Polymerization Mechanism of α,α’-bis(Tetrahydrothiophenio)-β-xylene Dichloride

Abstract

A experimental study was performed regarding the influence of the base nature and solvent on the reactive intermediate concentration in the base-promoted bissulfonium salts polymerization. Such polymerization reaction is part of a synthetic procedure used to prepare conjugated polymers. In addition, a theoretical study suggest that a one-electron transfer could be involved in the initiation step.

Introduction

The interest on poly(p-phenylene vinylene)s, PPV, lies on its unique photoconducting, electroactive, and non-linear optical properties. Although there are numerous ways to synthesize PAV's, the route through a precursor polyelectrolyte, IV, as show in Scheme 1, yields the highest molecular weight attainable for these systems and allows to cast films of very good optical quality. It is well known that the reactive intermediate III is formed in situ when the bissulfonium salts are treated with a base[1]. However, the polymerization reaction mechanism is still not known in detail, thus radical and anionic mechanisms were proposed; being the first mechanism the most accepted at present [2]. Moreover, the initiator nature and the termination mechanism step are not known.

Experimental

The appearance and decay of the intermediate III in the reaction mixture was observed through its band in the UV-Vis spectra; λ_max 312nm, using water or water:acetonitrile-(1:4) as solvents and a spectrophotometer equipped with temperature controlled sample chambers at 25°C. The molecular modeling was performed with the semiempirical programs PM3 and AM1.

Results and Discussion

The UV-Vis spectroscopy study showed that a decrease in solvent polarity accelerated the formation of the intermediate III as well as its decay. We also observed that higher concentrations of the ylid did not affect III decay rate. However, III decay rate was dependent on base concentration. These results may suggest that the base OH^- could be involved in the polymerization initiation step either as a electron-transfer agent or as polar group that promotes secondary reactions which produce freeradicals [3]. Nevertheless, additional studies are necessary to confirm these assumptions. As a initial step towards this objective, a computational study was carried out in order to determine if the base OH^- can act as a electron-transfer agent. Therefore, the products and reactants heat of formations of the reaction III + OH^- was calculated by semiempirical methods. These calculations indicated that the electron-transfer reaction was thermodynamically feasible and that the LUMO(III) has a lower energy than the HOMO(OH^-). In addition, the calculations were repeated in the solvent box, water.