3.3 Troubleshooting
3.3.3 EPR Signal of the Heating Element
It was known that the heating element in the cavity also induces an EPR signal. This signal is reproducible for the given conditions.
Especially, for the recorded spectra during the homopolymerization of styrene-d8 and the copolymerization of styrene-d8 and MMA, the signal of the heating element disturbs the measurement because of the low signal intensity of the sample radicals. To solve this issue, the spectra of copolymerization and styrene-d8 were averaged from up to 60 spectra (shown in black in Figure 3.8) and the backgrounds were averaged over 10 measurement (shown in red in Figure 3.8). In this way, the signal from the hating elements can be successful subtracted from the sample spectrum.
PnMA without oxygen PnMA with oxygen
20 G
Figure 3.7: Both spectra were recorded during a PnMA homopolymerization at 233 K in bulk under pseudo stationary conditions (p.r.r. 30 Hz). The black one shows an oxygen signal whereas no oxygen is observed in the red ones.
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homopolymerization of sty-d8 bachground
homopolymerization of sty-d8 after substraction of backgound
10 G
Figure 3.8: Spectrum recorded during a radical homopolymerization of styrene-d8 in bulk at 353 K (black). The background was measured before the irradiation (red). After subtracting the background, the spectrum of the homopolymerization of styrene-d8 can be obtained without the signal of the heating element.
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4 T ERMINATION K INETICS OF R ADICAL P OLYMERIZATION OF M ETHACRYLATES i
Methacrylates are one of the most important monomer families used for polymerizations. The most famous one is poly(methyl methacrylate) (PMMA) which is known as Plexiglass®.[3] Furthermore, several methacrylates are widely used for copolymerizations as can be seen in Chapter 5. Thus, the knowledge of the kinetics of this monomer family is essential for planning radical polymerizations, and moreover for further investigations into the kinetics of copolymerizations. The kinetics of some monomers within this monomer family is well investigated and a family behavior for the propagation kinetics could be found.[98] Nevertheless, the detailed kinetics of some methacrylates are still unknown and some questions about the crossover chain length were left open in previous studies, too.[55]
One interesting monomer within the methacrylate family is n-pentyl methacrylate (PnMA). Due to its glass transition temperature slightly below room temperature, poly-PnMA is an interesting matrix polymer for testing the mechanical properties of polymer–filler compounds.[54]
However, nothing was known about the kinetics of PnMA so far.
Therefore, the propagation kinetics of PnMA was investigated by A.
Nitschke and coworkers and the results are discussed in the relating publication.[99] Moreover, the chain-length dependent termination kinetics of PnMA was studied in detail within this work using the powerful SP–PLP–EPR method. The obtained results are discussed in this chapter.
iThe results of PnMA in this chapter were published in A. Nitschke, L. Riemann, L. Kollenbach, V. Braun, M. Buback, P.Vana, Macromolecular Chemistry and Physics 2019, 221(1), 1900345 and are reproduced with permission.[99]
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In a previous study of the termination kinetics of methacrylates, a significant temperature dependency of ic was observed for 2-ethyl hexyl methacrylate (2-EHMA) and dodecyl methacrylate (DMA).[55] This was surprising because such a temperature dependency of ic was not observed for other methacrylates such as butyl methacrylate (BMA)[31] and acrylates,[42] respectively. Nonetheless, the focus of the previous study was not on this temperature dependency. Therefore, more detailed investigations into the temperature dependency of ic for 2-EHMA and DMA in bulk were performed within this work and the results are discussed in this chapter.
Beside this interest into the termination kinetics of this monomer family, several parts of the experimental setup had to be replaced or repaired in comparison to previous works. Especially, the excimer laser was exchanged by an Nd:YAG laser which led to problems such as formation of a glass radical (Chapter 3.3.1). Hence, it was essential to verify that the new results are reliable. Because of the investigations into the termination kinetics of numerous methacrylates, this monomer family is an excellent candidate to validate the changes within the experimental setup.
4.1 EPR Spectrum of Pentyl Methacrylate
The experimental EPR spectrum recorded during a homopolymerization of PnMA in bulk at 233 K (black line in Figure 4.2) shows the characteristic thirteen signals for methacrylates.[31,55,100]
Due to the methyl group, the rotation around the Cα–Cβ-bond is hindered, and thus two conformers of methacrylates exist (Figure 4.1). As can be seen, the dihedral angles Θ between p-orbital containing the radical and the hydrogens of the methylene group differ. Θ can be correlated to the hyperfine coupling constant ahf by the Heller–McConnell equation (Equation 4.1).[101–103] Hence, the hydrogen atoms of the methylene group have inequivalent ahf.
ahf = A∙cos2(Θ) 4.1
43 A is a proportionality constant. The simulated spectra for both conformers of PnMA are shown in red and blue in Figure 4.2. From simulations, the hyperfine coupling constants for the three equivalent hydrogen atoms of the methyl group and for the two inequivalent hydrogen atoms of the methylene group were obtained and are given in Table 4.1.
Figure 4.1: Newman projection of the two conformers of methacrylates.
*
*
*
*
*
*
*
* *
experimental spectrum simulated spectrum conformer A simulated spectrum conformer B simulated overall spectrum
20 G 20 G
Figure 4.2: EPR spectrum (black) recorded during a PnMA polymerization at 233 K with DCP (9·10−2 mol·L−1) as the photoinitiator under laser pulsing with a repetition rate of 30 Hz. Red and blue are the simulated spectra for both conformers with the hyperfine coupling constants given in Table 4.1. Superposition of the two conformers spectra lead to the overall spectrum in green. The arrow indicates the magnetic field position used within the SP–PLP–EPR experiments.
44
Moreover, superimposing of both conformer spectra lead to the overall spectrum shown in green in Figure 4.2 which perfectly agrees with the experimental spectrum of PnMA. Furthermore, the ratio of the two conformers can be determined by simulation and is around 1:1 at 233 K. As can be seen in Figure 4.2, the small inner lines of the spectrum marked with asterisks only belongs to conformer A shown in red. With increasing temperature, these inner lines increase because of the improved rotation around the Cα–Cβ-bond as will be shown for MMA at 333 K in Chapter 5.1. However, the ratio of conformers is unimportant for determining the termination kinetics. The understanding of the spectra is important to be sure that no other radicals—such as oxygen for example (Chapter 3.3.2)—are formed which may influence the termination kinetics.