Using a first-principles total-energy procedure within the framework of density functional theory, we study the geometric and electronic structures of polymerized coronene in which additional C 2 units adjoin the coronene molecule forming a fused pentagon. We found that the polymer is energetically stable. Moreover, the energy per C atom is higher than that of an isolated graphene sheet by 0.2 eV. Our calculations also show that the polymer is a metal with a substantial density of states at the Fermi level. The wave functions at the Γ point near the Fermi level shows the similar distribution to that for the highest occupied and the lowest unoccupied states of an isolated coronene molecule. Our first-principles molecular dynamics simulation also showed that the polymer can spontaneously form from dense defects in the graphene at any temperature.