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

1. A polymer represented by formula (A1):

wherein in the formula (A1), R 0 represents a residue derived from a cationic polymerization initiator, or a functional group; L 1 represents a linker and L 1 may not be present; D 1 is represented by formula (d1-1); and Z 1 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), D 2 is represented by the formula (d1-1); and R 1 to R 4 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), R 1 to R 4 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

R 5 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), R 1 to R 4 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 L 1 in the formula (A1) or represents binding to Z 1 in the formula (A1) when L 1 is not present.

2. The polymer according to claim 1 , wherein the polymer is a random copolymer having the unit represented by the formula (A2) and the unit represented by the formula (A3).

3. The polymer according to claim 1 , wherein a ratio of the unit represented by the formula (A2) to repeating units carried by the polymer is 60% or less.

4. The polymer according to claim 1 , wherein a ratio of the unit represented by the formula (A2) to repeating units carried by the polymer is 0.01% to 10%.

5. The polymer according to claim 1 , wherein each of R 1 to R 4 is a hydrogen atom.

6. The polymer according to claim 1 , wherein L 1 has a structure represented by any of formulas (11) to (114):

wherein in the formulas (11) to (114), two * represent direct binding or indirect binding to D 1 and Z 1 , respectively, in the formula (A1); and in the formula (114), ω represents a residue derived from a terminal after completion of cationic polymerization reaction, or a functional group.

7. The polymer according to claim 1 , wherein R 5 is an ethyl group.

8. The polymer according to claim 1 , wherein a molecular weight of the polymer is 10000 to 200000, as measured by Gel Permeation Chromatography.

9. The polymer according to claim 1 , wherein a molecular weight of the polymer is 10000 to 50000, as measured by Gel Permeation Chromatography.

10. The polymer according to claim 1 , wherein R 0 is selected from a functional group derived from any of a dye, a reporter molecule, a target-binding molecule, and a polymer.

11. A contrast agent for photoacoustic imaging, comprising the polymer according to claim 1 , and a dispersion medium.

12. The polymer according to claim 1 , wherein R 5 is the isopropyl group.

13. The polymer according to claim 1 , wherein a number of bound dye per polymer molecule is 1.9 to 7.8.

14. The polymer according to claim 13 , wherein the number of bound dye per polymer molecule is 4.7 to 7.8.