Ring-Opening Polymerizations of 1,3-Dehydroadamantanes: Synthesis of Novel Thermally Stable Poly(1,3-adamantane)s

Abstract

Ring-opening polymerization of 1,3-dehydroadamantanes (DHAs) such as 1,3-dehydroadamantane (1), 5-butyl-1,3-dehydroadamantane (2), and 5,7-dibutyl-1,3-dehydroadamantane (3) was carried out under various conditions. Poly(1,3-adamantane)s were quantitatively obtained by thermal polymerization of DHAs in bulk. Radical polymerization of DHAs with α,α′-azobis(isobutyronitrile) (AIBN) proceeded in bulk at 80 °C. By contrast, virtually no reactions of DHAs occurred with n-BuLi in n-heptane and phenylmagnesium chloride in tetrahydrofuran (THF), indicating their negative anionic polymerizability. On the other hand, cationic ring-opening polymerization of DHAs smoothly proceeded with super Brønsted acids such as trifluoromethanesulfonic acid (TfOH) or trifluoromethanesulfonimide (Tf2NH) to afford corresponding polymers in good to quantitative yield. Although Tf2NH induced rapid polymerization of DHAs even at −78 °C, higher temperatures such as 0 or 30 °C were necessary to initiate polymerization with TfOH. In the presence of Lewis bases such as ethyl acetate, diethyl ether, or THF, retardation of cationic polymerization was apparently observed, indicating nucleophilic interaction of added bases toward a propagating 1-adamantyl cation derived from DHAs. The resulting poly(2) bearing a flexible n-butyl group was soluble in various solvents, while highly symmetrical poly(1) and poly(3) showed very poor solubility. The Mn value of the cationically obtained poly(2) reached 6900, and the degree of polymerization was estimated to be greater than 30. The resulting poly(1,3-adamantane)s showed high thermal stability in thermogravimetric analysis (TGA), and the 10% weight loss temperatures of poly(1), poly(2), and poly(3) were respectively 480, 488, and 477 °C. Poly(2) and poly(3) exhibited a glass transition temperature (Tg) at 205 and 139 °C, respectively, in differential scanning calorimetry (DSC) measurements.