Thermal and photochemical ring opening polymerization of benzoxazines

Abstract

Polymers as engineering materials have been used and created over the past 150 years, challenging traditional materials but enabling the creation of new goods that have helped to broaden human activity. Almost every element of modern life involves polymers. Most people come into contact with at least one polymer-containing product, such as car tires or contact lenses. Polymer materials are helpful and practical for many applications due to their vast range of properties. Modern polymer materials have drawn increased interest since they offer unique features that serve various purposes. Polymers are used in the technology of all levels—for instance, silicone polymer-based artificial skin or water desalination membranes. Polymers can be divided into thermoplastics and thermosets based on how they react to heat. By curing a solid or viscous liquid, thermoset materials are a class of materials that become permanently toughened. However, thermoplastic materials can be reused multiple times since they can be remelted. Because of their highly cross-linked structure, thermoset polymers cannot be recycled in large quantities. In addition to widely used thermoset plastics, including polyester, epoxy, vinyl esters, and polyurethane resins, phenol-formaldehyde resins are also significant to the polymer industry. Phenolic resins are widely used in a variety of industries, including high-tech aerospace, construction, and commodity materials. Phenolic resins offer a number of desirable characteristics, such as high mechanical strength, dimension stability, resistance to different solvents, and flame retardance, but they also have a number of disadvantages. Polybenzoxazines solve the shortcomings of conventional phenolic resins. Heat stability, zero volumetric change during curing, low moisture absorption, high char yield, least amount of hazardous material emission during cure, lack of catalyst requirement, and low cost are only a few of the features of polybenzoxazines. Polybenzoxazines are utilized in a variety of industrial areas, including composite construction, high-performance electronic circuit boards, and the aviation industry. Polybenzoxazines are synthesized from monomers of 1,3-benzoxazines. Among the many benzoxazine monomers, including 1,2- and 1,4-benzoxazine, only the 1,3-benzoxazine monomer is active for ring-opening polymerization (ROP). Typically, the appropriate monomers are polymerized at high temperatures without catalysts to produce polybenzoxazines. There has been research on catalyst-assisted benzoxazine curing, and the results show that by adding specific catalysts, it is possible to shorten the time required to start the polymerization and speed up the reaction. Although the maximal exotherm temperature was slightly lowered, no appreciable polymerization was seen below 100°C. Two components that constitute the benzoxazine ring, a six-membered heterocyclic ring, are N and O. According to the outcomes of chemical modeling, the oxazine ring in a benzoxazine molecule creates a distorted semi-chair structure. The resulting ring tension allows this kind of molecule to undergo ring-opening polymerization under particular reaction conditions. The ring is also expected to be opened by a cationic mechanism, according to Lewis' definition of basicity for the N and O atoms. Until present, several strategies have been investigated to lower the polymerization temperature of polybenzoxazines. For this goal, the use of initiators or catalysts such as Bronsted and Lewis acids, which can either be introduced as separate components to the polymerization mixture or directly integrated into the benzoxazine monomers in a latent state, has been examined. Even though various additives can catalyze ring-opening polymerization, heat treatment is necessary for synthesizing polybenzoxazine. Though with mixed results, photopolymerization of benzoxazines at room temperature has also been investigated. This method is more suited for the crosslinking of main chain polybenzoxazine precursors because, unfortunately, partial polymerization only yielded complicated, low molecular weight polymer structures. Evidently, an effective cure still requires thermal energy despite the attraction of the light-induced benzoxazines treatment. Thus, it seemed important to investigate whether successful ring-opening polymerization of benzoxazines could be accomplished by combining light and heat treatment. In this thesis, different advanced polybenzoxazines were acquired via a ring-opening polymerization process and were presented in three separate studies. In the first part of the thesis, simple benzoxazines were mixed and reacted with various phenolics such as phenol, p-cresol, p-nitrophenol, 1,3,5-trihydroxy benzene (phloroglucinol), 1,3-dihydroxy benzene (resorcinol), and N-(2-hydroxyphenyl)benzamide. The influence of these phenolic compounds on the temperature of ring-opening polymerization was examined by DSC analysis. Additionally, the mixtures of cresol-based monobenzoxazine and N-(2-hydroxyphenyl)benzamide gave poly(benzoxazine–benzoxazole)s through a new methodology, eliminating the need for synthesis of ortho-amide benzoxazines. The produced polymers were soluble and extensively studied using a variety of characterization techniques. In the second part of the study, sodium cyanoborohydride (NaCNBH3) and sodium borohydride (NaBH4) were used as catalysts to lower the ring-opening polymerization (ROP) temperature of 1,3-benzoxazines. Dynamic scanning calorimeter (DSC) and Fourier transform infrared (FTIR) showed that the onset of ROP temperatures was decreased by ca. 80 °C. Furthermore, 1H NMR and FTIR were used to reveal the room temperature behavior of the catalysts in benzoxazine, and it was found to be inactive at low temperatures and active after 170 °C. In addition to their catalytic activity, borohydrides also lessen the severe colorization of benzoxazines during the curing process. Colorimetry was used to analyze the color changes of catalyzed and uncatalyzed polybenzoxazine samples. Thermal gravimetric analysis (TGA) was also used to assess and contrast the thermal stabilities of catalyzed polybenzoxazine with those of their uncatalyzed counterparts. Finally, iodonium salts were used to produce photo-induced cationic ring-opening reactions on main-chain polybenzoxazine precursors. Upon photolysis at 300 nm, the precursors exhibited gelation in a comparatively brief period. In contrast to usual benzoxazines, the produced gels contained unreacted oxazines, and subsequent thermal curing was successfully applied at relatively lower temperatures. It has been demonstrated that the dual curable nature of the main chain polybenzoxazine precursors makes them useful for deep curing applications. Moreover, sensitizers such thioxanthone and camphorquinone were used in the formulations to increase the photocuring wavelength to higher wavelengths.