Patent ID: 7157207

Claim:
A polymer having a weight-average molecular weight of 1,000 to 50,000 and comprising at least one repeating unit of formula (1), at least one repeating unit of formula (2) and at least one repeating unit of formula (3) wherein formulas (1) to (3) are as follows: wherein R 1 , R 3 and R 4 each independently represents a hydrogen atom or a methyl group; R 2 represents a linear, branched or cyclic C 1-12 alkyl group; R 5 and R 6 each independently represents a hydrogen atom or a hydroxyl group; X represents a substituent containing a lactone structure; and n stands for 1 or 2, wherein said formula (3) is selected from one or more formulas (3-1) to (3-7): wherein R 7 represents a hydrogen atom or CO 2 R 9 in which R 9 represents a linear, branched or cyclic monovalent C 1-8 hydrocarbon group; R 8 represents a hydrogen atom or a linear, branched or cyclic monovalent C 1-10 hydrocarbon group or R 8 s may be coupled to form a ring, together with a carbon atom to which the R 8 s are attached; and Y represents a methylene group or oxygen atom.