Photopolymerizable recording composition containing acylphosphine oxide photoinitiator

A photopolymerizable recording composition for the preparation of printing plates and relief plates, which consists mainly of a mixture, containing (a) a photoinitiator, (b) one or more monomers possessing one or more photopolymerizable olefinically unsaturated double bonds and (c) one or more organic polymeric binders, the improvement wherein the photoinitiator is an acylphosphine oxide compound of the formula ##STR7## where R.sup.1 is alkyl of 1 to 6 carbon atoms, cycloalkyl of 5 or 6 ring carbon atoms, aryl which is unsubstituted or substituted by halogen, alkyl or alkoxy, or an S-containing or N-containing five-membered or six-membered heterocyclic radical,

• R.sup.2 has the same meanings as R.sup.1, or is alkoxy, aryloxy or aralkoxy, R.sup.1 and R.sup.2 being the same or different or together form a ring, and
• R.sup.3 is tertiary alkyl of 4 to 18 carbon atoms or tertiary cycloalkyl of 5 or 6 ring carbon atoms or is a cycloalkyl, aryl or 5-membered or 6-membered heterocyclic radical containing S, N or O as hetero atoms which contains substituents A and B bonded at least to the two ring carbon atoms, capable of substitution, which are adjacent to the point of attachment of the carbonyl group to the carbonyl group, A and B being identical or different and each being alkyl, alkoxy, alkoxyalkyl, alkylthio, cycloalkyl, aryl or halogen.