Polymeric Sulfur and Selenium

This polymer is obviously totally inorganic, and can be made directly by the ring-opening polymerization of rhombic sulfur, which consists of eight-membered sulfur rings. 

Rhombic sulfur is a brittle, crystalline solid at room temperature. Heating to 113 °C causes it to melt to a reddish-yellow liquid of relatively low viscosity. Above approximately 160 °C, the viscosity increases dramatically because of the free-radical polymerization of the cyclic molecules into long, linear chains.6,8 14 30 47-51 At this point, a degree of polymerization of approximately 105 is obtained. If the temperature is increased to above approximately 175 °C, depolymerization occurs, as evidenced by a decreasing viscosity. A similar type of depolymerization occurs with the polysiloxanes discussed in Chapter 4. In thermodynamic terms, the negative -TAS term overcomes the positive AH term for chain depolymerization. (The temperature at which the two terms are just equal to one another is called the ceiling temperature for the polymerization.) 

In any case, if this polymerized form of elemental sulfur is quenched (cooled rapidly), it becomes a solid. This solid is glassy at very low temperatures, but becomes highly elastomeric above its glass-transition temperature of approximately -30 °c.6 8 14 30 The situation is complicated by the presence of unpolymerized S8 molecules which would certainly act as plasticizers. So far, attempts to cross-link the elastomeric form into a network structure suitable for stress-strain measurements have not been successful. The polymer is unstable at room temperature, gradually crystallizing, and eventually reverting entirely to the S8 cyclics. 

The ability of sulfur to form chains is also important in the sulfur vulcanization of diene elastomers such as natural rubber. In this case, strings of sulfur atoms of varying length form the cross-links that tie one chain to another. In this sense, they can be thought of as the short chains of a short chain-long chain bimodal network, as was described in Chapter 4. 

Cyclic Se8 molecules have also been polymerized by heating.30 These chains are described in the following section. 

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Distribution functions for the end-to-end separation of polymeric sulfur and selenium are obtained from Monte-Carlo simulations which take into account the chains geometric characteristics and conformational preferences. Comparisons with the corresponding information on PE demonstrate the remarkable equilibrium flexibility or compactness of these two molecules. Use of the S and Se distribution functions in the three-chain model for rubberlike elasticity in the affine limit gives elastomeric properties very close to those of non-Gaussian networks, even though their distribution functions appear to be significantly non-Gaussian. 

Stress-strain isotherms have also been calculated with this approach. Examples are unimodal networks of polyethylene and poly(dimethylsiloxane) (226), polymeric sulfur and selenium (227), short n-alkane chains (228), natural rubber (229), several polyoxides (230,231), and elastin (232), and bimodal networks of poly(dimethylsiloxane) (233). It is possible to include excluded volume effects (1), in such simulations (234). In the case of the partially helical polymer poly-oxymethylene, the simulations were used to resolve the overall distributions into contributions from unbroken rods, once-broken rods, twice-broken rods, etc. (231). It was also shown how applying stresses to the ends of chains of this type can be used to bias the distributions in the direction of increased helical content and increased average end-to-end distances (231). In this sense, imposition of a stress has the same effect on the helix-coil equilibrium as a decrease in the temperature (6). 

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