Polymeric selenium

It has been shown recently that at 20 °C the unstable iodosulfanes (n = 1,2,...) decompose to give homocyclic sulfur molecules and iodine In a similar fashion SCjIj prepared from Se Clj and solid KI in carbon disulfide solution produces Seg and polymeric selenium Mixed sulfur-selenium ring molecules can be prepared by the reaction of mixtures of Se2Cl2 and chlorosulfanes (S Cl2 n = 1, 2, 4, 6-8) with potassium iodide in carbon disulfide solution. The subsequent decomposition of the unstable iodides results in the formation of mixed SeoS8 molecules. The general reaction scheme can be presented as follows ... 

Fig. 9. Melting kinetics and crystallization kinetics of polymeric selenium (right) and polyethylene (left). The equilibrium melting temperatures are 494.2 and 414. 6K. The dotted curve indicates that on crystallization of the macromolecule from small molecules Sc2 there is no molecular nucle-ation necessary as in the melt crystallization (see also ref. 43 for a more detailed discussion of Se crystallization and melting). Drawn after Ref. 4,)...

The crystalline state configuration of polymeric selenium is a helix in which all the rotational angles are of the same sign with a value of 78°. This is fairly close to the expected value of 90°.52 The barrier to rotation about the Se-Se bond is thought to be roughly the same as that about S-S bonds. No reliable experimental results are available on the statistical properties of these chains. Some values of the glass-transition temperature of polymeric selenium have been reported15 and these could at least provide a measure of the dynamic flexibility of the chains. However, these results are probably compromised by the presence of cyclic selenium molecules that act as plasticizer. 

Standard entropy of cyclization caldeg mo Polymeric selenium to cyclooctaselenium -5.5 (4)... 

Assistant Dean of the College of Engineering, and Director of the Polymer Research Center, University of Cincinnati, Ohio Poly(dimethylsiloxanes), cychc Polymeric selenium Polymeric sulfur... 

Polymeric selenium Polymeric sulfur Polyphosphates Poly (pho sphonate) Poly (sulfur nitride) Poly (y iny IfeiTOcene)... 

Poly (feiTocenyldimethylsilane) Polygermanes Polymeric selenium Polymeric sulfur Polyphosphates... 

The analysis of heat capacity of a given homopolymer thus starts with the evaluation of the experimental crystalline and amorphous heat capacities over as wide a temperature range as possible. For amorphous polymers, the glassy and liquid heat capacities are directly measurable. For crystallizing polymers, the crystalline and amorphous heat capacities may have to be extrapolated, as illustrated in the polyethylene example in Fig. 5.17. Only in rare cases are almost completely crystalline polymers samples available (as for example, for polyethylene, polytetrafluoroethylene, polymeric selenium, and polyoxymethylene). 

How much of these values is size dependent, and how much is a property of the polymer, is not known at present. The relatively low value for polymeric selenium is an indication of the size of the molecules in the solid state. 

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