Sustainable and Recyclable Polymers from CO2 and Lignin: Enabling Waste to Wealth Transformation

Abstract

Abstract The fast expansion of the plastics industry has intensified the global crisis of plastic waste pollution, ecological imbalance, and greenhouse gas emissions. This work highlights the conversion method of chaining up greenhouse gas CO2 with biomass lignin to develop new sustainable, recyclable polymers from abundant and non-food based renewable resources. A CO2 and lignin-based cyclic carbonate monomer has been synthesized using a cost-effective, non-phosgene-based and greener approach under atmospheric pressure and room temperature. The fully controllable synthesis of CO2 and lignin-based polymers is accomplished via ring opening polymerization (ROP) by varying the catalyst [1, 8-diazabicyclo (5.4.0) undec-7-ene (DBU), and 1, 5, 7-triazabicyclo [4.4.0] dec-5-ene (TBD)], catalyst loading (0.5–5.0%) and reaction time (2–40 min). Among the polymers, the best polymer obtained in 1% TBD with a reaction time of 30 minutes. The structural elucidations of the synthesized cyclic carbonate monomer and polymer are established using various spectroscopic analyses including 1H, 13C, and 2D HSQC NMR, FT-IR, and GPC. The high molecular weights (Mn: 120.34–154.58 kDa) and excellent thermal stabilities (Td5%: 244–277 ºC from TGA and Tg: 33–52 ºC from DSC) of the polymers are advantageous for practical applications. Significantly, the CO2 and lignin-based polymers have successfully recycled to the monomer for a circular plastic economy by heating at 90 ºC for 12 hours in presence of DBU. This process yields original structure of monomers that can be used for another polymerization without unwanted change of chemical structures, making it a sustainable solution. Therefore, this work illustrates an innovative hybrid approach of both the synthesis of 100% biomass-based polymer and infinite closed-loop recycling of polymer-monomer using abundant renewable resources, that further leads to a sustainable circular plastic economy alternative to the linear petroleum-based polymers.