Photochemical reactions of polystyrenes ring‐substituted with thiocyanato groups

The photochemistry of poly(2‐vinylphenyl thiocyanate) (P(2‐VPT)), poly(3‐vinylphenyl thiocyanate) (P(3‐VPT)) and poly(4‐vinylphenyl thiocyanate) (P(4‐VPT)) is investigated under 254 nm UV light. Poly(4‐bromostyrene) (P(4‐BrSt)) is used as reference material. Experiments with phenyl thiocyanate show that the main photoreaction is a cleavage into cyano and phenylthio radicals which also occurs in the polymers. Photo‐crosslinking data for the polymers P(2‐VPT), P(3‐VPT) and P(4‐VPT) are evaluated with the Charlesby‐Pinner relation. For P(2‐VPT), P(3‐VPT) and P(4‐VPT) the qunatum efficiencies Φ CL of crosslinking are 0.018, 0.029 and 0.021, the quantum efficiencies Φ CL of chain scission are 9 · 10 −4 , 37 · 10 −4 and 6 · 10 −4 . P(2‐VPT), P(3‐VPT) and P(4‐VPT) rank among the most sensitive negative resists in the polystyrene series. Hammett's σ constants for SCN substituents are determined (3‐SCN: σ = 0.50, 4‐SCN: σ = 0.45). However, a general correlation between the Φ CL or Φ CS values of ring‐substituted polystyrenes and Hammett's σ constants of the substituents is not found.