Insight into the Thermal Ring-Opening Polymerization of Phospha[1]ferrocenophanes

A mixture of cis/trans isomers of phospha[1]ferrocenophanes equipped with one iPr group at the α position to the bridging PhP moiety was prepared. Both isomers (cis-4 and trans-4) were obtained as racemates and could be separated so that their thermal properties were investigated individually. The molecular structure of cis-4 was determined by single-crystal X-ray analysis showing a tilt angle α=26.35(8)°. Interconversion between both isomers occurred in the melt at elevated temperatures and revealed that the trans isomer is thermodynamically more stable. Structural and thermodynamic data was complemented by DFT calculations (B3PW91/6-311+G(d,p) and B3PW91-D3(BJ)/6-311+G(d,p)). Performance of thermal ring-opening polymerization (ROP) of trans-4 at 230 °C gave polymers and cyclic oligomers. Gel permeation chromatography (GPC) of the sulfurized polymer resulted in a molecular weight of 62.5 kDa (M_w ) and a polydispersity index of 1.39 (PDI). Mass spectrometric analysis of the oligomers showed the presence of cyclic species from dimers to heptamers. After sulfurization, preparative thin layer chromatography led to the separation of three isomeric dimers. Structural characterization of these dimers by single-crystal X-ray analysis led to the conclusion that the Fe-Cp bond breaks during the thermal ROP process. A mechanism similar to the known mechanism of the photolytic ROP of ferrocenophanes is proposed.