Propagation, Termination and Transfer Kinetics of TMAEA
8.1 The EPR Spectrum in TMAEA Polymerization
Figure 8.1 shows the EPR spectra for TMAEA polymerization (20 wt.%) in aqueous solution at 21 and 95 °C recorded under pseudo-stationary conditions with a p.r.r. of 20 Hz which allows for an easy labeling of magnetic field positions assigned to fast terminating end-chain radicals (SPRs) and slowly terminating mid-chain radicals (MCRs). Bands due to SPRs at 21 °C (Figure 8.1, l.h.s.) reflect the applied p.r.r. as highlighted in the framed area due to the laser-induced oscillation in SPR concentration whereas the termination rate of MCRs is usually too small for a significant decay in MCR concentration within the time scale between two subsequent laser pulses. The effect of oscillating SPR concentration is less pronounced at lower SPR fractions, e.g., at higher temperatures, as illustrated by the spectrum for 95 °C (Figure 8.1, r.h.s.) where the SPR contribution to the spectrum is negligible. The positions assigned to SPRs correlate with a quartet hfc pattern of a(3H) = 20.6 G. In order to ensure an accurate determination of molar MCR fractions, xMCR, the experiments were carried out under stationary UV irradiation. Two experimental spectra including the associated simulations are given for 0 and 80 °C in Figure 8.2. The simulations reveal a significant contribution of mid-chain radicals even at low temperatures. In agreement with polymerizations of TMAEMA in aqueous solution, a remarkably high S/N quality is achieved which is due to the very high overall radical concentration of 1.38·10−4 mol·L−1 (0 °C) and 1.93·10−4 mol·L−1 (80 °C), respectively. The experimental determination of monomer-to-polymer conversion which relates to the applied sweep time of 5.12 s turned out to be difficult due to the long life-time of TMAEA radicals (see below). Estimates based on an effective rate coefficient of propagation according to eq ( 2.7) yield numbers for monomer-to-polymer conversion of 60 % and 45 % for the lowest and the highest temperature under investigation, respectively. EPR spectra recorded with either a lower monomer concentration, i.e., 10 wt.%, or a higher sweep time, i.e., yielding higher conversion, but under otherwise identical experimental conditions, did not show any variation in hfc pattern or composition of the spectra.
8.1 The EPR Spectrum in TMAEA Polymerization
139 Thus the impact of conversion on the EPR spectra can be considered as negligible for the presented spectra. The detailed understanding of EPR spectra is essential for systems with two or more species coexisting during polymerizations in order to ensure a correct band assignment due to a complicate band overlap.
8.1.1 Simulation of EPR Spectra in the Presence of MCRs
The simulations of experimental spectra were successful under the assumption of two MCR conformers yielding different hfc patterns (Table 8.1). One MCR conformer, in what follows denoted as MCR3, is a common species in acrylate MCR spectra and results in a triplet hfc pattern.165,167–169
The fraction of MCR3 which decreases from 0.15 at −5 °C to 0.07 at 95 °C turned out to be well below the fractions reported for butyl acrylate (in bulk and toluene) or for non-ionized acrylic acid polymerizations in aqueous solution.39,165,167–169,206
95 °C p.r.r. = 20 Hz
SPR SPR
SPR
21 °C p.r.r. = 20 Hz
20 G
19.33 Hz SPR
Figure 8.1: EPR spectra recorded during aqueous-solution polymerization of TMAEA (20 wt.%) at 21 and 95 °C under pseudo-stationary conditions with a p.r.r. of 20 Hz and with Darocur® (1.9∙10−2 mol∙L−1) acting as the photoinitiator. The magnetic field positions dominated by SPRs (red, l.h.s.) reflect the applied p.r.r. (framed area) which is not observed at lower SPR fractions, e.g., at 95 °C (r.h.s.).
The underlying quartet SPR spectrum refers to a(3H) = 20.6 G.
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The second MCR conformer (MCR5) exhibits a strong temperature dependence of the hfcc (Table 8.1) which has not been observed with the MCR3 conformer. While at lower temperatures, the MCR5 spectrum is adequately assigned to a hfc of four non-equivalent β-methylene-protons yielding a 16 line hfc pattern, the MCR spectrum at higher temperatures is assigned to two pairs of non-equivalent β-methylene-protons (9 lines) indicating that, according to Heller-McConnel equation (eq ( 5.1)), the dihedral angles in the MCR5 conformer change significantly with temperature. For the simulation, line broadening has to be taken into account which reduces the hfc patterns of the MCR5 species to a phenotypical quintet as shown in Figure 8.4 below. Although the co-existence of two MCR conformers has also been reported for other acrylate polymerizations165,167–169, the situation for TMAEA is different in that the MCR5 conformer at low temperature has not been observed with acrylate polymerization in organic and aqueous solution so far while, toward higher temperature, the MCR hfc pattern becomes similar the one reported for BA (1.5 M in toluene) between −40 °C and 60 °C.121 The temperature dependence of the hfcc indicates a significant change in MCR conformation of TMAEA which is perhaps due to the charged side group and to the associated intra- and intermolecular interactions.
Table 8.1: Hyperfine coupling constants (hfcc), a(H), for end-chain (SPR) and mid-chain radicals (MCR) deduced from fitting of experimental overall EPR spectra recorded during TMAEA (20 wt.%) polymerization in aqueous solution at the denoted temperatures. Two different MCR conformers, MCR3 and MCR5, are necessary for an adequate fitting (see text). The hfcc are identical for aqueous-solution polymerization of 10 wt.% TMAEA.
radical θ /°C a(H) / G
SPR −5 to +95 20.6 (3H, α-H)
MCR3 −5 to +95 27.4 (2H, β-H)
MCR5 −5 24.0 (1H, β-H) 3.5 (1H, β-H) 15.8 (1H, β-H) 8.2 (1H, β-H) MCR5 95 16.6 (2H, β-H) 11.0 (2H, β-H)
8.1 The EPR Spectrum in TMAEA Polymerization
141
0 °C experiment
simulation 80 °C
20 G
xMCR= 0.99 ± 0.02 xMCR5= 0.92 ± 0.02 xMCR3= 0.07 ± 0.02 xMCR= 0.90 ± 0.04
xMCR5= 0.78 ± 0.04 xMCR3= 0.12 ± 0.04
Figure 8.2: EPR spectra recorded during aqueous-solution polymerization of TMAEA (20 wt.%) at 0 and 80 °C. The spectra refer to a sweep time of 5.12 s and stationary UV irradiation with Darocur® (1.9∙10−2 mol∙L−1) acting as the photoinitiator. The simulations represented by the red lines reveal a high overall MCR fraction, xMCR, and the co-existence of two MCR conformers denoted as MCR3 and MCR5.
Under the assumption that according to Heller-McConnel equation (eq ( 5.1)) A(β-H) = 2·a(α-H), dihedral angles of Θ = 35.2° for the MCR3 species and Θ = 40,2° (a(β-H) = 24.0 G), Θ = 72.9° (a(β-H) = 3.5 G), Θ = 51.7°
(a(β-H) = 15.8 G) as well as Θ = 63.5° (a(β-H) = 8.2 G) result for the MCR5 species at −5 °C whereas at 95 °C numbers of Θ = 50.5° (a(β-H) = 16.6 G) and Θ = 58.8° (a(β-H) = 11.0 G) are obtained.
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8.1.2 Fraction of Mid-chain Radicals under Stationary Conditions
The molar fractions of MCRs, xMCR, which were deduced by fitting the experimental EPR spectra are shown from −5 to +95 °C in Figure 8.3. The numbers of xMCR for TMAEA (20 wt.%) are significantly higher than with BA and AAm. This finding in conjunction with the high absolute radical concentration indicates that MCR propagation rather than termination is the dominant degradation pathway for MCRs in TMAEA polymerizations even at low temperatures and high radical concentrations. The xMCR values should be adequately reproduced by eq ( 2.12).
-60 -40 -20 0 20 40 60 80 100 0.0
0.2 0.4 0.6 0.8 1.0
AAm (10 wt.% / H
2O) BA (1.52 M / toluene) TMAEA (20 wt.% / H
2O)
x
MCRθ / °C
Figure 8.3: Molar fraction of mid-chain radicals, xMCR, deduced from fitting the experimental EPR spectra recorded during polymerization of TMAEA (20 wt.%) in aqueous solution between −5 and +95 °C. The values of xMCR are identical for solutions containing 10 wt.% TMAEA. For comparison, reported experimental values of xMCR for BA polymerization (1.52 M in toluene)121 and AAm (10 wt.%) are included.
8.1 The EPR Spectrum in TMAEA Polymerization
143 8.1.3 Band Assignment used with the SP–PLP–EPR Experiment
In contrast to investigations into the termination and transfer kinetics of AAm (Section 5), the overlap of individual signal bands allows for simultaneously monitoring both SPRs and MCRs as illustrated for the two MCR5 hfc patterns in Figure 8.4. The magnetic field position for the SPR monitoring in both spectra is identical which ensures an accurate determination of SPR concentration irrespective of variations in the MCR5 spectrum with changing temperature. It goes without saying that the magnetic field positions used for SPR monitoring in Figure 8.4 agrees with the positions related to the oscillation of SPR concentration shown in Figure 8.1.
0 °C 80 °C
20 G
SPR MCR
SPR MCR
Figure 8.4: Simulated SPR and MCR5 spectra for 0 and 80 °C as deduced from fitting of experimental spectra shown in Figure 8.2 and according to the hfcc is listed in Table 8.1. The positions used for SPR and MCR monitoring are labelled. The baseline included as the red horizontal line in order to illustrate the negligible contribution of MCR band intensity at magnetic field positions used for SPR monitoring in SP–PLP–EPR investigations.
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