Cyclic and spirocyclic polyacetal ethers from lignin-based aromatics

Abstract

Monomers structurally resembling lignin were prepared by reacting 4-hydroxybenzaldehyde, vanillin, syringaldehyde, (each bio-available) or ethylvanillin (synthetic) with dibromoethane, yielding dialdehydes CHO–Ar–OCH₂CH₂O–Ar–CHO. Condensation copolymerization with tetraols catalyzed by para-toluene sulfonic acid yielded polyacetal ethers with cyclic acetals in the case of di-trimethylolpropane (di-TMP) and spirocyclic acetals in the case of pentaerythritol (PTOL). Number average molecular weights (M_n) were in the range of 10 600 to 22 200, although the insolubility of those polymers based on 4-hydroxybenzaldehyde precluded this measurement. The polymers are thermally robust and exhibit 5% mass loss via thermogravimetric analysis in the range of 307–349 °C. Those copolymers based on PTOL displayed glass transition (T_g) temperatures (108–152 °C) at least 40 °C higher than their di-TMP analogues (68–98 °C), highlighting the added rigidity conferred by spirocyclic acetals versus cyclic acetals. Preliminary degradation studies were conducted in dimethyl sulfoxide with 0.5% added aqueous HCl (concentrated or 2 M). Dynamic light scattering confirmed the facile hydrolysis of the polymers. Generally, polymer degradation was faster with a higher acid concentration and copolymers from the PTOL tetraol were more resistant to hydrolysis than those from the di-TMP tetraol.