Transition metal-catalyzed polymerization of 1,3,5-trioxane

The molybdenum complexes [(η5-C5Me5)Mo(CO)3OTf] (1a), [(η5-C5H5)Mo(CO)3X] (2a–2c) (X=F3CSO3 (a), F3CCO2 (b), BF4 (c)), and [(η5-C5H4CO2CH3)Mo(CO)3OTf] (3a) catalyze the cationic ring-opening polymerization (ROP) of 1,3,5-trioxane to polyoxymethylene (POM, 5). Hitherto unknown 3a is accessible by treatment of [(η5-C5H4CO2CH3)Mo(CO)3CH3] with CF3SO3H. The precipitation time of the polymerization, which is defined in the following as the time until precipitation starts, is dependent on the Lewis acidity of the transition metal center and the presence of formaldehyde or water. 13C-labeled formaldehyde was copolymerized with 1,3,5-trioxane and randomly incorporated into POM. This observation supports the conception that formaldehyde is reversibly formed during the polymerization. Catalyst 2a was successfully applied in the ROP of trioxane even in the presence of up to 3.6 mol% of water. Time-dependent 1H-NMR spectroscopic investigations of the trioxane polymerization revealed the formation of methoxymethyl formate (4) as a by-product, an isomer of trioxane. In the presence of 2a as a catalyst, 4 was degraded to methyl formate (6) and POM (5) in a ratio of about 2:1. This degradation was monitored by 1H-NMR spectroscopy. Catalyst 2a is also able to copolymerize 1,3,5-trioxane with 1,3-dioxepane leading to a thermally stable copolymer after treatment with an aqueous solution of Na2CO3.