Nippon Oil Fats Co., Ltd.

Minoru Yasuda

Masayuki Takemoto

Osamu Nagura

Yukio Nomura

A premix, an origomeric prepolymer obtainable from a limited addition polymerization effected with the premix and a thermosetting resin obtainable from such premix or prepolymer as well as fiber reinforced plastics made of such thermosetting resin which are excellent in the heat resistance, impact resistance and flexibility and exhibit a high toughness, wherein said premix comprises, as a first essential component, an amine-terminated amic acid expressed by the formula, ##STR1## in which R.sup.1 stands for a tetravalent aromatic residue having 6 to 20 carbon atoms and R.sup.2 denotes a divalent organic residue having 2 to 20 carbon atoms, said amic acid being produced by reacting a tetracarboxylic dianhydride with a diamine compound, and, as a second essential component, an N,N'-bis maleimide of the formula, ##STR2## wherein R.sup.3 denotes a divalent organic residue having 2 to 20 carbon atoms, the mole ratio of [N,N'-bismaleimide]/[amine-terminated amic acid] being in the range from 1 to 3.

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Yokohama, Japan

Minoru Yasuda