Sterically hindered polyiminophosphazene base. Prepn: R. Schwesinger, H. Schlemper, Angew. Chem. Int. Ed. 26, 1167 (1987); and properties: R. Schwesinger et al., Ann. 1996, 1055. Use as promoter for ethylene oxide polymerization: B. Esswein et al., Macromol. Symp. 107, 331 (1996); for copolymer formation: S. F"orster, E. Kr"amer, Macromolecules 32, 2783 (1999).