Ring-opening polymerization

Many cyclic monomers are amenable to ROP.{{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 }} These include epoxides,{{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}}{{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}} cyclic trisiloxanes,{{cn|date=December 2023}} some lactones{{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}} and lactides, cyclic anhydrides, cyclic carbonates,{{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}}

and amino acid N-carboxyanhydrides.{{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 }}{{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}} Many strained cycloalkenes, e.g norbornene, are suitable monomers via ring-opening metathesis polymerization. Even highly strained cycloalkane rings, such as cyclopropane{{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 }} and cyclobutane{{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 |bibcode= 1996MaMol..29.8053Y }} derivatives, can undergo ROP.

Ring-opening polymerization has been used since the beginning of the 1900s to produce polymers. Synthesis of polypeptides which has the oldest history of ROP, dates back to the work in 1906 by Leuchs.{{cite journal|title=Glycine-carbonic acid|last=Leuchs|first=H.|journal=Berichte der Deutschen Chemischen Gesellschaft|year=1906|volume=39|page=857|doi=10.1002/cber.190603901133|url=https://zenodo.org/record/1426172}} Subsequently, the ROP of anhydro sugars provided polysaccharides, including synthetic dextran, xanthan gum, welan gum, gellan gum, diutan gum, and pullulan. Mechanisms and thermodynamics of ring-opening polymerization were established in the 1950s.{{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|page=1710|doi=10.1039/TF9555101710}}{{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}} The first high-molecular weight polymers (M_n up to 10⁵) with a repeating unit were prepared by ROP as early as in 1976.{{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}}{{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}}

New research shows that ROP can be completed with cyclic esters with minimal to no use of solvents by using resonant acoustic mixing.{{Cite journal |last1=Fowler |first1=Harriet R. |last2=O’Shea |first2=Riley |last3=Sefton |first3=Joseph |last4=Howard |first4=Shaun C. |last5=Muir |first5=Benjamin W. |last6=Stockman |first6=Robert A. |last7=Taresco |first7=Vincenzo |last8=Irvine |first8=Derek J. |date=2025-02-10 |title=Rapid, Highly Sustainable Ring-Opening Polymerization via Resonant Acoustic Mixing |journal=ACS Sustainable Chemistry & Engineering |volume=13 |issue=5 |pages=1916–1926 |doi=10.1021/acssuschemeng.4c06330 |pmc=11816011 |pmid=39950108}}

An industrial application is the production of nylon-6 from caprolactam.

Ring-opening polymerization can proceed via radical, anionic, or cationic polymerization as described below.{{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}} Additionally, radical ROP is useful in producing polymers with functional groups 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, amides, and carbonates as functional groups along the main chain.{{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.}}

{{main article|Anionic polymerization}}

File:Wiki566665.tif

Anionic ring-opening polymerizations (AROP) involve nucleophilic reagents as initiators. Monomers with a three-member ring structure - such as epoxides, aziridines, and episulfides - undergo anionic ROP.

A typical example of anionic ROP is that of ε-caprolactone, initiated by an alkoxide.

{{main article|Cationic polymerization}}

Cationic initiators and intermediates characterize cationic ring-opening polymerization (CROP). Examples of cyclic monomers that polymerize through this mechanism include lactones, lactams, amines, and ethers.{{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}} CROP proceeds through an S_N1 or S_N2 propagation, chain-growth process. The mechanism is affected by the stability of the resulting cationic species. For example, if the atom bearing the positive charge is stabilized by electron-donating groups, polymerization will proceed by the S_N1 mechanism. The cationic species is a heteroatom and the chain grows by the addition of cyclic monomers thereby opening the ring system.

File:PTMEG synthesis.svg.{{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 }}]]

The monomers can be activated by Bronsted acids, carbenium ions, onium ions, and metal cations.

CROP can be a living polymerization and can be terminated by nucleophilic reagents such as phenoxy anions, phosphines, or polyanions. 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.

{{main article|Ring-opening metathesis polymerization}}

Ring-opening metathesis polymerisation (ROMP) produces unsaturated polymers from cycloalkenes or bicycloalkenes. It requires organometallic catalysts.

The mechanism for ROMP follows similar pathways as olefin metathesis. The initiation process involves the coordination of the cycloalkene monomer to the metal alkylidene complex, followed by a [2+2] type cycloaddition to form the metallacyclobutane intermediate that cycloreverts to form a new alkylidene species.{{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}}{{cite book|last=Hartwig|first=John F.| author-link = John F. Hartwig | title=Organotransition metal chemistry: from bonding to catalysis|year=2010|publisher=University Science Books|location=Sausalito, California|isbn=978-1-891389-53-5}}

File:Romp mechanism.png Commercially relevant unsaturated polymers synthesized by ROMP include polynorbornene, polycyclooctene, and polycyclopentadiene.{{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|bibcode=2017JAChS.13913644W |issn=0002-7863}}

The formal thermodynamic criterion of a given monomer polymerizability is related to a sign of the free enthalpy (Gibbs free energy) of polymerization:

$$\backslash Delta\; G\_p(xy)\; =\; \backslash Delta\; H\_p(xy)-T\backslash Delta\; S\_p(xy)$$

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));

:{{math|ΔH_p(xy)}} is the enthalpy of polymerization (SI unit: joule per kelvin);

:{{math|ΔS{{sub|p}}(xy)}} is the entropy of polymerization (SI unit: joule);

:{{mvar|T}} is the absolute temperature (SI unit: kelvin).

The free enthalpy of polymerization ({{math|ΔG_p}}) may be expressed as a sum of standard enthalpy of polymerization ({{math|ΔG_p°}}) and a term related to instantaneous monomer molecules and growing macromolecules concentrations:

$$\backslash Delta\; G\_p\; =\; \backslash Delta\; G^\backslash circ\_p\; +\; RT\backslash ln\backslash frac\{[\backslash ldots\; -\; (\backslash ce\{m\})\_\{i+1\}\; \backslash ce\{m\}^\backslash ast]\}\{[\backslash ce\{M\}][\backslash ldots-(\backslash ce\{m\})\_i\; \backslash ce\{m\}^\backslash ast]\}$$

where:

:{{mvar|R}} is the gas constant;

:{{math|M}} is the monomer;

:{{math|(m)_i}} is the monomer in an initial state;

:{{math|m^*}} is the active monomer.

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=ΔG_p° = ΔH_p° − TΔS_p°}} (where {{math|ΔH_p°}} and {{math|ΔS_p°}} indicate a standard polymerization enthalpy and entropy, respectively), we obtain:

:$\backslash Delta\; G\_p\; =\; \backslash Delta\; H^\backslash circ\_p\; -\; T(\backslash Delta\; S^\backslash circ\_p\; +\; R\backslash ln[M])$

At equilibrium ({{math|1=ΔG_p = 0}}), when polymerization is complete the monomer concentration ({{math|[M]_eq}}) assumes a value determined by standard polymerization parameters ({{math|ΔH_p°}} and {{math|ΔS_p°}}) and polymerization temperature:

$$\backslash begin\{align\}$$

{}[\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}

Polymerization is possible only when {{math|[M]₀ > [M]_eq}}. Eventually, at or above the so-called ceiling temperature ({{mvar|T_c}}), at which {{math|1=[M]_eq = [M]₀}}, formation of the high polymer does not occur.

$$\backslash begin\{align\}$$

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}

For example, tetrahydrofuran (THF) cannot be polymerized above {{mvar|T_c}} = 84 °C, nor cyclo-octasulfur (S₈) below {{mvar|T_f}} = 159 °C.{{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}}{{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}}{{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|bibcode=1959JAChS..81..780T }}{{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|bibcode=1960JAChS..82..289T }} However, for many monomers, {{mvar|T_c}} and {{mvar|T_f}}, 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|ΔG_p}} prevails (thus, when {{math|ΔH_p° < 0}} and {{math|ΔS_p° < 0}}, the inequality {{math|{{abs|ΔH_p}} > −TΔS_p}} is required). Therefore, the higher the ring strain, the lower the resulting monomer concentration at equilibrium.

• {{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}}
• {{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}}
• {{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 }}

Category:Polymerization reactions