To obtain a product in a simple manner without containing an impurity such as an ionic side-product, by esterifying an amic acid having a constituent unit having a specific type of structure as recurring units with an orthocarboxylic acid ester.
An amic acid having, recurring units, the structural unit of formula I is dissolved in an aprotic polar solvent such as N,N- dimethylforrnamide or the like, to which an orthocarboxylic acid ester of formula II is added for reaction in an amount of 0.5 to 10 equivalents relative to the carboxyl groups of the amic acid, thereby obtaining an amic acid ester. The amic acid is prepared by reacting, in an aprotic polar solvent, 1 mole of an aromatic tetracarboxylic acid dianhydride with 0.5 to 2 moles of an aromatic diamine. In the formulas, R represents a tetravalent aromatic group represented by formula III or the like and containing a six-membered carbon ring; four carboxyl groups are directly bonded to different carbon atoms, respectively; R' represents a divalent aromatic residue having a six-membered carbon ring; R" represents hydrogen or a 1-5C alkyl group; and R"' represents a 1-5C alkyl group.
IMAMURA SHIGEKI
KISHIMOTO SOICHIRO
ECHIGO YOSHIAKI
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