Preparation of polymer, preparation of chemically amplified resist composition, resist pattern forming process, polymer, and chemically amplified resist composition

A method for preparing a polymer comprising repeat units adapted to be decomposed under the action of acid and free of repeat units adapted to function as a photoacid generator, the method comprising the steps of

• (1) polymerizing starting monomers including a monomer having the formula (A1) in a solution through living radical polymerization using a radical initiator and a reversible addition-fragmentation chain transfer agent (RAFT agent), to form a polymer P-1 having an end structure selected from structures having the formulae (X-1) and (X-2) originating from the RAFT agent at the end of the backbone,
• (2) adding a radical generator and a thiol compound to the solution containing polymer P-1, and heating the solution to convert the end structure to hydrogen to form a polymer P, and
• (3) mixing the solution containing polymer P with a poor solvent and allowing polymer P to precipitate as solids for purification,
• wherein RA is hydrogen or methyl,
• RAL is an acid labile structure-containing group having any one of the following formulae (a-1) to (a-10):
• wherein R1 and R2 are each independently a C1-C4 alkyl group,
• R3 is a C1-C4 alkyl group, cyclopentyl, cyclohexyl, oxanorbornyl or thianorbornyl group, some —CH2— in the cyclopentyl and cyclohexyl groups may be replaced by —O— or —S—,
• R4 is a C1-C4 alkyl group, cyclopentyl group, or cyclohexyl group,
• R5 is —O—, —S—, methanediyl or ethane-1,2-diyl,
• R6 is a C3-C40 divalent alicyclic group which may contain at least one atom selected from oxygen and sulfur,
• R7 is an acid labile group having the formula (a-3), (a-5) or (a-6),
• m is 1 or 2, n is 0 or 1, the broken line designates a point of attachment,
• wherein RX1 and RX2 are each independently a C2-C20 saturated hydrocarbyl group which may contain a heteroatom, C7-C20 aralkyl group or C6-C20 aryl group, the broken line designates a point of attachment to the carbon atom in the backbone.