The present invention is a copolymer containing, as essential structural units, unit A represented by Formula (1) and unit B represented by Formula (2): (wherein R1 and R4 each independently represent a hydrogen atom or a methyl group, and R2 and R3 each independently represent a hydrogen atom or a C1-20 hydrocarbon group, excluding cases where two or more groups of R1 to R3 are hydrogen atoms.) (wherein R5 represents a hydrogen atom or a methyl group, and R6 represents a group selected from the group consisting of C1-20 hydrocarbon groups optionally containing an oxygen atom, C1-20 alkoxy groups, hydrogen atoms, oxygen radicals and hydroxyl groups.)