Silicone-polycarbonate block copolymers and polycarbonate blends having reduced haze, and method for making

A method for making a silicone polycarbonate block copolymer which can be blended with polycarbonate to produce a polycarbonate blend having a haze value of less than about 30, where the silicone polycarbonate block copolymer has from about 0.5% to about 80% by weight of chemically combined polydiorganosiloxane with an average block length of about 10 to about 100 chemically combined diorganosiloxy units, which method comprises,

• (A) adding phosgene to bisphenol under interfacial reaction conditions and at a pH in the range of from about 10 to about 12 in the presence of an amount of a phase transfer catalyst which is effective for catalyzing reaction between phosgene and the bisphenol until about 1 to about 99 mole percent of phosgene has been added, based on the total moles of available hydroxy groups of the bisphenol,
• (B) lowering the pH of the mixture of (A) to a value in the range of about 8.0 to about 9.0, and while maintaining such pH range, continuing to add phosgene to the mixture until there is present at least a sufficient amount, and up to about a 5 mole % excess of phosgene which is adequate to generate enough chloroformate end groups capable of reacting with available bisphenol hydroxy groups and any hydroxyaryl groups present in the terminal position of polydiorganosiloxane subsequently added to the mixture to satisfy the desired polydiorganosiloxane weight percent requirements in the resulting silicone polycarbonate block copolymer,
• (C) introducing into the mixture of (B), the predetermined weight percent of the hydroxyaryl polydiorganosiloxane without additional phosgene,
• (D) raising the pH of the resulting mixture of (C), to a value in the range of about 10 to about 12, and,
• (E) eliminating any excess chloroformate groups in the mixture of (D).