Hemiacetal compound, polymer, resist composition, and patterning process

A hemiacetal compound having the general formula (1b), (1c) or (1d):

• wherein R1 is hydrogen, methyl or trifluoromethyl, R2 to R4 are each independently hydrogen or a straight, branched or cyclic, C1-C15 monovalent hydrocarbon group in which a constituent —CH2— may be replaced by —O— or —C(═O)—, X1 is a straight, branched or cyclic, C1-C15 divalent hydrocarbon group in which a constituent —CH2— may be replaced by —O— or —C(═O)—, X2 is —CH2— or —O—, k1 is 0 or 1, and k2 is an integer of 0 to 3.

A polymer comprising recurring units of at least one type selected from the general formulae (2a) to (2d):

• wherein R1 is hydrogen, methyl or trifluoromethyl, R2 to R4 are each independently hydrogen or a straight, branched or cyclic, C1-C15 monovalent hydrocarbon group in which any constituent —CH2— may be replaced by —O— or —C(═O)—, X1 is a straight, branched or cyclic, C1-C15 divalent hydrocarbon group in which a constituent —CH2— may be replaced by —O— or —C(═O)—, X2 is —CH2— or —O—, the ring ZZ designates a non-aromatic mono- or polycyclic ring of 4 to 20 carbon atoms having a hemiacetal structure, k1 is 0 or 1, and k2 is an integer of 0 to 3.