Poly stannanes

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

As mentioned in the introduction, the simple alkyl find aryl stannylenes originally formulated as R2Sn are actually linear, branched, or cyclic poly-stannanes. One isomer of the compound diphenyltin is the cyclic hex-amer dodecaphenyl cyclohexastannane as shown by its X-ray crystallographic structure determination (62). 

The poly(stannane-co-germane) and poly(stannane-co-silane) materials exhibit kmax between those of the respective homopolymers. However, increasing the stannane contents in the copolymers results in a red shift of the UV absorption probably due to the effective overlap of orbitals ascribed to the increased atomic radius of tin over Ge(Si). As an example, for poly(di-n-butylstannane-ct -di-n-butylsilane) with an Sn Si ratio of 43 57, kmax = 375 nm, as compared to 314 and 384 nm for poly(di-n-butylsilane) and poly(di-n-butylstannane), respectively. 

The poly(dialkyl)stannanes absorb at Amax ca. 390-400 nm, and poly(diaryl)stannanes (with substituents in the aryl rings to increase the solubility) absorb at ca. 435 nm. This absorption is due to a transition, although the apparent... 

Sita and Babcock have also prepared high molecular weight poly(dibutyl-stannane) from dibutylstannane using Rh(H)(CO)(PPh3)3 as catalyst.147 As is common with late metal catalysts, branching of the polymer chains is observed as a result of alkyl group redistribution. 

Support-bound stannanes have been prepared from phenyllithium bound to macro-porous polystyrene and chlorostannanes [14,41], by treatment of support-bound alkyl chlorides with lithiated stannanes [21,41], and by radical or palladium-mediated addition of stannanes to alkenes and alkynes (Figure 4.7 [42-47]). The chloride of poly-styrene-bound chlorostannanes can be displaced by treatment with arylzinc reagents, thereby yielding resin-bound arylstannanes [46]. Polystyrene-bound stannanes have also been prepared by copolymerization of 4-[2-(dibutylchlorostannyl)ethyl]styrene with styrene and divinylstyrene [48],... 

TABLE 5. Representative examples for the preparation of poly(dialkyl)stannanes by Wurtz-type coupling A, Sml2-induced reduction B, electrochemical reduction C and dehydrogenative coupling D... 

Poly- and cyclic oligo-(dialkyl)stannanes S(n9Sn) Poly- and cyclic oligo-(diaryl)stannanes <5(119Sn)... 

The hyperbranched polymer, as obtained by the dehydropolymerization of n-Bu2SnH2 with a rhodium catalyst, has a kmax value of 394 nm98, which is slightly red-shifted with respect to high molecular weight poly(dibutyl)stannane (Table 7, entries 2-5). 

Poly(dioctyl)stannane and poly(dihexyl)stannane exhibit a reversible thermochromic behaviour as a result of a phase transition at ca 40 °C. A discolouration of the polymers was observed upon slightly warming above room temperature and variable-temperature UV-vis spectrometry showed a reduction of the >.max values of (n-Oct2Sn) in toluene solution from 384 to 369 nm and of (n-Hex2Sn) from 398 to 382 nm82. On the other hand, however, poly(dibutyl)stannane and poly(diaryl)stannane do not exhibit thermochromic behaviour in the temperature range between —10 to 90 °C and —20 to 90 °C82 100, respectively. 

The outcome is more complex in the naphthalene series, where both the position of attack and the competition by rearomatization depend on the reagent chosen and conditions. Monocyanonaphthalenes are benzylated only by />-methoxytoluene, or by m- or />-methoxyphenylacetic acids, not by (poly)methylbenzenes [23,199]. 1,4-Dicyanonaphthalene reacts easily with toluene and a variety of precursors of the benzyl radical (alkylbenzenes [200-205], benzylsilanes, stannanes [206], and borates [56], as well as phenylacetic acids) [207] giving three types of products, viz. l-benzyl-4-cyanonaphthalenes, 1- or 2-benzyl-l,2-dihydro-l,4-dicyanonaphthalenes and dibenzobicyclo[3.3.1]nonanes resulting from the formation of a second carbon-carbon bond (Schs. 12 and 13). 

Thermogravimetric analyses (TGA) of poly(dialkyl)stannanes under nitrogen showed onset temperatures for thermal decomposition in the temperature range of 250-280 °C, which is close to the values for related polydialkylsilanes. The TGA analyses of poly(diaryl)stannanes ° show onset temperatures for the decomposition in the range of 203-327 °C. Thermal decomposition of polystannanes leads to tin and tin oxide, and thus application of these materials for the preparation of conductors or coatings in semiconductors may be envisaged. It is worth noting that electronic conductivities of about 0.01 -0.3 S cm were observed for thin films of the polystannanes after exposure to SbFs vapor as an oxidant . ... 

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