Poly(diethyl trimethylene-1,1-dicarboxylate) oligomers with an average degree of polymerization (DP) of 20 and a polydispersity of 1.13 have been isothermally crystallized at 30 degreesC from benzene and also crystallized from the melt (T-m = 176 degreesC). The crystals were investigated using electron microscopy (imaging and diffraction), X-ray diffraction, and computational modeling. The diffraction signals index on an orthorhombic unit cell with parameters a = 1.554 +/- 0.002 nm, b = 1.136 +/- 0.002 nm, and c (chain axis) = 0.585 +/- 0.002 nm. A space group of P2(1)2(1)2(1) is assigned on the basis of systematic absences of h00, 0k0, and 00l for odd h, k, and l respectively; thus, two antiparallel chain segments pass through the basal ab plane of the unit cell. The calculated density of 1.20 g.cm(-3) is commensurate with the measured value of 1.17 +/- 0.03 g.cm(-3). The repetitive -CH2-CH2-CX2- backbone conformation, where the side groups X are ethyl esters (-COOC2H5), has the torsional angle sequence GTGGTG, with the trans bonds at the dimethylene C-CH2-CH2-C Sites. This conformation generates a 2-fold helix with a pitch that matches the measured crystallographic c-value of 0.585 rim. Ellipsoidal-shaped, lamellar-like crystals, with lateral dimensions on the micron scale but only 7.23 mn thick, are obtained by isothermal crystallization from benzene at 30 degreesC. Electron and X-ray diffraction data show that the molecular-axes are orthogonal to the lamellar surface and that the crystal thickness matches a DP of 20. Thus, a degree of selective crystallization of molecular length occurs in these solution-grown crystals at fixed temperature. Under appropriate conditions, the crystals also stack to generate a one-dimensional lattice. This feature does not occur in the melt-crystallized fibers; in this case, the lamellar-like character is suppressed as the polydisperse molecules interpenetrate to form a crystal with a greater coherence length in the c-direction but, of course, with chain-end defects.