The recent results on synthesis of polyimides containing highly flexib
le and highly rigid chains are surveyed. The principles of the formati
on of such polymers, their properties, and some aspects of their pract
ical use are considered. The value of the thermodynamic stiffness of m
acromolecules is shown to be one of the main factors determining the i
mportant properties of polyimides and of the materials derived. from t
hem.