PURPOSE: To produce polyolefin having low halogen contents, no-halogenated aromatics and normalized functional valence in the chain by polymerizing polymer chained monomers with the specified masked functional group-containing monomers.

CONSTITUTION: (A) At temp. less than 60°C, the corresponding masked functional group-containing monomer is produced by allowing (a) a functional-group containing monomer of the formula (R1 is ethylenic unsaturated hydrocarbyl radical; (n) ≥1, X is COOH, OH, CN, S, NH2, CO or NH group) to react with (b) one and more non-halogenated metal compounds. Then the polymer having masked functional group consisting of polymer chain monomer and functional group containing monomer is produced by polymerization of polymer chain monomer and the masked functional group containing monomer.