2,5-Diisopropenylthiophene by Suzuki-Miyaura cross-coupling reaction and its exploitation in inverse vulcanization: a case study

Abstract

A novel thiophene derivative, namely 2,5-diisopropenylthiophene (DIT) was synthetized by Suzuki-Miyaura cross-coupling reaction (SMCCR). The influence of reaction parameters, such as temperature, solvent, stoichiometry of reagents, role of the base and reaction medium were thoroughly discussed in view of yield optimization and environmental impact minimization. Basic design of experiment (DoE) and multiple linear regression (MLR) modeling methods were used to interpret the obtained results. DIT was then employed as a comonomer in the copolymerization with waste elemental sulfur through a green process, inverse vulcanization (IV), to obtain sulfur-rich polymers named inverse vulcanized polymers (IVPs) possessing high refractive index (n ≈ 1.8). The DIT comonomer was purposely designed to (i) favor the IV process owing to the high reactivity of the isopropenyl functionalities and (ii) enhance the refractive index of the ensuing IVPs owing to the presence of the sulfur atom itself and to the high electronic polarizability of the π-conjugated thiophene ring. A series of random sulfur-r-diisopropenylthiophene (S-r-DIT) copolymers with sulfur content from 50 up to 90 wt% were synthesized by varying the S/DIT feed ratio. Spectroscopic, thermal and optical characterizations of the new IVPs were carried out to assess their main chemical-physical features.

Scheme 1. Double condensation of Suzuki–Miyaura on 2,5-dibromothiophene substrate to give DIT then used as a comonomer in the IV process with sulfur to give S-r-DIT copolymers.

Scheme 2. Synthesis of DIT by SMCCR: substrate and reaction conditions were kept constant except the inorganic base.

Scheme 3. Possible mechanistic paths in the Suzuki–Miyaura reaction. The different intermediates of the catalytic cycle are highlighted with particular emphasis on the role of the base (here OH⁻). Referred to DIT synthesis: Ar = 2,5-thiophenyl; R = isopropenyl; X = Br; L = PPh₃; RBZ₂ = isopropenylboronic acid pinacol ester.

Scheme 4. Possible associative and diffusive phenomena of the ions and reacting species involved in the SMCCR in the presence of TBANP as PTC.

Fig. 1. ATR-FTIR spectra of selected S-r-DIT copolymers. Spectra of DIT comonomer and elemental sulfur are reported as well for comparison purpose.

Fig. 2. TGA curves of S-r-DIT copolymers at various compositions.

Fig. 3. (a) Absorbance (red line, and left scale) and photoluminescence (black line, and right scale) of a 10% toluene solution of the S-r-DIT copolymer with 70 wt% sulfur content. (b) Optical function spectra of the copolymer: refractive index (n, red line, and left scale) and extinction coefficient (k, black line, and right scale).

Fig. 4. Refractive index of polymers used for photonics as a function of the Abbe number. The red dot is for the data of IV S-r-DIT copolymer reported in Fig. 3. PC, PS, PET and PMMA from ref. ; Aquivion® from ref. ; all other materials from ref. and references therein reported.