Evaluation of Molecular Weight of Polymer via ¹³C CPMAS NMR Measurements:Terminal Carbon Signals of Polystyrene

¹³C CPMAS measurements may be capable of satisfactory quantitative evaluation in some cases. The figure shown below is a quantitative DDMAS spectrum and a CPMAS spectrum of polystyrene (Mw = 4,000, monomer unit number = 38). From the intensity ratio of the terminal carbons (α-CH₂ and CH₃) and the other carbons, the unit number is estimated to be ca 40, indicating the possibility of molecular weight evaluation via ¹³C CPMAS spectra.

The figure on the right shows CPMAS spectra of polystyrene samples of three different molecular weights Mw of 4,000, 25,000 and 50,000. Comparing with the signal intensities of the terminal carbons for the Mw = 4,000 sample, the intensity of the Mw = 25,000 sample is about 1/5 while the intensity of the Mw = 50,000 sample is about 1/10, demonstrating consistent molecular weights.