A cis-3,4-di­(furan-2-yl)­cyclobutane-1,2-dicarboxylic acid (CBDA-2) is readily prepared stereospecifically from trans-3-(2-furyl)­acrylic acid, a furfural-derived compound, through a solid-state [2 + 2] photocycloaddition in 95% isolated yield. The cyclobutane ring in CBDA-2 shows desired stabilities during thermal, sunlight, and chemical tests. The single crystal structure of CBDA-2 revealed the geometry of this molecule and orientation of the two dicarboxylic acid groups displaying its potential to serve as a unique, semirigid diacid building block in material science. A preliminary study showed that condensation of this diacid with glycerol yielded a green polymer with good stability. The diacid could also be used as a cross-linker for a biobased epoxy to yield an exceptionally hard and solvent-resistant thermoset.

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.

While ultraviolet light is hailed for its renewability, nontoxicity, and lack of resulting waste products, photochemistry is relatively out of reach for many researchers and industries because conventional ultraviolet radiation sources are not always accessible. For example, the availability of sunlight varies with weather conditions, geographical location, and daylight duration. The use of commercial incandescent light as an alternative to conventional ultraviolet radiation was explored in this article. The classic [2 + 2] photocycloaddition of trans-cinnamic acid was tested under controlled conditions in the solid state, and it was found that 150 W light bulbs provided satisfactory results and 1.5 cm away from the light source was a viable distance for the photoreaction. A [2 + 2] cycloaddition of 10 mg trans-cinnamic acid finished in as few as 4 h. Gram-scale synthesis was also achieved in 40 h by using a 500 W floodlight bulb. The incandescent floodlight also effectively facilitated a cutting-edge photopolymerization in 24 h to produce a two-dimensional (2D) polymer. The use of reliable, inexpensive, and nonhazardous incandescent light in place of sunlight or even ultraviolet lamps for certain photoreactions will allow for the wider study and development of photochemistry.

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.