Structural Studies of Polyesters. IV. Molecular and Crystal Structures of Poly(ethylene succinate) and Poly(ethylene oxalate)

Abstract

In the course of structural studies of aliphatic polyesters of the type [-O-(CH2)x-O-CO-(CH2)y-CO-],., the molecular and crystal structures of poly(ethylene succinate) (x=2, y=2) and poly(ethylene oxalate) (x=2, y=O) were determined by interpretation of X-ray diffraction patterns and complementary study of infrared spectra. The crystallographic data are: Poly(ethylene succinate); a=7.60 A, h= 10.75 A, c (fiber axis)=8.33 A, orthorhombic space group Pbnb-D~~. four molecular chains pass through the unit cell. Poly(ethylene oxalate); a=6.44 A, h=6.22 A, c (fiber axis)= 11.93 A, orthorhombic space group Pbcn-D~t. two molecular chains pass through the unit cell. The chain conformation of poly(ethylene succinate) is T3GT3G, in which the two C(H2)-C(H2) bonds in the chemical repeating unit are G or G, where T means trans form and G and G are used to discriminate right-handed and left-handed gauche forms, respectively. The chain conformation of poly(ethylene oxalate) is TsGTsG, in which the C(H2)-C(H2) bond is G or G. There are some close similarities between the molecular and crystal structures of poly(ethylene succinate) and poly(ethylene oxalate), and their structures are quite different from polyethylene-like structures for the higher members (x=2, y::::4) of the ethylene glycol series. The high melting points of poly(ethylene succinate) and poly(ethylene oxalate) among the homologous polymers may be related to these molecular and crystal structures. KEY WORDS X-ray Diffraction/Infrared Spectra/Crystal Structure/ Poly(ethylene succinate) / Poly(ethylene oxalate)/ Conformation/ Crystal structures of various kinds of aliphatic polyesters of the type [-O-(CH2).-O-CO-(CH2)y-CO-]n were first studied by Fuller and his collaborators .1-4 As for polyesters of the ethylene glycol series (x=2), they found that the observed lengths of chemical repeating units of the numbers of y higher than 4 were slightly shortened (about 0.5 A) from the calculated lengths of the fully extended zigzag forms, and the even members of y were crystallized in monoclinic forms while the odd members of y were in orthorhom-bic forms, though the lateral arrangements of the chains in these members were quite similar to that found in polyethylene. 5