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Thiol-ene coupling kinetics of D-limonene: a versatile 'non-click' free-radical reaction involving a natural terpene
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-0663-0751
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.ORCID iD: 0000-0003-3201-5138
2013 (English)In: RSC Advances, ISSN 2046-2069, Vol. 3, no 27, 11021-11034 p.Article in journal (Refereed) Published
Abstract [en]

The free-radical photoinduced thiol-ene reaction between D-limonene, as renewable diolefinic substrate, and two mono-/tri-functional thiols (iso-tridecyl 3-mercaptopropionate and trimethylolpropane tris(3-mercaptopropionate)), has been investigated kinetically to define a relationship between alkene structure and reactivity. Separate thiol-ene solutions of the appropriate thiol in d-chloroform, supplemented with 1.0 wt% of DMPA (Irgacure 651), were subjected to polychromatic UV-irradiation and the chemical changes monitored discontinuously via H-1 NMR spectroscopy to quantify double bond conversion. The kinetic concentration profiles were modeled analytically and simulated in the application software COPASI for parameter estimation and to verify if the experimental data explained a suggested mechanistic scheme. Empirical results demonstrate that the external vinylidene bond of limonene reacts about 6.5 times faster with thiol than the internal trisubstituted 1-methyl-cyclohexene unsaturation. The selectivity observed for the two unsaturations was successfully explained by means of a simplified steady-state equation derived from the sequential reaction mechanism accounting for propagation and chain-transfer elementary steps with estimated rate coefficients. Kinetic modeling results attribute the difference in selectivity partially to steric impediments controlling thiyl-radical insertion onto the double bonds and predominantly to differences in relative energy between the two tertiary insertion carbon radical intermediates. The rate-limiting step was identified as the third chain-transfer hydrogen-abstraction reaction promoted by the second insertion carbon radical intermediate. High thiol-ene conversions were obtained in a timely fashion without major influence of secondary reactions demonstrating the suitability of this reaction for network forming purposes. The mechanistic and kinetic information collected can be used as a quantitative predictive tool to assess the potential use of D-limonene in thiol-ene network forming systems involving multifunctional alkyl ester 3-mercaptopropionates.

Place, publisher, year, edition, pages
RSC Publishing, 2013. Vol. 3, no 27, 11021-11034 p.
Keyword [en]
Carbon-Centered Radicals, Acrylate Photopolymerizations, Unsaturated-Compounds, Vinyl Mechanisms, Thiyl Radicals, Cross-Linking, Chemistry, Polymerization, Termination, Copolymers
National Category
Chemical Sciences
URN: urn:nbn:se:kth:diva-124738DOI: 10.1039/c3ra40696bISI: 000320467500059ScopusID: 2-s2.0-84881446388OAI: diva2:638438
Swedish Research Council, 621-2007-5723

QC 20130730

Available from: 2013-07-30 Created: 2013-07-29 Last updated: 2013-10-02Bibliographically approved
In thesis
1. 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.
Trita-CHE-Report, ISSN 1654-1081 ; 2013:36
National Category
Chemical Sciences
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)

QC 20131002

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

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