The present invention relates to a novel photoresist composition sensitive to radiation in the deep ultraviolet and a process for imaging the composition. The photoresist composition comprises a) a novel polymer that is insoluble in an aqueous alkaline solution and comprises at least one acid labile group, and b) a compound capable of producing an acid upon irradiation. The novel polymer of the present invention comprises at least one unit with a bisester group, (--C(O)OWC(O)O--), attached on one side to a polymer backbone unit (A) comprising an aliphatic group, and attached on the other side to an adamantyl group. The invention also relates to the novel polymer and a novel monomer for obtaining the novel polymer.