Coumarines as masked phenols for amide functional benzoxazines

High performance polymers constitute an essential part of daily life because of their advanced properties. Their chemical structure and property is modifiable and desired properties can be obtained, so polymers are adaptable for many different areas such as aerospace, adhesives, electronic circuits, and coatings. Polymeric materials are divided into two main classes according to their reaction with increasing temperature which are thermoplastics and thermosets. A thermoplastic polymer shows fluid property above a definite temperature, however thermoset polymer demposes at high temperatures and it does not exhibit fluid property. Not only plastic materials are included in this categorisation, but also adhesives, coatings and various other classes show similar behaviour. Thermosetting polymers include constitutional repeats unit in their chemical structure and they are also named as thermosetting resins. They are examples of thermosetting polymers which are phenolic and urea-formaldehyde resins, epoxy resins, and unsaturated polyesters; also, polystyrene, polypropylene, polyethylene, and poly(vinylchloride) are the examples of thermoplastic polymers. Thermoplastic polymers are very big macromolecules, they possess linear or branched long chains in their structure, and these long chains can be held together by van der Waals forces, dipole–dipole forces, and hydrogen bonding. Thermosetting polymers have severalizing properties. Their cross-linked structure is resulted from repeating units with covalently bonded, so when they are exposed to high temperature or a reactive solvent they become insoluble and infusible. Although high temperatures can destroy the covalent bonds in thermosetting resins, these polymers will not flow. So, it can be deduced that the thermosetting structure is resulted from the existence of network formed by covalent bonds. The polymer will swell in a good solvent but will not dissolve. Thermosetting resins include various kind of materials and their properties gain interest in a last decade. However, phenolic resins are the most important one because they possess striking properties, so they are used in many applications such as commodity and construction, adhesive industry, printed circuit boards, conductive polymer structures, material capsules and electronics, and aerospace industry. They exhibit outstanding characteristics such as heat and flame resistance, dimensional stability, high mechanical strength and high resistance against various solvents, acids and water. They have low smoke density while burning. Phenolic resins draw interest in market for thermo-structural applications and aerospace industry because of their low cost and good heat and flame resistance. The synthesis of phenolic resins occurs with the reaction of formaldehyde with phenol or substituted phenol. Phenol-formaldehyde resins are the first completely synthetic and commercially existing thermosetting plastics. There are many advantages about phenolic resins, but also some drawbacks are exist for example, high curing temperatures, void formation during curing, water absorption, short shelf life, and being brittle. Also, acid or base catalyst used for the synthesis of resol and novolac, so using these catalysts results in the corrosion on processing equipment. During curing process, volumetric shrinkage occurs and side products are released. In order to surpass these problems, a new class of polymers have been developed called as polybenzoxazines. Polybenzoxazines are developed as an alternative of phenolic resins in the last decade because of their striking characteristics compared to many traditional resins. Polybenzoxazines exhibit outstanding properties such as (i) good thermal stability and mechanical strength, (ii) high Tg, (iii) near-zero volumetric change during curing, (iv) low water uptake and dielectric constants, (v) high char yield, (vi) release of limited toxic by-product during curing, (vii) no strong acid catalysts required for curing, (viii) low cost materials, and (ix) storage at room conditions. Because of these remarkable features, polybenzoxazines are preferred in many industrial areas especially as composites in aerospace industry, blends and high performance electronic circuit boards. Another remarkable property of polybenzoxazines is their easy synthesis procedure from their corresponding monomers.Generally, phenols, primary amines, and formaldehyde are used for traditional synthesis method, but there are also various ways to synthesize polybenzoxazines. Classical synthesis method is mostly preferred because phenols and primary amines can be found easily. Also, elimination of side products is easy after synthesis. Therefore, new benzoxazine monomers have been developed in last ten years. However, polybenzoxazines possess some drawbacks such as (i) brittleness after cure, (ii) cold flow, (iii) film and fiber formation difficulties. In order to overcome these problems, various main-, side- and end-chain polymers have been synthesized. Polybenzoxazines have blended or modified with other commercial polymers to surpass the drawbacks. Some of modified polybenzoxazines have used as smart thermosets for shape memory, self-healing and electrochromic materials.The block copolymers are a kind of polymer blends. Modification of polybenzoxazines with Jeffamines is useful for obtaining high performance thermosets. The Jeffamine, which is also named as polyetheramine, includes primary amines with the end of polyether backbone. To obtain polyetheramine Jeffamine, monoamines, diamines, and triamines can be used as core structure. Jeffamines are generally used for epoxy curing, developing of rigid, impact resistant coatings, castings, and adhesives. Besides, it can affect water absoprtion of surface, thus electrical resistivity of surface can be altered. Various polymerization methods with benzoxazines have been studied and benzoxazine based polyesters, polysiloxanes, polyethers, polyamides, polystyrenes, polyacrylates, main-chain polybenzoxazine prepolymers, polytriazoles, polyurethanes, poly(phenyl acetylene) derivatives were successfully obtained. Some of modified polymers were used in several smart material applications. These studies shows that benzoxazines have high potential for the applicability of different areas; for example, ordinary thermosets, composite materials, and stimuli responsive polymers. Among all the studies about substituents and positions in benzoxazine molecule, amide functional benzoxazines gain interest in recent times because such structures can be comparable to anilide based polymers. Although, there are many benzoxazine monomers in literature, benzoxazines with amide linkages were barely synthesized. Due to the multi-stage synthesis, primary amine-functional benzoxazine monomers are hard to synthesize. During the monomer synthesis, the main difficulty is the selectivity problems of primary amine components. Aminomethylol formation may be inevitable. In order to synthesize amine functional benzoxazines, using protecting groups and further hydrolysis are needed. However, purification is also a problem because of the many side reactions resulted from reduction methods. In this study, benzoxazines with amide linkages were successfully prepared. Initially, 3,4-dihydrocoumarine (DHC) was used as reagent to synthesize phenolic amides via ring-opening reaction using primary amines. The phenolic amides were then reacted with formaldehyde and primary amines to produce amide group containing benzoxazines. Moreover, polymeric amide functional benzoxazine precursor was prepared with bisamine terminated poly(propylene oxide) by Mannich type polycondensation. The obtained polymer exhibited good film forming properties and flexible films were solvent casted. All the monomers and polymeric precursor were characterized by 1H-NMR and FT-IR spectroscopies. Moreover, the curing behaviors and thermal stabilities of the compounds were analyzed by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). Besides, tensile tests were performed to characterize both cured and uncured polymeric benzoxazine precursors.