AbstractHighcis‐1,4 ‐polybutadiene has been used to prepare toughened polystyrene in an attempt to improve its low temperature impact properties. A range of physical and mechanical properties was obtained by keeping the amount of rubber and the polymerization conditions constant, and varying the rate of agitation in a purpose ‐built reactor system. Although a good balance of tensile and impact properties is obtained at room temperature, the rubber partially crystallizes when the polyblends are cooled to below −40°C. This should decrease the efficiency of rubber particles to create and terminate crazes. However, it is significant that the developed crystallinity decreases with the rubber phase volume, and is suppressed almost completely at about 21 rubber phase volume (RPV). The factors influencing the RPV are discussed, and a study of the phase inversion with three different types of rubber shows that its duration depends on the viscosity of the styrene/rubbe
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