Process for the production of poly (diorganosiloxane)/polycarbonate block copolymers

A process for the production of thermoplastic poly(diorganosiloxane)/polycarbonate block copolymers with average molecular weights M.sub.W (weight average, determined by gel chromatography) of 18000 to 60000 by reacting aromatic, Si-free diphenol, carbonic acid diaryl ester and polydiorganosiloxane in the presence of catalyst at a temperature of between 80.degree. C. and 320.degree. C. and pressures of 1000 mbar to 0.01 mbar, wherein the catalyst is a quaternary ammonium compound and/or quaternary phosphonium compound of the formulae (I) and/or (II) ##STR11## in which R.sub.1-4 are identical or different C.sub.1 -C.sub.30 alkyl, C.sub.6 -C.sub.10 aryl or C.sub.5 -C.sub.6 cycloalkyl, wherein at least one of R.sub.1-4 is a C.sub.8 -C.sub.30 alkyl and in which X.sup.- is an anion in which the corresponding acid/base pair H.sup..sym. +X.sup..crclbar. .revreaction.HX has a pKB of 1 to 11, in quantities of 10.sup.-1 to 10.sup.-8 mol, relative to 1 mol of Si-free diphenol, and further wherein the polydiorganosiloxane component comprises .alpha.-.omega.-bishydroxyaryl-, .alpha.-.omega.-bishydroxyalkyl-, .alpha.-.omega.-bisacyl-, or .alpha.-.omega.-bishydroxyacyl-polydiorganosiloxanes in a quantity by weight of 30 wt. % to 0.5 wt. % relative to the total weight of Si-free diphenol and polydiorganosiloxane.