Disclosed are: a polyolefin polymer having a polar group represented by the general formula (i), which is produced by modifying, with a high degree of modification rate, a double bond in a polymer that has the double bond at one terminal, at both terminals, or in the inside of its polymer chain and is produced by a known method; a method for producing the polyolefin polymer; and a water dispersion material and a mold release agent composition each comprising the polyolefin polymer. (i) wherein PO represents a polyolefin polymer produced by the polymerization of a monomer comprising an ethylene or an olefin having 3 to 20 carbon atoms, wherein a group inside the parentheses represents a polar group; A and B independently represent any one selected from R1, a cyano group, C(O)OR1 and C(O)NR1R2, provided that A and B may together form a cyclic structure; M represents a metal cation or an onium cation; j represents the number of the metal cation(s) or the onium cation(s), and is an integer of 0 to 4; i represents the number of valency of the metal cation or the onium cation, and is an integer of 0 to 4; D and E independently represent any one selected from an oxygen atom, a nitrogen atom, a sulfur atom, R3, OR3, SR3 and NR3R4, provided that D and E may together form a cyclic structure; d and e represent the number of valency of the substituent D and the number of valency of the substituent E, respectively, and independently represent an integer of 0 to 4; R1, R2, R3 and R4 independently represent any one selected from a hydrogen, an optionally substituted alkyl, alkenyl, alkynyl, aralkyl or aryl group having 20 or less carbon atoms, and a group having a heterocyclic ring; and n represents the average number of the functional groups, and is a number of 0.80 to 10.0.