EPR spectra recorded under variable temperature heating conditions
In order to monitor the distribution of radical adduct formation over a given temperature range, following AIBN thermal decomposition, and to compare this product distribution for conventional heating versus dielectric heating, a series of variable temperature EPR measurements were performed. In the initial series of experiments, aerobic solutions of AIBN containing a ST (either PBN or DMPO) were heated to various temperatures, and in all cases the CW-EPR spectra were simultaneously recorded at these elevated temperatures. Sample heating was achieved either using the conventional methods or the dielectric heating methods as described above in the experimental section. The resulting CW-EPR measurements are shown in Fig. 1. The overall spectral profiles recorded under both conventional and dielectric heating conditions suggest that, within a tolerance difference of ± 5 °C, both heating methods produce a similar distribution of trapped radicals (CP· or CPO·). Qualitative analysis of the EPR spectra and identification of the resulting trapped radicals is achieved by comparing the experimental EPR parameters (isotropic 1H and 14N hyperfine coupling constants, aiso) to previously published values (Table 1).
As previously mentioned, the carbon-based CP● radicals formed by initial decomposition of the AIBN molecule react quickly with atmospheric oxygen to form a peroxyl radical, that readily decomposes to produce a propoxyl CPO· radical, as shown in Scheme 2. The spin-trap adduct of the 2-cyano-2-propylperoxyl (CPOO·) radical was never detected in this work, as expected since this reactive adduct is known to be very temperature sensitive and has never been observed by EPR above 230 K.[24]
The EPR spectra recorded using PBN as the ST indicate that the EPR signal intensity grows with increasing temperature up to 85 °C (under conventional heating) and up to 80 °C (under MW heating) (Fig. 1 a,c), with only a minor change observed in the line shapes due to the simultaneous presence of the two radical adducts (i.e., ST·-CP and ST·-CPO) (Table 2). The values of the hyperfine coupling constants in Table 1 confirm that the radical species detected at 80 oC by conventional heating (or at 75 oC by MW heating) can be easily assigned to a PBN●-CPO adduct. As the temperature is raised above 90 °C, only the PBN●-CP adduct is observed, regardless of whether conventional or dielectric heating was employed.
Table 1. Spin Hamiltonian parameters (giso and aiso) for the PBN/DMPO-radical adducts formed via AIBN decomposition; the parameters were extracted by simulation of the EPR spectra.
Spin Trap
|
Radical Trapped
|
aN
|
aH
|
aOther
|
giso
|
Solvent
|
Ref.
|
PBN
|
CP●
|
1.433
|
0.312
|
13C = 1.075
|
2.0058
|
Toluene
|
t.w
|
PBN
|
CP●
|
1.430
|
0.322
|
-
|
2.0059
|
Benzene
|
[24]
|
PBN
|
CP●
|
1.460
|
0.307
|
-
|
-
|
THF
|
[28]
|
PBN
|
CPO●
|
1.387
|
0.206
|
-
|
2.0062
|
Toluene
|
t.w
|
PBN-d9
|
CPO●
|
1.387
|
0.206
|
-
|
2.0061
|
Benzene
|
[24]
|
PBN-nitronyl-13C
|
CPO●
|
1.393
|
0.216
|
13C = 0.470
|
-
|
Benzene
|
[25]
|
DMPO
|
CP●
|
1.390
|
1.875
|
14N = 1.485
|
2.0059
|
Toluene
|
t.w
|
DMPO
|
CP●
|
1.460
|
2.040
|
-
|
-
|
Xylene
|
[28]
|
DMPO
|
CPO●
|
1.275
|
0.827
|
1H = 0.165
|
2.0058
|
Toluene
|
t.w
|
DMPO
|
CPO●
|
1.266
|
0.837
|
1H = 0.189
|
-
|
Benzene
|
[24]
|
*MNPE
|
CP●
|
1.485
|
-
|
-
|
2.0058
|
Toluene
|
t.w
|
‘DMPO-deg’
|
N/A
|
1.480
|
-
|
-
|
2.0040
|
Water
|
[29]
|
‘DMPO-deg’
|
N/A
|
1.490
|
-
|
-
|
-
|
DMSO
|
[30]
|
All hyperfine values are given in units of mT. *MNPE = 4-methyl-4-nitroso-pentanal. The MNPE●-CP adduct is formed through the breakdown of DMPO●-CPO, as shown in Scheme 4.
|
An identical series of results were obtained using the second ST (DMPO) as shown in Fig. 1 b,d, with a mixture of both ST·-CP and ST·-CPO observed. In this case, the EPR spectra contained the highest contribution from the DMPO●-CPO adduct at temperatures up to 90 °C (via conventional heating) or 85 °C (via microwave heating). At higher temperatures, T ≥ 90 °C, the DMPO●-CP adduct becomes most dominant (Tables 1 and 2). Interestingly, simulations of EPR spectral contributions (reported in Table 2) indicate a greater contribution of CP● radicals compared to CPO● radicals for samples treated to dielectric heating at each interrogated temperature.
Table 2. EPR spectral contributions of radical adducts detected in the VT EPR study shown in Fig. 1. The percentages of adducts present were calculated through the simulated weighting of contributions to the EPR spectra.
Heating Method
|
Spin Trap
|
Temp. / °C
|
% of ST●-CPO
|
% of ST●-CP
|
% degraded DMPO
|
Conventional
|
PBN
|
80
|
100
|
0
|
N/A
|
|
|
85
|
100
|
0
|
N/A
|
|
|
90
|
60
|
40
|
N/A
|
|
|
100
|
0
|
100
|
N/A
|
Dielectric
|
PBN
|
75
|
100
|
0
|
N/A
|
|
|
80
|
90
|
10
|
N/A
|
|
|
85
|
37
|
63
|
N/A
|
|
|
90
|
13
|
87
|
N/A
|
|
|
100
|
0
|
100
|
N/A
|
Conventional
|
DMPO
|
90
|
100
|
0
|
0
|
|
|
100
|
0
|
50
|
50
|
Dielectric
|
DMPO
|
85
|
98
|
1
|
1
|
|
|
90
|
0
|
80
|
20
|
|
|
100
|
0
|
50
|
50
|
EPR measurements performed under anaerobic conditions
The reaction pathway for generating the CP● and CPO● radicals (see Schemes 1 and 2), and the reason why the CPO● radicals dominate the EPR spectra at lower temperatures must be carefully considered. In the previous experiments (Fig. 1), the observations were made using aerobic solutions. However, the mechanism for radical formation, and in particular the role of molecular oxygen, can be further interrogated by performing the conventional heating experiments under anaerobic conditions (either with partially or fully degassed solutions of AIBN/DMPO/Toluene). The results of the thermal decomposition experiments performed using anaerobic conditions are presented in Fig. 2. In these experiments, the DMPO ST was chosen over PBN, simply because DMPO affords better spectral profiles differences and hyperfine couplings between the two adducts (i.e., easier to discriminate between the two radicals using DMPO compared to PBN).
Under fully anaerobic conditions (Fig. 2 b) only the DMPO●-CP adduct was observed at all temperatures as expected (i.e., with no oxygen available in solution, the carbon-based radicals cannot react with O2 to produce CPO●). However, under partially degassed conditions, the product distribution over the interrogated temperature range was notably different due to the now limited supply of O2. Under fully aerobic conditions, the signal from the DMPO●-CP adduct could not been seen at temperatures below 95 °C (Fig. 2 a), as these lower temperature spectra are dominated by contributions from DMPO●-CPO. However, in the partially degassed solutions, this DMPO●-CP adduct signal was now clearly visible at the lower temperatures (65 °C and 55 °C, in Fig. 2 c,d respectively) owing to the smaller intensity of the DMPO●-CPO. The DMPO●-CPO signal completely disappears at 70 °C after 3 mins of Ar bubbling through the solution and at 65 °C after 4 mins (Fig. 2 c,d), where previously in fully aerobic conditions (Fig. 2 a), this signal disappeared above 90 °C.
Rapid heating (T-jump) measurements
The relative abundances of the carbon and oxygen-based radicals, namely CP● and CPO●, is importance for radical polymerisation, as the former act as radical initiators whilst the latter act as chain terminators. The relative kinetics of radical formation is very different in this case, as is the experimental temperature window in which they can be observed. One must therefore be careful when drawing any conclusions on radical abundances based on the available technique used to study or quantify them. To explore this issue further, the heating experiments for AIBN decomposition were subjected to a sudden and rapid rise in temperature (i.e., a temperature jump) as opposed to the slow continuous heating conditions applied earlier. This experiment enables one to explore any changes in observed radical adduct signal intensities following the rapid temperature jump in the solution.
In these rapid heating (T-jump) experiments, the solution was allowed to equilibrate to room temperature for 2 mins before initiating the mode of heating (Fig. 3). The target temperature of 95 °C was reached after ca. 3 mins using conventional heating (Fig. 3 a,b), but after only 30 secs using dielectric heating (Fig. 3 c,d). This target temperature was chosen since only the signal of the trapped ST●-CP adduct should remain visible at these high temperatures (as shown earlier in Fig. 1). The integrated EPR signal intensity is plotted in Fig. 3 (labelled Normalised Maximum Signal Intensity). It should be stated, that although the overall EPR signal is dominated by contributions from the trapped CPO● radical, a small contribution will nevertheless come from the trapped CP● radical (i.e., both radicals contribute to the overall integrated intensity). Owing to the faster heating rates achieved using dielectric heating compared to conventional heating, the lag time between the temperature initiation point and the first appearance of an EPR signal is correspondingly shorter using microwaves (ca. 7 and 6 secs for PBN and DMPO adducts respectively, Fig. 3 c,d) compared to conventional heating (ca. 75 secs for both adducts, Fig. 3 a,b). On a technical point, separate multifrequency dielectric measurements on the solution permittivity were conducted and these revealed that the loss of the DMPO solution is greater than the PBN solutions by ca. 6% (see Supporting information for details). This explains the small discrepancy in applied power and thus heating rates for the PBN vs DMPO solutions.
The results in Fig. 3 clearly indicate an increase in signal intensity up to the target temperature point, and beyond this the signal intensity rapidly drops off. At this point, only the small residual component from the trapped CP● radical remains and contributes to the signal intensity. The drop off in signal intensity is clearly associated with the loss in ST●-CPO adduct abundance. As discussed later, various reactions are responsible for the loss of the ST●-CPO adduct. Fig. 3 c,d indicates that, although achieving a faster rate of heating when utilizing dielectric heating, the ST●-CPO adduct is still visible. This point was further illustrated by enhancing the target temperature to 135 °C, over a similar time scale. For these experiments, solutions were prepared in o-xylene (as opposed to toluene, which will boil above 110 °C). Under these conditions no signal of the PBN/DMPO●-CPO adducts were detected (Fig. 4). Despite the normal radical chemistry of CPO● still occurring, the EPR signals are completely absent in Fig. 4 (and only moderately visible in Fig. 3) simply because the rate of heating was so fast. The target temperature of 135 °C is reached within ca. 20 secs utilizing AIBN/PBN/o-xylene whilst for AIBN/DMPO/o-xylene, this time is slightly reduced to ca. 15 secs. The lag time of the PBN- and DMPO●-CP adducts in o-xylene were 4 secs and 7 secs respectively. In other words, conventional heating enables the ready observation of these radicals, as their abundance builds up in solution, whereas rapid dielectric heating does not facilitate their detection by EPR (i.e., their formation and subsequent decay occurs quickly, so no signal can be observed).