Abstract
The increasing pursuit for bio-based plastic materials led us to investigate the potential use of the monoterpene limonene in thermoset synthesis using the free-radical mediated thiol–ene reaction. The high efficiency of this reaction to prepare multifunctional ene-terminated resins, as intermediary macromolecular precursors, for thermosets synthesis was demonstrated under thermal and photoinitiated conditions. Although an excess of terpene favors formation of well-defined macromonomers in organic solution, the characteristic low-vapor pressure of limonene hinders its simple removal (or recycling) via evaporation after synthesis. Alteration to an initial thiol–ene stoichiometry of 1 : 0.5 enables production of high molecular weight resins in the form of ‘hyperbranched oligomeric-like’ structures having moderate polydispersity. UV-curing of these polyfunctional resins combined with equal mole compositions of multifunctional alkyl ester 3-mercapto propionates yields highly sticky, amorphous and flexible elastomers with different thermo-mechanical properties. These can be further modulated by varying the amount of unreacted thiol occluded within the networks working as a plasticizer. Introduction of a renewable cycloaliphatic structure into the materials offers a convenient way to enhance the glass-transition temperature and stiffness of traditional thiol–ene networks. The materials synthesized may be considered potentially useful as sealants and adhesives in a wide variety of applications including organic coatings. The versatility of UV-irradiation over thermal initiation makes this method particularly suitable for green industrial synthesis processes via thiol–ene chemistry using limonene and multifunctional thiols. The thiol–ene system evaluated herein serves as a model example for the sustainable incorporation of natural diolefinic monomers into semi-synthetic thiol–ene networks exhibiting a range of thermo-mechanical properties.
Highlights
IntroductionScheme 1 Stepwise thiol–ene reaction scheme involving the two unconjugated double-bonds of limonene with: initiation (i), reversible propagation (insertion–elimination) (ii), chain-transfer (hydrogen-abstraction) (iii), and thiyl self-termination (homocoupling) (iv) steps
The development of plastic materials derived from renewable resources for replacement of petroleum-based ones continues gaining growing interest both in industry and in the scienti c research community
The notable features inherent to free-radical thiol–ene addition prompted us to investigate the synthesis of bio-based thermosets using R-(+)-limonene 1, as a renewable diole nic substrate, together with TMPMP 2 as the primary synthetic thiol crosslinker
Summary
Scheme 1 Stepwise thiol–ene reaction scheme involving the two unconjugated double-bonds of limonene with: initiation (i), reversible propagation (insertion–elimination) (ii), chain-transfer (hydrogen-abstraction) (iii), and thiyl self-termination (homocoupling) (iv) steps. Inclusion of (CTA)s enabled molecular weight reduction of the copolymers while retaining low polydispersities.[47] In another study, partially bio-based polymers bearing reactive epoxy groups in the side-chains were synthesized by Morinaga and co-workers (2013) via thermally induced radical copolymerization of LO with methyl acrylate.[48] A pioneering study by Bahr et al (2012) employed LDO as a difunctional precursor of cyclic limonene dicarbonate via halide catalysis with CO2 This newly derivatized carbonated monomer was further reacted with di- and poly-functional amines to prepare linear, non-toxic (isocyanate-free), oligohydroxyurethanes and thermosetting polyurethanes, respectively.[49] Diepoxides of limonene have been obtained by catalytic oxidation using immobilized Mn(Salen)Cl complexes, the 1,2-epoxide is formed primarily due to a higher regioselectivity.[50,51,52] Coates et al (2004) reported on the synthesis of alternating polycarbonate copolymers issued from the cis and trans diastereomers of D-limonene monoxide (LO) and CO2 using b-diiminate zinc acetate complexes as catalysts; the reaction proceeded selectively on the trans monomer affording a regioregular polycarbonate structure.[53] This work represents an excellent example of combination of two very different sources of renewable substances in the creation of linear polymers. The thermal and viscoelastic properties of the semisynthetic photocured materials are reported and discussed thoroughly
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