PET versus PEN: what difference can a ring make?

Poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) are structurally related polyesters. In each polymer, the ethylene glycol diesters are separated by rigid rings and are attached to the 1,4-positions of the phenyl and the 2,6-positions of the naphthyl rings in PET and PEN, respectively. Because neighboring ethylene glycol units of each polyester a re separated by phenyl or naphthyl rings, their conformations are independe nt of each other. As a consequence, their RIS conformational models should be identical, with the same populations of trans, gauche +, and gauche- con formations about the -O-CH2-, -CH2-CH2-, and -CH2-O- bonds. This means that PET and PEN are equally flexible as judged by their conformational partiti on functions. However, because they differ geometrically, properties such a s the mean-square end-to-end distance (<r(2)> (0)) or characteristic ratio (C-r = <r(2)> (0)/n <l(2)>), though averaged over identical conformations, are not expected to be coincident. The terephthaloyl portion of PET can be considered to consist of the [GRAPHICS] bonds, which are collinear and only the conformations about the carbonyl ca rbon to phenyl ring carbon bonds may be altered. This results in the tereph thaloyl unit acting as a freely rotating link in both the statistical and d ynamic senses. In the naphthaloyl residue, on the other hand, the carbonyl carbon to C-2 and C-6 to carbonyl carbon bonds are connected to a collinear , non-rotatable virtual bond between C-2 and C-5 and to the non-collinear, non-rotatable real bond between C-5 and C-6, respectively. These geometrica l differences between PET and PEN result in distinctly different values for properties like <r(2)> (0) and C-D even though they are averaged over the same conformational populations. Additionally, volumes occupied by their se gments when confined to extended conformations and interconversions between these extended conformers were found to be particularly sensitive to the g eometrical distinctions between PET and PEN and several differences in thei r physical properties are discussed in this context. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.