PURPOSE: To provide a co-polymer useful as a coating component and having many arms radiated from a hub portion and exhibiting improved stability, a low viscosity and good coating function.

CONSTITUTION: The co-polymer having a hub portion from which from 3 to 8 arms are radiated where the hub portion is the residue of a tri-functional to octal-functional thiol and each arm is a functional unit mutually compatible with structural units and comprising the functional units including crosslinking substituents capable of undergoing a crosslinking reaction with a crosslinking agent or crosslinking substituent in functional units in another molecule of the same polymer and optional compatible auxiliary functional units that contain wetting groups is provided. The polymer has from 3,000 to 150,000 of molecular weight and a hub portion which is a residue of thiol ester formed with from 3 to 8 functional alcohol and thio-2-6C alkane acid.