11.1. Deactivation kinetics in CuII-mediated polymerization of butyl acrylate 11.1.1. Deactivation rate coefficient of SPRs in BA polymerization
As detailed in chapter 9.1, the kinetics of SPRs in acrylate polymerizations may be studied without formation of MCRs at temperatures below −30 °C. For determination of kdeact in ATRP of butyl acrylate, SPRs were produced by a single laser pulse in the presence of Cu(II)Br2(HMTETA) dissolved in BA and 15 wt% acetonitrile as a co-solvent. The cR vs.
time trace was fitted via PREDICI simulation to the kinetic model detailed in section 5.3.6.
The chain-length-dependent termination rate coefficient was measured from copper-free SP-PLP-EPR of BA containing 15 wt % acetonitrile and was found to be similar to the data depicted in Table 11 for bulk BA. Average concentration of the Cu(II) species was implemented into PREDICI as obtained by the mean value of the copper(II) signal before and after SP-PLP from EPR measurements. The best fits of the model and experimental data obtained at initial Cu(II) concentrations of 1 and 2 mM are compared on the l.h.s. of Figure 45 to SP-PLP-EPR data (and associated fit) from the copper-free system.
Figure 45. L.h.s.: SP-PLP-EPR data of butyl acrylate at −40 °C without Cu(II) (upper black line) and in presence of 1 mM and 2 mM Cu(II)Br2HMTETA (dark and light gray lines, respectively). The dashed black lines refer to a fit of the data by the kinetic model described in chapter 5.3.6. R.h.s.: Associated decay of the EPR line from the Cu(II)-complex during the course of an SP-PLP. Measurement of the copper-line before and after the SP-PLP-EPR experiment is required for estimation of the actual concentration of Cu(II) during SP-PLP.
The decay of cSPR measured vs. time after an SP is accelerated by the presence of Cu(II)Br2HMTETA as becomes obvious by comparing the SPR traces depicted on the l.h.s
138 DEACTIVATION IN ATRP of Figure 45 measured with an without pre-mixed Cu(II). Fitting of the data via PREDICI
simulation (black dashed lines) yields kdeact ≈ (1±0.3)·107 L·mol−1·s−1. Fitting requires implementation of the actual concentration of Cu(II)Br2(HMTETA) present during SP-PLP.
This information is obtained from measurement of the Cu(II) EPR-signal before and after SP-PLP, the intensity measured before SP-PLP is associated with the initial concentration of Cu(II).
11.1.1. Deactivation rate coefficient of MCRs in BA polymerization
Model MCRs produced by initation of BA macromonomers (MMs) may be used to study kinetics of MCRs without the complexity introduced by the occurrence of SPRs and by the presence of BA monomer as detailed in chapter 9.2. Time-resolved EPR detection of model MCRs produced by an SP irradiated on a MMMP / MM sample in presence of a Cu(II)-complex is used for determination of kdeact for MCRs during ATRP mediated by Cu(II)Br2(HMTETA). Since propagation of model MCRs (with macromonomer) is found to be negligible, termination between two MCRs is described via a chain-length averaged quantity, 〈kt〉 as detailed in chapter 9.2. The recorded MCR traces were fitted by eq. (29) as explained in section 5.3.6. The fitting procedure is illustrated in Figure 46 for an SP-PLP-EPR trace recorded at 30 °C from a sample containing 85 wt.% MM, 15 wt.% acetonitrile (as a co-solvent) and 1 mM Cu(II)Br2HMTETA. The concentration of the copper complex implemented into the fitting procedure was corrected by the conversion of Cu(II)Br2(HMTETA) made during the experiment, as determined from the Cu(II) EPR line before and after SP-PLP. Fitting of eq. (29) does not perfectly match the experimental data.
Deviation between experiment and eq. (29) may be caused by different addition reactivity (associated with different kts) of model MCRs produced by an SP as discussed in detail in chapter 9.2.
DEACTIVATION IN ATRP 139
Figure 46. Experimental cR vs. t traces obtained from initiation of macromonomer (containing 15 wt.% acetonitrile) at t = 0 without Cu(II) (black line) and in presence of 1 M
Cu(II)Br2(HMTETA) (gray lines). The dashed lines refer to best fits of eq. (29) to the experimental data taking the actual concentration of Cu(II) into account as well as 〈kt〉 obtained from copper-free MM (containing 15 wt.% acetonitrile) as given in chapter 9.2.
The kdeact obtained by the best fit of eq. (29) to experimental cR vs. t in presence of Cu(II)Br2(HMTETA) are plotted in an Arrhenius representation in Figure 47 (black squares).
The 〈kt〉 implemented into the fitting procedure refers to SP-PLP-EPR on copper-free MM (containing 15 wt.% acetonitrile) as given in chapter 9.2.
Figure 47. Rate coefficients kdeact obtained for model MCRs from BA macromonomer and Cu(II)Br2(HMTETA) dissolved in BA macromonomer and 15 wt.% acetonitrile. Open square symbols refer to averages from up to seven independent experiments carried out at
140 DEACTIVATION IN ATRP different amounts of the copper(II)-species (small gray squares). The black line refers to straight line fitting to the data which yields an Arrhenius-activation energy of 30 kJ·mol−1. Arrhenius fitting of the data depicted in Figure 47 yields an activation energie Ea(kdeact) = 30 kJ·mol−1. The rate coefficient may be expressed via: kdeact = 2.75·109·exp(−1549/(T/K)). The value obtained at 30 °C, kdeact(30 °C) = 1.7·104 L·mol−1·s−1, is by one order of magnitude below the associated 〈kt〉(30 °C) (= 2.3·105 L·mol−1·s−1) for two model MCRs in macromonomer solution containing 15 wt.% acetonitrile at the same temperature (see chapter 9.2). The pronounced effect of deactivation rate on the decay of MCRs (see Figure 46) is remarkable. Even though the kdeact is found to be an order of magnitude below 〈kt〉 at same temperature, deactivation rate exceeds termination by roughly an order of magnitude due to the high concentration of Cu(II) as compared to radical concentration. Diffusion control of the deactivation step during ATRP is discussed in literature.[18, 174] The measured kdeact for model MCRs does not exlude a diffusion control of the deactivation step. Similar activation energies (32 and 30 kJ·mol−1, respectively) are found for termination and for deactivation of model MCRs which similarity may indicate, that deactivation (as is assumed for termination) runs under control of solvent friction which is indicative for diffusion controlled reactions (see eqs. (19) and (20)). Termination between MCRs in a 15 wt.%
macromonomer sample is associated with a rather high activation energy of 32 kJ·mol−1 which is presumably caused by an associated high Ea(η−1) of the macromonomer mixture with acetonitrile (see chapter 9.2). It appears to be a matter of priority, to measure kdeact in solvents which exhibits a weaker temperature dependence of fluidity and thus allow to check for a difference in Eas for kdeact and 〈kt〉 in future studies. Assuming diffusion control of the deactivation step, the ratio kdeact / 〈kt〉 ≈ 0.1 represents a reduced capture radius for transfer of bromine from Cu(II)Br2HMTETA to the radical functionality as compared to termination between two MCRs. The hydrodymanic radius for the copper complex should not be too different from rg of the MM.
CLOSING REMARKS AND OUTSTANDING CHALLENGES FOR SP-PLP-EPR 141