Controlling the microstructure of polymers through chemical reactivity is key to control the material properties of synthetic polymers. Herein we investigate the ring-opening copolymerization of a mixture of lactide and 2-ethyl-2-oxo-1,3,2-dioxaphospholane, promoted by an aluminum pyrrolidine monophenolate complex or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). This monomer mixture provides fast access to amphiphilic copolymers. The reaction conditions control the copolymer microstructure, which has been determined via a combination of 1H and 31P NMR spectroscopy. The choice of initiator has a profound impact: both initiators produce tapered block copolymers but with reverse monomer selectivity. While the aluminum initiator favors the cyclic phosphonate monomer, DBU favors lactide polymerization. Moreover, a sequential control of temperature facilitates the preparation of block copolymers in one pot. Thermal properties measured by TGA and DSC correlate to copolymer architectures. This methodology is the first report of copolymerization between cyclic phosphonates and lactide and opens the possibility to tune the thermal properties, solubility, and degradation rates of the resulting materials.