TECHNOLOGY FOCUS - ELECTRONICS - Preferred Membrane: The electrolyte membrane further comprises a proton conductor. Preferred Method: In the method (M1) and (M2), the curing the mixture is performed at 50-250degreesC. The forming the membrane involves: tape casting the mixture of the first benzoxazine monomer, the second benzoxazine monomer, and the crosslinkable compound; casting the mixture of the first benzoxazine monomer, the second benzoxazine monomer, and the crosslinkable compound on a support. The method (M2) further involves: removing the cured resultant from the support. Preferred Components: In the method (M1) and (M2), the amount of the proton conductor is 100-1000 pbw.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: Preferred Components: The proton conductor comprises at least one of a phosphoric acid and 1-10C alkyl phosphoric acid.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The second benzoxazine monomer is present in an amount of 0.5-50 pbw. Preferred Compound: 5 Second benzoxazine monomers (II) is given in the specification e.g. a compound of formula (IIa) or (IIb).R2a=T2;T2=-C(CH3)2-, -C(CF3)2-, -C(=O)-, -SO2-, -CH2-, -C(CCl3)-, -CH(CH3)-, -CH(CF3)- or group of formula (i);R4=T3;T3=phenyl, -CH2-CH=CH2, phenyl-1-yl, -CH2CH2CH2-1H-imidazol-1-yl, 1H-benzoimidazol-2-yl, 3-trifluoromethyl-benzen-1-yl, 1-fluoro-benzen-4-yl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl, or pyrimidin-2-yl    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The crosslinkable compound (10-90 pbw), comprises at least one compound selected from polybenzimidazole, polybenzthiazole, polybenzoxazole, and polyimide. In the method (M1), the crosslinkable compound is present in an amount of 5-95 pbw. Preferred Definitions:    R1=1-10C alkyl, allyl, 6-20C aryl, tert-butyl, 2-10C alkenyl or 2-10C alkynyl;    R2=T3;    R3=T2.    SPECIFIC COMPOUNDS - 10 Compounds are specifically claimed as the compound (I) e.g. 3-phenyl-3,4-dihydro-2H-benzo(e)(1,3)oxazine (IA); 6-tert-butyl-3-phenyl-3,4-dihydro-2H-benzo(e)(1,3)oxazine; 6-tert-butyl-3-(3-imidazol-1-yl-propyl)-3,4-dihydro-2H-benzo(e)(1,3)oxazine; 3-(1H-benzoimidazol-2-yl)-6-tert-butyl-3,4-dihydro-2H-benzo(e)(1,3)oxazine; and 6-tert-butyl-3-(3-trifluoromethyl-phenyl)-3,4-dihydro-2H-benzo(e)(1,3)oxazine.    EXAMPLE - tertiary Butylphenol (1 mol), p-formaldehyde (2.2 mol), and aniline (1.1 mol) were mixed and stirred without a solvent at 100degreesC for 1 hour to produce a crude product. The crude product was washed twice with a 1N NaOH aqueous solution and once with distilled water. The washed crude product was then dried with magnesium sulfate. Then, the resultant was filtered. The resultant was dried in a vacuum to obtain 6-tert-Butyl-3-phenyl-3,4-dihydro-2H-benzo(e)(1,3)oxazine as benzoxazine monomer (A1). Benzoxazine monomer of the formula (IIa) (where R2a is phenyl) was prepared as follows: 4,4'-hexafluoroisopropylidene diphenol (4,4'-HFIDPH) (1 mol), p-formaldehyde (4.4 mol), and benzene (2.2 mol) were mixed and stirred without a solvent at 100degreesC for 1 hour to produce a crude product. The crude product was washed twice with 1N NaOH aqueous solution and once with distilled water. The washed crude product was dried with magnesium sulfate. Then, the resultant was filtered. The resultant was dried in a vacuum to obtain the benzoxazine monomer (IIa) (where R2a is phenyl) (A2). Electrolyte membrane was prepared as follows: (A1) (60 pbw), (A2) (3 pbw) and polybenzimidazole (PBI) (37 pbw) were blended, and heated to 220degreesC at a heating rate of 20degreesC/hour. Then the mixture was cured at the same temperature to synthesize a crosslinked material of polybenzoxazines. The crosslinked material of polybenzoxazines was impregnated with 85 wt.% of phosphoric acid at 80degreesC for about 2.5 hours to prepare an electrolyte membrane. The amount of the phosphoric acid was 500 pbw. PBI electrode was prepared by preparing a catalyst layer slurry using PBI and polyvinylidene difluoride (PVDF) as binder. The catalyst layer slurry was coated on carbon paper coated with a microporous layer using a bar coater. The amount of the loaded platinum in the prepared electrode was 1.0-1.4 mg/cm2. When a Pt-Co alloy, which was known to improve catalyst activity by preventing phosphoric acid anion adsorption, was used as a catalyst, polyvinylidene fluoride was used as a binder to prepare a catalyst ink. The catalyst ink was coated on carbon paper coated with a microporous layer using a bar coater. The amount of the loaded platinum in the prepared electrode was 2.2-3.5 mg/cm2. The crosslinked polybenzoxazines impregnated with phosphoric acid were disposed between the electrodes to produce a membrane electrode assembly (MEA). The MEA was then disposed between two bipolar plates to form a fuel cell.