Reactive blending of isosorbide-based polycarbonates: Catalytic selectivity and transesterification mechanism

Catalytic activities of metal laurates with different coordination capabilities and sodium salts with different ligand alkalinities toward the transesterification between isosorbide (ISB)-based polycarbonate (I c C-PC) and bisphenol-A polycarbonate (BPA-PC) are investigated to accelerate asymmetric chain exchange without aggravating chain degradation. Only catalysts capable of direct reaction with PC chains to form metal alkoxide containing active species (PC–O-M) can initiate and thus accelerate chain transesterification . Sodium salts exhibit the highest catalytic activity and can be further improved by complex formation of 15-crown-5, demonstrating the key role played by alkalinity of reactive ligands. Different from weak catalytic selectivity and serious chain scissions by tin-based catalysts, it is found that, for sodium-catalyzed systems, BPA-PC chains prefer to bond to ISB segments in endo position and the high reactivity of sodium-based catalysts not only significantly reduces essential loading but also suppresses chain degradation. We confirm that these differences must arise from whether active PC-O-M acts as a direct nucleophilic attacker or works with coordination process.