Polyalphaolefin Hydrogenation – Residual Olefin Analysis – 1H NMR versus Bromine Number

1H NMR is an excellent tool for monitoring the residual olefin content of polymers after hydrogenation reactions. The fact that the olefin fall in a unique region of the spectrum means that it is a straightforward measurement to quantify the %H present as olefin or to correlate that olefin content with other analyses such as bromine number. Here is an example of a polyalphaolefin residual olefin analysis. The olefin proton content (%H) was plotted against bromine number values obtained on each of the samples. A linear correlation was obtain but two different correlations were observed that were dependent on the viscosity index of the polyalphaolefin being analyzed. Figure 1 shows the 1H NMR spectra obtained on neat samples on a Picospin-80 spectrometer operating at 82.3 MHz. The methyl and methlene protons of the polymer backbone are plainly seen and the olefin and alpha-olefin protons are observed.

1H NMR - Polyalphaolefins - Residual Olefin Analysis

Figure 1: 1H NMR – Polyalphaolefins – Residual Olefin Analysis

 

Figure 2 shows the linear correlation between %H olefin and bromine number with the two correlations caused by different VI grade being indicated. The analysis shows that for the two viscosity grades the grade can be identified from the linear correlation that the data falls onto and the %H olefins content can directly yield the bromine number. This NMR method provides an alternative to the following ASTM standards:                                                            D1159 Test Method for Bromine Numbers of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration                                      D1491 Test Method for Test for Bromine Index of Aromatic Hydrocarbons by Potentiometric Titration                                                                         D1492  Standard Test Method for Bromine Index of Aromatic Hydrocarbons by Coulometric Titration                                                                             D2710 Test Method for Bromine Index of Petroleum Hydrocarbons by Electrometric Titration                                                                                      D5776 Test Method for Bromine Index of Aromatic Hydrocarbons by Electrometric Titration

Correlation of Olefin Content obtained by 1H NMR with Bromine Number in Polyalphaolefins

Figure 2: Correlation of Olefin Content obtained by 1H NMR with Bromine Number in Polyalphaolefins