NMR Measurement of Identical Polymer Samples by Round Robin Method V. Determination of Degree of Polymerization for Isotactic Poly(methyl methacrylate) Having a t-Butyl End Group

Abstract

Research Group on NMR, The Society of Polymer Science, Japan, has continued assessments of the reliability of NMR measurements of polymers. In the present study, 1H and 13C NMR data for isotactic poly(methyl methacrylate) (it-PMMA) having a t-butyl end group were collected on 27 NMR spectrometers by a round-robin method, to survey the accuracy of NMR spectroscopic determination of degree of polymerization (DP) of the polymer by the end group analysis as well as other basic NMR parameters such as chemical shift and spectral resolution. Five samples of it-PMMAs whose DP’s range from 53 to 5200 were prepared with t-BuMgBr in toluene. 1H and 13C NMR spectra were measured in nitrobenzene-d5 at 110 °C. The standard deviations (σ) for chemical shift measurements of 1H and 13C NMR signals were 0.001–0.004 ppm and 0.04–0.18 ppm, respectively. A singlet signal due to t-butyl group at initiating chain end of the it-PMMA is separately observed from the signals due to the monomeric units in 1H NMR spectra measured in nitrobenzene-d5. Thus DP of the polymer can be determined by the equation of DP = 3 × [OCH3]/[t-C4H9]. The value of σ for DP determination by 1H NMR increased as DP increased (6.4–23.4%). However, the averaged values of DP agreed well with those determined by SEC. 13C NMR analysis of DP was possible for the it-PMMA with DP up to ca. 250. The values agreed with those determined by 1H NMR spectroscopy. 1H NMR determination of DP by 5 runs using single spectrometers (100 and 500 MHz) gave much better reproducibility; σ 3.9–9.2% and 0.7–3.4% for the measurements with 100 and 500 MHz instruments, respectively.