wherein m and n are an integer of 1 or more; R1 to R7 are each independently selected from a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, an aralkyl group, a functional group containing an ester group, a functional group containing a ketone group, and a functional group containing an ether group; and R1 to R7 may be different in a repetition of a monomer,