PURPOSE: To contain a protective group convertible to a reactive functional group at high temperature by subjecting an olefin compound to a free radical additive reaction with a specified polymer.

CONSTITUTION: (B) An olefin compound (e.g.: t-butyl methacrylate) having a group of the formula (X is NH, a 1-4C alkylene, a 6-10C arylene; R1 is H, a halogen, a 1-4C alkyl, or a 6-10C aryl; R2 is a 4-10C tertiary alkyl; (m) is 0-1, (n) is 1-5) is subjected to a free radical additive reaction with (A) a polymer (e.g.: PE) having a base polymer chain substantially formed of a structural unit derived from an olefin unsaturated hydrocarbon not substituted or having a substantially non-reactive substituent at a temperature of 225°C or lower, preferably 150-200°C under the presence of an initiator to provide a polymer containing 5wt.% or less of a grafted part.