R5, R6 and R7 represent hydrogen, iso-C3 -C5 -alkyl, one of the substituents R5, R6 or R7 not being hydrogen, in quantities of from 100 to 1 mol-%, preferably in quantities of from 100 to 5 mol-% and, more preferably, in quantities of from 100 to 20 mol-%, based on the total quantity of difunctional carbonate structural units in the polycarbonate.