Rigid biobased polycarbonates with good processability based on a spirocyclic diol derived from citric acid

Introducing biobased polymers from renewable sources for use as high-performance thermoplastics with high demands on mechanical rigidity, transparency, thermal stability, as well as good processability, is a significant challenge. In the present work we have designed and prepared a rigid biobased bis-spirocylic diol by di-cycloketalization of a bicyclic diketone (cis-bicyclo[3.3.0]octane-3,7-dione, obtained from citric acid) using trimethylolpropane. This spiro-diol monomer has two reactive primary hydroxyl groups and the synthesis from inexpensive biobased starting materials is straightforward and readily upscalable, involving no chromatographic purification. In order to explore the usefulness of the new monomer, it was employed in melt polycondensations with diphenylcarbonate at up to 280 degrees C to form rigid fully amorphous polycarbonates (PCs). Molecular weights (MWs) up toM(n)= 28 kg mol(-1)were achieved, and thermal and dynamic mechanical measurements showed glass transitions up toT(g)= 100 degrees C, with no thermal decomposition untilT(d)similar to 350 degrees C. Solvent cast films had excellent mechanical flexibility and strength, as well as a high transparency with only slight coloration. Results by dynamic melt rheology implied that the high-MWPCs had a good processability at 170 degrees C, with a stable shear modulus over time, but started to degradeviachain scission reactions when the temperature approached 200 degrees C. In conclusion, the present work demonstrates the straightforward preparation of the citric acid-based spiro-diol, and indicates that it is an efficient building block for the preparation of rigid biobased PCs and other condensation polymers.