Patent ID: 7662897

Claim:
A process for producing a polymer for photoresist, the process comprising a step of subjecting a mixture of monomers containing: a monomer (a) having a lactone skeleton represented by the following formula (1a), (1b) or (1c): wherein R 1 is a group including (meth)acryloyloxy group, each of R 2 , R 3 and R 4 is a lower alkyl group, n denotes an integer of 0 to 3, and m denotes an integer of 0 to 5; a monomer (b) having a group which becomes soluble in alkali by elimination with an acid; and a monomer (c) having an alicylic skeleton, having a hydroxyl group and represented by the following formula (3a): wherein R is a hydrogen atom or a methyl group, R 10 is a methyl group, a hydroxyl group, an oxo group or a carboxyl group, and k denotes an integer of 1 to 3, where at least one of the R 10 s is a hydroxyl group, to dropping polymerization with a polymerization solvent to obtain a polymer solution having: the polymer formed by the dropping polymerization; and the polymerization solvent, wherein the process further comprises: (i) an extraction step (B) of subjecting the polymer to extraction procedure using an organic solvent and water to partition the polymer into a layer of the organic solvent and a metal component as an impurity into an aqueous layer; or (ii) a step (I) of passing said polymer solution which contains a metal component with a metal content of 250 ppb by weight or less on the basis of the polymer weight, through a filter constituted by a porous polyolefin membrane having a cation-exchange group, to obtain the polymer with a sodium content of 95 ppb by weight or less on the basis of the polymer weight; wherein said polymerization solvent is a glycol or ester solvent, and the organic solvent used in the extraction step (B) has a density of 0.95 or less and a solubility parameter (SP value) of 20 Mpa 1/2 or less.