Counterion effect of amine salts on ring-opening polymerization of 1,3-benzoxazines

Polymers are advanced materials with a wide range of use in our daily lives, as a remarkable area where many researchers have been working. Recently, applications of polymers in science, technology and different areas of the industry and simple synthesis with the help of many different processes for these applications have highlighted the important features of the polymers. According to the changes by heat, polymers can be divided into two classes as thermoplastic and thermoset polymers. Thermoplastics are polymers which can be reshaped by heat. They have a wide range of uses depending on the structural differences such as poly(hexamethylene adipamide), poly(ethylene terephthalate), poly(vinyl chloride). As thermoplastics can be re-melted, they can also be used many times. Thermoset polymers can not re-melt because of their cross-linked structure. Due to these cross-linked structures, they are rigid and insoluble in solvents. Although there are types of phenolic resins such as phenol-formaldehyde, melamine-formaldehyde, and urea-formaldehyde, when looked the world-wide production schemes, phenol-formaldehyde resins appear to have a great importance. These phenolic resins, which are the first man-made thermoset plastics, are used in many areas, especially in the wood and wood industry, and are synthesized by the reaction of phenol, formaldehyde solutions. Although the traditional phenolics have excellent thermal and mechanical properties, there are disadvantages such as the formation of voids during vulcanization which prevents the formation of a void-free composite product. As since the first days of development of polybenzoxazines have eliminated almost all deficient due to their properties such as near-zero volumetric change upon curing, low water absorption, for some polybenzoxazines Tg much higher than cure temperature, high char yield, release of no toxic by-product during curing and no strong acid catalysts required for curing, they have became a good competitor against to traditional phenolic resins. When synthesized the first benzoxazine monomer by Holly and Cope in 1944, phenol, formaldehyde and primary amine groups used in the synthesis were varied and different benzoxazine monomers were developed. The polymerization of the benzoxazines is accomplished by heat, light and with catalysts. The polymerization occurs via ring opening reaction of the oxazine ring of benzoxazine. In addition to the many properties that make the benzoxazine structure in the polymer field superior to the classical resins and which are superior to the conventional resins, the high opening temperature (curing temperature) in the polymerization, which is one of the disadvantages of the industrial applications, requires new processes in production and increases the energy consumption significantly. Benzoxazines can be cured with acid catalysts at room temperature. However, the use of these catalysts is not preferred as it requires extra equipment and production scheme in terms of production. Many studies have been carried out based on the lower curing temperature benzoxazine synthesis due to the temperature sensitive side groups they have and the reducing of the curing temperature of the benzoxazines. In this thesis, the counterion effect of amine salts as catalyst has been investigated via the proposed mechanism for the purpose of reducing the high ROP of 1,3 classical benzoxazines. Aliphatic and aromatic different amine salts with Cl-, Br-, I-, NO-3 that provided the reduction on curing temperature for the synthesized P-a, P-Bn, B-a benzoxazine monomers was followed by DSC. In our results, it was observed that the amine salts decreased the ring-opening temperature and these decreases were interpreted according to the effect of the nucleophilic character of the counterion species of the amine and ammonium salts. As well as the nucleophilic character of counterions, the solubility and ionization of the amine salts in the melted benzoxazines have also been shown to play an important role on catalytic activity. The thermal properties of the cured polymers were analyzed by TGA. The latent (hidden) catalyst character of the amine salts was analyzed by using 1H NMR in solvent and at room temperature. This analysis showed that even after 24 hours the oxazine ring of benzoxazines was protected and the HX salts could be stored in the same container together with the benzoxazines in the long storage period. Accordingly, at the end of the study, it has been shown that the HX salts in the formulations can be used as strong and cost effective latent catalysts for the ring-opening polymerization of 1,3-benzoxazines.