Synthesis and chemical recycling investigations of polythioureas

Abstract

There is currently intensive research on the development of biobased polymers as potential alternatives to the environmentally hazardous isocyanate-based polyurethanes. In this context, polythioureas (PTUs) form a particularly attractive target because they can be synthesized by using isothiocyanates, a class of molecules that can be found in nature with expected low toxicity. Herein, a series of 16 PTUs with varied chemical structures, mostly being new polymers, have been synthesized and their possible chemical recycling pathways via thermally induced and acid-catalyzed depolymerizations have been investigated. The obtained PTUs showed a varied range of molecular weights (up to M_n ∼ 69.5 kDa), intrinsic viscosity (up to ∼6 dL/g), and glass transition temperatures (T_g ∼ 59–128 °C). Notably, we observed that the presence of aromatic segments lowered the thermal stability of the polymers, so they were generally easier to depolymerize (compared to those without aromatic groups), forming oligomers with controlled end-groups (i.e. telechelic polymers) that could be repolymerized. The obtained aliphatic PTUs were generally resistant against thermochemical depolymerizations, but they could be effectively depolymerized by sulfuric acid. The repolymerization methods depended on the end groups of the depolymerized products, which in this work included direct repolymerization of polythioureas (if the end groups contain ∼1:1 of isothiocyanates and amines) and copolymerizations with another monomer terephthaldehyde (if the end groups contain only amines). Our results provided a first comprehensive molecular insight into the synthetic and recycling possibilities of using isothiocyanates and polythioureas in the exploration of potential alternatives for isocyanates and polyurethanes.