The synthesis of ultra-high-molecular-weight (UHMW) ethylene–propylene (EP) polar elastomers with high functional degree represents a great challenge. The modification of UHMW EP–5-ethylidene-2-norbornene (ENB) terpolymers (EPDM) with a high content of ENB structural units provides a viable way. The synthesis of UHMW EPDM with a high content of ENB structural units is critical for the preparation of UHMW polar EP elastomers with high functional degree. Herein

the copolymerization of ethylene with propylene and the terpolymerization of ethylene and propylene with ENB were investigated by using N-heterocyclic carbene vanadium complex NHC·VOCl3 (PhCH2NCH=CHN(2

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6-Me3C6H2R)C:VOCl3). The UHMW EPDM (Mw = 1059–1586 kg·mol−1) were successfully prepared with high catalytic activity of ≤2.65 × 105 g polymer·mol−1 of V·h−1. The content of ENB structural units in resulting EPDMs can be increased to 12.5 mol% (35.4 wt%) by increasing the ENB concentration in monomer feed. The epoxidation of UHMW EPDM with m-chloroperbenzoic acid in tetrahydrofuran results in an efficient conversion of C=C double bonds in the ENB structural units into epoxy groups. The functionalized EP elastomers containing epoxy groups of ≤11.2 mol% have been successfully synthesized. The epoxy groups in the functionalized EP elastomers can be successfully and quantitatively converted into carbonyl groups

affording functionalized EP elastomers with a high content of norcamphor structural units of ≤11.2 mol%. The UHMW EPDM with a high content of polar epoxy or carbonyl groups can behave with good compatibility with inorganic fillers

which is a benefit for the preparation of high-performance composites of UHMW EPDM with inorganic fillers.