Various approaches for main-chain type benzoxazine polymers

Main-chain type benzoxazine polymers (MCBPs) as a new class of polybenzoxazines are very widely used in the recent years due to their unique properties as thermoplastic linear polymers that can undergo thermoset conversion when they are further polymerized and cross-linked. MCBPs are unique polymers with a thermoplastic/ thermoset crossover nature attributed to the presence of cross-linkable sites in the polymer backbone, while the uncross-linked polymer itself has properties comparable to those of many thermoplastics. The cross-linking mechanism for such polymers as a function of various processing parameters is of great importance. MCBPs have an edge over cross-linked polymers derived from monomeric benzoxazine. MCBPs are developed as a new generation of polybenzoxazine precursors. The properties of MCBPs derived from the polymeric Mannich condensation are much better compared to those of the monomeric type dibenzoxazines. This chapter argues that the polycondensation via Mannich reaction approach has some disadvantages. The resultant MCBPs has relatively low molecular weight. This is because it is difficult to maintain the stoichiometry during the condensation reaction due to the many side reactions and branching that are caused by the partial ring-opening of the benzoxazine structure. Additionally, the poor solubility of some aromatic diamines and bisphenols in solvents used for MCBP synthesis limits specific molecular design or achievement of higher molecular weight. To overcome these difficulties, benzoxazine monomers with reactive end groups other than phenols and amines are used in copolymerization reactions with other functional groups (e.g., ethynyl, propargyl, allyl, alcohol, and carboxyl) to form linear polymers with benzoxazine rings as repeating units in the main chain. © 2011