This invention relates to an extrusion method preparing polycarbonates from
a solution of an oligomeric polycarbonate. A mixture of bis(methyl
salicyl)carbonate (BMSC), BPA and a transesterification catalyst are first
equilibrated at moderate temperatures to provide a solution of
polycarbonate oligomer in methyl salicylate. The solution is then fed to a
devolatilizing extruder, where the polymerization reaction is completed
and the methyl salicylate solvent is removed. The solution comprising the
oligomeric polycarbonate can also be pre-heated under pressure to a
temperature above the boiling point of methyl salicylate and subsequently
fed to a devolatilizing extruder equipped for rapid flashing off the
solvent. The method provides polycarbonate with greater efficiency than
the corresponding process in which unreacted monomers are fed to the
extruder. Additionally, the method of the invention does not require the
isolation of a precursor polycarbonate comprising ester-substituted
phenoxy terminal groups.
