Advances in Trimethylolpropane Tris(3-mercaptopropionate) for Sustainable High-Performance Polymer Materials

Abstract

Trimethylolpropane tris(3-mercaptopropionate) (TMPMP) is a multifunctional thiol that underpins rapid, efficient “click” chemistry for advanced polymer networks. Its trifunctionality enables high conversions in thiol–ene and thiol–epoxy reactions, even with internal alkenes where thiyl radical–driven cis/trans isomerization typically limits reactivity. TMPMP has recently advanced sustainable photopolymerization, enabling solvent-free, bio-based resins with over 97% renewable content and excellent performance in 3D printing. Incorporation into photoresponsive thiol–epoxy systems further allows visible-light curing and UV-triggered degradability, enabling dynamic modulation of network integrity, surface wettability, and solubility through controlled bond cleavage.

Thiol–ene Radical Coupling

As reported previously, internal C=C bonds, i.e. 1,2-disubstituted alkenes, exhibit diminished reactivity in radical thiol–ene reactions compared to terminal alkene bonds. This is due, in part, to a competing cis/trans isomerisation process that is very efficiently mediated by thiyl radicals via an addition–isomerisation–elimination pathway. This was further highlighted by Claudino, Johansson and Jonsson in their studies of radical thiol–ene coupling between the trifunctional thiol trimethylolpropane tris(3-mercaptopropionate) (TMPMP) and the two 1,2-disubsti-tuted alkenes methyl oleate (MO) and methyl elaidate (ME). The authors demonstrated that thiol–ene coupling can proceed to high conversions under bulk conditions with an equimolar ratio of functional groups. Consistent with previous studies and reports, the internal cis species, MO, was shown to undergo an immediate isomerisation reaction to the corresponding trans species with the cis/trans isomerization process significantly affecting the rate of thioether product formation. [1]

3D painting

The photopolymerization process in 3D printing is considered greener once it involves a fast reaction and low energy consumption. Various reactions for photopolymerization can be used nowadays, but a special one is the thiol-ene “click” reaction that occurs in equimolar concentrations of thiol and alkene groups. In this sense, solvent-free photopolymerizable formulations were prepared to contain non-modified castor oil, Trimethylolpropane tris(3-mercapto propionate), and cellulosic fillers from hemp, tagua, and walnut. All formulations presented conversions higher than 70% and fast polymerization rates. Moreover, the filled formulations presented excellent curing depths in fewer seconds of light exposition, an important factor for their applicability in 3D printing. Furthermore, the hemp filler formulation presented the highest cross linking density as determined by the DMTA, and was selected for printing two complex structures (pyramid and honeycomb shape). The rheology analysis showed that the formulations had adequate viscosities for the printer. Lastly, all polymers presented at least 97% bio-based contents, with gel contents superior to 96%. [2]

Photoresponsive Degradable Thiol–Epoxy Network System

Following the idea of a photolatent thiol-epoxy system with photo-responsive properties, trimethylolpropane tris(3-mercaptopropionate), a resin formulation was developed containing a multi-functional thiol monomer, a bi-functional epoxy crosslinker (epoxy-NBE) with a photosensitive o-nitrobenzyl ester (o-NBE) group and a photolatent base PLB. Since the UV absorption profile of the photolatent base and the photosensitive o-NBE group are over lapping, a photosensitizer (ITX) was added to the formulation to shift the absorption window of the photolatent base to the visible light region. In the presence of ITX, the base-catalyzed curing reaction was triggered upon visible light exposure (λ > 400 nm) without inducing a premature cleavage of the photolabile o-NBE groups. Once formed, the thiol-epoxy networks were cleaved across the o-NBE links upon irradiation with light in the UV-A spectral region. The photo-induced cleavage reaction led to a distinctive increase in the solubility and wettability of the polymer networks as polar cleavage products are formed. Previous work on photocleavable networks showed that the UV-induced increase in solubility is more pronounced in lower crosslinked networks since less crosslinks have to be cleaved to form soluble species. Thus, in the current study a mono-functional epoxy monomer (epoxy-GME), which can act as chain extender, since it increases the molecular weight between two crosslink points. [3]

References:

[1] Lowe, A. B. (2014). Thiol–ene “click” reactions and recent applications in polymer and materials synthesis: a first update. Polymer Chemistry, 5(17), 4820-4870.

[2] Alarcon, R. T., Bergoglio, M., Cavalheiro, é. T. G., & Sangermano, M. (2025). Thiol-Ene Photopolymerization and 3D Printing of Non-Modified Castor Oil Containing Bio-Based Cellulosic Fillers. Polymers, 17(5), 587. https://doi.org/10.3390/polym17050587

[3] Romano, A., Roppolo, I., Giebler, M., Dietliker, K., Mo?ina, ?., ?ket, P., Mühlbacher, I., Schl?gl, S., & Sangermano, M. (2018). Stimuli-responsive thiol-epoxy networks with photo-switchable bulk and surface properties. RSC advances, 8(73), 41904–41914. https://doi.org/10.1039/c8ra08937j

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