Acetal modifier, polymer, chemically amplified positive resist composition, and resist pattern forming process

A polymer adapted to turn alkali soluble as a result of deprotection under the action of acid, the polymer comprising repeat units A1 having on side chain a structure including an aromatic hydroxy group protected with an acid labile group having the formula (AL-1) or (AL-2):

• wherein RL1, RL2, RL, RL6 and RL7 are each independently hydrogen or a C1-C15 hydrocarbyl group, RL1 and RL2 may bond together to form a ring with the carbon atom to which they are attached, and any two of RL5 to RL7 may bond together to form a ring with the carbon atom to which they are attached,
• RL3, RL4, RL8 and RL9 are each independently hydrogen or a C1-C20 hydrocarbyl group which may contain a heteroatom, RL3 and RL4 may bond together to form a ring with the carbon atom to which they are attached, and RL8 and RL9 may bond together to form a ring with the carbon atom to which they are attached,
• W1 is each independently a group having an oxygen or sulfur-containing cyclic saturated heterocycle,
• m1 and m2 are each independently an integer of 0 to 3, and
• the broken line designates a point of attachment to the oxygen atom of the aromatic hydroxy group.