Oligomer mixture characterized in that it comprises at least three poly (alkylene phosphates) of the formula, in which R 1, R 2, R 3 and R 11 mutually independently respectively are a straight-chain or branched C 1 -C 8 -alkyl moiety or a straight-chain or branched C 1 -C 4 -alkoxyethyl moiety, A is a straight-chain, branched and/or cyclic C 4 -C 20 -alkylene moiety, or A is a moiety of the formula -CH 2 -CH=CH-CH 2 -, a moiety of the formula -CH 2 -C≡C-CH 2 -, a moiety of the formula -CHR 5 -CHR 6 - (O-CHR 7 -CHR 8) a -, a moiety of the formula -CHR 5 -CHR 6 -S b -CHR 7 -CHR 8 - or a moiety of the formula - (CHR 5 -CHR 6 -O) c -R 9 - (O-CHR 7 -CHR 8) d -, in which a is an integer from 1 to 5, b is an integer from 0 to 2, c and d are mutually independently an integer from 1 to 5, R 5, R 6, R 7 and R 8 are mutually independently H or methyl, R 9 is a moiety of the formula -CH 2 -CH=CH-CH 2 -, a moiety of the formula -CH 2 -C≡C-CH 2 -, a 1, 2-phenylene moiety, a 1,3-phenylene moiety, a 1,4-phenylene moiety, or a moiety of the formula a moiety of the formula a moiety of the formula or a moiety of the formula -C (=O) -R 12 -C (=O)-, where R 10 and R 11 are mutually independently respectively H or C 1 -C 4 -alkyl or R 10 and R 11 together are an optionally alkyl-substituted ring having from 4 to 8 carbon atoms, R 12 is a straight-chain, branched and/or cyclic C 2 -C 8 -alkylene moiety, a 1,2-phenylene moiety, a 1,3-phenylene moiety, or a 1,4-phenylene moiety, and n is an integer from 0 to 100, with the proviso that the at least three poly(alkylene phosphates) of the formula differ from one another at least in the number n of the repeating units, and the average number of the repeating units n of the at least three poly (alkylene phosphates) of the formula is greater than 1.10 and smaller than 2.00. R 1, R 2, R 3 and R 11 mutually independently respectively are a straight-chain or branched C 1 -C 8 -alkyl moiety or a straight-chain or branched C 1 -C 4 -alkoxyethyl moiety, A is a straight-chain, branched and/or cyclic C 4 -C 20 -alkylene moiety, or A is a moiety of the formula -CH 2 -CH=CH-CH 2 -, a moiety of the formula -CH 2 -C≡C-CH 2 -, a moiety of the formula -CHR 5 -CHR 6 - (O-CHR 7 -CHR 8) a -, a moiety of the formula -CHR 5 -CHR 6 -S b -CHR 7 -CHR 8 - or a moiety of the formula - (CHR 5 -CHR 6 -O) c -R 9 - (O-CHR 7 -CHR 8) d -, in which a is an integer from 1 to 5, b is an integer from 0 to 2, c and d are mutually independently an integer from 1 to 5, R 5, R 6, R 7 and R 8 are mutually independently H or methyl, R 9 is a moiety of the formula -CH 2 -CH=CH-CH 2 -, a moiety of the formula -CH 2 -C≡C-CH 2 -, a 1, 2-phenylene moiety, a 1,3-phenylene moiety, a 1,4-phenylene moiety, or a moiety of the formula a is an integer from 1 to 5, b is an integer from 0 to 2, c and d are mutually independently an integer from 1 to 5, R 5, R 6, R 7 and R 8 are mutually independently H or methyl, R 9 is a moiety of the formula -CH 2 -CH=CH-CH 2 -, a moiety of the formula -CH 2 -C≡C-CH 2 -, a 1, 2-phenylene moiety, a 1,3-phenylene moiety, a 1,4-phenylene moiety, or a moiety of the formula R 10 and R 11 are mutually independently respectively H or C 1 -C 4 -alkyl or R 10 and R 11 together are an optionally alkyl-substituted ring having from 4 to 8 carbon atoms, R 12 is a straight-chain, branched and/or cyclic C 2 -C 8 -alkylene moiety, a 1,2-phenylene moiety, a 1,3-phenylene moiety, or a 1,4-phenylene moiety, and n is an integer from 0 to 100,