A description is given of a photoreactive polymer preparable by radical polymerization using at least one photoreactive monomer, where the photoreactive monomer has at least one radically polymerizable C—C double bond, at least one hydrophilic group selected from an ethylene glycol group and polyethylene glycol groups having at least 2 ethylene glycol units, and at least one photoreactive group, the photoreactive group being a photoenolizable carbonyl group.