New 2,5-norbornadiene-type monomers bearing 1-adamantyl and cyclohexyl ester groups on their 2-position polymerized with azobisisobutyronitrile to form the polymers consisting of two types of polymer unit structures. The major part had a saturated nortricyclene framework, which was formed by 2,6-addition along with intramolecular cyclization on the norbornadiene moiety. The minor part consisted of 2-norbornene-type units constructed via 2,3-addition. A series of norbornadiene-based monomers spontaneously polymerized in the presence of oxygen. Because a radical inhibitor, namely hydroquinone, could suppress this spontaneous reaction, it was indicated that the oxygen-induced polymerization proceeds via free-radical polymerization mechanism. Changing a quantity of provided oxygen gas (O-2) to a norbornadiene monomer significantly affected on polymerization results, in specific, molecular weight of the formed polymer, which indicated that oxygen serves as one of the key reagents for the formation of free-radical initiating species. It was proven that the combination of norbornadiene ethyl ester with O-2 was applicable as a new free-radical initiator for polymerization of methyl methacrylate. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2528-2536