On the structure of poly(tetramethylene terephthalate). Structural, infrared, and Raman studies of three tetramethylene glycol dibenzoate derivatives, models for poly(tetramethylene terephthalate)

The crystal structures of tetramethylene glycol dibenzoate (1), tetramethylene glycol di-pora-chlorobenzoate (2), and tetramethylene glycol di-para-nitrobenzoate (3) have been solved by direct methods in order to establish the possible conformations of the methylenic sequence in poly(tetramethylene terephthalate), poly(4GT). Compound 1 has a triclinic unit cell of dimensions a = 7.870(3), b = 8.574(3), c = 12.993(4) Å α = 83.76(3), β = 89.92(3), γ = 64.68(3)°; Z = 2, space group [Formula: see text]. For 2 the unit cell is also triclinic and has dimensions a = 5.916(3), b = 7.599(2), c = 10.404(2) Å; α = 67.81(2), β = 77.47(2), γ = 81.63(3)°; Z = 1, space group [Formula: see text]. The unit cell dimensions of 3 are a = 6.086(1), b = 11.475(3), c = 13.162(3) Å and β = 101.93(2)°, Z = 2. The space group is P21/c. The structures were solved by direct methods using 2682, 969, and 781 observed reflections for 1, 2, and 3, respectively. The refinements were concluded when R reached the values of 0.042, 0.046, and 0.045, respectively. The conformation of the methylenic sequence varies depending on the substituent on the benzoate group. The conformation observed in 1 is tg−t t t, it is t t t t t for 2, and tg−tgt for 3 (t = trans, g = gauche). Although the equivalent fiber repeat of 3 coincides with the observed fiber repeat for α-poly(4GT), this does not imply that 3 and α-poly(4GT) have the same conformation. Compound 2 allows for the modelling of the β-form of poly(4GT). However, this does not lead to a clear choice between the two structures reported for this form. The two distinct conformations proposed for the β-form of poly(4GT) are distinguished on the basis of their ir and Raman spectra. The comparison of the spectra of the model compounds reveals that, in ir, for an all-trans conformation there is no absorption band at 1383 cm−1 but one is present at 1395 cm−1. This situation that is observed in the ir spectra of β-poly(4GT) led us to propose that this form has the all-trans conformation as proposed by Hall and Pass. This choice is further supported by the presence of three bands at 1047, 1347, and 1405 cm−1 in the Raman spectra of β-poly(4GT). These three bands are only observed in the model compounds having the all-trans conformation.