STRUCTURAL INVESTIGATIONS OF UREA ALIPHATIC POLYESTER ADDUCTS

Linear aliphatic polyesters and urea form crystalline complexes of the host-guest type. Crystals of such complexes were obtained and the structures of the urea-poly(trimethylene adipate) (PTA) and the urea-poly(trimethylene glutarate) (PTG) adducts were established from three-dimensional X-ray data. The crystals belong to the hexagonal system, space group P6(1), and the unit cells have the following dimensions: a = 8.222(6) angstrom and c = 11.004(2) angstrom for the urea-PTA adduct and a = 8.191(5) angstrom and c = 11.041(2) angstrom for the urea-PTG adduct. The crystal structures were established by direct methods from 416 unique reflections in the first case and from 406 unique reflections in the second one. The stoichiometries of these adducts are: [urea]6.[O-(CH2)3-O-CO-(CH2)4-CO-]0.82 and [urea]6.[O-(CH2)3-O-CO-(CH2)3-CO-]0.90. The urea molecules forming the host structure are linked to one another through H bonds, and the resulting network consists of hexagonal channels much like a honeycomb. The PTA or the PTG aliphatic polyester chain, the guest, is located within the hexagonal channel. As in the case of the urea-PTHF adduct, which is ''isostructural'', the C and O atoms are indistinguishable. The PTA and PTG polyester chains are in the nearly all-trans conformation. The morphology of the crystals, as revealed by scanning electron microscopy, consists of stacks of hexagonal platelets. DSC measurements indicate that there is no phase change as the temperature is lowered to -150-degrees-C. The melting temperatures are clearly higher than that of pure urea while the enthalpies of melting, DELTAH(m), expressed per atom in the main chain, indicate a certain dependency upon the chain parity. The values of M(n)BAR, M(w)BAR, and the ratio M(w)BAR/M(n)BAR measured before and after adduct formation indicate that the polydispersity indices decrease significantly, from 1.40 to 1.23 for PTA and from 1.90 to 1.37 for PBA. Thus the complexation is selective and has a purification effect on the considered polymer.