Sulfophthalide cycle cleavage and formation of fluorenyl structures upon poly(diphenylene sulfophthalide) thermolysis

Abstract

Thermolysis of poly(diphenylene sulfophthalide) (PDSP) in the temperature range from 100 to 500 °C was studied by IR and UV-Vis spectroscopy and thermogravimetric analysis. A series of absorption bands in the IR spectrum of PDSP were assigned on the basis of the theoretical calculations of the IR spectrum of diphenyl sulfophthalide used as a model compound, in particular, ν_as(S=O) = 1352 cm⁻¹, ν_s(S=O) = 1196 cm⁻¹, ν(C-O) ∼ 920 cm⁻¹, ν(S-O) = 824 cm⁻¹, and δ(SO₂) = 576 cm⁻¹. The sulfophthalide cycle (SPC) in PDSP decomposes at the thermolysis temperatures in a range of 260–400 °C. An analysis of the IR spectra of the thermolyzate and the quantum chemical calculations of the IR spectra of the model compounds confirmed the predominant formation of fluorenyl structures in the thermolyzed polymer. The changes in the UV-Vis spectra observed upon the thermolysis of thin films of PDSP (the hypsochromic shift of the long-wavelength absorption band from 271 to 263 nm and the appearance a shoulder at ∼310 nm) and the results of TD-DFT calculations of the UV-Vis spectra of the model compounds are consistent with the hypothesis about the formation of fluorenyl structures. The general scheme of PDSP thermolysis at 260–400 °C was proposed in which the major process is the formation of fluorenyl fragments in macromolecules of the polymer due to the intramolecular ring closure in biradicals formed by the SPC cleavage.