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 str uctures of the urea-poly(trimethylene adipate) (PTA) and the urea-poly (trimethylene glutarate) (PTG) adducts were established from three-dim ensional X-ray data. The crystals belong to the hexagonal system, spac e 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 an d a = 8.191(5) angstrom and c = 11.041(2) angstrom for the urea-PTG ad duct. The crystal structures were established by direct methods from 4 16 unique reflections in the first case and from 406 unique reflection s 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 hexagon al channels much like a honeycomb. The PTA or the PTG aliphatic polyes ter chain, the guest, is located within the hexagonal channel. As in t he case of the urea-PTHF adduct, which is ''isostructural'', the C and O atoms are indistinguishable. The PTA and PTG polyester chains are i n the nearly all-trans conformation. The morphology of the crystals, a s revealed by scanning electron microscopy, consists of stacks of hexa gonal platelets. DSC measurements indicate that there is no phase chan ge as the temperature is lowered to -150-degrees-C. The melting temper atures 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 f ormation indicate that the polydispersity indices decrease significant ly, 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 conside red polymer.