A sulfur copolymer with a pyrrole compound for the crosslinking of unsaturated elastomers

Abstract

Effective applications must be found for sulfur, a widely available and inexpensive element. Over the last decade, copolymers with unsaturated comonomers have been prepared via so-called inverse vulcanization. In this work, a sulfur copolymer with a circular and biosourced di-pyrrole compound was obtained for the first time and was used as the sole crosslinking agent of an unsaturated elastomer. Pyrrole compounds (PyCs) were synthesized via the Paal–Knorr reaction of 2,5-hexanedione (HD) and hexamethylenediamine (HMD) or ethylenediamine (EDM). The PyCs were obtained without using solvents or catalysts in high yield and with water as the only co-product. Poly(S-co-HMDP) and poly(S-co-EDP) copolymers were prepared under the typical conditions of inverse vulcanization. Throughout the entire synthetic pathway, the overall yield was up to 92% and the atom efficiency was up to 73%. The E-factor evaluated for organic compounds was almost null. The sulfur weight content in the copolymers ranged from 40% to 80% and the average number of sulfur atoms in the sequences ranged from 3 to 17. The copolymers were found to be amorphous with a glass transition temperature ranging from −2 to 38 °C, increasing with the content of the pyrrole ring. The number average molecular weight was found to be in the range from 1500 to 9000 g mol⁻¹. The molecular weight distribution was pretty narrow, with values lower than 2. NMR investigation suggested that the β position of the pyrrole ring reacted with sulfur atoms. A poly(S-co-HMDP) copolymer with an average sequence of 3 sulfur atoms was used as the sole crosslinking agent in a composite based on an unsaturated elastomer such as poly(styrene-co-butadiene) from anionic polymerization. More efficient crosslinking was obtained by promoting the ionic reaction of sulfur with elastomer chains by using 1,5-diazabicyclo(5.4.0)undec-7-ene. These results pave the way for the synthesis of a wide variety of sulfur copolymers with comonomers containing pyrrole rings for the sustainable crosslinking of elastomers, avoiding the use of oil-based accelerators.