Nonionic Water-Soluble Polysilynes. Synthesis and Properties of a Novel Class of Functionalized Materials

Abstract

The nonionic water-soluble polysilynes poly(4,7,10-trioxaundecylsilyne) (1) and poly(4,7,-10,13-tetraoxatetradecylsilyne) (2), which are inaccessible using the conventional Wurtz-type coupling with Na in refluxing toluene, have been prepared in reasonable yields using graphite potassium C 8 K as the reducing agent in THF at 0 °C. A water-insoluble analogue of 1, viz. poly(4,7,10-trioxahexadecylsilyne) (3), is obtained in nearly quantitative yield under similar conditions. Despite the fact that 1 and 2 possess all the characteristic polysilyne-like (photo)physical properties, aqueous solutions of 1 and 2 unexpectedly exhibit thermoresponsive behavior; i.e., at 49 °C a lower critical solution temperature (LCST) is found. The presence of an LCST, which has to originate from folding/unfolding processes of the polysilyne backbone, suggests that polysilynes have a hybrid structure with a predominantly one-dimensional overall appearance consisting of linear fragments with small branches and/or incorporated (branched) cyclics, instead of the previously proposed extended sheetlike and/or hyperbranched/dendritic structures. Additional support for a hybrid structure was given by semiempirical PM3 calculations on a variety of oligomeric model compounds. The PM3 results suggest that Si-Cl moieties incorporated in oligomers will be more reactive than monomeric Si-Cl groups. The calculations further indicate that linear chain extension is preferred over branching. Cyclic voltammetry in combination with absorption/excitation spectroscopy reveals that in going from the related polysilane to the polysilyne the valence band edge shifts ca. -0.7 V, while the conduction band edge remains virtually unchanged. Furthermore, it is demonstrated that polysilynes 1 and 2 are effective photoinitiators for radical polymerizations upon excitation at λ 400 nm. This is exemplified for the conversion of methylacrylate into poly(methylacrylate).