Nonionic water-soluble polysilynes. Synthesis and properties of a novel class of functionalized materials

The nonionic water-soluble polysilynes poly(4,7,10-trioxaundecylsilyne) (1) and poly(4,7,10,13-tetraoxatetradecylsilyne) (2), which are inaccessible u sing the conventional Wurtz-type coupling with Na in refluxing toluene, hav e been prepared in reasonable yields using graphite potassium C8K as the re ducing agent in THF at 0 degrees C. A mater-insoluble analogue of 1, viz. p oly(4,7,10-trioxahexadecylsilyne) (3), is obtained in nearly quantitative y ield under similar conditions. Despite the fact that 1 and 2 possess all th e characteristic polysilyne-like (photo) physical properties, aqueous solut ions of 1 and 2 unexpectedly exhibit thermoresponsive behavior; i.e., at 49 degrees C a lower critical solution temperature (LCST) is found. The prese nce 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 linea r fragments with small branches and/or incorporated (branched) cyclical ins tead of the previously proposed extended sheetlike and/or hyperbranched/den dritic structures. Additional support for a hybrid structure was given by s emiempirical PM3 calculations on a variety of oligomeric model compounds. T he PM3 results suggest that Si-Cl moieties incorporated in oligomers will b e more reactive than monomeric Si-Cl groups. The calculations further indic ate that linear chain extension is preferred over branching. Cyclic voltamm etry 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 unchang ed. Furthermore, it is demonstrated that polysilynes 1 and 2 are effective photoinitiators for radical polymerizations upon excitation at lambda 400 n m. This is exemplified for the conversion of methylacrylate into poly(methy lacrylate).