Scalable synthesis of CBDA-4, a thermocleavable diacid building block, was accomplished by capturing and photodimerizing a metastable crystalline solid of trans-cinnamic acid.
A previously overlooked building block, cyclobutane-1,3-diacid (CBDA), is introduced to materials synthesis due to its great potentials. As an example of CBDA, α-truxillic acid or 2,4-diphenylcyclobutane-1,3-dicarboxylic acid, was readily synthesized from commercially available trans-cinnamic acid. This CBDA showed outstanding stability both in sunlight and upon heating. While its two carboxylic acid groups can be readily utilized in connecting with other molecules to form new materials, the cyclobutane ring was able to tolerate acid and base treatments showing good chemical stability. A series of cyclobutane-containing polymers (CBPs), namely poly-α-truxillates, were obtained by condensation between α-truxillic acid and diols including ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-petanediol, and 1,6-hexanediol. The structures of these poly-α-truxillates were analyzed by NMR, FT-IR, and HRMS. Powder X-ray diffraction results of the poly-α-truxillates indicated that they are semi-crystalline materials. Preliminary thermal, chemical, and photochemical tests showed that the poly-α-truxillates exhibited comparable stabilities to PET.
A novel
renewable building block with a semirigid structure, (1α,2α,3β,4β)-2,4-di(furan-2-yl)cyclobutane-1,3-dicarboxylic
acid (CBDA-5), was synthesized from furfural. The synthesis started
with a Knoevenagel condensation between furfural and malonic acid,
and a subsequent decarboxylation in the same pot, which produced 2-furanacrylic
acid. The corresponding ester, ethyl-2-furanacrylate, was prepared
by the Fisher or Steglich esterification of 2-furanacrylic acid with
ethanol. Solvent-free [2 + 2] photodimerization of crystalline ethyl
2-furanacrylate was then carried out at −20 °C under blacklight
to give diethyl 2,4-di(furan-2-yl)cyclobutane-1,3-dicarboxylate (CBDE-5).
Afterward, CBDE-5 was hydrolyzed to give the corresponding dicarboxylic
acid, CBDA-5. Both CBDE-5 and CBDA-5 were confirmed by NMR spectroscopy,
Fourier transform infrared (FT-IR) spectroscopy, high-resolution mass
spectrometry (HRMS), and single-crystal X-ray diffraction (XRD). A
preliminary study showed that CBDE-5 and CBDA-5 could be used as renewable
building blocks to produce fully biobased polyesters. In this study,
several “green” techniques were applied to prepare renewable
building blocks, including solvent-free crystallization, solvent-
and metal-free photodimerization, and the use of residential blacklight
as an energy-efficient, cost-effective, and operator-friendly UV (ECO-UV)
irradiation source in the stereoregular photocycloaddition.
The ability of diol monomers, such as 1,4-cyclohexanedimethanol (CHDM) and 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD), to replace the potentially hazardous BPA in the production of polyesters and polycarbonates with excellent thermal, optical, and...
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