PURPOSE: To provide a copolymer which, in a magnetic recording medium, can enhance the packing ratio of a magnetic pigment in a binder dispersion, improve the dispersibility, lower the viscosity of the dispersion, and improve the hardness of a magnetic tape, and to provide an iniferter for preparing the copolymer.

CONSTITUTION: This iniferter compd. is represented by the formula I(T)n [wherein I represents the free radical initiator portion of the iniferter compd. and comprises a hydroxy-functional polyurethane polymer which comprises: (i) a polyurethane backbone; (ii) two or more cross-linkable hydroxy groups pendant from the polyurethane backbone; (iii) at least n org. groups X pendant from the polyurethane backbone wherein each org. group X is also bonded to a T group; and T represents the terminator portion of the iniferter; in T and X, the T-X bond is capable of dissociating upon being subjected to an appropriate energy source of forming a terminator free radical of the formula nT. and an initiator free radical of the formula I(.)n.