Molecular weight determination of silicone oils via 29Si on Spinsolve NMR systems

In both academic polymer research and industrial polymer production, it is crucial to regulate the molecular weight of polymers. This necessity arises from the fact that the macroscopic properties of polymers are predominantly dictated by their molecular structure and chain length. Therefore, understanding the molecular weight itself becomes imperative. While size-exclusion chromatography (SEC) is frequently employed as the prevailing standard, Nuclear Magnetic Resonance (NMR) serves as a valuable complementary method. NMR not only furnishes information on the molecular weight but also facilitates a comprehensive structural elucidation of polymer samples.

Figure 1: General molecular structure of dimethylpolysiloxane.

In the present study, we examined two trimethyl-terminated dimethylpolysiloxane-type silicone oils with distinct viscosities (10 and 100 cs, respectively), corresponding to different molecular weights. Although the 1D 1H spectra of the samples appear nearly identical, and the end groups cannot be differentiated from the signals of the polymer backbone, molecular weight determination through end-group analysis is straightforward using 29Si NMR.


Figure 2: 1D 29Si spectra measured on a Spinsolve 80 MHz Multi-X Ultra system, a) silicon oil with 10 cs, b) silicon oil with 100 cs viscosity.

Figure 2 displays the 1D 29Si spectra of a) the 10 cs and b) the 100 cs silicone oil measured on a Spinsolve 80 MHz Multi-X Ultra system. Both samples were prepared in a 1:1 dilution with CDCl3 doped with Cr(acac)3 and spectra were acquired with 4096 scans. Two regions were integrated—one for the end groups at around 6.7 ppm and the polymer backbone at around -22 ppm.

To calculate the molecular weight of both samples, we referred to the following formula, where DP represents the degree of polymerization, and Mru is the molecular weight of the repeating unit:

Here, DP is calculated with the next formular, where N is the number of Si atoms of the respective integral and Int is the corresponding absolute integral of the signal:

With a value of 74 g/mol for the repeating unit we determined the respective Mn information with 963 g/mol for the 10cs sample (Figure 2 a) and 5975 g/mol for the 100cs sample (Figure 2 b), respectively.