Renewable Elastomers of Ethylene with Propylene Based on Triple Reversible Cross-Linking Sites

A six-coordination stable structure of Ni2+ with two carboxylate ligands and two sulfur atoms in side chains of ethylene-propylene copolymers can be constructed to form 3D physically cross-linked networks according to a density functional theory simulation. The reversibly cross-linked polar ethylene/propylene/5-ethylidene-2-norbornene copolymer elastomers (rcEPDMs) with triple noncovalent cross-linking sites of hydrogen bonds, ionic aggregation, and 6-coordination Ni2+ bonds can be successfully in situ prepared by the reaction of carboxyl functionalized EPDM with NiCl2·6H2O in the presence of tetrabutylammonium hydroxide. The rcEPDM networks with Ni2+ having a coordination number of 6 (rcEPDM-Ni2+) show much higher cross-linking density than their analogues with Zn2+ having a coordination number of 4 (rcEPDM-Zn2+), which is confirmed by swelling experiments and Mooney–Rivlin analysis. The rcEPDM-Ni2+ networks exhibit a much higher tensile strength of 19.0 MPa at an elongation of 587% than rcEPDM-Zn2+ networks (6.8 MPa at 777%). The mechanical properties of rcEPDM-Ni2+ networks containing triple cross-linking sites are superior to those of networks containing hydrogen and coordination bonds as well as those containing ionic aggregation and coordination bonds. The hysteresis energy of rcEPDM-Ni2+ networks increases by 19 times, and the strain recovery reaches 95% when the strain gradually increases from 50% to 400%, which is attributed to the synergy of noncovalent cross-linking sites. The hysteresis energy of rcEPDM-Ni2+ network is 66% higher than that of rcEPDM-Zn2+ networks at 400% strain, due to the formation of a more stable and dense six-coordination structure of rcEPDM-Ni2+ networks. To the best of our knowledge, this is the first example of renewable triple noncovalently cross-linked polar EPDM with high strength and outstanding strain recovery using Ni2+-carboxylate coordination bonds. The renewable cross-linked EPDM elastomers would have potential applications in organic–inorganic composite materials, bonding agents, sealing parts, and reusable elastomers.