New poly(ester-siloxane)urethane structures, which exhibit the alternation of the flexible and hard segments in the polymer chain, using the 'prepolymer' polyaddition procedure were obtained. These networks were analyzed on the basis of molecular theory of rubber elasticity from stress-strain data. The cumulated effect of the physical and chemical crosslinkings induced by a tetrafunctional compound was pointed out by the average critical molecular weight of the linear segments existing among the junctions of copolymer networks. The correlation between the structure of soft and hard segments and tensile properties, density of chemical crosslinks and the hardness of these polymers was established.