We herein describe a simple and efficient one-pot synthesis approach to prepare crystalline polycarbonate-polyester diblock copolymers by copolymerizing tetrachlorophthalic anhydride, CO2, and ethylene oxide using a metal-free catalyst. The block copolymers possess a melting point as high as 169 °C and two distinct glass transition temperatures. It is also possible to control the length and composition of the copolymers, thereby customizing their crystallinity and physical performance.