Alkylaluminophosphonate-catalyzed ring-opening homopolymerization of epichlorohydrin and propylene oxide

Reactions of an aluminum alkyl, AlR3 (R=Me, Et, iBu, tBu), with a phosphonic acid, R′P(O)(OH)2 (R′=H, Me, tBu, Ph), in molar ratios of 3:1, 2:1, and 1:1 were conducted in toluene–THF. Resulting aluminophosphonate solutions were screened for catalytic activity for the ring-opening homopolymerization of epichlorohydrin (ECH) and propylene oxide (PO). A highly active catalyst solution, 3a, was obtained from reaction of tBu3Al with MeP(O)(OH)2 in a 2:1 molar ratio in toluene–THF. A more active catalyst, 3b, was obtained by removing volatiles, specifically THF, from solutions of 3a and reconstituting with toluene. Catalyst 3a polymerized ECH at 60°C in 1–3 h to an elastomer with molecular weight, Mn, of 103 000 and polydispersity, Mw/Mn, of 1.9. Catalyst 3b yielded poly-ECH with Mn of 130 000 and Mw/Mn of 1.9 within 15 min. Catalysts 3a and 3b polymerized PO to oily oligomers with Mn of 3588 and 4046 and Mw/Mn of 1.1 and 1.2, respectively. Known dimeric and tetrameric aluminophosphonates such as [tBu2AlO2P(OSiMe3)Ph]2 and [RAlO3PR′]4 (R=Me, R′=Me; R=tBu, R′=Me, Ph) do not account for the observed activity of the catalyst solutions. Comparisons of catalytic activity with that for well-defined catalysts and a structural analogy of alkylaluminophosphonates to cyclic and cage tert-butylaluminoxanes are presented.