Polycarbonates (PC) are synthetic thermoplastic polymers derived from the reaction of dihydroxy compounds and phosgene, or their derivatives. They are essentially linear polyesters of carbonic acid and can be formed interfacially from reaction of dihydroxy compounds with phosgene or via ester interchange by reaction of dihydroxy compounds with a carbonate diester. The desired properties of polycarbonates include clarity or transparency (i.e. 90% light transmission or more), high impact strength and toughness, heat resistance, weather and ozone resistance, good ductility, being combustible but self-extinguishing, good electrical resistance, noncorrosive, and nontoxic. They are useful for forming a wide array of products, such as by molding, extrusion, and thermoforming processes.
In producing a polycarbonate, a homopolycarbonate is generally formed from the polymerization of a dihydroxy compound such as bisphenol A. However, when a dicarboxylic acid is copolymerized with the dihydroxy compound, a poly(ester-carbonate) (also known as a copolyester-carbonate, polycarbonate-ester, or a polyestercarbonate) results. A variety of dicarboxylic acids have been copolymerized with dihydroxy compounds to form various poly(ester-carbonate)s. Such poly(ester-carbonate)s generally have lower glass transition temperatures, improved melt flow rates, and better ductility relative to a bisphenol A homopolycarbonate.
Short chain dicarboxylic acids can be particularly desirable for the production of poly(ester-carbonate)s. However, short chain dicarboxylic acids, such as, adipic acid, can be difficult to incorporate into poly(ester-carbonate)s. Poor incorporation of the dicarboxylic acid can create undesirable quality issues, while incomplete incorporation leaves residual adipic acid in process streams that can adversely affect downstream operations. Short chain dicarboxylic acids have been successfully incorporated via interfacial polymerization when used in an activated form, such as the acid chloride or anhydride form. See, for example, U.S. Pat. No. 5,015,720. However, these activated forms are more expensive than the simple dicarboxylic acid. In addition, they can contain color contaminants that cause the final polymer to be highly colored. This is undesirable for applications in which a transparent or low color polymer composition is sought.
Accordingly, there remains a need for new processes that allow for incorporation of high levels of a dicarboxylic acid into a poly(ester-carbonate) while avoiding the above-described technical limitations of presently known methods.