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Thiol-ene coupling of 1,2-disubstituted alkene monomers: The kinetic effect of cis/trans-isomer structures
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.ORCID iD: 0000-0003-3201-5138
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.ORCID iD: 0000-0003-0663-0751
2010 (English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 46, no 12, 2321-2332 p.Article in journal (Refereed) Published
Abstract [en]

The free-radical induced reaction between a tri-functional thiol (2-ethyl-(hydroxymethyl)-1,3-propanediol trimercapto propionate) and two 1,2-disubstituted alkenes (methyl oleate and methyl elaidate) has been investigated under photochemical conditions. The photoreaction was monitored via time-resolved FUR, Raman and NMR spectroscopy to provide insights about the kinetics and efficiency in end-product formation. The information collected was subjected to numerical modelling using the GEPASI software using pre-established literature values for the rate coefficients in order to verify the proposed reaction scheme. The results confirm the thiol-ene reaction mechanism showing a very fast cis/trans-isomerization (<1.0 min) when compared with the total disappearance of unsaturations, indicating that the rate-limiting step controlling the reaction is the hydrogen transfer from the thiol involved in the formation of product. High thiol-ene conversions can be obtained at reasonable rates without major influence of side-reactions when performed in bulk indicating that this reaction is suitable for network forming purposes with mono-unsaturated fatty acid methyl esters derivatives. The kinetic and mechanistic information collected provides a basis for the design of new thiol-ene systems aiming at material and coating applications.

Place, publisher, year, edition, pages
2010. Vol. 46, no 12, 2321-2332 p.
Keyword [en]
Thiol-ene chemistry, Real-time FTIR, Photopolymerization, Fatty acids, Isomer/isomerization, Coatings
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-28194DOI: 10.1016/j.eurpolymj.2010.10.001ISI: 000285128700013Scopus ID: 2-s2.0-79954618129OAI: oai:DiVA.org:kth-28194DiVA: diva2:384713
Funder
Swedish Research Council, 621-2007-5723
Note
QC 20110110Available from: 2011-01-10 Created: 2011-01-10 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Thiol−ene Coupling of Renewable Monomers: at the forefront of bio-based polymeric materials
Open this publication in new window or tab >>Thiol−ene Coupling of Renewable Monomers: at the forefront of bio-based polymeric materials
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Plant derived oils bear intrinsic double-bond functionality that can be utilized directly for the thiol–ene reaction. Although terminal unsaturations are far more reactive than internal ones, studies on the reversible addition of thiyl radicals to 1,2-disubstituted alkenes show that this is an important reaction. To investigate the thiol–ene coupling reaction involving these enes, stoichiometric mixtures of a trifunctional propionate thiol with monounsaturated fatty acid methyl esters (methyl oleate or methyl elaidate) supplemented with 2.0 wt.% Irgacure 184 were subjected to 365-nm UV-irradiation and the chemical changes monitored. Continuous (RT– FTIR) and discontinuous (NMR and FT–Raman) techniques were used to follow the progress of the reaction and reveal details of the products formed. Experimental results supported by numerical kinetic simulations of the system confirm the reaction mechanism showing a very fast cis/trans-isomerization of the alkene monomers (<1.0 min) when compared to the total disappearance of double-bonds, indicating that the rate-limiting step controlling the overall reaction is the hydrogen transfer from the thiol involved in the formation of final product. The loss of total unsaturations equals thiol consumption throughout the entire reaction; although product formation is strongly favoured directly from the trans-ene. This indicates that initial cis/trans-isomer structures affect the kinetics. High thiol–ene conversions could be easily obtained at reasonable rates without major influence of side-reactions demonstrating the suitability of this reaction for network forming purposes from 1,2-disubstituted alkenes. To further illustrate the validity of this concept in the formation of cross-linked thiol–ene films a series of globalide/caprolactone based copolyesters differing in degree of unsaturations along the backbone were photopolymerized in the melt with the same trithiol giving amorphous elastomeric materials with different thermal and viscoelastic properties. High thiol–ene conversions (>80%) were easily attained for all cases at reasonable reaction rates, while maintaining the cure behaviour and independent of functionality. Parallel chain-growth ene homopolymerization was considered negligible when compared with the main coupling route. However, the comonomer feed ratio had impact on the thermoset properties with high ene-density copolymers giving networks with higher glass transition temperature values (Tg) and a narrower distribution of cross-links than films with lower ene composition. The thiol–ene systems evaluated in this study serve as model example for the sustainable use of naturally-occurring 1,2-disubstituted alkenes at making semi-synthetic polymeric materials in high conversions with a range of properties in an environment-friendly way.

Abstract [sv]

Vegetabiliska oljor som innehåller dubbelbindningar kan användas direkt för thiolene reaktioner. Trots att terminala dubbelbindningar är mycket mer reaktiva än interna visar dessa studier att den reversibla additionen av thiyl radikaler till 1,2-disubstituerade alkener är en viktig reaktion. För att undersöka tiol–ene reaktionerna, som ivolverar dessa alkener förbereddes stökiometriska blandningar av en trifunktionell propionat tiol och enkelomättade fettsyrametylestrar (metyloleat eller metyl elaidat) samt 2.0 vikt.% Irgacure 184. Dessa blandningar utsattes för 365-nm UV strålning och de kemiska förändringarna studerades. De kemiska förändringarna analyserades med olika kemiska analysmetoder; realtid RT–FTIR, NMR och FT–Raman. Dessa användes för att analysera de kemiska reaktionerna i realtid och följa bildandet av produkterna. Reaktionsmekanismen bekräftades med hjälp av experimentella data och beräkningar av numeriska och kinetiska simuleringar för systemet. Resultaten visar en mycket snabb cis/trans-isomerisering av alkenmonomeren (<1.0 min) jämfört med den totala förbrukningen av dubbelbindningarna, vilket indikerar att det hastighetsbegränsande steget kontrolleras av väteförflyttningen från tiolen till slutprodukten. Förbrukningen av den totala omättade kolkedjan är lika med tiolförbrukningen under hela reaktionen, även om bildandet av produkten gynnas från trans-enen. Detta indikerar att den första cis/trans-isomerstrukturen påverkar kinetiken. Höga tiol-ene utbyten kan enkelt erhållas relativt snabbt utan inverkan av sidoreaktioner. Detta innebär att denna reaktion kan användas som nätverksbildande reaktion för flerfunktionella 1,2-disubstituted alkenmonomerer. Vidare användes fotopolymerisation i smälta på en serie globalid/kaprolaktonbaserade sampolyestrar med varierad grad av omättnad med samma tritiol vilket resulterade i bildandet av amorfa elastomeriska material med olika termiska och viskoelastiska egenskaper. Hög omsättning (>80%) uppnåddes relativt enkelt för samtliga blandningar oberoende av den initiala funktionaliteten. Homopolymerisation av alkenen var försumbar i jämförelse med den tiol–en-reaktionen. Mängden alkengrupper har inverkan på härdplastsegenskaperna där en hög andel alken ger en nätstruktur med högre glastransitionstemperatur (Tg). Tiol–ene reaktionen utvärderades i modellsystem baserade på naturlig förekommande 1,2-disubstituterade alkener för att demonstrera konceptet med tiol-förnätade halvsyntetiska material.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. 49 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2011:49
Keyword
Thiol-ene chemistry, monounsaturated fatty-acids, renewable resources, cis/trans-isomerization, photopolymerization, networks, polyesters, thermal/viscoelastic properties, thin-films, unsaturated (macro)lactones, coatings
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-40267 (URN)978-91-7501-094-6 (ISBN)
Presentation
2011-09-30, K1, KTH, Teknikringen 56, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2007-5723
Note
QC 20110915Available from: 2011-09-15 Created: 2011-09-13 Last updated: 2012-04-02Bibliographically approved
2. Macromolecular design: UV-curable thiol-ene networks based on renewable resources
Open this publication in new window or tab >>Macromolecular design: UV-curable thiol-ene networks based on renewable resources
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Plant oils and terpenes are ubiquitous natural renewable compounds. The double bonds contained in most of these monomers can be utilized via the photo-induced free-radical thiol–ene reaction to create novel bio-derived polymer thermosets representing a valuable ‘green’ alternative to petrochemical olefins and resulting synthetic plastic materials. Nevertheless, there are several factors limiting their applicability, the first one being the relatively slow reaction rates towards thiol–ene coupling and many times the need to modify these natural olefins to make them more reactive. The latter process necessarily introduces additional pre-synthesis steps which has implications related both to cost and synthetic routes employed thereafter, those of which may or may not follow the principles of Green Chemistry. Therefore, this thesis intends to gain primary insight about the thiol–ene mechanism, kinetics and reactivity involving these multi-substituted olefins and then use the resulting knowledge to design semi-synthetic thermosets by incorporating these natural monomers into thiol–ene networks in the most environmentally friendly way possible. Mechanistic kinetic results show that internal 1,2-disubstituted enes found in mono-unsaturated vegetable oils and some macrolactones undergo a fast reversible cis/trans-isomerization process in favour of trans-isomer formation coupled with the thiol–ene mechanism. The slow reactivity of these enes has been accredited not just to the isomerization itself, but predominantly to the chain-transfer hydrogen-abstraction step. This rate-limiting step, however, does not seem to compromise their use in the creation of thiol–ene networks as demonstrated by photopolymerization in the melt of a series of linear globalide/ε-caprolactone-based copolyesters differing in amount of unsaturations along the backbone crosslinked with a tri-functional thiol propionate ester monomer. The resulting thermoset films were amorphous elastomers exhibiting different thermal and mechanical properties depending on the comonomer feed ratio. D-limonene, a renewable diolefinic substrate, proved to be an important terpene in free-radical thiol–ene additions. Empirical results show that the 1,1-disubstituted exo-vinylidene bond is about 6.5 times more reactive than the endocyclic 1,1,2-trisubstituted 1-methyl-cyclohexene moiety when reacting with mercapto propionate esters in organic solution conditions. Kinetic modeling results suggest that the differences in double bond reactivity are partially ascribed to steric impediments coupled with differences in electron-density controlling thiyl-radical insertion onto the two unsaturations but predominantly to differences in relative energy between the two tertiary insertion carbon-centered radical intermediates. Off-stoichiometric manipulations in the thiol–limonene mole ratio, assisted by numerical model simulations, offer a convenient method to visualize and assess the overall reaction system kinetics irrespective of time, thus being regarded as an important guiding tool for organic and polymer chemists aiming at designing thiol–ene reaction systems based on limonene. Multifunctional limonene-terminated thiol–ene macromonomer resins were synthesized in ethyl acetate solution and then reacted in different combinations with polyfunctional mercapto propionate esters to afford semi-synthetic thiol–ene networks with different thermo-viscoelastic properties depending on functionality, crosslink density, homogeneity and excess ofthiol occluded into the networks. The bulky cycloaliphatic ring structure of limonene locked between thioether linkages introduce a certain degree of rigidity to the final networks and increase the glass-transition temperature when compared tomore standard thiol–allyl systems. In all cases evaluated, high thiol–ene conversions were achieved with minimum or no side-reactions such as chain-growth homopolymerization and at reasonable reaction rates.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 79, v p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:36
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-129549 (URN)978-91-7501-845-4 (ISBN)
Public defence
2013-10-03, F3, Lindstedtsvägen 26, KTH, Stockholm, 14:30 (English)
Opponent
Supervisors
Note

QC 20131002

Available from: 2013-10-02 Created: 2013-10-02 Last updated: 2013-10-07Bibliographically approved

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