Abstract: Quantum-chemical density functional theory (DFT) calculations at B3LYP/TZVP//B3LYP/SVP level were performed to find the most stable aggregates of n-butyllithium (n-BuLi) and polystyryllithium (PSLi) living species during anionic bulk polymerization of styrene. Results showed that the hexamers of n-BuLi were the most stable aggregates in both gas and solution. In addition, dimeric PSLi was the most stable aggregates according to the DFT calculations using HStLi as the model of PSLi. To interpret the effect of P-ligands on the structure of n-BuLi and PSLi aggregates during anionic polymerization of styrene, DFT calculations on the possible geometrical structures and relative stabilities of the living species and initiators in the presence of P-ligands were carried out. The presence of P-ligands facilitated the deaggregation of PSLi dimers and n-BuLi hexamers to form more stable mixed aggregates of P-ligands→monomeric n-BuLi and P-ligands→monomeric PSLi. The higher the mole fraction of P-ligands coordinated with n-BuLi (or PSLi) was, the higher their stabilities to bind n-BuLi (or PSLi) and their energies required to disaggregate n-BuLi (or PSLi) from the mixed aggregates were.