Abstract: One shot mouldable and curable homogeneous rubber composition consisting mainly of polyisoprene rubber and a monomer mixture composed of divinylbenzene and .alpha.-methylstyrene in specifically limited weight ratios of the rubber/the monomer mixture and of divinylbenzene/.alpha.-methylstyrene and containing small amounts of organic peroxide and at least one of amine compound and phenol compound. The cured product of the rubber composition has high Young's modulus, tear energy and tensile strength retention. More improved cured rubbers having these physical properties well balanced in a high level can be easily obtained by additionally adding a small amount of sulfur or p-quinonedioxime to the above rubber composition.

Abstract: An ethylene-propylene copolymer rubber composition having a high Young's modulus, which comprises 100 parts by weight of a mixture of the following ingredients (a) and (b) in a weight ratio of ingredient (a)/ingredient (b) of 67/33-50/50, said ingredient (a) being an ethylene-propylene copolymer having a Mooney viscosity of at least 75, said ingredient (b) being divinylbenzene, (c) 0.3-2.5 parts by weight of an organic peroxide, and (d) 0.01-0.25 part by weight of sulfur, and the homogeneous rubber composition cured at 120.degree.-180.degree.C having a Young's modulus of 200-500 Kg/cm.sup.2, a tensile strength of at least 200 Kg/cm.sup.2, an elongation of at least 300%, a tear energy of at least 10 Kg/cm and retentions of Young's modulus and tensile strength at 70.degree.C of at least 50%.