Compounds of the formula I which are very particularly preferred are those in which X is --O--, --Ca H2a -- is linear, R2 is 1,2-phenylene, R3 is hydrogen, R4 is a radical of the formulae VI or VII in which f or g is 0, Y is --O--, a is 5, b is an integer from 1 to 12, c and d are 1 and e is 2.