Chain-Length-Dependent Termination of Sodium Methacrylate

Article pubs.acs.org/Macromolecules

Chain-Length-Dependent Termination of Sodium Methacrylate Polymerization in Aqueous Solution Studied by SP-PLP-EPR Hendrik Kattner, Patrick Drawe, and Michael Buback* Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstr. 6, D-37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Via the single pulse−pulsed laser polymerization−electron paramagnetic resonance (SP-PLP-EPR) technique, the chain-length-dependent termination of 5 and 10 wt % sodium methacrylate (NaMAA) in aqueous solution was measured from 5 to 60 °C. The rate coefficients kt(i,i) for termination of two ionized radicals of identical size i were analyzed by the composite model. Three out of the four composite-model parameters behave similarly to nonionized monomers, whereas the fourth parameter, the rate coefficient for termination of two radicals of chain length unity, kt(1,1), exhibits a distinctly different behavior. The temperature dependence of kt(1,1) is significantly below the one of fluidity (inverse solution viscosity). Moreover, absolute kt(1,1) increases with NaMAA concentration, i.e., toward higher viscosity. Both observations indicate that the termination kinetics of ionized radicals largely differs from the Smoluchowski-type behavior.

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INTRODUCTION The accurate knowledge of termination rate coefficients, kt, is essential for modeling the kinetics of conventional and reversible-deactivation radical polymerizations. Among the powerful pulsed-laser-induced polymerization (PLP) techniques for the accurate measurement of rate coefficients,1−5 single-laser-pulsed (SP)-PLP in conjunction with electron paramagnetic resonance spectroscopy (SP-PLP-EPR) has recently emerged as the perfect method for detailed investigations into the termination and transfer kinetics.6−10 Via online EPR spectroscopy, the type and the concentration of radicals are measured with a time resolution of microseconds after applying a laser pulse.11,12 The radical species may be safely identified by their individual hyperfine coupling pattern.13−17 In the case of several radical species being present, the SP-PLP-EPR technique even allows for monitoring the time evolution of individual concentrations in the absence of serious overlap of the associated EPR bands.8,18,19 Moreover, information about different conformers becomes accessible.11,20−26 As is characteristic of SP techniques, the radical chain length, i, increases linearly with time, t, after applying the laser pulse at t = 0, according to eqs 1 and 2, unless transfer reactions come into play. Propagation rate coefficients, kp, are mostly known from PLP experiments carried out in conjunction with size-exclusion chromatographic analysis of the polymeric product.27 The preselected monomer concentration is also known. i = k pc Mt

The term (+1) in eq 2 accounts for the photoinitiator-derived primary fragment which starts chain growth, thus providing a better description of chain-length-dependent termination at small i.4 The instantaneous initiation by the laser pulse ensures a narrow size distribution of growing chains. Termination at time, t, after pulsing occurs between macroradicals of more or less identical size. Because of the linear correlation between radical size and t, the time-dependent analysis of radical concentration after SP initiation allows for investigations into chain-length-dependent termination (CLDT) kinetics, represented by the rate coefficient kt(i,i) for termination of two radicals both of chain length i.4,7 At the low and moderate degrees of monomer conversion studied so far, kt(i,i) decreases toward larger chain lengths because of decreasing diffusional mobility of the larger radicals. This dependence is adequately described by the composite model introduced by Smith, Russell, and Heuts (eq 3).28 k t(i,i) = k t(1,1)i−αs

i ≤ ic

k t(i,i) = k t(1,1)ic−αs + αli−αl = k t0i−αl i > ic

(3)

According to eq 3, kt(i,i) decreases toward larger i with the power-law exponents αs and αl for short and long chains, respectively. The two regions are separated by the crossover chain length, ic. The highest termination rate coefficient, kt(1,1), refers to the termination of two radicals of chain length unity. The decay of kt(i,i) is more pronounced at small chain lengths, below ic, i.e., αs > αl, than for larger radicals, where the

(1)

i = k pc Mt + 1

Received: December 7, 2016 Revised: January 23, 2017

(2) © XXXX American Chemical Society

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DOI: 10.1021/acs.macromol.6b02641 Macromolecules XXXX, XXX, XXX−XXX

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including the calibration procedure for radical concentration with the stable radical 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxyl, (TEMPOL, Aldrich, 98%) under otherwise identical experimental conditions, have been detailed elsewhere.6,46 Ultrapure water (Milli-Q, type 1) was degassed by several pump− freeze−thaw cycles. Sodium methacrylate (NaMAA, 99%, SigmaAldrich) was used as received. 2.1 mM of an aqueous solution of the photoinitiator Darocur 1173 (2-hydroxy-2-methyl-1-phenylpropan-1one, Aldrich, 97%) in degassed water were added under an argon atmosphere. Initial monomer concentrations of 5 and 10 wt % referring to the total amount of the water−monomer mixture were chosen, corresponding to molar concentrations of 0.47 and 0.97 mol L−1 at 25 °C. Monomer contents below 5 and above 10 wt % were not accessible to EPR investigations with the currently available equipment due to the poor signal-to-noise quality and the large dielectric loss associated with the highly polar samples, respectively. A sample volume of 100 μL was filled into an EPR flat cell (Suprasil TE102 aqueous cell, sample volume 100 μL) and used immediately after preparation. To improve signal-to-noise quality, up to 20 individual EPR traces of radical concentration vs time t after pulsing were coadded. During the application of this pulse sequence, monomer concentration decreases by less than 3%. The monomerto-polymer conversion induced by several pulses was measured by near-infrared (NIR) spectroscopy after applying a series of laser pulses. Density and fluidity (see Supporting Information) of the monomer solutions prior to polymerization were determined on a DPR 2000 instrument (Anton Paar) and by a viscosity meter AMVn (Anton Paar, 1569), respectively. The fitting of experimental EPR spectra was achieved by using MATLAB in conjunction with the software package Easyspin. The IUPAC-recommended expression dcR/dt = −2ktcR2 was used as the termination rate law.3,47

terminating radicals get entangled and have to undergo segmental diffusion prior to reaction. Theory predicts αs to be in the range 0.5−1.0,7,29−34 depending on the shape of the radicals, random coil or rod-like, respectively, whereas αl is predicted to be around 0.16 for large macroradicals with the radical functionality sitting at the chain end.35−37 The coefficient kt(1,1) may be correlated with the hydrodynamic radius, r1, the self-diffusion coefficients of both monomeric radical species, DA1 and DB1, the Avogadro number, NA, the spin factor, Pspin = 0.25, and the capture radius, Rc, according to the Smoluchowski equation, kt(1,1) = 2πPSpinNA(DA1 + DB1) RC.38 Studies into nonionized monomers revealed that the product of kt(1,1) and viscosity, η, is almost insensitive toward temperature, as is expected for a diffusion-controlled process.16,39,40 Under the assumption of capture radius and hydrodynamic radius being close to each other, the product term kt(1,1)η = const·Rc/r1 is more or less identical for small radicals and is insensitive toward chemical functionality, e.g., is very similar for methyl acrylate (MA), methyl methacrylate (MMA), vinyl acetate, and styrene in bulk as well as for acrylamide in aqueous solution. The product kt(1,1)η is, however, systematically lower for larger radicals including (meth)acrylates with a long alkyl side chain. The highly shielded chain-end radicals of n-dibutyl itaconate (DBI) exhibit kt(1,1)η values which are by about 2 orders of magnitude below the associated data for MA and MMA but are close to kt(1,1)η of midchain radicals formed by intramolecular transfer (backbiting) of acrylate chain-end radicals.41 The kt(1,1) values of DBI, however, scale with temperature in the same way as does solution fluidity. This behavior may be understood as a special type of diffusion control which refers to two small radicals approaching each other at the sites of radical functionality, which encounter is however far less likely than an arbitrary contact. With the exception of studies into trimethylaminoethyl methacrylate chloride (TMAEMA),42,43 investigations into CLDT in aqueous solution were restricted to nonionized radicals, e.g., to methacrylic acid and acrylamide, so far.8,44 Early EPR studies into the termination kinetics of sodium acrylate could only be analyzed by adopting composite-model parameters measured for acrylates in organic solution.19 The present investigation provides the first comprehensive study into the chain-length-dependent termination of the monomeric salt sodium methacrylate (NaMAA), which is a strong electrolyte. The growing radicals and the poly(sodium methacrylate) species, which are also strong electrolytes, are not fully dissociated because of counterion condensation limiting the maximum degree of ionization for poly(NaMAA) in aqueous solution to about 0.36.45 Propagation rate coefficients, kp,, of NaMAA have already been reported by Laciḱ et al.27

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RESULTS AND DISCUSSION The experimental EPR spectrum for polymerization of 10 wt % NaMAA in aqueous solution at 60 °C is given in Figure 1 and is

Figure 1. Experimental EPR spectrum of NaMAA radicals during the polymerization of 10 wt % monomer in aqueous solution at 60 °C. The spectrum was recorded under stationary UV irradiation with Darocur (2.1 mmol L−1) acting as the initiator. The magnetic field position used for the SP-PLP-EPR experiments is indicated by the arrow.

entirely due to propagating (tertiary) radicals. The hyperfine splitting pattern of the observed EPR spectrum is characteristic for methacrylates and has been reported, e.g., for (nonionized) methacrylic acid (MAA) and tert-butyl methacrylate.17,20,23,25,48 The arrow in Figure 1 indicates the magnetic field position used for the SP-PLP-EPR studies. Time-resolved experiments have also been performed at other field positions with however no measurable impact on the radical concentration vs time traces. The particular field position indicated by the arrow in Figure 1 was selected, as no radical species from side reactions are expected to occur in this range. Concentration vs time traces of MAA− radicals are shown in Figure 2 for three temperatures and initial monomer concentrations of 5 and 10 wt %. The data reveal three remarkable trends: First, radical lifetime largely exceeds the one of nonionized radicals, which is

EXPERIMENTAL PART

A continuous-wave X-band EPR spectrometer system (Elexsys-II 500T, Bruker) equipped with an ER 41122SHQE-LC cavity (Bruker) was synchronized to a XeF laser (LPX 210 iCC, Lambda Physik) by a Quantum Composers 9314 pulse generator (Scientific Instruments). In stationary measurements, the sample was irradiated by a mercuryarc lamp (LAX 1450/SH2/5, 500 W, Müller). A modulation amplitude of 3 G in conjunction with a field modulation frequency of 100 kHz and a receiver gain of 84 with an attenuation of 13 dB were selected for enhanced signal-to-noise quality. The microwave power was close to 10 mW. Temperature was controlled within ±0.05 °C by an ER 4131VT control unit (Bruker). Further experimental details, B


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deduced by a two-step procedure. First, the crossover chain length, ic, and the power-law exponent αl are determined from a double-log plot of the EPR-derived relative radical concentrations measured as a function of time t after pulsing, according to eq 4. ⎛ 2k 0c 0 t αl ⎞ ⎛ c0 ⎞ t R p ⎟ + (1 − αl) log(t ) log⎜ R − 1⎟ = log⎜⎜ ⎟ 1 ⎝ c R (t ) ⎠ ⎝ − αl ⎠

i > > ic (4)

Such plots are shown in Figure 4. The slope of the straight-line fit at large t yields (1 − αl) and thus the exponent αl for long

Figure 2. Absolute radical concentration vs time traces for NaMAA radicals measured at three temperatures for 5 and 10 wt % monomer via the SP-PLP-EPR technique. Data measured at 5 °C were not included in order to avoid heavy signal overlap.

typically of the order of a few milliseconds.39,49,50 Similar observations made with radicals of trimethylaminoethyl methacrylate chloride (TMAEMA) polymerization in aqueous solution 51 were assigned to a significant reduction of termination rate as a consequence of electrostatic repulsion.51 Second, no clear enhancement of termination rate with temperature is seen at a given monomer concentration, which suggests that the activation energy of termination is small and, moreover, is at least partially compensated by CLD termination. It should be noted with Figure 2 that the radical chain lengths at a given time t after pulsing are larger at higher temperature. Third, the direct comparison of primary data for 5 and 10 wt % monomer, highlighted in Figure 3, reveals a novel finding in

Figure 4. Double-log plots of the relative radical concentration measured by SP-PLP-EPR experiments on 10 wt % NaMAA in aqueous solution at 21 and 60 °C. The intersection point of the two fitted straight lines and the slope to the straight line at large i yield the composite-model parameters ic and αl, respectively.

radicals. The intersection of the two straight lines provides the characteristic time tc, from which the crossover chain length, ic, is obtained via ic = kpcMtc.7,39,52 The same type of plot for the 5 wt % NaMAA data at 60 °C (Figure 5) provides no indication of two intersecting straight lines and thus yields no ic and αl values. The reason behind this difference compared to the 10 wt % NaMAA data may be due to the fact that the SP-PLP-EPR experiment at the lower NaMAA content essentially covers the range of smaller chain

Figure 3. Comparison of the absolute radical concentration vs time profiles measured during SP-PLP-EPR experiments on NaMAA at 5 and 10 wt % monomer in aqueous solution at 60 °C. The figure highlights the unexpected effect of monomer concentration on the termination kinetics of NaMAA in aqueous solution.

that the termination rate is higher for the more viscous system at 10 wt % NaMAA. This is in contrast to Smoluchowski-type diffusion control of termination as reported for nonionized monomers/radicals where kt(1,1) and kt scale with fluidity, i.e., with inverse solution viscosity.8 The viscosity of an aqueous solution containing 10 wt % NaMAA exceeds the one of a solution with 5 wt % NaMAA by 20% (see Figure S1 of the Supporting Information). Because of the higher monomer concentration and the enhanced kp upon increasing NaMAA concentration, as reported by Laciḱ and co-workers,27 larger macroradicals are produced at 10 wt % monomer. Thus, CLD termination offers no explanation for the higher kt at higher NaMAA concentration as depicted in Figure 3. The linear correlation of time after laser pulsing with the chain length of propagating radicals (eqs 1 and 2) enables the analysis of the SP-PLP-EPR traces for CLD kt on the basis of the composite model.28 The four parameters of this model are

Figure 5. Double-log plot of relative radical concentration measured by an SP-PLP-EPR experiment on 5 wt % NaMAA in aqueous solution at 60 °C. No intersection point and hence no values for ic and αl are observed at this lower monomer content. The number of ic found at 10 wt % is represented by the dashed blue line. The regime under investigation essentially refers to short chains, i.e., to i < ic. C


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Macromolecules lengths. The crossover chain length, as obtained for 10 wt % NaMAA, is indicated by the dashed blue line. This point is probably too close to the region of significant experimental scatter, where an intersection of two straight lines may not be safely detected. The data in Figure 5 thus primarily refer to the kinetics of small radicals at i < ic, which is also supported by the fact that the slope to the fitted straight yields an exponent which is typical of αs. On the basis of the existing data, it may however not be ruled out that the crossover behavior of termination may disappear at very low NaMAA content, perhaps because of a weaker tendency for coil formation. Within the early time regime at chain lengths below ic, eq 2 rather than eq 1 should be used,4 which is associated with eq 5 being the suitable expression for analysis of αs and kt(1,1)c0R.7 2k t(1, c R0 −1= c R (t )

1)c R0 ((k pcMt

1 − αs

+ 1)

k pcM(1 − αs)

− 1)

Figure 7. Composite-model exponents for the short-chain regime, αs, and for the long-chain regime, αl, deduced from the entire range of experimental temperature. The temperature-averaged mean value of αs for the monomer concentrations of 5 and 10 wt % in aqueous solution is represented by the black line.

1 ≤ i < ic (5)

The fit of the experimental data for chain lengths below ic to eq 5 is shown in Figure 6 for SP-PLP-EPR experiments at 10 wt % NaMAA and 60 °C.

Figure 8. Crossover chain lengths for 10 wt % NaMAA in aqueous solution (triangles) measured from 5 to 60 °C. The temperatureaveraged mean value of ic is represented by the full line.

NaMAA is below the one reported for methyl methacrylate, ic(MMA) = 100,53 but is well above the value for vinyl acetate, ic(VAc) = 20 ± 10,50,7 as well as for methyl acrylate, ic(MA) = 35 ± 1049 and for styrene, ic(Sty) = 30 ± 10.39 Wittenberg et al. performed batch polymerizations under chain-length control by 2-mercaptoethanol with 30 wt % (nonionized) MAA and estimated ic = 68, which is close to the number for the ionized monomer NaMAA.54 The rate coefficient for termination of two ionized radicals of chain length unity, kt(1,1), which is the fourth compositemodel parameter, is represented by the Arrhenius plots for 5 and 10 wt % NaMAA in Figure 9. The associated Arrhenius expressions read

Figure 6. Plot of relative radical concentration for 10 wt % NaMAA in aqueous solution at 60 °C according to eq 5 (red line). The fit yields αs and kt(1,1)c0R. With c0R being accessible by calibration (see below), the product term provides kt(1,1).

No calibration is required for the plots in Figures 4−6, as the EPR signal intensity is proportional to radical concentration. Calibration with a stable radical species is however needed for the determination of the initial radical concentration at t = 0, c0R, and thus for deducing kt(1,1) from kt(1,1)c0R. Illustrated in Figure 7 are the composite-model exponents, αs and αl, for the experimental temperature range 5−60 °C. No variation with temperature is observed for either αs or αl. The arithmetic mean value of αl for 10 wt % NaMAA is found to be 0.18 ± 0.08. The power-law exponent for small chain lengths has been determined at the two NaMAA concentrations to be αs (5 wt %) = 0.62 ± 0.05 and αs (10 wt %) = 0.56 ± 0.05. In view of the experimental uncertainty, the mean value of αs = 0.59 ± 0.08 appears to be an adequate measure of CLDT for small radicals in NaMAA polymerizations at low monomer contents and low degrees of monomer conversion. This number fully meets the expectations from theory and is close to the one of nonionized MAA (αs = 0.62 ± 0.06).44 Also with ic, no clear dependence on polymerization temperature was found at 10 wt % NaMAA (Figure 8). The mean value of ic turns out to be 80 ± 10. The number for

Figure 9. Arrhenius plot of kt(1,1) for 5 and 10 wt % along with the diffusion-limited value estimated for 10 wt % NaMAA from kt(1,1) = 1/3RTη−1 via the separately measured fluidities (inverse solution viscosities), which are associated with an activation energy of EA(η−1) = 20.3 kJ mol−1 (see Supporting Information). D


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than for systems without ionic charges being involved, where termination is essentially controlled by viscosity.57 A rather low activation energy of EA(kt(1,1)) = 8 ± 2 kJ mol−1 has also been measured for TMAEMA. The low EA(kt(1,1)) with ionized monomers, however, does not result in absolute kt(1,1) being above the rate coefficient of nonionized systems. The pre-exponential A(kt(1,1)), e.g., for NaMAA, is by about 4 orders of magnitude below the one for nonionized MAA because of the large impact of repulsion on the reaction of the two identically charged radicals. Although being a strong electrolyte, NaMAA must not be fully dissociated into free ions in the concentration region under investigation, and contact-ion-pair formation according to Scheme 1 should be considered.42 The equilibria proposed in Scheme 1 should be shifted toward the left-hand side upon increasing monomer concentration, as the ratio of sodium cations to anionic radicals is enhanced. The associated reduction of free ionic species accounts for the increase in kt upon passing from 5 to 10 wt % NaMAA, which is evidenced by the SP-PLP-EPR data in Figures 3 and 9. As an illustration of the widely differing types of termination rate behavior observed so far, the Arrhenius plots of the SPPLP-EPR data for kt(1,1) of aqueous solutions of MAA and of NaMAA as well as of n-dibutyl itaconate (DBI) radical bulk polymerizations are presented in Figure 10. The MAA data are

ln(k t(1, 1)/L mol−1 s−1) = 19.1 − 999(T −1/K−1) for 5 wt % NaMAA in aqueous solution

and ln(k t(1, 1)/L mol−1 s−1) = 20.4 − 1049(T −1/K−1) for 10 wt % NaMAA in aqueous solution

The kt(1,1) data for the MAA− radicals (Figure 9) clearly demonstrate three distinct differences compared to the associated nonionized MAA radicals: (1) The termination rate coefficients of two small MAA− radicals are far below the diffusion-controlled limiting values whereas in the case of nonionized small monomers, e.g., of MAA and AAm, kt(1,1) approaches the diffusion limit.8,44,55 A low kt(1,1) value has also been measured for the cationic radicals of trimethylaminoethyl methacrylate chloride.42,43 (2) kt(1,1) at 10 wt % NaMAA exceeds the value at 5 wt % NaMAA by approximately a factor of 3, which is unexpected, as the viscosity of the aqueous solution increases by 20% in passing from 5 to 10 wt % NaMAA (see Supporting Information). (3) The activation energy of kt(1,1), EA(kt(1,1)) = 8.7 ± 0.2 kJ mol−1 for 10 wt % NaMAA and EA(kt(1,1)) = 8.4 ± 0.2 kJ mol−1 for 5 wt % NaMAA, is significantly below the one of fluidity, which transport property has been separately measured for 5 and 10 wt % NaMAA to be EA(η−1) = 19.4 kJ mol−1 and EA(η−1) = 20.3 kJ mol−1, respectively. Thus, the assumption of EA(η−1) = EA(kt(1,1)) which turned out to hold for nonionized species in organic and in aqueous phase does not apply with ionized NaMAA.8 The relatively low EA(kt(1,1)) appears to be the signature of termination with ionized monomers.42,43 The results from Figure 9 demonstrate that termination with NaMAA does not behave according to Smoluchowski-type diffusion control. Obviously, the charge which sits close to the radical functionality plays a major role. Termination of two equally charged species, i.e., of two MAA− radicals, being successful, requires mediation by counterions in the vicinity of the radicals.56 In simplified form, the situation of NaMAA in aqueous solution may be understood by an equilibrium between contact ion pairs, solvent-separated ion pairs, and free ions (Scheme 1).

Figure 10. Arrhenius plot of kt(1,1) for nonionized methacrylic acid (MAA), sodium methacrylate (NaMAA), and n-dibutyl itaconate (DBI).

Scheme 1. Illustration of a Dynamic Equilibrium between Contact-Ion Pairs, Solvent-Separated Ion Pairs, and Free Ions in Aqueous Solution for Radical Species of NaMAA

close to the Smoluchowski prediction and thus refer to diffusion control in the sense of encounter control, meaning that termination takes place once the reacting species contact each other. The straight line for DBI has more or less the same slope as the MAA Arrhenius line, which is what one would expect in the case of EA(kt(1,1)) being identical to EA(η−1). For MAA and DBI, the EA(η−1)’s are indeed both close to 20 kJ mol−1. The large discrepancy between the MAA (10 wt %) and the DBI data is thus indicative of a temperature-independent factor, below 0.01, which reduces the encounter efficiency of DBI radicals. This situation may be referred to as retarded termination with the reaction being strongly affected by steric repulsion, while scaling with solution viscosity. The data for dilute aqueous solutions of NaMAA exhibit a clearly different behavior and demonstrate the strong impact of electric repulsion on the termination kinetics. As a consequence, EA(kt(1,1)) is well below EA(η−1) which, as with the other two systems, is close to 20 kJ mol−1. Moreover, higher NaMAA concentration enhances kt(1,1) which cannot be understood in terms of the Smoluchowski picture, as solution viscosity

These species differ in ion distance and hence in the strength of electrostatic interactions. Termination occurs more rapidly with electronically neutral contact-ion pairs than with free ions. The fraction of free ions should be around 60% in 1 M aqueous NaMAA solution at 50 °C, as may be adopted from conductivity experiments on aqueous sodium acrylate solutions between 5 and 60 °C. The formation of free ions is favored toward higher temperature.56,57 The low activation energy, EA(kt(1,1)), observed for termination in NaMAA polymerization may thus be understood as a consequence of both diffusivity and dissociation to free ions being enhanced toward higher temperature. As the resulting effects are opposed to each other, the change of kt(1,1) with temperature should be smaller E


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increases simultaneously. Although termination of NaMAA radicals does not behave according to the Smoluchowski equation, the reaction exhibits features of diffusion control, e.g., is chain-length dependent. Termination in the case of ionized species should be is referred to as a counterion-mediated process, whereas retarded termination refers to situations with strong steric hindrance affecting the diffusion-controlled process. The classical picture of diffusion control being associated with reaction upon each radical−radical contact is termed encountercontrolled termination. The significant lowering of kt(1,1) in passing from 10 to 5 wt % NaMAA may serve as an indication of kt(1,1) becoming very small in the absence of counterions, i.e., toward even lower NaMAA concentration. Thus, it would be rewarding to try and measure kt(1,1) under conditions where the sodium counterions are shielded by crown ether or by cryptand compounds. Such experiments are however not easily carried out, in particular, as also propagation will be significantly retarded in the presence of naked charged radicals and monomer molecules.58 Applying the arguments outlined in Scheme 1 to the reaction of ionized monomer with ionized macroradicals, i.e., to the propagation reaction, may also explain why kp of fully ionized monomer increases toward higher monomer concentration and why the activation energy, EA(kp), increases upon passing from an ionized to the associated nonionized monomer. The impact of counterions on termination and propagation rate will probably operate with all ionized monomer systems and thus constitute a characteristic distinction in kinetic behavior of nonionized and ionized monomers.27,59 It appears rewarding to investigate further ionized systems and to include the termination kinetics of other strong electrolyte monomers, e.g., of 2-acrylamido-2-methylpropanesulfonic acid in aqueous solution.60

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.B.). ORCID

Michael Buback: 0000-0002-8617-919X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.K. gratefully acknowledges a Ph.D. fellowship granted by the Deutsche Forschungsgemeinschaft.

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REFERENCES

(1) Barner-Kowollik, C.; Buback, M.; Egorov, M.; Fukuda, T.; Goto, A.; Olaj, O. F.; Russell, G. T.; Vana, P.; Yamada, B.; Zetterlund, P. B. Critically evaluated termination rate coefficients for free-radical polymerization: Experimental methods. Prog. Polym. Sci. 2005, 30, 605−643. (2) Buback, M.; Egorov, M.; Gilbert, R. G.; Kaminsky, V.; Olaj, O. F.; Russell, G. T.; Vana, P.; Zifferer, G. Critically Evaluated Termination Rate Coefficients for Free-Radical Polymerization, 1 The Current Situation. Macromol. Chem. Phys. 2002, 203, 2570−2582. (3) Buback, M.; Gilbert, R. G.; Russell, G. T.; Hill, D. J. T.; Moad, G.; O’Driscoll, K. F.; Shen, J.; Winnik, M. A. Consistent Values of Rate Parameters in Free Radical Polymerization Systems. II. Outstanding Dilemmas and Recommendations. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 851−863. (4) Smith, G. B.; Russell, G. T. Theoretical Validation of Single-Pulse Pulsed-Laser Polymerization as a Method for Investigating ChainLength-Dependent Termination. Z. Phys. Chem. 2005, 219, 295−323. (5) Beuermann, S.; Buback, M. Rate coefficients of free-radical polymerization deduced from pulsed laser experiments. Prog. Polym. Sci. 2002, 27, 191−254. (6) Kattner, H.; Buback, M. Detailed Investigations into Radical Polymerization Kinetics by Highly Time-Resolved SP-PLP-EPR. Macromol. Symp. 2013, 333, 11−23. (7) Barth, J.; Buback, M.; Russell, G. T.; Smolne, S. Chain-LengthDependent Termination in Radical Polymerization of Acrylates. Macromol. Chem. Phys. 2011, 212, 1366−1378. (8) Kattner, H.; Buback, M. Termination and Transfer Kinetics of Acrylamide Homopolymerization in Aqueous Solution. Macromolecules 2015, 48, 7410−7419. (9) Meiser, W.; Barth, J.; Buback, M.; Kattner, H.; Vana, P. EPR Measurement of Fragmentation Kinetics in Dithiobenzoate-Mediated RAFT Polymerization. Macromolecules 2011, 44, 2474−2480. (10) Soerensen, N.; Barth, J.; Buback, M.; Morick, J.; Schroeder, H.; Matyjaszewski, K. SP-PLP-EPR Measurement of ATRP Deactivation Rate. Macromolecules 2012, 45, 3797−3801. (11) Kamachi, M. Search for Highly Resolved Electron Spin Resonance Spectra of the Transient Radical in Radical Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 269−285. (12) Yamada, B.; Westmoreland, D. G.; Kobatake, S.; Konosu, O. ESR spectroscopic studies of radical polymerization. Prog. Polym. Sci. 1999, 24, 565−630. (13) Willemse, R. X. E.; van Herk, A. M.; Panchenko, E.; Junkers, T.; Buback, M. PLP−ESR Monitoring of Midchain Radicals in n-Butyl Acrylate Polymerization. Macromolecules 2005, 38, 5098−5103. (14) Kajiwara, A. Studying the fundamentals of radical polymerization using ESR in combination with controlled radical polymerization methods. Macromol. Symp. 2007, 248, 50−59. (15) Barth, J.; Buback, M.; Hesse, P.; Sergeeva, T. EPR Analysis of nButyl Acrylate Radical Polymerization. Macromol. Rapid Commun. 2009, 30, 1969−1974. (16) Kattner, H.; Buback, M. EPR Study of Backbiting in the Aqueous-Solution Polymerization of Acrylamide. Macromol. Rapid Commun. 2015, 36, 2186−2191.

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CONCLUSIONS The SP-PLP-EPR technique was applied to the detailed kinetic study of termination in aqueous solution polymerizations of 5 and 10 wt % sodium methacrylate between 5 and 60 °C. The highly time-resolved radical concentration profiles were analyzed for chain-length dependent (CLD) termination, kt(i,i), i.e., for rate coefficients of termination of two radicals with identical chain length, i. The composite model allows for an adequate fit of kt(i,i), although the size of kt(1,1) as well as the associated dependence on temperature and monomer concentration clearly show that conventional Smoluchowskitype diffusion control is not capable of describing the termination of ionized radicals. The other three compositemodel parameters are, however, similar in size to the ones measured for nonionized monomers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02641. Figure S1: temperature dependence of fluidity for aqueous solutions of 5 and 10 wt % NaMAA; Figure S2: density, ρ, for aqueous solutions of 5 and 10 wt % NaMAA between 31 and 61 °C (PDF) F


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