Synthesis and Characterization of Linear Poly(dialkylstannane)s
journal contributionposted on 30.10.2007, 00:00 by Fabien Choffat, Susanne Käser, Pascal Wolfer, Daniel Schmid, Raffaele Mezzenga, Paul Smith, Walter Caseri
A comprehensive study was made of the synthesis of a spectrum of poly(dialkylstannane)s by catalytic dehydropolymerization of dialkylstannanes (dialkyltin dihydrides, R2SnH2, prepared by reduction of R2SnCl2), with R = ethyl, propyl, butyl, pentyl, hexyl, octyl, and dodecyl. The polymerization reactions were followed by 1H and 119Sn NMR spectroscopy, IR spectroscopy (disappearance of the Sn−H vibration), and quantitative measurement of H2 which evolved during the reaction. Among the numerous metal complexes employed as catalyst, [RhCl(PPh3)3] was found to be particularly suited for the preparation of these inorganic polymers. The reaction parameters temperature, solvent, R2SnH2 concentration, and [RhCl(PPh3)3]/R2SnH2 ratio were varied, with the most prominent influence on the monomer conversion being the temperature. The number-average molar masses of the polystannanes were in the range of 1 × 104 to 1 × 105 g/mol, depending on the reaction conditions. For the generic case of the polymerization of Bu2SnH2 with [RhCl(PPh3)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(PPh3)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.