Mono- and di-acylphosphine oxides

A compound of the formula I ##STR16## in which R.sub.1 is C.sub.1 -C.sub.18 alkyl, C.sub.2 -C.sub.18 alkenyl, C.sub.1 -C.sub.8 alkyl which is monosubstituted or polysubstituted by phenyl, (C.sub.1 -C.sub.12 alkyl)-phenyl, halogenophenyl, (C.sub.1 -C.sub.12 alkoxy)-phenyl, cyano, C.sub.2 -C.sub.5 alkoxycarbonyl, C.sub.1 -C.sub.12 alkoxy or/and halogen, C.sub.5 -C.sub.8 cycloalkyl, C.sub.6 -C.sub.12 aryl, which is unsubstituted or mono- or poly-substituted by halogen, C.sub.1 -C.sub.12 alkyl or/and C.sub.1 -C.sub.12 alkoxy, a 5-membered or 6-membered heterocyclic monovalent radical which contains one or more O, S or/and N atoms and which may contain a fused benzo radical or/and which is unsubstituted or substituted by C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy or/and halogen, or is a radical of the formula ##STR17## in which X is phenylene, xylylene, cyclohexylene or C.sub.1 -C.sub.6 alkylene which may be unsubstituted or monosubstituted or polysubstituted by halogen, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy or phenyl, R.sub.2 is C.sub.1 -C.sub.18 alkyl, C.sub.2 -C.sub.18 alkenyl, C.sub.1 -C.sub.8 alkyl which is monosubstituted or polysubstituted by phenyl, (C.sub.1 -C.sub.12 alkyl)-phenyl, halogenophenyl, (C.sub.1 -C.sub.12 alkoxy)-phenyl, cyano, C.sub.2 -C.sub.5 alkoxycarbonyl, C.sub.1 -C.sub.12 alkoxy or/and halogen, C.sub.5 -C.sub.8 cycloalkyl, C.sub.6 -C.sub.12 aryl which is unsubstituted or mono- or poly-substituted by halogen, C.sub.1 -C.sub.12 alkyl or/and C.sub.1 -C.sub.12 alkoxy, a 5-membered or 6-membered heterocyclic monovalent radical which contains one or more O, S or N atoms and which may contain a fused benzo radical or/and is unsubstituted or substituted by C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy or/and halogen, or is a radical --CO--R.sub.3 or --OR.sub.4, or R.sub.1 and R.sub. 2 together with the phosphorus atom form a bicyclic phosphabicyclohexane or phosphabicyclononane ring, R.sub.3 is C.sub.1 -C.sub.18 alkyl, C.sub.2 -C.sub.6 alkenyl, phenylvinyl, C.sub.1 -C.sub.8 alkyl which is mono- or poly-substituted by phenyl, C.sub.2 -C.sub.5 alkoxycarbonyl, halogen, C.sub.1 -C.sub.12 alkoxy, phenoxy, C.sub.1 -C.sub.12 alkylthio or/and phenylthio, C.sub.6 -C.sub.12 aryl which is unsubstituted or mono- or poly-substituted by C.sub.1 -C.sub.12 alkyl, C.sub.1 -C.sub.12 alkoxy, C.sub.1 -C.sub.12 alkoxyalkyl, C.sub.1 -C.sub.4 alkylthio or/and halogen, or a monovalent 5-membered or 6-membered heterocyclic radical containing one or more O, S or N atoms, which radical can be mono- or poly-substituted halogen, C.sub.1 -C.sub.4 alkyl or C.sub.1 -C.sub.4 alkoxy, and R.sub.4 is C.sub.1 -C.sub.18 alkyl, phenyl-C.sub.1 -C.sub.4 alkyl, C.sub.5 -C.sub.8 cycloalkyl or C.sub.6 -C.sub.10 aryl, with the proviso that at least one of the radicals R.sub.1, R.sub.2 and R.sub.3 is a substituted alkyl radical or that R.sub.1 and R.sub.2 together with the phosphorus atom form a bicyclic ring.