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, RL6, RL7 and RL8 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 RL6 to RL8 may bond together to form a ring with the carbon atom to which they are attached,
• RL3, RL1, RL9 and RL10 are each independently hydrogen or a C1-C20 hydrocarbyl group which may contain a heteroatom, RL3 and RM may bond together to form a ring with the carbon atom to which they are attached, and RL9 and RL10 may bond together to form a ring with the carbon atom to which they are attached,
• RL5 and RL11 are each independently hydrogen or a C1-C15 hydrocarbyl group,
• m1 and m2 are each independently an integer of 1 to 3, and
• the broken line designates a point of attachment to the oxygen atom of the aromatic hydroxy group.