Abstract
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 via1H 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.
Highlights
The pursuit of viable compounds from renewable resources in replacement of petroleum-derived ones is currently seen as one of the major challenges in polymer science on grounds of depleting fossil-oil reserves, environmental awareness and issues of sustainable development
This study aims to gain primary insight of the free-radical induced thiol–ene coupling of limonene in CDCl3 solution promoted by photogenerated thiyl radicals of the 3-mercaptopropionate ester type (RCH2OCLOCH2CH2S?) in order to establish a direct relationship between alkene structure and reactivity
Numerical time-course simulations of the reaction system revealed to be a valuable modeling tool both in describing the overall kinetics and estimating the individual rate parameters for propagation and chain-transfer steps, those of which are still unknown for this particular thiol–ene system
Summary
The pursuit of viable compounds from renewable resources in replacement of petroleum-derived ones is currently seen as one of the major challenges in polymer science on grounds of depleting fossil-oil reserves, environmental awareness and issues of sustainable development. Paper polymerization (i.e., chain growth)[5] they constitute the ideal alkene for the thiol–ene coupling reaction This intrinsic feature has been explored recently by Meier and coworkers in the synthesis of monofunctional, homodifunctional and hetero-difunctional limonene-modified monomers via the thiol–ene reaction which were polymerized into linear polymers by use of other chemistries and natural (or bio-derived) monomers.[6,7] It would be more interesting, if the monoterpene could be directly reacted without any intermediate modifications. This would allow direct incorporation of an abundant and inexpensive bio-renewable monomer into thiol–ene networks by appropriate choice of multifunctional thiol cross-linkers to create novel thermosetting structures not accessible via petrochemistry (say, for instance, chirality)
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