If (oligomer) denotes a radical (iv) of formula (3c), R4 and R4??? are each preferably ethyl or in particular methyl; v is preferably an integer from 2 to 500, more preferably from 5 to 500, even more preferably from 5 to 250 and particularly preferably from 10 to 100; Q??? is for example hydrogen; and An is as defined before.