Synthesis of Thioamide containing polybenzoxazines by Willgerodt-Kindler reaction

Abstract

Advanced polymers are used in quite wide areas to increase our daily comfort. The processable and improvable structures of polymers have been the focus of interest for researchers, eventually fields of application in science, technology, and industry are increasing day by day. The terms thermoplastic and thermoset determine by how a material can be processed under a changed temperature. Thermoplastic polymers can be melted at elevated temperatures and can be shaped and by heating can be reshaped according to the mold. Thermoset polymer becomes rigid when heated and it can not be reshaped by heating. Due to their cross-linked structure, they are insoluble in solvents. Phenol-formaldehyde, melamine-formaldehyde, urea-formaldehyde are some examples of thermoset materials. Looking at the worldwide production in recent years, phenol-formaldehyde resins are among the most produced thermoset resins. Phenolic resins can be applied in many fields such as construction, adhesive industry, conductive polymers, aviation. The properties of phenolic resins such as heat and flame resistance, dimensional stability, high mechanical strength, resistance to solvents and low cost enable these resins to be used in these application areas. Phenol-formaldehyde resins are the first synthetic and commercially available thermoset polymer. Benzoxazine resins termed as polybenzoxazines, have been developed to overcome the disadvantages of phenolic resins such as high curing temperatures, short shelf life, brittleness, by-product formation during curing and catalyst requirement. Polybenzoxazines, have become superior to traditional phenolic resins thanks to their near-zero volumetric changes during curing, low moisture absorption, no strong catalyst requirement during curing, no toxic by-products, high char yield properties. Another advantage is the easy synthesis of benzoxazine monomers using phenol, primary amines, and formaldehyde. The formation of polybenzoxazine is carried out by the ring-opening reaction occurring in the oxazine ring of the benzoxazine monomer. Although there are several oxazine isomers, including 1,2-, 1,3- and 1,4-, polybenzoxazines are synthesized from the 1,3-benzoxazine monomer. Because only 1,3-benzoxazine monomers are suitable for the ring-opening reaction. The design flexibility of benzoxazines enables monomer synthesis with various functional groups. Polymers which are obtained from benzoxazine monomers with different functional groups may exhibit different physical and chemical properties, such as high efficiency, self-healing, shape-memory, etc. As it is well known, thioamides, which are carboxylic acid derivatives, can be synthesized by the Willgerodt-Kindler reaction. In this study, the first examples of thioamide-containing benzoxazines were successfully produced by using the design flexibility of benzoxazine through the Willgerodt-Kindler reaction. In these syntheses, primarily phenolic mono and difunctional thioamides were obtained. Three different benzoxazine monomers were synthesized using the obtained phenolic thioamides and primary amines. In addition, while thioamides are obtained in these reactions, an environmentally friendly approach has been applied by using elemental sulfur, which is released as a by-product from the coal and petroleum industry. Then, the difunctional benzoxazine monomer containing phenolic thioamide and the main chain polybenzoxazine containing thioamide by Mannich type polycondensation were successfully synthesized. This thioamide-containing polybenzoxazine exhibited good film-forming properties and flexible films were obtained. All starting phenolic reagents, benzoxazine monomers and polybenzoxazines obtained in this study were characterized by 1H-NMR and FT-IR spectroscopy. Curing behavior and thermal stability of the compounds were analyzed by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).