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

✍ Scribed by Yuxia Liu; Charles U. Pittman Jr.

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 304 KB
• Volume: 35
• Category: Article
• ISSN: 0887-624X
• DOI: 10.1002/(sici)1099-0518(199712)35:17<3655::aid-pola5>3.0.co;2-r

✦ Synopsis

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 1,2addition 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 (T c ) 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 138ЊC, 2-methylene-1,3dioxane, 2a, underwent 1,2-addition polymerization. Insoluble poly(2-methylene-1,3-dioxane) 100% 1,2-addition was obtained. At above 150ЊC, 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 30ЊC. At 67-80ЊC, an insoluble polymer was obtained, which contained mostly 1,2-addition units but small amounts of ester units were detected. At 133ЊC, a soluble polymer was obtained containing a substantial fraction of ring-opened ester units together with 1,2addition cyclic ketal units. 2-Methylene-1,3-dioxepane, 3a, underwent partial ROP even at 20ЊC to give a soluble polymer containing ring-opened ester units and 1,2-addition cyclic ketal units. At 020ЊC, 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 RuCl 2 (PPh 3 ) 3 , BF 3 , TiCl 4 , H 2 SO 4 , H 2 SO 4 supported on carbon, (CH 3 ) 2 CHCOOH, and CH 3 COOH. The initiation by Lewis acids or protonic acids probably occurs through an initial protonation. The propagation step of the ROP proceeds via an S N 2 mechanism. The chain transfer and termination rates become faster at high temperatures, and this may be the primary reason for the low molecular weights (M n °10 3 ) observed for all ringopening polymers. The effects of temperature, monomer and initiator concentration, water content, and polymerization time on the polymer structure have been investigated during the Ru(PPh 3 ) 3 Cl 2 -initiated polymerization of 2a. High monomer concentrations ([M]/[ln]) increase the molecular weight and decreased the amount of ring-opening. Higher initiator concentrations (Ru(PPh 3 ) 3 Cl 2 ) and longer reaction times increase molecular weight in high temperature reactions. Successful copolymerization of 2a with hexamethylcyclotrisiloxane was initiated by BF 3 OEt 2 . The copolymer obtained displayed a broad molecular weight distribution; M V n Å 6,490, M V w Å 15,100, M V z Å 44,900. This polymer had about 47 mol % of ({Me 2 SiO{) units, 35 mol % of ring-opened units, and 18 mol % 1,2-addition units of 2a.