A new polymer composition comprising (i) a polymer having a main polymer chain derived from (ia) at least 30% by weight to 100% by weight based on the polymer of a first monomer which introduces at least one of a secondary carbon and a tertiary carbon to the main polymer chain, and (ib) in the range of from 0 to 70% by weight based on the polymer of at least one other monomer; (ii) at least one lithium salt; and (iii) at least one antioxidant is provided, as well as a process for producing a polymer vulcanizate comprising the step of vulcanizing the polymer composition mentioned above additionally comprising a vulcanization system. A method for improving the hot air aging and aqueous fluid resistance characteristics of the polymer mentioned above comprises the step of admixing said polymer with at least one lithium salt and at least one antioxidant is also provided.