Cationic ring-opening polymerizations of cyclic ketene acetals initiated by acids at high temperatures

Abstract

Three unsubstituted cyclic ketene acetals (CKAs), 2-methylene-1,3-dioxolane, 1a, 2-methylene-1,3-dioxane, 2a, and 2-methylene-1,3-dioxepane, 3a, undergo exclusive 1,2-addition polymerization at low temperatures, and only poly(CKAs) are obtained. At higher temperatures, ring-opening polymerization (ROP) can be dominant, and polymers with a mixture of ester units and cyclic ketal units are obtained. When the temperature is raised closer to the ceiling temperature (Tc) of the 1,2-addition propagation reaction, 1,2-addition polymerization becomes reversible and ring-opened units are introduced to the polymer. The ceiling temperature of 1,2-addition polymerization varies with the ring size of the CKAs (lowest for 3a, highest for 2a). At temperatures below 138"C, 2-methylene-1,3-dioxane, 2a, underwent 1,2-addition polymerization. Insoluble poly(2-methylene-1,3-dioxane) 100% 1,2-addition was obtained. At above 150°C, a soluble polymer was obtained containing a mixture of ring-opened ester units and 1,2-addition cyclic ketal units. 2-Methylene-1,3-dioxolane, 1a, polymerized only by the 1,2-addition route at temperatures below 30°C At 67-80°C an insoluble polymer was obtained, which contained mostly 1,2-addition units but small amounts of ester units were detected. At 133°C, a soluble polymer was obtained containing a substantial fraction of ring-opened ester units together with 1,2-addition cyclic ketal units. 2-Methylene-1,3-dioxepane, 3a, underwent partial ROP even at 20°C to give a soluble polymer containing ring-opened ester units and 1,2-addition cyclic ketal units At -20°C, 3a gave an insoluble polymer with 1,2-addition units exclusively. Several catalysts were able to initiate the ROP of 1a, 2a, and 3a, including RuCl2(PPh3)3, BF3, TiCl4, H2SO4, H2SO4 supported on carbon, (CH3)2CHCOOH, and CH3COOH. The initiation by Lewis acids or protonic acids probably occurs through an initial protonation. The propagation step of the ROP proceeds via an SN2 mechanism. The chain transfer and termination rates become faster at high temperatures, and this may be the primary reason for the low molecular weights (Mn ≤ 103) observed for all ring-opening polymers. The effects of temperature, monomer and initiator concentration, water content, and polymerization time on the polymer structure have been investigated during the Ru(PPh3)3Cl2-initiated polymerization of 2a. High monomer concentrations ([M]/[ln]) increase the molecular weight and decreased the amount of ring-opening. Higher initiator concentrations (Ru(PPh3)3Cl2) and longer reaction times increase molecular weight in high temperature reactions. Successful copolymerization of 2a with hexamethylcyclotnsiloxane was initiated by BF3OEt2. The copolymer obtained displayed a broad molecular weight distribution; M̄n = 6,490, M̄w = 15,100, M̄z = 44,900. This polymer had about 47 mol % of (-Me2SiO-) units, 35 mol % of ring-opened units, and 18 mol % 1,2-addition units of 2a. © 1997 John Wiley & Sons, Inc.