Electron spin resonance study of radical formation in cis-poly(phenylacetylene)/ferric acetylacetonate-triethylaluminum (PPA/Fe-Al). 2. Results and interpretation

Abstract

The line-shape and saturation analysis for inhomogeneously broadened ESR spectra, developed in the previous paper, is used to study the change of the PPA/Fe-A1 resonance signal with temperature and polymer aging. A mechanism for radical formation, involving backbone cyclization reactions, is proposed to interpret the irreversible increase in spin concentration observed above -70 "C. As before, the spectra of the soluble and insoluble fractions of the polymer are interpreted as arising largely from triphenylcyclohexadienyl radicals formed upon cyclization of a cis-cisoid hexatriene precursor. An analysis of the resonance signal of the catalyst revealed that a low A1:Fe ratio a t the active polymerization site leads to a high concentration of the cis-cisoid conformation. The presence of oxygen-complexed radicals with shorter spin-lattice relaxation times is inferred from saturation experiments.

Diminished Helical Character in Para-Substituted Cis-Transoidal Polyphenylacetylenes Due to Intramolecular Cyclization

“…14 In solution and in bulk, 1,3-cis-5-hexatriene sequences in PPA and its substituted analogues undergo electrocyclization to form cyclohexadiene repeat units. 1,[15][16][17] We expect a dramatic influence on the observed chiroptical properties of helical cis-transoidal PPAs with different cis content. 1 The mole fraction of cis (92%), trans (4%), and cyclohexadiene (4%) repeats units 18 Even at the highest temperature measured (75 °C, C 6 H 6 ), a helical structure was apparent.…”

“…Helical c is- and trans -poly(alkylacetylenes) exhibit different CD spectra in solution . In solution and in bulk, 1,3- cis -5-hexatriene sequences in PPA and its substituted analogues undergo electrocyclization to form cyclohexadiene repeat units. ,− We expect a dramatic influence on the observed chiroptical properties of helical cis-transoidal PPAs with different cis content is the same in both cis -(−)-poly[4-(menthO 2 C)PA] and cis -(−)-poly[4-(menthO 2 CN)PA].…”

Independent Electrocyclization and Oxidative Chain Cleavage along the Backbone of cis-Poly(phenylacetylene)

“…49,51,55 Bulk thermal treated PPA produced oxidation products. 52,57 By contrast, bulk samples annealed at 180 °C for 1 h in air are indistinguishable from those under vacuum. 51 Additives and impurities complicate efforts to draw connections between experiments.…”

Dramatic decrease of the cis content and molecular weight of cis‐transoidal polyphenylacetylene at 23 °C in solutions prepared in air

“…The decrease in molecular weight at room temperature affected the determination of the molecular weight of cis ‐transoidal PPA both by gel permeation chromatography (GPC) and by light scattering experiments 24–26. An electrocyclization mechanism was suggested27 for the intramolecular cyclization process in solution and in bulk 1, 3…”