Online NMR Analysis

This section includes work from a jointly authored publication.¹⁸⁵ The work presented here was equally contributed to by Sam Parkinson and Dr. Stephen Knox.

NMR spectroscopy is one of the most powerful spectroscopic techniques available to polymer chemists due to its ability to generate large detail on chemical structure especially in increas-ingly complex polymer systems or formulations.⁷ Traditional NMR instruments are often large, costly and limited in terms of operating conditions, stifling their uptake into on-line monitor-ing platforms. However, newer “benchtop” NMR systems that utilise lower field permanent magnets, have allowed for much more rapid uptake into on-line monitoring platforms.^7,58,186 After the NMR was attached to the reactor outlet a protocol was developed that minimised acquisition time whilst maintaining spectra quality.

2.3.1.1 Spectrum Acquisition Time

The time taken to acquire a spectrum is proportional to the number of scans acquired per spectrum. The fewer the number of scans required the faster the spectrum will be acquired however fewer scans will lead to a lower signal to noise ratio (S/N), which may impact spectrum quality. Typically, 32 scans were collected for NMR spectra acquired on traditional high-field instruments such as those obtained in chapter 3. This parameter was optimised by collecting NMR spectra of a low conversion PDMAm_x sample whilst varying the number of scans (2-32). The subsequent conversion calculated from these spectra indicate very little change is observed with conversion always consistently around 35 % (Figure 2.8). Therefore, two scans were performed for all further NMR spectra allowing for the fastest spectra collection without compromising spectra quality.

2.3. Reactor C

Figure 2.8: Conversion data obtained from NMR spectra of a PDMAm₁₀₀ kinetic sample using either 2, 4, 8, 16 or 32 scans.

2.3.1.2 Pulse Techniques

One major benefit of the incorporated NMR system is the ability to acquire NMR spectra in non-deuterated solvents, which is not possible for traditional high field systems. Therefore re-action solution can be passed directly from the reactor outlet through the NMR for analysis.

However, the solvent peaks observed in the spectra are much more intense than with deuter-ated solvents. This results in significantly broader solvent peaks which can lead to overlap or convolution of other signals in the spectra. One method of removing these large solvent peaks is a technique called presaturation (Figure 2.9), where a long low-power pulse is applied on the solvent resonance before the normal pulse sequence. This long pulse saturates the solvent resonance preventing it from being detected during the following pulse sequence. A minor draw-back to using presaturation is the slightly increased time required to collect spectra reducing temporal resolution.

2.3. Reactor C

Figure 2.9: NMR pulse sequence used during a presaturation method.

In order to determine if there were any differences in kinetic data using normal and presatu-ration pulse sequences, transient profiles were obtained using both methods (Figure 2.10). A notable improvement in the kinetic profile when using the presaturation method was observed, particularly after 5 minutes when a sudden deviation from first order kinetics occurred using the normal pulse sequence. This is because as conversion increases, the vinyl peak intensity decreases and thus any phase issues brought about by the large solvent peak, adjacent to the vinyl peaks, are likely to have a greater effect.

Figure 2.10: a) Conversion vs time and b) Semi-logarithmic rate plots obtained for the RAFT polymerisation of dimethylacrylamide using standard and presaturation NMR pulse sequences. All reactions were performed for at 30 % w/w, 80 °C and [DMAm]:[CCTP]:[VA-044]

= 100:1:0.02.

2.3. Reactor C

2.3.1.3 Selection of NMR Flow Cell

A way to increase the S/N ratio without lengthening acquisition time, is to increase the sample volume and thus the number of protons observed by the magnet. For the experiments in section 2.3.1.1 PFA tubing (1.6 mm I.D) was used as the flow cell through the NMR. To determine if the quality of NMR spectra could be improved a glass flow cell with a larger internal diameter (4 mm I.D ) was used (Figure 2.11).

Figure 2.11: PFA and glass flow cells used to collect online NMR spectra

Initially, NMR spectra of a dimethylacrylamide solution were obtained over multiple flow rates in both flow cells (Figure 2.12) and the area of the vinyl proton region were compared relative to a static DMAm spectra obtained in the glass flow cell. A 5-fold increase in vinyl integral area can be observed in the glass flow cell. As well as a higher maximum flow rate (3 mL min^-1) before signal is lost compared to the PFA tubing (1 mL min^-1). This is expected as a larger internal diameter will reduce the fluid velocity allowing excited material to be in the detection window of the NMR for longer. Increasing the available flow rate ranges would be beneficial when using the transient profiling method developed in chapter 3 as the flow rate limit also limits the time scales which can be monitored. However, at high flow rates (> 1 mL min^-1) the relative integral of DMAm increases above 1 in the glass flow cell indicating an increase in the concentration of DMAm in the cell. This may be due to the expansion and contraction of the fluid as it enters and exits the larger area of the flow cell.

2.3. Reactor C

Figure 2.12: Relative integral area for DMAm vinyl protons over multiple flow rates using either a glass or PFA flow cell. Integral areas are relative to static sample in the glass flow cell.

To determine if this expansion and contraction will have an effect on the polymerisation transient kinetic profiles were obtained using both PFA and glass flow cells (Figure 2.13). The kinetic profile obtained using the glass flow cell showed a much reduced rate of reaction compared to PFA tubing. If the concentration of DMAm increased relative to the rest of the reaction solution in the glass flow cell, a stronger peak in the vinyl region would be present leading to a decrease in conversion. As the glass flow cell appeared to lead to inaccurate kinetics when compared to offline kinetic profiles obtained for a similar polymerisation (Figure 4.2), PFA was kept as the NMR flow cell.

Figure 2.13: a) Conversion vs time and b) semi-logarithmic rate plots obtained for the RAFT polymerisation of dimethylacrylamide using both glass and PFA flow cells. All reactions were performed for at 30 % w/w, 80 °C and [DMAm]:[CCTP]:[VA-044] = 100:1:0.02.