Radiopaque polymers

1. A polymer having a backbone comprising a polyhydroxylated polymer cross linked by a C4 or C5 alpha keto acid, wherein the cross linked polymer is in the form of a microsphere having a diameter between 30 μm and 700 μm when swollen.

2. The polymer according to claim 1 , wherein the C4 or C5 alpha keto acid, preferably is alpha ketoglutarate.

3. The polymer according to claim 1 , wherein the cross linking groups are of the formula 1:

wherein

*is the point of attachment to the polyhydroxylated polymer; and

wherein Q is a group of the formula 1a:

wherein

n is 1 or 2.

4. A polymer according to claim 1 , wherein the polyhydroxylated polymer is or comprises PVA.

5. A polymer according to claim 1 , wherein the polyhydroxylated polymer is or comprises PVA and comprises cross linking groups of formula 3 which cross link the PVA

wherein

Q is a group of the formula 1a

wherein

n is wherein n is 1 or 2.

6. A polymer according to claim 4 , wherein the PVA has a weight average molecular weight of 2,000 Da to 180,000 Da.

7. A polymer according to claim 4 , wherein the PVA has a weight average molecular weight of 10,000 Da to 32,000 Da.

8. A polymer according to claim 4 , wherein the PVA has a weight average molecular weight of 2,000 Da to 32,000 Da.

9. A polymer according to claim 1 , wherein the polymer is a hydrogel.

10. A polymer according to claim 1 , wherein the polymer further comprises an imagable agent.

11. A polymer according to claim 1 , wherein the polymer is ionically charged at pH7.4.

12. A polymer according to claim 1 , wherein the polymer comprises a covalently bound ionic group which is ionized at pH 7.4.

13. A microparticle or microsphere comprising a polymer according to claim 1 .

14. A microparticle or microsphere according to claim 13 , for use in the embolization of a blood vessel.

15. A pharmaceutical composition comprising one or more microparticles or microspheres according to claim 13 , and a pharmaceutically acceptable carrier or diluent.