Provided is a novel material which can be used suitably for, for example, a precursor of a polymer coating material that enables the coating of the surface of a base material with a polymer to modify the properties of the surface or impart a functionality to the surface. A composite material characterized in that a compound having a polymerization initiation site containing a halogen group is incorporated in a crosslinked structure comprising a catechol derivative or a phenol derivative each represented by formula (I). In formula (I), R represents a hydrocarbon group having 2 to 20 carbon atoms which may be interrupted by an oxygen atom and has at least one double bond site, and A represents a hydrogen atom, a hydroxy group or an alkoxy group having 1 to 20 carbon atoms.