Semi-renewable latex via step-photopolymerization of dithiol and dienic terpenes

Abstract

Abundance, low-cost and ene functionality are the key assets of terpenes in view of a large-scale use as renewable monomers in radical polymerization. However, their sterically hindered ene structure has been mostly ineffective in a chain-growth mechanism, leading to sluggish activity. To address this challenge, the alternative of a step-growth diene–dithiol linear photopolymerization is investigated here. Four non-modified dienic terpenes – geranyl acetate, linalool, limonene and geraniol – are reacted with ethylene glycol dithiol in various conditions. In bulk and solution, limited conversions prevent the formation of high-molecular-weight polymers. By contrast, a mini-emulsion containing the two difunctional comonomer yields higher conversions (>98%) in less than 10 min of ultraviolet irradiation, making the formation of polythioether chains possible. The authors assume that a thiol–ene polymerization confined in nanodroplets may drive high conversion thanks to a radical compartmentalization effect. The resultant aqueous polymer dispersions show an average diameter ranging from 100 to 140 nm, low glass transition temperature (less than −30°C), but only moderate molecular weights between 4 and 5 kDa for the fraction soluble in tetrahydrofuran. These promising results pave the way for a novel semi-synthetic and eco-efficient approach to polymer latexes based on natural terpene monomers.