Polymerization and mechanistic studies of benzoxazine

Studies have been carried out with the aim of increasing the molecular weight of Mannich base polymer from the polymerization of benzoxazine monomer. High molecular weight polybenzoxazine has been synthesized from p-cresol-aniline based benzoxazine monomer (pC-a) initiated by PCl5 at low temperature, followed by high temperature polymerization. The melting point of this polymer is observed for the first time at 190°C and crystallization temperature at 160°C. In addition, high molecular weight polymer from polymerization of p-cresol-methylamine based benzoxazine (pC-m) initiated by Yb(OTf)3 at low temperature has been obtained. However, there are many potential structures occurring in this polymerization system. Therefore, its initiation mechanism has been investigated by utilizing the final products from the dimer systems with high performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR). Combination of low yield of desired Mannich based benzoxazine and lot of other unexpected compounds from aromatic-based benzoxazine is obtained. The initiation mechanism of benzoxazine has been proposed that protonation firstly takes place at the nitrogen atom and then migrates to the oxygen atom. The proposition is based on the experimental result. Although, the aromatic-based benzoxazine monomer has higher tendency to form desired Mannich base structure, it also has higher tendency to form other compounds. Therefore, in order to obtain the desired Mannich base structure in the high yield, methyl-based benzoxazine should be used. Further investigation has been done on end group studies of the product from polymerization of benzoxazine monomer by utilizing 31P NMR spectroscopy to investigate both the main chain and how the chain is terminated. The small model phenolic compound and also the benzoxazine oligomer are phosphorylated with 1-chloro-dioxaphospholane in order to assign the 31P resonance peaks in the benzoxazine system. The structure of polybenzoxazine is discovered. 31P NMR spectroscopy is also applied to the piperazine based Mannich base polymer, which has a number of attractive properties such as its white color and a high degradation temperature, which is comparable to the difunctional polybenzoxazine. While conventional phenolic resin, including polybenzoxazine are colored product when undergo a reaction, this novel polymer possesses pure white color after purification. By utilizing this facile technique, the end group and main chain of the polymer, which is important to know in order to control the desired properties is proposed.