Question

Show the cationic poymerization of tetrahydrofuran, using phosphorous pentafluiride/water as catalyst.Prove, based on the mechanism, that phosphorous pentafluride/water is indeed a catalyst and not an initiator.

Answer 1

The cationic polymerization of tetrahydrofuran (THF) with the aid of catalysts was described by Meerwein et al. (Angew. Chem. 72 (1960), 927). Either preformed catalysts are used or the catalysts are produced in situ in the reaction mixture. This is done by producing oxonium ions in the reaction medium with the aid of strong Lewis acids, such as boron trichloride, aluminum chloride, tin tetrachloride, antimony pentachloride, iron(III) chloride or phosphorus pentafluoride, or by means of strong Bronstedt acids, such as perchloric acid, tetrafluoboric acid, fluorosulfonic acid, chlorosulfonlc acid, hexachlorostannic acid , iodic acid, hexachloroantimonic acid or tetrachloroferric acid, and with the aid of reactive compounds referred to as promoters, such as alkylene oxides, eg. ethylene oxide, propylene oxide, epichlorohydrin or butylene oxide, oxetanes, orthoesters, acetals, ?-haloethers, benzyl halides, triarylmethyl halides, acyl chlorides, ?-lactones, carboxylic anhydrides, thionyl chloride, phosphoryl chloride or halosulfonic acids, said oxonium ions initiating the polymerization of the THF. However, out of the large number of these catalyst systems, only a few have become important industrially since some of them are highly corrosive and/or, in the preparation of the PTHF, lead to discolored PTHF products of only limited use. In addition, many of these catalyst systems do not have a catalytic action in the strict sense but must be used in stoichiometric amounts, based on the macromolecule to be prepared, and are consumed during the polymerization. For example, in the preparation of PTHF with fluorosulfonic acid as the catalyst according to U.S. Pat. No. 3,358,042, two molecules of fluorosulfonic acid must be used as the catalyst per molecule of PTHF. A particular disadvantage of the use of halogen-containing catalyst systems is that they lead to the formation of halogenated byproducts in the preparation of PTHF and said byproducts are difficult to separate from the pure PTHF and have an adverse effect on its properties.

In the preparation of PTHF in the presence of the stated promoters, these promoters are incorporated as telogens in the PTHF molecule, so that the primary product of the THF polymerization is not PTHF but a PTHF derivative, for example a PTHF diester or sulfonate, from which PTHF must be liberated in a further reaction, for example by hydrolysis or transesterification (cf. U.S. Pat. No. 2,499,725 and DE-A 27 60 272). When alkylene oxides are used as promoters, they also act as comonomers and are incorporated into the polymer, with the result that THF/alkylene oxide copolymers having different properties, in particular performance characteristics differing from those of PTHF, are formed