Two eugenol-based aromatic dimethyl terephthalate (DMT)-like monomers were prepared via a thiol–ene click reaction and a subsequent nucleophilic substitution reaction with methyl chloroacetate or 1,4-dibromobutane. Two series of thermoplastic polyesters derived from renewable eugenol and linear aliphatic α,ω-diols HO–(CH2)n–OH (n = 2, 3, 4, 6, 10, 12) were successfully synthesized. These prepared polyesters have weight-average molecular weights in the range of 18 500–90 500 g mol−1, and polydispersity indexes (PDIs) between 1.8 and 2.2. Their chemical structures were all accurately characterized by 1H NMR spectroscopy, 13C NMR spectroscopy and FTIR spectroscopy. The random microstructures of the synthetic polyesters were also explored by 13C NMR spectroscopy. The obtained polyesters all exhibit a thermal stability above 330 °C. More importantly, the thermal stability, the maximum degradation rate and the residue weight are intimately associated with the length of the linear aliphatic α,ω-diol. Their thermo-mechanical properties were studied by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The length of the linear aliphatic α,ω-diol crucially influences the glass transition temperature (Tg). With the gradual increase in the α,ω-diol length, the Tg of the synthesized polyester exhibits a tendency to decrease. The polyesters are all amorphous materials at room temperature, with Tg values ranging from −28.4 to 7.6 °C. The incorporation of aromatic eugenol into the polyester chains reduces the crystallinity significantly. The Young's modulus and ultimate strength are in the range of 1.2–6.9 MPa and 0.96–3.37 MPa, respectively. On the contrary, the elongation at break reaches up to 840–1000%, indicating the excellent viscosity properties for such unmanageable viscous materials.
An overall picture of the reaction pathways towards the side products and final polymers during the transesterification synthesis of biodegradable aliphatic carbonates (APCs) were delineated for the first time. Catalysed by sodium ethoxide, the normal reaction between α,ω-diols (carbon number n = 2–6) and diethyl carbonate generates APC oligomers with propagating terminal alkoxide anions. These anions can backbite at the nearest carbonate carbons or the nearest carbonate α-CH 2 carbons in the same APC chains, eliminating corresponding cyclic carbonates or cyclic ethers. Also, each alkoxide anion can randomly attack a carbonate α-CH 2 carbon in another APC chain, generating ether linkages in the final polymers whose percentages are about 100%, 1.35%, 0.08% and 0% for n = 2, 3, 4 and 5, respectively. The content of cyclic carbonates shows a similar decreasing trend; ethylene carbonate (n = 2) even predominates over the final polymer while trimethylene carbonate (n = 3) and 1,4-butylene carbonate (n = 4) are only minor by-products. Moreover, the latter two cyclic carbonates are not stable enough and can further decarboxylate, forming relatively high amount of allyl alcohol and tetrahydrofuran, respectively. However, trimethylene oxide (n = 3) and tetrahydropyran (n = 5) are minor cyclic ethers. The thorough understanding of the chemical process is essential to improve the APC synthesis.
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