No abstract

Colloids of TiO2, where rutile was the only crystal modification which could be detected, with ca. 2.5 nm average particle diameter were synthesized by hydrolysis of TiCl4 in acidic solutions. The as‐prepared particles were incorporated in polymers such as poly(vinyl alcohol) (PVAL), partially hydrolyzed poly(vinyl acetate) (PVAC88), polyvinylpyrrolidone, and poly(4‐vinylpyridine). Nanocomposites transparent in the visible range were obtained. The highest TiO2 contents in such materials were achieved with PVAL and PVAC88, with TiO2 contents of ca. 35 wt.‐% (i.e. 10.5 vol.‐%). In particular, the nanocomposites with TiO2 contents above 24 wt.‐% acted as efficient UV filters for radiation up to ca. 360 nm. At very low TiO2 contents, an absorption maximum of the embedded TiO2 particles was observed at 225 nm with an extinction coefficient of 140 000 cm−1 and a full width at half maximum of 45 nm, i.e. not only the absorption at the maximum at 225 nm but also at the flank of this band contributed significantly to the broadband UV absorption in the nanocomposites at higher TiO2 fractions. The incorporation of TiO2 enhanced the refractive index of the nanocomposites: for instance a refractive index of 1.609 was measured for a nanocomposite comprising 10.5 vol.‐% TiO2 in PVAL, compared with 1.521 for the pristine polymer. TEM image of a section of a nanocomposite of poly(vinyl alcohol) and 11 wt.‐% TiO2 (appearing dark).magnified imageTEM image of a section of a nanocomposite of poly(vinyl alcohol) and 11 wt.‐% TiO2 (appearing dark).

We present a facile, high-yield synthetic route to linear poly(dibutylstannane) of relatively high molecular weight (M w = 20 6 10 3 g mol 21 ) through dehydropolymerization of dibutylstannane with the catalyst [RhCl(PPh 3 ) 3 ]. Unlike previously synthesized polystannanes, the present materials were entirely free of cyclic oligomers. A reversible phase transition was observed at a temperature of y1 uC, which was found to be associated with a marked change in crystalline order. The material appeared to be in a liquid-crystalline-like state at room temperature and could be readily oriented by simple shearing processes, which resulted in highly ordered films.

A comprehensive study was made of the synthesis of a spectrum of poly(dialkylstannane)s by catalytic dehydropolymerization of dialkylstannanes (dialkyltin dihydrides, R 2 SnH 2 , prepared by reduction of R 2 SnCl 2 ), with R ) ethyl, propyl, butyl, pentyl, hexyl, octyl, and dodecyl. The polymerization reactions were followed by 1 H and 119 Sn NMR spectroscopy, IR spectroscopy (disappearance of the Sn-H vibration), and quantitative measurement of H 2 which evolved during the reaction. Among the numerous metal complexes employed as catalyst, [RhCl(PPh 3 ) 3 ] was found to be particularly suited for the preparation of these inorganic polymers. The reaction parameters temperature, solvent, R 2 SnH 2 concentration, and [RhCl(PPh 3 ) 3 ]/R 2 SnH 2 ratio were varied, with the most prominent influence on the monomer conversion being the temperature. The numberaverage molar masses of the polystannanes were in the range of 1 × 10 4 to 1 × 10 5 g/mol, depending on the reaction conditions. For the generic case of the polymerization of Bu 2 SnH 2 with [RhCl(PPh 3 ) 3 ] as catalyst, it was demonstrated that poly(dibutylstannane) did not form by a random polycondensation, but by growth at a rhodium complex, whereby only a minor part of [RhCl(PPh 3 ) 3 ] was converted to catalytically active species by reaction with tin hydrides. The polymers featured phase transitions into liquid-crystalline states, on occasion at remarkably low temperatures. A particularly high phase transition temperature was observed for poly(dipropylstannane), which also was characterized by a high density, indicative of a particularly favorable packing of the propyl groups.