STRUCTURE OF THE HARD SEGMENTS IN NONHYDROGEN BOND-FORMING POLYURETHANE ELASTOMERS AS DERIVED FROM OLIGOMERIC MODEL COMPOUNDS

Several molecularly uniform oligourethane model compounds for the hard segment of segmented polyurethane elastomers with piperazine (PIP) and tetramethylene bischloroformate based hard segments have been synthesized. X-ray structure analysis was performed on single crystals with both compounds representing different parts of the piperazine-1,4-diylcarbonyloxy-tetramethyleneoxycarbonyl repeating unit or containing at least one complete repeating unit and methylurethane end groups. A common feature of the model compounds investigated was that the PIP rings are in the chair conformation; the oxytetramethyleneoxy unit is in the g+ tg- conformation, which is also the conformation of the higher melting modification of the two hard segment model polymorphs. The conformation and packing model of the hard segment was derived from the crystal structure of the model compound containing one repeating unit by connecting the two methyl end groups of successive molecules by a CH2CH2 unit without shifting the molecules, i.e., by replacement of two opposing OCH3 groups by an oxytetramethyleneoxy chain in the g+ tg- conformation. The resulting cell for the polyurethane model structure is triclinic and contains two units of half a polymer repeat, and is in excellent agreement with the experimental X-ray powder diffraction pattern of the hard segment polymer.