Up-down disorder in the crystal structure of form III of isotactic of poly(4-methylpentene).

The crystal structure of form III of isotactic poly(4-methyl-1-pentene) is refined with the full-profile X-ray powder diffraction refinement procedure (Rietveld method). Chains with 4/1 helical conformation are packed in a tetragonal unit cell according to the space group I4(1). In the limit-ordered regions of the crystal every chain is surrounded by four enantiomorphous and isoclined chains. A small amount of disorder, characterized by statistical substitution of anticlined helices, could be present at boundary regions which separate pieces of ordered crystals, composed by all up helices and all down helices, coherently juxtaposed