In an example, a process includes polymerizing a mixture that includes an ethylene monomer, a propylene monomer, and a diene monomer to form an ethylene-propylene-diene (EPDM) terpolymer using ring-opening metathesis polymerization (ROMP). The process further includes chemically reacting the EPDM terpolymer with a norbornene-based phosphinate cross-linking material to form a flame-retardant, cross-linked EPDM rubber.