2,2,4,4-Tetramethyl-1,3-cyclobutanediol (CBDO) is an aliphatic diol. This diol is produced as a mixture of ''cis''- and ''trans''- isomers, depending on the relative stereochemistry of the hydroxyl groups. It is used as a monomer for the synthesis of polymeric materials. CBDO is currently being researched as an alternative to bisphenol A (BPA). BPA is a precursor used in the production of a wide range of

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s including polycarbonates, polyesters,

polysulfone Polysulfones are a family of high performance thermoplastics. These polymers are known for their toughness and stability at high temperatures. Technically used polysulfones contain an aryl- SO2-aryl subunit. Due to the high cost of raw material ...

s, and polyester ketones.

Replacement for BPA

The controversies associated with BPA in large quantities are ultimately related to its endocrine disrupting abilities. Like BPA, CBDO is a diol with a structure suitable for making polyesters. CBDO’s C₄ ring is sufficiently rigid to prevent the two OH groups from forming cyclic structures. Unlike BPA, there is no current evidence of carcinogenic or toxic effects from CBDO-based consumer products. There are, however, few studies on the toxicology of CBDO for both long term and short term effects. CBDO has potential advantages relative to BPA as a building block for production of polyesters. CBDO is very stable thermally and mechanically. Polyesters prepared from CBDO are rigid materials, but the combination of CBDO with flexible diols results in materials with high impact resistance, low color, thermal stability, good photooxidative stability and transparency.Hoppens, Nathan C., Hudnall, Todd W., Foster, Adam, Booth, Chad J. Aliphatic-aromatic copolyesters derived from 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Journal of Polymer Science: Part A: Polymer Chemistry. Volume 42, 2004, pp. 3473-3478. As an added bonus, CBDO-derived polymers have high ductility.Kelsey, Donald R., Scardion, Betty M., Grebowicz, Jamusz S., Chuah, Hoe H. High Impact, Amorphous Terephthalate Copolyesters of Rigid 2,2,4,4-Tetramethyl-1,3-cyclobutanediol with Flexible Diols. Macromolecules. Volume 33, 2000, pp. 5810-5818. The thermal and mechanical properties of CBDO-derived polyesters are often superior to conventional polyesters.

Preparation

Synthesis of CBDO involves pyrolysis of isobutyric anhydride followed by hydrogenation of the resulting 2,2,4,4-tetramethylcyclobutanedione. This synthesis resembles the method used to produce CBDO today. The first step involves conversion of the

isobutyric acid Isobutyric acid, also known as 2-methylpropanoic acid or isobutanoic acid, is a carboxylic acid with structural formula ( CH3)2CH COOH. It is an isomer of ''n''-butyric acid. It is classified as a short-chain fatty acid. Deprotonation or esterif ...

or its anhydride into the

ketene In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule). The name may also refer to the specific compound ethen ...

. This ketene then dimerizes to form a four-membered ring with two ketone groups. The product ring is hydrogenated to give a diol. The last step commonly involves catalytic

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organ ...

with

ruthenium Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemical ...

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

, or

rhodium Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring i ...

catalysts Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

. Hydrogenation of the diketone ring results in both cis and trans isomers. A simplified scheme for the production of CBDO is presented below. CBDO Reaction Scheme

Structure and properties

The C₄ ring of the cis isomer of CBDO is non-planar. For simple non-planar cyclobutanes, dihedral angles range from 19 to 31°. CBDO’s cis isomer crystallizes as two conformers with an average dihedral angle of 17.5° in the solid state. However, the trans isomer has a dihedral angle of 0°.

Polyesterification

The current economic method for the production of polyesters is direct esterification of dicarboxylic acids with diols. This condensation polymerization adds monomeric units to a chain. Individual chains react with one another through carboxyl and hydroxyl terminal groups. Finally, transesterification occurs within the chain.Horst Köpnick, Manfred Schmidt, Wilhelm Brügging, Jörn Rüter, Walter Kaminsky "Polyesters" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. Although CBDO is most often used in polyesters, mixed copolycarbonates of CBDO and a series of bisphenols have also been synthesized. The differing reactivities of the cis and trans isomers have not been studied in depth.

References

{{DEFAULTSORT:Tetramethyl-1,3-cyclobutanediol, 2,2,4,4- Monomers Diols Cyclobutanes