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.