Constitutive equations are derived for the mechanical behavior of rubbery polymers at finite strains. The model is based on the concept of rigid-rod networks, where breakage of chains is treated as bond scission. Adjustable parameters in the stress-strain relations are found by fitting observations in tensile tests for ethylene-octene copolymers. It is revealed that the constitutive equations correctly describe stress strain curves up to the break points. Young's modulus and the critical strength per bond monotonically decrease with temperature and increase with molecular weight.