Benzoxazine Structure Research Articles

Methylol‐functional benzoxazines as precursors for high‐performance thermoset polymers: Unique simultaneous addition and condensation polymerization behavior

AbstractA new class of high‐performance resins of combined molecular structure of both traditional phenolics and benzoxazines has been developed. The monomers termed as methylol‐functional benzoxazines were synthesized through Mannich condensation reaction of methylol‐functional phenols and aromatic amines, including methylenedianiline (4,4′‐diaminodiphenylmethane) and oxydianiline (4,4′‐diaminodiphenyl ether), in the presence of paraformaldehyde. For comparison, other series of benzoxazine monomers were prepared from phenol, corresponding aromatic amines, and paraformaldehyde. The as‐synthesized monomers are characterized by their high purity as judged from 1H NMR and Fourier transform infrared spectra. Differential scanning calorimetric thermograms of the novel monomers show two exothermic peaks associated with condensation reaction of methylol groups and ring‐opening polymerization of benzoxazines. The position of methylol group relative to benzoxazine structure plays a significant role in accelerating polymerization. Viscoelastic and thermogravimetric analyses of the crosslinked polymers reveal high Tg (274–343 °C) and excellent thermal stability when compared with the traditional polybenzoxazines. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Facile, one‐pot synthesis of aromatic diamine‐based phosphinated benzoxazines and their flame‐retardant thermosets

AbstractThree aromatic diamine‐based, phosphinated benzoxazines (7–9) were prepared from three typical aromatic diamines—4,4′‐diamino diphenyl methane (1), 4,4′‐diamino diphenyl sulfone (2), and 4,4′‐diamino diphenyl ether (3) by a one‐pot procedure. To clarify the reaction mechanism, a two‐pot procedure was applied, in which the reaction intermediates (4–6) were isolated for characterization. The structures of intermediates and benzoxazines were confirmed by high resolution mass, IR, and 1D and 2D‐NMR spectra. In addition to self‐polymerization, (7–9) were copolymerized with cresol novolac epoxy (CNE). After curing, the homopolymers of P(7–9) are brittle while the copolymers of (7–9)/CNE are tough. Dynamic mechanical analysis shows the Tgs of (7–9)/CNE copolymers are 187, 190, and 171 °C, respectively. Thermal mechanical analysis shows the CTEs of (7–9)/CNE copolymers are 46, 38, and 46 ppm, respectively. All the (7–9)/CNE copolymers belong to an UL‐94 V‐0 grade, demonstrating good flame retardancy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Cationic Ring-Opening Polymerization of 1,3-Benzoxazines: Mechanistic Study Using Model Compounds

The benzoxazine monomer is used to simplify the study of the benzoxazine initiation mechanism. The HPLC retention time and the 1H NMR spectra of crude products from the benzoxazine reaction are compared with the results from a vast number of pure model compounds, which are synthesized based on the hypothesized mechanisms. Products involved in the process are identified, with species having benzoxazine structures, Mannich base and other components (acetal, nonacetal phenoxy structures, and methylene bridge structure). Initiation mechanisms of benzoxazine, e.g., the oxygen protonation and the nitrogen protonation, are proposed.

Primary Amine-Functional Benzoxazine Monomers and Their Use for Amide-Containing Monomeric Benzoxazines

Amino-functional benzoxazine monomers have been successfully prepared. Several routes have been applied to incorporate amino group into benzoxazine structure. These approaches include reduction of the corresponding nitro-functional benzoxazines and deprotection of protected amino-functional benzoxazine monomers. Various approaches that allow primary amine groups to be prepared without damaging the existing benzoxazine groups have been evaluated. Tetrachlorophthalimide and trifluoroacetyl are found to be suitable protecting groups. In addition, a model compound of amide-functional benzoxazines is prepared from primary amine-functional benzoxazine. Fourier transform infrared spectroscopy (FTIR) and 1H and 13C nuclear magnetic resonance spectroscopy (NMR) are used to characterize the structure of the monomers. The polymerization behavior of amino-functional monomers and model compound are studied by differential scanning calorimetry (DSC).

31P NMR spectroscopy in benzoxazine model compounds and benzoxazine chemistry – main chain and end group studies

Using 31P NMR spectroscopy after phosphorylation, different phenols are easily discriminated. Proposed phenolic model compounds from benzoxazine reaction/polymerization are phosphorylated with 2-chloro-1,3,2-dioxaphospholane directly in an NMR tube. The dimer and oligomer structure of benzoxazine is different from that of monomer, that is, the breaking of the oxazine ring upon polymerization results in the formation of Mannich bridge structure linking phenolic groups together. Different electronic environment in the phosphorus derivatives of the methylamine-based benzoxazine model dimer and oligomers allows comparison of these 31P NMR spectra with the 31P-NMR spectra of the compounds from traditional benzoxazine polymerization provides useful end group information. Phenolic functional groups in benzoxazine dimer and oligomers, e.g. phenolic end groups and phenolic groups on the backbone, are studied.

Synthesis and characterization of nanomagnetite thermosets based on benzoxazines

AbstractNanomagnetite thermosets were obtained by thermally activated ring opening copolymerization of benzoxazine groups coated on the surface of the nanomagnetite with bare benzoxazine. For this purpose, carboxylic acid containing 1,3‐benzoxazine was synthesized and covalently bonded on magnetite nanoparticles by postcoating method. The average size of benzoxazine coated nanoparticles was 40–100 nm as determined by Dynamic Light Scattering (DLS), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) measurements. The crystal structure of benzoxazine coated nanoparticles was shown to be magnetite by X‐ray diffraction (XRD) analysis. Thermally activated curing behavior of nanomagnetite‐benzoxazines has also been studied by differential scanning calorimetry (DSC). Magnetic and thermal properties of the cured samples were investigated. It was shown that the precursor nanomagnetite benzoxazine and cured samples exhibited typical ferromagnetic character with low coercivities between 1.5 and 2.5 Oe. The cured samples showed high thermal stability. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6780–6788, 2008

Preparation and cure behavior of organoclay‐modified allyl‐functional benzoxazine resin and the properties of their nanocomposites

AbstractA new type of polybenzoxazine‐clay nanocomposites were prepared by the in‐situ polymerization of allyl‐functional benzoxazine monomer, bis(3‐allyl‐3,4‐dihydro‐2H‐1,3‐benzoxazinyl)isopropane (B‐ala), in the presence of two different types of organoclay, allyldimethylstearylammonium‐montmorillonite and propyldimethylstearylammonium‐montmorillonite. The organoclays were mixed with molten B‐ala, followed by pouring into glass mold and then gradual curing up to 250°C. DSC and IR were used to follow the cure behavior of B‐ala in the presence of organoclay, indicating that organoclays catalyzed the ring opening of cyclic benzoxazine structure. The XRD of the nanocomposites showed featureless patterns, suggesting the exfoliation of the organoclay into the matrix. The viscoelastic properties of the hybrids showed that the glass transition temperatures (Tg) of the nanocomposites shifted to lower temperature in the presence of small amount of organoclay, but Tg started to increase with the increase of the organoclay content. This result suggests that, in the presence of organoclay, the curing reaction of ally and benzoxazine occurred in a different way, resulting in a different network structure. However, the presence of dispersed layered silicates into the matrix enhanced the thermal stability over the neat thermoset resin. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers

High‐molecular‐weight AB‐type benzoxazines as new precursors for high‐performance thermosets

AbstractNovel high‐molecular‐weight polybenzoxazine precursors, namely AB‐type benzoxazine precursors, were synthesized from aminophenols and formaldehyde. Both 1H NMR and IR confirmed the structure of the precursors, indicating the presence of a cyclic benzoxazine structure in the backbone of the precursors. The weight‐average molecular weight was estimated by size exclusion chromatography to be to in the range of 1300–4500. The precursors gave self‐standing thin films when their solutions were cast in dioxane over glass plates and dried, and upon a gradual thermal cure up to 250 °C, they afforded polybenzoxazine films. The viscoelastic analyses showed that the glass transition temperatures of the polybenzoxazine films obtained from these novel precursors were as high as 260–300 °C. Thermogravimetric analysis results indicated that the onset of decomposition and the char yield of the thermosets derived from these AB‐type precursors were higher than those of traditional polybenzoxazine. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1878–1888, 2007.

Preparation of conductive polybenzoxazines by oxidative polymerization

AbstractSoluble and thermally curable conducting high molecular weight polybenzoxazine precursors were prepared by oxidative polymerization 3‐phenyl‐3,4‐dihydro‐2H‐benzo[e][1,3] oxazine (P‐a) alone and in the presence of thiophene (Th) with ceric ammonium nitrate in acetonitrile. The structure of the precursors was confirmed by FTIR, 1H NMR, and DSC measurements, indicating the presence of a cyclic benzoxazine structure, together with small but varying amount of a ring opened phenolic structure. The resulting polymers exhibit conductivities around 10−2 S cm−1 and undergo thermal curing at various temperatures. Attempts to copolymerize P‐a with another electroactive monomer, pyrrole (Py), by a similar redox process were unsuccessful, which was attributed to the unfavourable oxidation potential of Py. The cured products exhibited high thermal stability but lower conductivity, than those of the precursors. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 999–1006, 2007

The curing reactions of ethynyl-functional benzoxazine

The model compounds 3-(3-ethynyl-phenyl)-6-methyl-3,4-dihydro-2H-benzoxazine (EPMB) and 6-methyl-3-phenyl-3,4-dihydro-2H-benzoxazine (PMB) were synthesized and the influence of ethynyl group on the thermal cure reaction of benzoxazine compound was investigated. The bond parameters of benzoxazine ring calculated at the level of B3LYP/6-31G(d) showed that the variations of benzoxazine structures due to the introduction of ethynyl were negligible. The cure reaction of EPMB and PMB was studied by DSC and kinetic parameters were determined by an autocatalytic model. The results revealed that the apparent activation energy for cure reaction of EPMB was 103.90 kJ/mol which was lower than that of PMB (124.14 kJ/mol). The kinetic parameters of cure reactions of PMB and EPMB obtained by the Kissinger and Ozawa methods agreed well with the results by the autocatalytic model. The structures of the cured EPMB were characterized by 1H NMR and solid-state 13C NMR. The results showed there are polyene structures and hydrogen bonds in the cured EPMB.

Mannich reaction of phenolphthalein and synthesis of a novel polybenzoxazine

Phenolphthalein (3,3-bis(4-hydroxyphenyl)-1(3H)-isobenzofuranone, B-hpi), reacted with paraformaldehyde and allylamine by the Mannich reaction, was investigated as the acid ingredient for the first time, and a novel benzoxazine was synthesized. In order to confirm the structure of benzoxazine, benzyl-terminated B-hpi (3,3-bis(4-benzyloxyphenyl)-1(3H)-isobenzofuranone, B-bpi) was prepared. The chemical structures of B-hpi, B-bpi and benzoxazine were compared by FT-IR, 1H-NMR and 13C-NMR; it was demonstrated that the lactone structure remains in the molecular structure of benzoxazine (3,3-bis(3-allyl-3,4-dihydro-2H-1,3-benzoxazine)-1(3H)-isobenzofuranone, B-adi). B-adi transformed into a quinoid structure in basic medium similar to that of B-hpi. Software of Accelrys' materials studio (DMol3), FT-IR and UV-Vis spectrophotometry were employed to investigate the properties of the quinoid structure of B-adi and B-hpi. In situ FT-IR spectra showed the thermal curing behavior of B-adi. The char yield of polyB-adi was about 40% by weight at 800°C in nitrogen atmosphere, as determined by TGA. T g values for this polyB-adi were as high as 273°C and 295°C from the maximum of loss modulus and tan δ by DMTA, respectively.

Synthesis and thermal cure of high molecular weight polybenzoxazine precursors and the properties of the thermosets

High molecular weight polybenzoxazine precursors have been synthesized from aromatic or aliphatic diamine and bisphenol-A with paraformaldehyde. The precursors were obtained as soluble white powder. Molecular weight was estimated from the size exclusion chromatography to be several thousands. The structure of the precursors was confirmed by IR, 1H NMR and elemental analysis, indicating the presence of cyclic benzoxazine structure. The ratio of the ring-closed benzoxazine structure and the ring-opened structure in the high molecular weight precursor was estimated from 1H NMR spectrum and also from the exotherm of DSC, showing that the ratio of the ring-closed benzoxazine structure was 77–98%. The precursor solution was cast on glass plate, giving transparent and self-standing precursor films, which was thermally cured up to 240 °C to give brown transparent polybenzoxazine films. The toughness of the crosslinked polybenzoxazine films from the high molecular weight precursors was greatly enhanced compared with the cured film from the typical low molecular weight monomer. Tensile measurement of the polybenzoxazine films revealed that polybenzoxazine from aromatic diamine exhibited the highest strength and modulus. While, polybenzoxazine from longer aliphatic diamine had higher elongation at break. The viscoelastic analyses showed that the glass transition temperature of the polybenzoxazines derived from the high molecular weight precursors were as high as 238–260 °C. Additionally, these novel polybenzoxazine thermosets showed excellent thermal stability.

The kinetics of B‐a and P‐a type copolybenzoxazine via the ring opening process

AbstractThe structure of benzoxazines is similar to that of phenolic resin through thermal self‐curing of the heterocyclic ring opening reaction that neither requires catalyst nor releases any condensation byproduct. These polybenzoxazine resins have several outstanding properties such as high thermal stability and high glass transition temperature. To better understand the curing kinetics of this copolybenzoxazine thermosetting resin, dynamic and isothermal differential scanning calorimetry measurements were performed. Three models, the Kissinger method, the Flynn–Wall–Osawa method, and the Kamal method, were used to describe the curing process. Dynamic kinetic activation energies based on Kissinger and Flynn–Wall–Osawa methods are 72.11 and 84.06 KJ/mol, respectively. The Kamal method based on an autocatalytic model results in a total order of reaction between 2.66 and 3.03, depending on curing temperature. Its activation energy and Arrhenius preexponential are 50.3 KJ/mol and 7959, respectively. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 730–737, 2005

The chemistry of novolac resins — V. Reactions of benzoxazine intermediates

As part of our study on the curing reactions between novolac resins and hexamethylenetetramine (HMTA), a model benzoxazine, 3-(3,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine, was heated under carefully controlled conditions, and the structural changes were studied by 13C and 15N n.m.r. spectroscopy. The benzoxazine structure is relatively stable, and detectable decomposition only occurred about 185°C with the formation of methylene linkages between phenolic rings. Various nitrogen-containing structures, such as amides, amines and imines, together with hydroxybenzyl alcohol, bis(ortho-hydroxybenzyl) ether, hydroxybenzaldehyde and ortho-hydroxybenzoic acid, are also formed as the side-products of the decomposition. The dominant product after heating the sample to 240°C is 2,2′-methylene-4,4′,6,6′-tetramethyldiphenol. These findings when applied to novolac/HMTA systems provide an explanation for reaction mechanisms/reaction pathways from the benzoxazine intermediates to the final cross-linking network. © 1997 Elsevier Science Ltd.

The chemistry of novolac resins - VI. Reactions between benzoxazine intermediates and model phenols

The reaction between 3-(,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine and either 2,4-xylenol or 2,6-xylenol was studied by 13C n.m.r. techniques, to model the reactions between benzoxazine intermediates and free ortho- and para-phenolic sites in the curing of novolac resins with hexamethylenetetramine (HMTA). The results indicate that 2,4-xylenol can directly react with benzoxazine intermediate at low temperature via several pathways to form methylene linkages between phenolic rings. 2,2′-Methylene-4,4′,6,6′-tetramethyldiphenol is the dominant product after heating benzoxazine and 2,4-xylenol to 205°C. However, no reaction occurs between the benzoxazine and 2,6-xylenol before the decomposition of the benzoxazine structure. At higher temperatures, 2,6-xylenol reacts with the decomposition species of the benzoxazine to form para-para and ortho-para methylene linkages between phenolic rings, together with ortho-ortho methylene linkages generated from the decomposition of the benzoxazine. Minor amounts of nitrogen-containing structures, such as amines, amides and imines, were also formed. The results when applied to novolac/HMTA systems provide possible reaction mechanisms and pathways from the benzoxazine intermediates and vacant ortho- and para-phenolic reactive sites leading to the final cross-linking network. © 1997 Elsevier Science Ltd.