Degradation and cis-to-trans isomerization of poly[(2,4-difluorophenyl)acetylene]s of various initial molecular weight: SEC,NMR, DLS and EPR study

Molecular weight (MW) and configurational stabilities of a series of high-cis poly[(2,4-difluorophenyl)acetylene]s (PdFPhA, M_w from 2.2 × 10⁴ to 9.7 × 10⁵) exposed to the atmosphere and diffuse daylight are reported. Polymers dissolved in THF-d₈ undergo simultaneous cis-to-trans isomerization and degradation under formation of a bimodal MW distribution. Degradation and isomerization rates are tightly connected and decrease with decreasing MW of the parent PdFPhAs. This finding may be interesting for the functional applications of substituted polyacetylenes. The partly aged PdFPhAs contain two separable fractions: higher-MW high-cis macromolecules containing a low amount of unpaired electrons and lower-MW deeply isomerized cis/trans macromolecules enriched in the unpaired electrons content. The difference in MW of these fractions reflects the various degradation rates of isomerized and high-cis macromolecules. Only a slow degradation without changes in the configurational structure proceeds if PdFPhA is allowed to age in the solid state. The suppression of the cis-to-trans isomerization reflects the suppression of motion of the polymer chains segments in the solid state.