Complex sentence: Seleucus made Seleucia his new capital, thus imitating Lysimachus, Cassander and Antigonus, all of whom had named cities after themselves.
Split into simpler sentences: Seleucus made Seleucia his new capital. Seleucus was thus imitating Lysimachus, Cassander and Antigonus, who all had named cities after themselves.