All-gauche conformation

It is known that polyoxymethylene in the crystalline phase takes the all gauche conformation with a 9/5 helix [21]. However, the amorphous phase has a distribution of gauche and frans-conformations. Figures 7.10 and 7.11 show the CPMAS NMR spectra and powder pattern spectra of polyoxymethylene, respectively [22]. Sample A is a polyoxymethylene single crystal. To produce Sample B, Sample A is heated to 200°C and then quenched in ice water. Sample C is a melt-quenched sample of bulk polyoxymethylene and sample D is a bulk polyoxymethylene heated and cooled at a rate of... 

The latter process may cause major distortions of the torsion an es about C-O bonds whereas in the complexes, C-C bonds are essentially invariant in being synclinal ( all-gauche conformation ) in order to place the ether oxy s close to the cation. It is noteworthy that if C-O bonds are forced into the synclinal conformation they usually adopt torsion angles larger than 70° to avoid short 1,4CH. .. HC contacts... 

Figure 2.4a presents as a further example a helix of poly(oxymethylene) (POM). It corresponds to an all-gauche-conformation. Different from poly-(tetrafluoroethylene), here a trans-rotational isomeric state also exists, but it does not represent the energy minimum. 

Several polymers form helices that are simple in the sense that AC is a fraction of 360°, like 180°, 120° or 90°. These are called m/l-helices, m giving the number of monomeric units arranged along one 360°-turn. Examples are polypropylene, polystyrene or poly-1-butene, which all form 3/1-helices. The all-trans-conformation of polyethylene and the all-gauche-conformation of poly(oxymethylene) correspond to 2/1-helices. The next general helical form is given by the m/n-helices. The name is meant to indicate that m monomeric units are equally distributed over n turns. 

Contrary to the case of polyethylene an all-gauche conformation is energetically preferred in polyoxymethylene. The first... 

The chemical shift between the resonance maxima in PE presumably arises from constraint to trans bond conformations in the crystalline region as opposed to trans/gauche conformations in the amorphous regions. Assuming the chemical shift difference (6.4 ppm) between the crystalline peak in PE and the resonance of cyclohexane which exists in an all gauche conformation can be attributed entirely to conformational effects, Earl and VanderHart have concluded that the 2.3 ppm shift between resonances for the crystalline and... 

---