Coordination copolymerization of tetrahydrofuran and oxepane with oxetanes and epoxides

Abstract

Abstract The chelate catalyst, as typified by the Et 3 Al‐0.5 H 2 O‐0.5 acetylacetone product, usually prepared with Et 2 O or tetrahydrofuran (THF) present, has all the known characteristics of a coordination catalyst for polymerizing epoxides and uniquely for oxetanes. We have found that the chelate catalyst gives fairly good copolymerization of THF (54% in monomer charge) with 3‐(trimethylsilyloxy) oxetane which, after hydrolysis, is a water‐soluble, moderate molecular weight copolymer of THF (36%) with 3‐hydroxyoxetane (HO). This apparent coordination copolymerization of THF has been extended to trimethylene oxide (TMO), 3,3‐bis(trimethylsilyoxymethyl) oxetane, 3,3‐bis(chloromethyl)oxetane (BCMO), trans ‐2,3‐epoxybutane (TBO), and propylene oxide, listed in order of decreasing copolymerizability with THF. Presumably, this is the first known coordination copolymerization of THF which hitherto has only been polymerized with cationic catalysts. Oxepane also copolymerizes coordinately with TMO and BCMO, but less readily than THF, with the chelate catalyst. TBO polymerizes slowly with the chelate catalyst to form stereoregular polymer which can be separated into an acetone‐insoluble, highly stereoregular fraction and an acetone‐soluble, somewhat less stereoregular fraction. The soluble fraction can be eliminated by using 1.0 acetyl acetone per Al in the catalyst or by adding a small amount of a very strong base (0.09 quinuclidine per Al). The copolymerization of TBO with THF (39%) gives insoluble stereoregular homopolymer and soluble copolymer containing about 23% THF, reflecting the varied steric hindrance of the sites. Some anomalous results appear to be related to the mechanism: (1) steric and electronic factors of the monomers and of the polymerization site. For example, the fourth coordination position of Al is needed to achieve homopolymerization of BCMO and TMO‐THF copolymerization. (2) The aggregation state of the catalyst, since a nonpolar diluent as toluene is unfavorable for coordination copolymerization of THF. (3) The greater ring strain of epoxides causes a greater ease of polymerization, compared to oxetanes. Thus, Et 2 O often present in the chelate catalyst lowers the molecular weight of the polymer considerably with oxetanes compared to epoxides where Et 2 O has little or no effect.