Model Experiments Implicate a Benzoquinoneketene Intermediate in LCP Synthesis

LCP synthesis from 4-hydroxybenzoic acid (HBA) and comonomers involves the formation of phenolic anion end groups at the later stages of polymerization. Model reactions show that these end groups can undergo cleavage to form 4-oxo-2,5-cyclohexadienylideneketene OCK (a benzoquinoneketene intermediate) with loss of a resonance-stabilized phenolic anion. With base, ethyl 4-(4′-hydroxybenzoyloxy)benzoate formed ethyl 4-hydroxybenzoate and poly-HBA, while ethyl 4-(4′-methoxybenzoyloxy)benzoate did not react under the same conditions, indicating an E1cB mechanism involving OCK as an intermediate. Attempted trapping of the OCK intermediate using cycloadditions failed, but it was successfully trapped with secondary amines. These results make it likely that LCP synthesis occurs at least partly via OCK. It is a ubiquitous intermediate in LCP polymerization and characterization.