High-temperature polymers ring structures

The newest data show that in polydime thylsiloxanes, the molecular chains possess a helical structure with three to six silicon atoms in the helix [16, 17]. Under the action of high temperatures, such a structure creates favorable conditions for the closing of rings with chain cleavage. This is also confirmed by the experimental observations. Polydimethyl-siloxane decomposes into low-molecular cyclic polymers upon heating under vacuum at high temperatures, without appreciable cleavage of the Si—C bonds ... 

The opening of the caprolactam ring for nylon 6 involves an equilibrium reaction which is easily catalysed by water. In the case of nylon 12 from dodecanelactam, higher temperatures, i.e. above 260°C, are necessary for opening the ring structures but since in this case the condensation is not an equilibrium reaction the process will yield almost 100% of high polymer. ... 

As a result of needs for plastics and elastomers which are more stable to high temperatures and more resistant to degrading chemical reaction than presently available polymers, a study has been made of polymer structure with units consisting of oxadicizole rings connected by per-fluoroalkyl chains 17a). 

Attempts to reduce PPEs 12 under moderate pressures (2-3 bar H2) at temperatures of around 70-80 °C were unsuccessful. PPEs seem to be very stable chemically, and only when the reduction was performed in an autoclave under drastic conditions successful hydrogenation was observed. Hydrogen pressures of 300-500 bar at temperatures of around 300 °C are necessary for complete reduction of the PPE to the hydrogenated polymer 60 [40]. The yield of 60 is high the aromatic rings are not affected under these conditions and side products are not detected in the colorless materials formed. Attempts to use shorter reaction times, lower temperatures, or lower hydrogen pressures failed, and partially reacted material of undefined structure resulted. 

The high dielectric constant is characteristic of hydrogen-bonded liquids. Since HF forms only a two-dimensional polymer, it is less viscous than water. In the vapor, HF is monomeric above 80°C, but at lower temperatures the physical properties are best accounted for by an equilibrium between HF and a hexamer, (HF)6, which has a puckered ring structure. Crystalline (HF) has zigzag chains (Fig. 2-1). 

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