September 4. 2017 - HNBR is an elastomer with excellent heat and oil resistance, outstanding mechanical properties at elevated temperatures and good low temperature flexibility. Nevertheless, there is still a demand for further reduction in Tg while maintaining excellent media resistance. These modifications are not achievable with copolymers, which implies the application of terpolymers. While for many potential termonomers, the Tg of the homopolymer is reported in literature, values for their volume swell are difficult to find. In this paper, a model calculation based on solubility parameters is described. Based on this model, a new terpolymer generation could be synthesized with substantially improved low temperature flexibility while maintaining excellent media resistance.