Challenges and Methods for Polycondensation of Camphoric Acid Based Polyesters

As the market for biobased polyesters continues to grow, so does the need for more mechanically robust biobased polyesters. Cyclic terpenes found in plant oils are a potential source of rigid monomers for preparing polyesters. One cyclic terpene derivative of interest is camphoric acid, derived from the oxidation of camphor or α-pinene. This cyclic diacid contains a 5 membered ring with one secondary and one tertiary carboxylic acid. This study investigates the polymerization of polyesters containing camphoric acid. Initial attempts at polymerizing camphoric acid and ethylene glycol were unsuccessful. Thermogravimetric analysis of camphoric acid indicated its thermal stability was slightly lower than temperatures typically used for polycondensation, with a T_d95 of 176 °C. Polymerizations were attempted at lower temperatures of 160 to 180 °C with several catalysts, resulting in low conversion of the tertiary carboxylic acid group by NMR. As an alternative strategy, the camphoric acid monomer was reacted with ethylene carbonate and catalytic Na₂CO₃ at 160 °C. After 3 hours full conversion of the monomer was observed, and short oligomers comprised of hydroxyl end groups were obtained. Several polycondensation catalysts were screened at small scale for polymerization of the end capped prepolymer, with Sn(2-ethylhexanoate)₂ performing the best. The polymerization conditions were utilized on prepolymers prepared from other cyclic carbonates, and the thermal and mechanical properties of the resultant polymers are discussed.