New photocleavable structures. II. α‐Cleavable photoinitiators based on pyridines

A series of novel pyridine‐based type I photoinitiators were synthesized. UV spectroscopy revealed absorption characteristics very similar to those of a commercially available hydroxyalkylphenone [2‐hydroxy‐2‐methyl‐1‐phenyl propan‐1‐one (1b)]. Photo‐differential scanning calorimetry was used to study the polymerization kinetics of lauryl acrylate photoinitiated by the new pyridines. Under a nitrogen atmosphere, a 3‐substituted derivative [2‐hydroxy‐2‐methyl‐1‐pyridin‐3‐yl‐propan‐1‐one (3b)] gave results comparable to those obtained with 1b. Measurements under air indicated decreased sensitivity toward oxygen quenching reactions. Additionally, pyridine derivatives showed good water solubility (4.9 g/100 mL). Preliminary steady‐state photolysis experiments with 2,2,6,6‐tetramethyl‐1‐piperidinyloxy as a radical scavenger were carried out, and the α‐cleavage mechanism was found to be the main reaction pathway. The quantum yields of α‐cleavage were approximately 0.30 ± 0.04 for 3b and 0.18 ± 0.03 for 1b.