Copolyester sequence distribution by 60, 100, and 220 MHz PMR

The proton magnetic resonance spectra at frequencies of 60, 100, and 220 MHz of two bisphenol A–neopentyl glycol–terephthalate copolyesters having the same overall composition but different sequence distributions are correlated. Analysis of the terephthalate proton resonances indicates the usual singlets for the two symmetrical diester sequences. However, the dissimilar protons in the unsymmetrical diester give rise to an apparent quartet originating from an incompletely resolved AA′BB′ spectrum. The apparent quartet coupling constant is accurately determined to be 8.03 Hz by a 220 MHz separation of the appropriate peaks and is found to be approximately 8.3 Hz by employing a shift reagent [Pr(fod)3] in conjunction with suitable model compounds at 100 MHz. This permits an approximate calculation of the entire quartet intensity in those cases where the outside resonances of the latter and the singlets are superimposed, resulting in a more accurate calculation of sequence distribution. In particular, a copolyester prepared by a normal two-stage, interfacial polycondensation yields a degree of randomness B estimated to be slightly greater than 1, indicating a nearly random distribution of the comonomers. A second copolyester, prepared so as to enhance an alternating comonomer sequence, exhibits a B value of about 1.85.