Aliphatic-Aromatic Polyesters from Naturally Occurring Sinapic Acid through Acyclic-Diene Metathesis Polymerization in Bulk and Green Solvent Cyrene

Abstract

Green synthesis of renewable alternatives to fossil fuel-based (macro)molecules/polymers is more than ever a necessity. We recently developed a sustainable pathway to produce 6-hydroxy-5,7-dimethoxy-2-naphthoic acid (DMNA), which resembles the fossil-derived 6-hydroxy-2-naphthoic acid, from sinapic acid. To investigate the potential of DMNA as a building block for polymer syntheses, three novel DMNA-derived α,ω-dienes (M1–M3) were synthesized and engaged in acyclic-diene metathesis (ADMET) polymerization in a three-step study to prepare renewable aliphatic-aromatic polyesters (P1–P3). The first step aimed to evaluate the activity of seven commercial metathesis catalysts for the solvent-free ADMET polymerization of M3. Although most of the studied catalysts exhibited good reactivity, the second-generation Hoveyda–Grubbs catalyst (C4) proved the best. The second step was then started by varying the catalyst loading and testing M1 and M2 toward ADMET polymerization. Aliphatic-aromatic polyesters with a number-average molecular weight (Mn) up to 19.4 kDa (Đ = 1.88) were obtained. Furthermore, the results showed that the properties can be finely tuned depending on the monomer and catalyst loading. Thermal analysis demonstrated that the glass transition temperature (Tg) and the temperature at which 5% of the mass is lost (Td5%) varied depending on the alkene chain length. A general thermal trend was established: Tg(P1) > Tg(P2) > Tg(P3) and Td5%(P1) < Td5%(P2) < Td5%(P3). The third step of the ADMET study was to evaluate the tolerance of M1–M3 and C4 toward Cyrene, a green and high boiling point solvent derived from cellulose. The results revealed that Cyrene merits further investigation as a “general” non-toxic solvent for ADMET polymerization of other monomers, particularly those with high melting points.