Theoretical ratios of important kinetic characteristics for brittle polymers with cracks under mechanical and thermal effects are obtained, which underlie the study of the thermokinetics of the process of polymer fracture in terms of the theory of the time dependence of strength-durability. Rectilinear (internal and surface) cracks in plate-type specimens and internal circular (disk-shaped) cracks in polymer fibers are considered. Two test modes are considered successively: constant tensile stress, constant absolute temperature, unchanging structure, inactive medium, as well as a more complex mode of purely thermal loading - the case least developed in the theory of destruction. The calculated ratios of a number of limiting characteristics and parameters of the destruction process are given: safe and critical stress; initial length of a microcrack and its relative critical length; safe and critical voltage; local stress at the crack tip (in the fluctuation volume); the value of the free surface energy. The above relations are the basis for the development of the theory of the time dependence of strength-durability.