Synthesis of Poly(4-hydroxystyrene) for Photoresist: Nitroxide-Mediated Polymerization and Homogeneous Acidic Hydrolysis

Abstract The growing demand for high-precision chemical materials has intensified the need for stringent control over polymer structures in advanced photolithography. Poly(4-hydroxystyrene) (P(pHS)) is a key base polymer for chemically amplified photoresists. Its narrow molar mass distribution is crucial for minimizing line-edge roughness in photoresist patterns. In this study, we identified the origin of molar mass distribution broadening during the conversion of precursor polymers synthesized by nitroxide-mediated polymerization (NMP) into P(pHS) and established a homogeneous acidic hydrolysis method to suppress it. NMP using N-tert-butyl-N-(2-methyl-1-phenylpropyl)-O-(1-phenylethyl)hydroxylamine (PhEt-TIPNO) afforded TIPNO-terminated poly(4-acetoxystyrene) and poly(4-tert-butoxystyrene) (T-P(pAS) and T-P(pTBS), respectively) with controlled number-average molar masses (Mn) and low dispersities (Đ). During deprotection with hydrochloric acid (HCl), proton-induced C–ON bond cleavage generated carbocationic chain ends, which underwent intermolecular reactions to form branched structures. To minimize these side reactions, a homogeneous organic–aqueous medium was established to facilitate rapid quenching of the carbocations by water. Of the two precursor polymers, T-P(pAS) was more suitable for quantitative deprotection and end-group removal while minimizing branching. This process afforded P(pHS) products with controlled molar masses (Mn = 7.45–21.9 kDa, Đ = 1.08). The same strategy was successfully extended to the synthesis of poly(4-hydroxystyrene-co-styrene) (P(pHS-co-S)) with controlled molar mass and composition (Mn = 6.75 kDa, Đ = 1.07, FpHS = 0.769). The combination of NMP and homogeneous acidic hydrolysis provides an effective strategy for producing well-defined P(pHS)-based materials for photoresist applications.