Polymer, and contrast agent for photoacoustic imaging, including the polymer

A polymer represented by formula (A1):

• wherein in the formula (A1), R0 represents a residue derived from a cationic polymerization initiator, or a functional group;
• L1 represents a linker and L1 may not be present;
• D1 is represented by formula (d1-1);
• and Z1 represents a structure comprising at least one unit represented by formula (A2), at least one unit represented by formula (A3), and at least one unit represented by formula (A4):
• wherein in the formula (A2), D2 is represented by the formula (d1-1);
• and R1 to R4 each independently represents a hydrogen atom, or a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms, in which, when a substituent is present, the substituent is a functional group including at least one selected from the group consisting of a halogen atom, an oxygen atom and a nitrogen atom;
• wherein in the formula (A3), R1 to R4 each independently represents a hydrogen atom, or a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms, in which, when a substituent is present, the substituent is a functional group including at least one selected from the group consisting of a halogen atom, an oxygen atom, and a nitrogen atom;
• and
• R5 represents any of a methyl group, an ethyl group, an isopropyl group, a n-propyl group, a n-butyl group, a n-octyl group, a n-heptyl group, a phenyl group, a butenyl group, and a pentynyl group;
• wherein in the formula (A4), R1 to R4 each independently represents a hydrogen atom, or a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms, in which, when a substituent is present, the substituent is a functional group including at least one selected from the group consisting of a halogen atom, an oxygen atom, and a nitrogen atom, and
• wherein a ratio of a unit represented by the formula (A4) to repeating units of the polymer is 12.6% or less as measured by electric conductivity:
• wherein, in the formula (d1-1), * represents binding to L1 in the formula (A1) or represents binding to Z1 in the formula (A1) when L1 is not present.