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Chapter 3
Time-Resolved Electron Spin Resonance Observations of the Initial Stages of Conventional and Controlled Radical Polymerization Processes Atsushi Kajiwara* Department of Materials Science, Nara University of Education, Takabatake-cho, Nara 630-8528 Japan *E-mail:
[email protected].
Structural and kinetic investigations of radicals formed in the early stage of polymerizations have been conducted by the ESR technique with various time resolutions. Time-resolved ESR observations of water-soluble (meth)acrylate radicals formed in aqueous phase free-radical polymerization and controlled radical polymerization systems were conducted. These measurements had not been examined previously in spite of their fundamental importance in understanding the initiation procedures in radical polymerizations. Clear and well-resolved TR ESR spectra of sodium (meth)acrylates in water were observed and the structures and molecular dynamics of the radicals formation and reactions were discussed. TR ESR spectroscopy was also applied to the RAFT controlled radical polymerization and the sequential radical addition reactions in the initial stage of the RAFT polymerizations were observed.
Introduction Electron spin resonance (ESR) spectroscopy can reveal both structural and kinetic details of radical reactions during initiation of radical polymerizations (1–7) and results of structural and kinetic investigations of radicals in polymerizations conducted by ESR technique with various time resolutions have been reported (5–7). © 2018 American Chemical Society Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Step by step observation of propagation processes of radical polymerizations can be achieved by ESR with various time resolutions. Initiation involving the first radical addition reaction can be detected exclusively by time-resolved ESR (TR ESR) spectroscopy (8–15). Propagating radicals with long propagated chain under steady state are observed by steady-state ESR (SS ESR). The spectra between oligomerization and polymerization can be observed by single-pulse pulsed laser polymerization electron paramagnetic resonance (SP-PLP-EPR) spectroscopy (16). As shown in Figure 1, these three methods can observe different phase of radical polymerization processes based on different time-resolution.
Figure 1. Schematic diagram of ESR techniques with different time resolution.
TR ESR spectroscopy has provided information especially on the initial stage of radical polymerization reactions by exclusively observing the first radical addition reaction of the generated radical to the monomer. This spectroscopic method is based on a phenomenon called Chemically Induced Dynamic Electron-spin Polarization (CIDEP) (5–7). The spin state of the radicals formed by laser pulse is highly polarized and the relaxation process from the initial state to the Boltzmann distribution can be observed as ESR signals. The polarized state radicals relax within several micro seconds and transient radicals and their reactions can be detected within the time range. On the other hand, radical formed during conventional radical polymerizations of typical monomers can be observed by TR ESR. The TR ESR spectra of 1, 3-butadiene, di-tert-butyl fumarate, and vinyl pivalate at 25°C are shown in Figure 2 as examples. The structures of the radicals can be clearly 64 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
determined from these spectra. The relatively large doublet splitting due to phosphorous nuclei is clear, exclusive evidence for the observation of the chain initiating radical. Structures of the radicals are shown in the Figure along with their spectra. Addition rate constants and activation energy of the radical addition reaction can also be determined by the examination of the TR ESR spectra.
Figure 2. TR ESR spectra of chain initiating radicals of (a) 1,3-butadiene, (b) di-tert-butyl fumarate, and (C) vinyl pivalate initiated with TMDPO at 25°C. The spectra were measured in the range of 500-600 nsec from laser pulses.
TR ESR spectroscopy still has several unsolved problems. New aspects of TR ESR spectroscopy can be considered to be applicable to two additional fields of radical polymerization. One is radical reactions in aqueous phase and the other is controlled radical polymerization reactions. Formation of chain initiating radicals in the presence of various monomers was observed via TR ESR spectroscopy in organic solvent usually benzene, toluene, and n-hexane, since non-polar or lesspolar solvents are suitable for the measurements. Since radical polymerizations in the aqueous phase has been considered to be commercially important, mainly for industrial aspects of environmental protection, several propagation and termination kinetic studies have been reported for water soluble (meth)acrylates and acrylamides in aqueous phase (17–22). TR 65 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
ESR spectroscopy can potentially provide important information on the initiation procedure for radical polymerizations in the aqueous phase. On the other hand, observation of radicals by ESR spectroscopy in water is usually very difficult due to its highly polar nature. The polarity of water, or other polar solvents like alcohol, nitrile, ester, and ketone, absorb microwave energy and decrease the sensitivity of the ESR measurements. Moreover, water soluble initiators that can provide highly polarized radicals are essential for the measurements and there have been no suitable initiators commercially available. Fortunately, a water soluble acyl phosphinate, lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate (LiTMPO), has recently become available. LiTMPO provides the possibility of ESR measurements in aqueous media.
Scheme 1. Structures of Compounds Used
Although intermediate radicals formed during reversible addition fragmentation chain transfer (RAFT) polymerization systems have been observed by SS ESR spectroscopy (23–25), the initial stages of the RAFT polymerizations have not been examined. TR ESR spectroscopy would be able to provide detailed information on the reactions. In RAFT polymerization systems, the initial stage of the polymerization is considered to be very complicated because several kinds of radical addition reactions would be happening especially in the initial stage of the polymerization reaction. In this paper, results of TR ESR spectroscopy of the initial stages of both conventional polymerizations in aqueous phase and controlled radical polymerizations especially RAFT systems are discussed. Previously direct observations of TR ESR spectra of these polymerizations have been very difficult or even impossible. Structures of the initiators, monomers, and RAFT agent used in this paper are shown in Scheme 1. 66 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Results and Discussion TR ESR in Aqueous Phase The TR ESR spectrum of LiTMPO only (in the absence of monomer) in water was observed and both phosphorous centered and carbon centered radicals were clearly detected. The phosphorous centered radical signal is a doublet with 46.6 mT hyperfine splitting and the carbon centered radical signal is a singlet. The intensity of the lower field signal is weaker than that of the higher field one. This is probably due to the different mechanism of spin polarization (6, 7). The TR ESR spectrum of TMDPO only in toluene was also measured for comparison. In this case, the hyperfine splitting constant of phosphorous centered radical is observed at 35.6 mT and this value is smaller than that of P-centered radical generated from LiTMPO. Sodium acrylate (NaA) and sodium methacrylate (NaMA) were used as water soluble monomers for TR ESR measurements in the aqueous phase. When a solution of NaA and LiTMPO, in water was put into a flat cell (inner space is 0.1 mm x 5 mm x 30 mm), ESR results were very noisy and no clear signal was observed. Therefore a flow system was employed for improvement of sensitivity. 10 mL of a mother solution of LiTMPO (0.1 M) and NaA in water was circulated in the flat cell using a tubing pump. This procedure means that a fresh sample of the solution would be continuously supplied to the measurement area of the sample cell. The result of the flow measurement was dramatically changed and showed a well resolved spectrum with high S/N ratio. The TR ESR spectrum of NaA in water at 25°C is shown in Figure 3 along with that of tert-butyl acrylate (tBA) initiated by diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TMDPO) in toluene at 25°C. While the features of the both spectra are very similar the S/N ratio of the spectra obtained by the flow system was better than that of a non-flow system. Precise values of hyperfine splitting constants can be determined from the spectrum. The values are shown in the Figure 3 with the structure of the chain initiating radicals. In the case of NaMA, a well-resolved spectrum with very good S/N ratio was also observed in the flow system. The TR ESR spectrum of NaMA initiated by LiTMPO in water at 25°C is shown in Figure 4 along with that of tert-butyl methacrylate initiated by TMDPO in toluene at 25°C. The ESR spectra of methacrylates are usually sensitive to molecular dynamics (2, 26–29) as the splitting pattern and line widths of the spectroscopic lines reflect the molecular motion. In these spectra, line width of some inner spectroscopic lines, indicated by arrows in Figure 4, is broader than other lines. Intensity of the broader line is strongly influenced by the molecular motion and the molecular motion is connected to the bulkiness of the ester side groups. These relationships indicate that the bulkiness of Na cation in NaMA in water is similar to that of tert-butyl ester group in tBMA in toluene. Further detailed analysis of the splitting patterns would provide interesting information of the nature of the radicals especially from temperature dependent measurements. 67 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. TR ESR spectrum of chain initiating radical of sodium acrylate initiated with LiTMPO in water at 25°C along with its structure and hyperfine splitting constants (a). TR ESR spectrum of chain initiating radical of tert-butyl acrylate initiated with TMDPO in toluene at 25°C along with its structure and hyperfine splitting constants (b). These spectra were measured in the range of 500-550 nsec from laser pulses.
68 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 4. TR ESR spectrum of chain initiating radical of sodium methacrylate initiated with LiTMPO in water at 25°C along with its structure and hyperfine splitting constants (a). TR ESR spectrum of chain initiating radical of tert-butyl methacrylate initiated with TMDPO in toluene at 25°C along with its structure and hyperfine splitting constants (b). These spectra were measured in the range of 550-570 nsec from laser pulses.
These results are the first examples of TR ESR spectra of water soluble (meth)acrylates. The observations should provide useful information for aqueous phase polymerizations in both laboratory and industrial research works.
TR ESR of RAFT Polymerization Systems TR ESR spectroscopy can also be applied to the controlled radical polymerization systems and TR ESR results of reversible addition fragmentation chain-transfer (RAFT) polymerization systems are shown. The schematic reaction diagram of the observed reactions are shown in Scheme 2. 69 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Scheme 2. Structures of Adduct Radicals
A TR ESR spectra of an initiator (TMDPO) in the presence of a RAFT agent (BTBA) was measured before observation of the actual RAFT polymerization systems. Three different kinds of secondary formed adduct radicals were observed, as shown in Figure 5. The numbers in the figure correspond to the structures of the radicals shown in Scheme 2. TR ESR spectra obtained at three different reaction times, 0.5, 0.6, and 1.0 μsec, are shown. Within the time scale, the observed radicals were changed according to their life time and timing of formation. The strongest peak measured after 0.5 μsec of the laser pulse is considered to be due to a 2,4,6-trimethyl benzoyl radical. At 0.6 μsec, the trimethyl benzoyl radial signal was still observed and new doublet signals appeared at lower and higher field of the trimethyl benzoyl radical. Judging from the doublet feature and hfc patterns, these signals are considered to be due to radical addition of the diphenyl phosphinoyl radical to BTBA. The structure of the radical is 6 in Scheme 2. At 1.0 μsec, the signal of trimethyl benzoyl radical had disappeared and a new signal appeared between the doublet signals of the addition radical of diphenyl phosphinoyl radical to BTBA. This new signal is due to an addition radical of trimethyl benzoyl radical to BTBA. The observed results seem reasonable because the addition rate of a phosphorous centered radical is faster than that of a carbon centered radical. When vinyl monomers were added to the TMDPO solution instead of the BTBA, the formation of the adduct radicals of carbon centered radical to the monomers was too slow to be observe in the time range of measurement for a TR ESR. In the case of BTBA, the addition reaction of the carbon centered radical is fast enough to observe in the TR ESR time resolution. 70 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 5. TR ESR spectra observed in solution of BTBA with TMDPO in toluene at 25°C. Spectra observed at 0.5 μsec, 0.6 μsec, and 1.0 μsec from laser pulse are shown. Numbers in the figure are corresponding to the radical structures shown in Scheme 2.
Then, the TR ESR of radical polymerization of tBMA in the presence of BTBA was measured. The results under three different conditions are shown in Figure 6. Figure 6a is TR ESR spectrum of BTBA and TMDPO, Figure 6b is the spectrum of tBMA initiated with TMDPO, and the spectrum shown in Figure 6c is a result of tBMA polymerization initiated by TMDPO in the presence of BTBA. Figure 6c displays overlapped signals of 6a and 6b. From these results, it can be concluded that in the initial stage of a RAFT polymerizations, the addition reaction of the generated radical to the RAFT agent and monomer is competitive and both adduct radicals were observed until the 2.0 μsec region from the laser pulse. 71 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 6. TR ESR spectra observed under different conditions. (a) BTBA and TMDPO in toluene at 25°C. (b) tBMA polymerizations initiated with TMDPO in toluene at 25°C. (c) RAFT polymerization system of tBMA in the presence of BTBA initiated with TMDPO in toluene at 25°C. Numbers in the figure are corresponding to the radical structures shown in Scheme 2.
Direct observation of radicals in the initial stage of a RAFT controlled radical polymerization processes revealed that the first radical addition of generated phosphorous centered radical to monomer or RAFT agent occurs randomly without any selection. Radical addition of generated trimethyl benzoyl radical to BTBA was also observed. Since the addition rate of a benzyl radical to vinyl monomers is one or two order slower than that of phosphinoyl radical, in the polymerizations of styrene, (meth)acrylates, and dienes, the adduct radicals of the trimethyl benzoyl radicals were not observed. In the next step, control of the polymerization would start. Next subject of this observation study is the detection of second, third, and fourth radical addition reaction in the next time range (several ten’s of μsec) by SP-PLP-EPR or other ESR techniques. 72 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
TR ESR observation of RAFT polymerization systems with 2-phenyl-2propyl benzodithioate did not show any ESR signals under almost identical conditions to that employed with BTBA. These results indicate that various kinds of RAFT agent would show various kinetics even in the initial stage of the polymerizations.
Conclusion The structural and kinetic investigations of radicals in aqueous free radical and RAFT polymerizations have been conducted by ESR techniques with various time resolutions. Time-resolved ESR observations of both water-soluble (meth)acrylates in aqueous phase and in controlled radical polymerization systems showed clear and well-resolved TR ESR spectra. The structures and molecular dynamics of the chain initiating radicals of water-soluble sodium (meth)acrylates were discussed in comparison with corresponding tert-butyl esters. TR ESR spectroscopy was also applied to the RAFT controlled radical polymerizations. Radical addition reactions in the initial stage of the RAFT polymerizations were observed. Step-by-step formation of adduct radicals of both phosphorous- and carbon-centered radicals to the RAFT agent were clearly observed.
Experimental TR ESR Spectroscopy Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TMDPO, Aldrich) was purified by recrystallization from ethanol before use. Lithium phenyl(2,4,6trimethylbenzoyl)phosphinate (LiTMPO, TCI) was used as received. Benzyl benzodithioate (BTBA, Aldrich) was used without further purification. (Meth)acrylate monomers were purified by distillation just before use. A toluene or benzene solution of TMDPO (0.1 M) containing various concentrations of monomers was taken in an ESR sample cell. JEOL JES-LC11 flat flow cell and micro tube pump (Tokyo Rikakikai Co. Ltd., EYELA) were used for flow method. Laser pulses were irradiated by using a Q-switched Nd:YAG laser (Spectra Physics Quantaray DCR-2) operated at the third harmonic (10 mJ/flash at 355 nm with a 6-ns fwhm). For the measurements of the time-resolved ESR, a JEOL JES RE-2X spectrometer, equipped with a WBPA2 wide band pre-amplifier, was operated without magnetic field modulation, and the data were stored in a Tektronix TDS520A digital oscilloscope. Magnetic fields at resonance signals were determined by an Echo Electronics ES-FC5 NMR field meter. Measurement temperature was controlled by a JEOL DVT2 variable-temperature accessory. Data analysis was conducted by CIDEP software provided by JEOL Ltd. 73 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Size Exclusion Chromatography (SEC) Molecular weights and molecular weight distributions were estimated using a TOSOH CCP&8020 series GPC system with TSK-gel columns. Lenear combination of two G2000HHR and two GMHXL colums was employed. Polystyrene standards were used to calibrate the columns.
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