomer polymerize through this mechanism. For example, 4, 6 and 7-membered rings of cyclic [[esters]] polymerize through CROP.<ref name="controlled rop">{{cite book|last=Stridsberg|first=Kajsa M.|title=Controlled ring-opening polymerization : polymers with designed macromolecular architecture|year=2000|publisher=Tekniska högsk.|location=Stockholm|isbn=91-7170-522-8}}</ref> When considering the ring size of the monomer, the reactivity toward polymerization is dictated by the ability to release the [[ring strain]]. Therefore, cyclic monomers with small or lacking ring strain will not polymerize.<ref name="poly mat encyclo">{{cite book|title=Polymeric materials encyclopedia|year=1996|publisher=CRC Press|location=Boca Raton|isbn=0-8493-2470-X|page=1931|editor=Joseph C. Salamone}}</ref>
 
====Initiation====
[[File:CROP initiation..png|thumb|700px|Initiation of CROP.]]
The monomers can be activated by [[Brønsted–Lowry acid–base theory|Bronsted acids]], [[carbenium ion]]s, [[Onium compound|onium ions]], [[photoinitiator]]s, and [[covalent bond|covalent]] initiators.<ref name=nuyken />
 
====Propagation====
The [[ion|cationic]] species is an [[heteroatom]] and the chain grows by the addition of [[cyclic compound|cyclic monomers]] thereby opening the ring system.
 
In CROP, three mechanisms are distinguished by the propagating species.<ref name=nuyken />
* When the [[ion|cationic]] species is a secondary ion, polymerization proceeds by ring expansion. This mechanism is observed when the monomer is in low concentration.
* When it is a tertiary ion, polymerization proceeds by [[step-growth polymerization|linear growth]].
* The monomer can likewise be activated (i.e. cationic) and the propagation step will proceed via [[electrophilic addition]] of the activated monomer to the growing chain.
 
====Termination====
CROP can be considered as a [[living polymerization]] and can be terminated by intentionally adding termination reagents such as [[Alkoxy group|phenoxy anions]], [[phosphine]]s or [[Polyelectrolyte|polyanions]].<ref name=nuyken /> When the amount of monomers becomes depleted, termination can occur intra or intermolecularly. The active end can "backbite" the chain, forming a [[macrocycle]]. [[Alkyl]] chain transfer is also possible, where the active end is quenched by transferring an alkyl chain to another polymer.
 
===Ring-opening metathesis polymerization===
{{main article|Ring-opening metathesis polymerization}}
[[Ring-opening metathesis polymerisation]] (ROMP) produces [[Saturated and unsaturated compounds|unsaturated]] polymers from [[cycloalkene]]s or bicycloalkenes. It requires [[Organometallic chemistry|organometallic catalysts]].<ref name=nuyken />
[[File:ROMP of olefin..png|thumb|400px|Ring opening metathesis polymerization of olefin.]]
[[Ring-opening metathesis polymerisation|Ring-opening metathesis polymerization]] (ROMP) is used for making [[Saturated and unsaturated compounds|unsaturated]] polymers from [[Alkene|olefin]] monomers that are typically [[cycloalkene]]s or bicycloalkenes. It involves [[Organometallic chemistry|organometallic catalysts]] of [[transition metals]] such as W, Mo, Re, Ru, and Ti [[Transition metal carbene complex|carbenes complexes]].<ref name="cowie romp">{{cite book|last=Arrighi|first=J.M.G. Cowie, Valeria|title=Polymers chemistry and physics of modern materials|year=2007|publisher=Taylor & Francis|location=Boca Raton|isbn=978-0-8493-9813-1|pages=181–183|edition=3rd ed / J. M. G. Cowie and Valeria Arrighi}}</ref> Similarly, ROMP occurs for strained [[cyclic compound|cyclic monomers]]. The [[enthalpy]] for relieving the [[ring strain]] must be very favorable for ROMP to occur because the [[entropy]] decreases during polymerization (see [[Gibbs free energy]]). Cyclic [[alkene]]s of 5, 7, and 8 member rings, for example, undergo ROMP at room temperature, whereas the 6 member ring analog does not.<ref name=nuyken />
 
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 Thisname=hartwig>{{cite speciesbook|last=Hartwig|first=John canF.| thenauthor-link propagate as= shownJohn in the figureF. Hartwig The| growingtitle=Organotransition chainmetal canchemistry: be terminated by addingfrom an [[alkene]], usually ethyl vinyl ether,bonding to removecatalysis|year=2010|publisher=University the polymer fromScience the metal [[CatalysisBooks|catalyst]].<ref namelocation=sutthasupaSausalito, California|isbn=978-1-891389-53-5}}</ref>
[[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 poly[[norbornene]], poly[[cyclooctene]], and poly[[cyclopentadiene]].<ref>{{Cite journal|last1=Walsh|first1=Dylan J.|last2=Lau|first2=Sii Hong|last3=Hyatt|first3=Michael G.|last4=Guironnet|first4=Damien|date=2017-09-25|title=Kinetic Study of Living Ring-Opening Metathesis Polymerization with Third-Generation Grubbs Catalysts|journal=Journal of the American Chemical Society|language=EN|volume=139|issue=39|pages=13644–13647|doi=10.1021/jacs.7b08010|pmid=28944665|issn=0002-7863}}</ref>
[[File:Romp mechanism.png|thumb|center|850px|General scheme of the mechanism for ROMP.]]
 
Some commercially relevant [[Saturated and unsaturated compounds|unsaturated]] polymers are synthesized by ROMP, such as Norsorex ([[Norbornene|polynorbornene]]), Vestenamer (polycyclooctene), and Metton (polycyclopentadiene).
 
====Catalysts for ROMP====
{{See also|Olefin metathesis}}
 
The choice of [[catalysis|catalysts]] for ROMP will depend on their properties:<ref name=hartwig>{{cite book|last=Hartwig|first=John F.| authorlink = John F. Hartwig | title=Organotransition metal chemistry : from bonding to catalysis|year=2010|publisher=University Science Books|location=Sausalito, California|isbn=9781891389535}}</ref>
*ability to control the polymer's [[molecular mass|molecular weight]] and [[Molar mass distribution|molecular weight distribution]],
*tolerance to high temperatures,
*ability to polymerize monomers with different [[functional group]]s,
*activity of the catalyst to sustain a [[living polymerization]]
 
Different catalysts have different properties. Choosing the most suitable catalyst depends on the intended features of the resulting polymer. For example:
* [[Olefin metathesis#Schrock catalysts|Schrock catalyst]]: tungsten- and molybdenum-based [[Homogeneous catalysis|homogeneous catalysts]] provide faster initiation and good control over [[Dispersity|polydispersity]] and chain [[tacticity]], but are limited by type of functional groups, thus type of monomers available.<ref name=allthingmetathesis>{{cite web|last=Boothe|first=Paul|title=Ring-opening metathesis polymerization|url=http://allthingsmetathesis.com/ring-opening-metathesis-polymerization/|work=All things metathesis|accessdate=14 February 2014}}</ref>
* [[Grubbs' catalyst|Grubbs catalyst]]: slower initiation and results in higher polydispersity but it's air-stable and a wider range of [[functional groups]] can be used.<ref name=allthingmetathesis />
 
==Copolymerization==
{{main article|Copolymerization}}
[[File:Wiki333.tif|thumb|600px|Stoichiometric equation for ring-opening copolymerization]]
[[Copolymerization]] is the process of combining two polymers that are different. This is an industrial process that creates a substance that has long chains of molecules. In terms of ROP, the stoichiometric equation for copolymerization includes two or more of comonomers.
 
The following figure shows an example of such a copolymerization. By varying the ratio of monomers and the mode of initiation, many and varied polymers can be obtained, optimized for their use in agricultural, medicinal or pharmaceutical fields.
{{double image|center|Chakyoun_wikiproject_image1.png|350|Chakyoun_wikiproject_image2.png|400|Copolymerization of nonhomopolymerizable monomers, γ-butyrolactone (BL) and ε-caprolactone (CL).|Radical ring-opening copolymerization of a ketene acetal.}}
 
As an example of copolymerization of non-homopolymerizable monomers, [[γ-butyrolactone]] (BL) and [[ε-caprolactone]] (CL) show that the copolymerization provides high molar mass polymers: The BL/CL copolymer synthesis is viable despite the fact that BL monomer addition to its own –bl* active chain ends was highly reversible, as the –bl* unit could be blocked via a practically irreversible CL addition.<ref>{{cite journal|last=Duda|first=Andrzej|author2=Penczek, Stanislaw |author3=Dubois, Philippe |author4=Mecerreyes, David |author5= Jérôme, Robert |title=Oligomerization and copolymerization of γ-butyrolactone — a monomer known as unable to homopolymerize, 1. Copolymerization with ɛ-caprolactone|journal=Macromolecular Chemistry and Physics|date=April 1996|volume=197|issue=4|pages=1273–1283|doi=10.1002/macp.1996.021970408}}</ref><ref>{{cite journal|last=Ubaghs|first=Luc|author2=Waringo, Michel |author3=Keul, Helmut |author4= Höcker, Hartwig |title=Copolymers and Terpolymers of Tetramethylene Urea, γ-Butyrolactone, and Ethylene Carbonate or 1,2-Propylene Carbonate|journal=Macromolecules|date=September 2004|volume=37|issue=18|pages=6755–6762|doi=10.1021/ma049668e}}</ref><ref>{{cite journal|last=Agarwal|first=Seema|author2=Xie, Xiulan |title=SmI/Sm-Based γ-Buyrolactone−ε-Caprolactone Copolymers: Microstructural Characterization Using One- and Two-Dimensional NMR Spectroscopy|journal=Macromolecules|date=May 2003|volume=36|issue=10|pages=3545–3549|doi=10.1021/ma0258713}}</ref>
 
Similarly, the earlier studies of S<sub>8</sub> copolymerization with [[thiiranes]] (propylene sulfide; PS), at temperatures below T<sub>f</sub> for S<sub>8</sub> homopolymerization, revealed that the average [[sulfur]] rank in the copolymer was increased from 1 to 7 when 8[S<sub>8</sub>]<sub>0</sub>/[PS]<sub>0</sub> ratio was increasing from 0 to 10.<ref>{{cite journal|last=PENCZEK|first=STANISŁAW|author2=ŚLAZAK, ROMUALD |author3=DUDA, ANDRZEJ |title=Anionic copolymerisation of elemental sulphur|journal=Nature|date=29 June 1978|volume=273|issue=5665|pages=738–739|doi=10.1038/273738a0}}</ref><ref>{{cite journal|last=Duda|first=Andrzej|author2=Penczek, Stanislaw |title=Anionic copolymerization of elemental sulfur with propylene sulfide|journal=Macromolecules|date=January 1982|volume=15|issue=1|pages=36–40|doi=10.1021/ma00229a007}}</ref>
 
==Thermodynamics==
 
The ability of a [[cyclic compound|cyclic monomer]] to polymerize, using ROP is determined by two integral factors: the conversion of monomer molecules into [[macromolecules]] must be allowed both thermodynamically and kinetically. By practice, this means that: (i) monomer-macromolecule equilibrium must shift to the right-hand (macromolecules) side; and (ii) the corresponding polymerization mechanism should exist, which could enable conversion of the monomer molecules into the polymer repeating units, within the operable polymerization time.
 
The formal thermodynamic criterion of a given monomer polymerizability is related to a sign of the [[free enthalpy]] ([[Gibbs free energy]]) of polymerization:
:<math display=block>\Delta G_p(xy) = \Delta H_p(xy)-T\Delta S_p(xy)</math>
where:
where x and y indicate monomer and polymer states, respectively (x and/or y = l (liquid), g ([[gaseous]]), c ([[amorphous solid]]), c’ ([[crystalline solid]]), s ([[solution]])), ΔH<sub>p</sub>(xy) and ΔSp(xy) are the corresponding [[enthalpy]] (SI unit: joule per kelvin) and [[entropy]] (SI unit: joule) of polymerization, and T is the absolute temperature (SI unit: kelvin).
:{{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);
where R is the [[gas constant]], M is the monomer, (m)<sub>i</sub> is the monomer in an initial state, and m<sup>*</sup> is the active monomer.
:{{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 enthalpy of polymerization ({{math|Δ''G<sub>p</sub>''}}) may be expressed as a sum of standard enthalpy of polymerization ({{math|Δ''G<sub>p</sub>''°}}) and a term related to instantaneous monomer molecules and growing [[macromolecules]] concentrations:
:<math>\Delta G_p = \Delta H^\circ_p - T(\Delta S^\circ_p + R\ln[M])</math>
<math chem display=block>\Delta G_p = \Delta G^\circ_p + RT\ln\frac{[\ldots - (\ce{m})_{i+1} \ce{m}^\ast]}{[\ce{M}][\ldots-(\ce{m})_i \ce{m}^\ast]}</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|''DP{{sub|i}}''}}), and taking in to account that {{math|1=Δ''G<sub>p</sub>''° = Δ''H<sub>p</sub>''° &minus; ''T''Δ''S<sub>p</sub>''°}} (where {{math|Δ''H<sub>p</sub>''°}} and {{math|Δ''S<sub>p</sub>''°}} indicate a standard polymerization enthalpy and entropy, respectively), we obtain:
:<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&nbsp;°C, nor cyclo-octasulfur (S<sub>8</sub>) below T<sub>f</sub> = 159&nbsp;°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}}</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=126–126|doi=10.1002/pol.1958.1203112214}}</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]] ({{math|1=Δ''G<sub>p</sub>'' = 0}}), when polymerization is complete the monomer concentration ({{math|[M]<sub>eq</sub>}}) assumes a value determined by standard polymerization parameters ({{math|Δ''H<sub>p</sub>''°}} and {{math|Δ''S<sub>p</sub>''°}}) and polymerization temperature:
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}
{}[\ce{M}]_{\rm eq} &= \exp\left(\frac{\Delta H^\circ_p}{RT} - \frac{\Delta S^\circ_p}{R}\right) \\[4pt]
\ln\frac{DP_n}{DP_n - 1}[\ce{M}]_{\rm eq} &= \frac{\Delta H^\circ_p}{RT} - \frac{\Delta S^\circ_p}{R} \\[4pt]
[\ce{M}]_{\rm eq} &= \frac{DP_n - 1}{DP_n} \exp\left(\frac{\Delta H^\circ_p}{RT} - \frac{\Delta S^\circ_p}{R}\right)
\end{align}</math>
Polymerization is possible only when {{math|[M]<sub>0</sub> > [M]<sub>eq</sub>}}. Eventually, at or above the so-called [[ceiling temperature]] ({{mvar|T<sub>c</sub>}}), at which {{math|1=[M]<sub>eq</sub> = [M]<sub>0</sub>}}, formation of the high polymer does not occur.
<math chem display=block>\begin{align}
T_c &= \frac{\Delta H^\circ_p}{\Delta S^\circ_p + R\ln[\ce{M}]_0} ; \quad (\Delta H^\circ_p<0,\ \Delta S^\circ_p<0) \\[4pt]
T_f &= \frac{\Delta H^\circ_p}{\Delta S^\circ_p + R\ln[\ce{M}]_0} ; \quad (\Delta H^\circ_p>0,\ \Delta S^\circ_p>0)
\end{align}</math>
For example, [[tetrahydrofuran]] (THF) cannot be polymerized above {{mvar|T<sub>c</sub>}}&nbsp;=&nbsp;84&nbsp;°C, nor cyclo-octasulfur (S<sub>8</sub>) below {{mvar|T<sub>f</sub>}}&nbsp;=&nbsp;159&nbsp;°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|page=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, {{mvar|T<sub>c</sub>}} and {{mvar|T<sub>f</sub>}}, for polymerization in the bulk, are well above or below the operable polymerization temperatures, respectively.
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 {{math|Δ''G<sub>p</sub>''}} prevails (thus, when {{math|Δ''H<sub>p</sub>''° < 0}} and {{math|Δ''S<sub>p</sub>''° < 0}}, the inequality {{math|{{abs|Δ''H<sub>p</sub>''}} > &minus;''T''Δ''S<sub>p</sub>''}} is required). Therefore, the higher the ring strain, the lower the resulting monomer concentration at [[Chemical equilibrium|equilibrium]].
 
==SeeAdditional alsoreading==
*{{Cite book |title=Expanding Monomers: Synthesis, Characterization, and Applications |title-link=Expanding Monomers |publisher=CRC Press |year=1992 |isbn=978-0-8493-5156-3 |editor-last=Luck |editor-first=Russel M. |editor-last2=Sadhir |editor-first2=Rajender K. |location=Boca Raton, Florida}}
* [[Ring opening metathesis polymerization]]
*{{cite journal|title=Organocatalytic Ring-Opening Polymerization|author=Nahrain E. Kamber |author2=Wonhee Jeong |author3=Robert M. Waymouth |author4=Russell C. Pratt |author5=Bas G. G. Lohmeijer |author6=James L. Hedrick |journal=Chemical Reviews|year=2007|volume=107|issue=12|pages=5813–5840|doi=10.1021/cr068415b|pmid=17988157}}
* [http://www.pslc.ws/macrog/meta.htm Olefin Metathesis Polymerization]
*{{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 />
 
{{DEFAULTSORT:Ring-Opening Polymerization}}
[[Category:Polymerization reactions]]
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