METAL-CATALYZED DEHYDROPOLYMERIZATION OF SECONDARY STANNANES TO HIGH-MOLECULAR-WEIGHT POLYSTANNANES

The first high molecular weight polystannanes, H(SnR(2))(n)H (R = (n)B u, (n)Hex, (n)Oct), result from dehydropolymerization of secondary sta nnanes R(2)SnH(2) by zirconocene catalysts. Good catalysts include zir conocenes based on both CpCpZr (Cp* = eta(5)-C(5)Me(5)) and Cp(2)Zr f ragments, and the most active catalyst with respect to production of h igh molecular weight polystannanes was Me(2)C(eta(5)-C5H4)(2)Zr[Si(SiM e(3))(3)]Me. The latter catalyst produced H(Sn(n)Bu(2))(n)H chains (M( w)/M(n) = 66 900/20 300) that were contaminated by ca. 18% (by weight) low molecular weight cyclic oligomers. Lower molecular weights result ed from dehydropolymerizations of Me(2)SnH(2), PhMeSnH(2), and Ph(2)Sn H(2). At room temperature, H(SnR(2))(n)H (R alkyl group) polystannanes have lambda(max) values at ca. 380-400 nm, attributed to sigma --> si gma transitions. Thermal gravimetric analyses on the polystannanes re veal that these polymers are as thermally stable as related poly(dialk ylsilane)s and have onset temperatures for thermal decomposition in th e range 200-270 degrees C, under both nitrogen and air. The H(Sn(n)Bu( 2))(n)H polymer has been shown to be a good precursor to SnO2, as show n by bulk pyrolysis in air (ceramic yield: 56%). Preliminary results a lso indicate that these polymers may be useful as precursors to elemen tal tin. The polystannanes are photosensitive, and their photobleachin g behavior has been characterized by UV-vis spectrometry and Sn-119 NM R spectroscopy, which demonstrated that H(Sn(n)Bu(2))(n)H is photochem icaly depolymerized to a 2:1 mixture of cyclo-(Sn(n)Bu(2))(5) and cycl o-(Sn(n)Bu(2))(6). The polymers H(Sn(n)Hex(2))(n)H and H(Sn(n)Oct(2))( n)H exhibit thermochromic behavior which is visibly evident as a disco loration from yellow to colorless upon warming above room temperature. This reversible behavior is associated with an abrupt change in lambd a(max) (e.g., from 402 to 378 nm for films of H(Sn(n)Oc(2))(n)H) and a phase change at ca. 40 OC (by differential scanning calorimetry). Thi n films of H(Sn(n)Bu(2))(n)H and H(Sn(n)Oct(2))(n)H on glass slides we re doped by exposure to SbF5 vapor to conductivities of 10(-2) and 0.3 S cm(-1), respectively. Preliminary experiments suggest that the dehy dropolymerization occurs by a sigma-bond metathesis mechanism involvin g four-center transition states. A previous report on production of hi gh molecular weight poly(dibutylstannane) by the reductive coupling of (n)Bu(2)SnCl(2) by Na/15-crown-5 was reinvestigated and found to prod uce only low molecular weight material with M(w)/M(n) = 2400/ 1200.