CO2-based polycarbonates from biobased cyclic terpenes with end-of-life usage potential

Abstract

Biobased polymers have garnered increasing attention in recent years, aiming at more sustainable materials. This study focuses on the synthesis of polycarbonates sourced from cyclic terpenoid-based monomers and CO₂, representing polymers derived from a biobased feedstock. Menthyl, thymyl, and carvacryl glycidyl ethers, synthesized from menthol, thymol, and carvacrol and epichlorohydrin were copolymerized with CO₂ using catalytic systems such as (R,R)-(salcy)-Co(III)Cl (Co(Salen)Cl) and bis(triphenylphosphine)-iminium chloride ([PPN]Cl) or triethylborane (TEB)/[PPN]Cl. Moderate to high molar mass polymers (up to 60 kg mol⁻¹) were obtained with low dispersities (M_w/M_n < 1.24) via solvent-free bulk copolymerization. Despite the sterically demanding nature of these monomers, the cobalt-based catalyst system exhibited high monomer conversion, polymer selectivity, and carbonate linkage content. The resulting polycarbonates exhibited glass transition temperatures (T_g) ranging from 41 to 58 °C, when the polymer backbone consisted solely of polycarbonate linkages. However, with decreasing polycarbonate linkage content, the T_g value dropped to 0 °C for the menthol based polycarbonate. The aromatic side chain polycarbonates displayed not only the highest T_g values, but also the highest thermal stability, with T_5% reaching 260 °C. The thymol-based polycarbonate exhibited a Young's modulus (E) of 645 ± 43 MPa and an elongation at break (ε) of 5 ± 2%, as determined by tensile testing. All three biobased polymers underwent complete degradation under strong basic conditions (5 M KOH) within 30 hours, yielding their respective diols and CO₂, thus offering potential for end-of-life usage. CO₂ generated by thermal decomposition can be recycled for copolymerization, while the diols could find application for other purposes.