Achieving a glass transition temperature (T_g) above room temperature (except PLA) is one of the main obstacles for the current bio-based thermoplastics. Even polylactic acid (PLA) has mostly been used in the disposable packaging market among bio-based thermoplastics but it cannot compete with petroleum-based polystyrene and polyethylene terephthalate thermoplastics because of its lower T_g, toughness, and moisture resistance. Functionalized δ-hexalactone (FDHL) monomers are hypothesized to be synthesizable from hydroxymethyl furfural (HMF) and lignin-derived pendent groups (Figure 1a, route A), generating a variety of aliphatic polyesters and potentially overcome current polymer challenges. Different bulky, lignin derivatives were incorporated as pendant groups in the monomer to increase the glass transition temperature (T_g) beyond that possible from poly(δ-valerolactone). A successful FDHL monomer synthesis used commercially available methyl cyclopentanone-2-carboxylate as a starting material (Figure 1a, route B). Different alkyl (methoxy and cyclohexanol) and aromatic (phenol, 1-naphthol, and 2-phenyl phenol) groups were attached as pendant groups. ROP of these six-membered monomers resulted in new polymers ranging from 5 to 30 kg mol^-1with narrow dispersity (Figure 1b). The polymerizations were carried out at room temperature using acidic to super-basic organocatalysts. Typical equilibrium polymerization behavior was observed at room temperature, and the reaction was observed to be pseudo-first-order to monomer concentration in solution. The T_g for poly(MDHL) was found -44 °C, while T_g observed for poly(PDHL) was 6 °C due to the presence of the bulky phenolic group pendant to the main polymer chain. Other recently synthesized polymers bearing bulkier groups (cyclohexanol, naphthols, and phenyl phenols) further increased the T_g due to the presence of bulkier pendant groups.

a) Schematic diagram for the synthesis of FDHL monomers and polymerization path to get different polyesters. b) Structures of the synthesized new polyester polymers bearing different bulky pendant groups.

Atik Faysal

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