Thermosettable prepolymers or resins are prepared by reacting (1) a pyrazine compound containing at least two substituent groups which have a hydrogen atom attached to a carbon atom which is attached to the ring such as tetramethylpyrazine, (2) a material containing at least two aromatic aldehyde groups, and (3) at least one of (a) a nitrogen containing aromatic heterocyclic compound containing at least one hydrogen atom attached to a carbon atom attached to the ring and a polymerizable unsaturated group such as 2-methyl-5-vinylpyridine or (b) an aldehyde containing at least one polymerizable unsaturated group such as 4-isopropenyl-1-cyclohexene-1-carboxaldehyde. These prepolymers or resins are curable to thermoset products having good mechanical and thermal properties by heat and pressure or by homopolymerization or copolymerization with N,N'-bis-imides such as 1,1'-(methylenedi-4,1-phenylene)bismaleimide. The prepolymers are suitable for preparing composites.