Monomer, polymer, positive resist composition, and patterning process

A polymer comprising recurring units (a) having the formula (A):

• wherein R1 is hydrogen or methyl, R2 is hydrogen or a straight, branched or cyclic C1-C15 monovalent hydrocarbon group in which any constituent —CH2— may be replaced by —O—, —C(═O)—, —C(═O)—O— or —O—C(═O)—, R3 is each independently hydrogen, cyano, nitro, or a straight, branched or cyclic C1-C6 monovalent hydrocarbon group in which any constituent —CH2— may be replaced by —O—, —C(═O)—, —C(═O)—O— or —O—C(═O)—, R4 is each independently hydrogen or an acid labile group, X1 is a single bond, a C1-C12 linking group having an ester moiety, ether moiety or lactone ring, a phenylene group or naphthylene group, m is an integer of 1 to 4, and a is a positive number in the range:
• 0<a≦1.0.