The dependence of physical and mechanical properties of oligoether-based foam polyurethanes on the molecular mass (Me) of polymer chains between the nodes of the polymer network and on the content of rigid segments in theRigid foam polyurethanes (FPUs) and their different modifications are efficient insulants in a wide interval of operating temperatures. They are widely used either at elevated temperatures as a heat insulation or at low temperatures as a cryogenic insulation of tanks for condensed gases [1][2][3][4][5][6][7].In order to find the upper bound of operating temperatures for foam plastics, the heat resistance, shape stability, and other characteristics are used. At the same time, unfortunately, no similar conventional characteristics for obtaining the lower bound of operating temperatures for foam plastics are known to date. The possibility of using a certain material as a cryogenic insulation is generally judged by the mechanical properties of the material in tension and the coefficient of linear thermal expansion at a respective temperature. The necessity of determining just these properties is dictated by the results of analyzing the distribution of temperattires, stresses, and strains in the layer of external insulation during freezing and thawing tanks. A similar analysis is, in particular, performed in [ 1 ], where it is shown that, upon freezing, in the foam plastic layer contacting with the wall of the tank, large tensile stresses arise due to a difference in the coefficients oftbermal expansion, which are responsible for cracking of the insulation while in service. The value of these stresses depends not only on the final temperature, but also on the rate of cooling. Meanwhile, the compressive stresses arising in the external layer of foam plastics are not as dangerous.The majority of studies reporting on the successful application of FPUs of different types at low temperatures generally present the mechanical characteristics and the coefficients of thermal conductivity and linear thermal expansion at the indicated temperatures, as well as the variation of these indices during aging and after cyclic temperature loads [1][2][3][4][5][6]. However, for known reasons, the complete composition and structure of FPU, as a rule, are not given in these studies. Only in [7] it was reported that, in developing materials resistant to cryogenic temperatures, the key factor is the type of rigid and soft segments in the structure of FPU and their mutual ratio. Therefore, the purpose of the present study is to estimate the effect of the chemical structure of a polymer matrix on the properties of FPU at low temperatures.As is known [8,9], the properties of rigid FPUs, which are cross-linked polymers, are determined by the concentration of urethane groups in the polymer chains, their flexibility, and the frequency of their cross-linking. Urethane groups and the residues Latvian State Institute of Wood Chemistry, Riga, LV-IO06, Latvia.