An important new class of polymers, the thermoplastic elastomers, was announced by the Shell Chemical Company (U.S.A.) in 1965. These new products may be formed into useful articles by modern rapid thermoplastics processing techniques, such as injection molding, and without any chemical vulcanization step provide most of the useful physical properties of vulcanized rubber. High resilience, high tensile strength, highly reversible elongation, and abrasion resistance are obtained. The thermoplastic elastomers consist of ordered, block copolymers of the general structure A‐B‐A, where A is a thermoplastic block polymer and B is an elastomeric block polymer. Choice of monomers, block length, and the weight fractions of A and B are crucial in achieving elastomeric performance. An example is the polystyrene–polybutadiene–polystyrene block copolymer (S‐B‐S) where the molecular weights of S and B and the weight fraction of S are restricted. A two‐phase system is formed, with the middle‐block phase constituting a continuous three‐dimensional elastomeric network and the dispersed end‐block phase serving as multijunction points for the ends of the middle blocks. These systems, without vulcanization, have rubber‐like properties similar to those of conventional rubber vulcanizates but flow as thermoplastics at temperatures above the glass transition of the end block. The behavior is fully temperature reversible. Melt viscosity behavior, measured as a function of shear and temperature, is similar to that of conventional thermoplastics. Activation energies obtained at constant shear stress vary with temperature: at high temperatures they are between those of the pure homopolymers and at low temperatures they approach that of the thermoplastic part of the molecule. Melt viscosities, however, are very much higher than those of either homopolymer of the same total molecular weight. An additional energy term is indicated in the flow process which arises from the transfer of the end block from one aggregate to another, in the process being forced to pass through the elastomeric matrix with which it is thermodynamically incompatible. The classical kinetic theory of rubber elasticity can be applied to these polymers, treating the end‐block phase as hard discrete particles which do not contribute to the elastic network For example, equilibrium modulus or swelling measurements are used to calculate the concentration of elastically effective chains or effective crosslink density. The resulting effective elastic chain length (Mc) is thus identified, not with the middle‐block segment length, but with the normal entanglement length. Hence, normal entanglement junctions in the elastomeric matrix behave as strong effective crosslinks because the ends of the middle block are securely anchored in the end‐block aggregates. High tensile strengths, in the absence of reinforcing fillers or crystallization, may be attributed to a highly perfected network (Case theory) or to the inertial masses of the discrete end‐block aggregates (Bueche the...