Imidazolium-Catalyzed Formation of Bisphenol A Polycarbonate with a Reduced Level of Branching

The melt-phase polymerization of bisphenol A (BPA) and diphenyl carbonate (DPC) catalyzed by alkaline metal catalysts produces polycarbonates with high branching, which impairs the product's properties during weather resistance, ductility, and rheology. The use of an imidazolium-type catalyst can result in a reduced amount of branching relative to the benchmark Cs2CO3 catalyst. Modification of the imidazolium structure, especially by introducing a substitution at the C2 position, definitely improves the catalyst performance in enhancing the catalyst stability and reducing the branching level in the polycarbonate product. For the best catalyst identified (1,3-Ad(2)-2-Ph-Im-BPA), we have shown that at a DPC/BPA ratio of 1.075 and a catalyst loading of 7 ppm, the specifications can be met at the lab scale: polymerization time = 125 min, M-n = 11.0 K, OH = 660 ppm, branching = 460 ppm (similar to 75% reduction relative to the Cs2CO3 benchmark), and yellowness index = 6.3.