Termination and Transfer Kinetics of Sodium Acrylate Polymerization

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Termination and Transfer Kinetics of Sodium Acrylate Polymerization in Aqueous Solution Johannes Barth and Michael Buback* Institute for Physical Chemistry, University of Göttingen, Tammannstraße 6, D-37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Rate coefficients of termination and backbiting of secondary chain-end radicals (SPRs) and of propagation of tertiary midchain radicals (MCRs) have been measured for 20 wt % sodium acrylate (NaA), i.e., fully ionized acrylic acid, in aqueous solution at temperatures from 0 to 60 °C. Highly time-resolved SPR and MCR concentration vs time traces, measured after applying an intense laser pulse at t = 0, were analyzed via PREDICI simulation. Cross- and homotermination of MCRs have no major impact on the kinetics. The termination rate coefficient of SPRs in dilute solution of NaA is by more than 1 order of magnitude below the associated value of nonionized AA. The reduction of backbiting rate coefficient upon full ionization is close to the one associated with enhancing the concentration of nonionized AA from 10 to 50 wt %.

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INTRODUCTION Acrylic acid (AA) is widely used for production of hydrophilic homo- and copolymers by radical polymerization in aqueous phase. A particularly attractive feature of (meth)acrylic acid polymerization relates to the fact that reactivity may be tuned by partial or full ionization. Despite the technical importance of aqueous-solution AA polymerization, the understanding of kinetics and mechanism of the reaction is still rather incomplete, although progress has been achieved during recent years. It has turned out that the propagation kinetics of AA is largely affected by the aqueous environment,1−3 by the degree of ionization,4,5 and by the occurrence of midchain radicals (MCRs) in addition to secondary propagating chain-end radicals (SPRs).6,7 Propagation reactivity of (meth)acrylic acid chain-end radicals decreases by up to 1 order of magnitude in passing from dilute solution to bulk polymerization.1 The propagation rate coefficient, kp, of dilute aqueous solution may also be lowered by about 1 order of magnitude in passing from nonionized AA to sodium acrylate, i.e., to fully ionized AA.4,5 Both effects are associated with friction of rotational mobility within the transition state structure for propagation due to hydrogen-bonded interactions of carboxylic acid moieties in nonionized AA and to ionic interactions in NaA polymerization. The formation of MCRs during AA polymerization has been demonstrated by EPR spectroscopy using a specially designed flow cell.7 Termination during single pulse−pulsed laser polymerization (SP-PLP) of acrylic acid in aqueous solution has been studied via online monitoring of monomer conversion via near-infrared spectroscopy (SP-PLP-NIR).8 This technique provides access toward coupled rate coefficients ⟨kt⟩/kp, from which chainlength-averaged termination rate coefficients, ⟨kt⟩, may be deduced by implementing kp from independent experiments. © 2012 American Chemical Society

The preferred method for kp determination is PLP carried out in conjunction with size-exclusion chromatography (PLP-SEC). The application of this technique is however not trivial with AA polymerization, as both SPRs and MCRs are present. Upon variation of pulse laser repetition frequency, the propagation rate coefficient of chain-end radicals, ksp, is determined together with an effective rate coefficient, keff p , which includes the reactivity of MCRs. From keff p , the ratio of rate coefficients of backbiting of SPRs to propagation of MCRs, kbb/ktp, may be obtained. Such frequency-tuned PLP-SEC experiments have been carried out for 10 wt % AA in aqueous solution at 6 °C.6 The estimate of kbb/ktp data from keff p is based on the validity of the so-called “long-chain hypothesis”.9 The problems encountered with deducing rate coefficients from coupled quantities which include SPR and MCR kinetics may be circumvented by directly measuring the time evolution of SPRs and MCRs, preferably by quantitative electron paramagnetic resonance (EPR) after initiation by an intense excimer laser pulse. We have recently performed such SP-PLP-EPR investigations on aqueous solutions containing 10 and 50 wt % nonionized AA, respectively. The concentrations of SPRs and MCRs were measured via highly time-resolved EPR after almost instantaneous production of a large amount of small radicals, by photoinitiator decomposition with a laser single pulse.10 From measured radical concentration vs time traces, the rate coefficients kbb, ktp, and ks,s (for homotermination of two t SPRs) were studied between 5 and 40 °C. The SP-PLP-EPR method is unrivalled in its capacity of providing detailed insight into the termination kinetics of Received: January 30, 2012 Revised: April 11, 2012 Published: May 2, 2012 4152

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Based on PLP-SEC studies into the propagation rate coefficient of SPRs as a function of temperature, of monomer concentration, and of degree of ionization, kp(SPR) of 20 wt % NaA was estimated to be 1/5 of the associated kp(SPR) for 20 wt % nonionized AA.1,4,5 It has been checked by PREDICI modeling that variation of kp(SPR) within a factor of 2 results in no significant effect on the rate coefficients obtained from fitting the measured radical concentration vs time data. The rate coefficients for backbiting, propagation from MCRs, and termination were optimized by PREDICI fitting of the experimental SPR and MCR concentration vs time traces. Termination rate coefficients were implemented into the kinetic scheme as a function of chain length i, according to the composite model:14

radical polymerization. It is the purpose of the present study to apply this technique to the analysis of termination during polymerization of sodium acrylate (NaA). SP-PLP-EPR experiments have been carried out for polymerizations of 20 wt % NaA in aqueous solution between 0 and 60 °C at monomer conversions to a maximum of 75%. To ensure homogeneity of the polymerizing system, the concentration was not increased above 20 wt % NaA. Lower sodium acrylate concentrations occur during polymerization to higher degrees of monomer conversion. Rate coefficients for backbiting, for propagation of MCRs, and for termination are deduced from the individually measured SPR and MCR concentration vs time traces.

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EXPERIMENTAL SECTION

k ti , i = k t1,1i−αs ,

Sodium acrylate (NaA, Aldrich, 97%) and the photoinitiator 2hydroxy-2-methylpropiophenone (Darocur, 98%, Aldrich) were used as received. The solutions for polymerization were prepared with demineralized water. The SP-PLP-EPR experiment and the procedure for calibration of the EPR signals have been detailed elsewhere.11,12 Polymerization temperatures were between 0 and 60 °C at 20 wt % initial NaA concentration. Monomer conversion was measured via near-infrared (NIR) spectroscopy before and after the experiment as reported elsewhere.13 The arithmetic mean value of monomer concentration before and after the SP-PLP-EPR was introduced into PREDICI modeling as the relevant NaA concentration.

k ti , i = k t1,1(ic)−αs + α1 i−α1 = k t0i−α1 ,

THE KINETIC MODEL The kinetic scheme in Table 1 was used for PREDICI modeling of the experimental SPR and MCR vs time traces, cSPR(t) and cMCR(t). Table 1. Kinetic Scheme Implemented into the Model Used for PREDICI Simulation of the cSPR(t) and cMCR(t) Curves Obtained from SP-PLP-EPR Measurements reaction step

backbiting termination of two SPRs termination of an SPR and an MCR termination of two MCRs

ref for the rate coeff

• ki

monomer + R 0 → SPR1 kp

SPR i + monomer + → SPR i + 1 k pt

MCR i + monomer + → SPR i + 1

see text refs 2, 4, 5 and see text this study

k bb

this study

k ts,s(i , i)

this study

SPR i + ⎯→ ⎯ MCR i SPR i + SPR i ⎯⎯⎯⎯⎯⎯⎯→ polymer k ts,t(i)

SPR + MCR i ⎯⎯⎯⎯⎯→ polymer k tt,t

MCR + MCR ⎯→ ⎯ polymer

(1)

i > ic

(2)

where kt(i,i) is the termination rate coefficient of two SPRs, both of chain length i. The parameters αs and αl represent the power-law exponent of chain-length dependence below and above a crossover chain length ic, respectively. For acrylate SPRs these composite-model parameters have been determined to be αs = 0.80, αl = 0.16, and ic = 30.11,15 These numbers provide an excellent representation of chain-length-dependent termination for aqueous-solution polymerization of nonionized AA.10 Freezing of the NaA−water mixture prevents SP-PLPEPR measurements at temperatures well below 0 °C, where SPRs are the dominant radical species. Thus, separate direct measurements of αs, αl, and ic for SPRs are not easily carried out. We adopted the composite-model parameters of acrylate SPRs for NaA. It has been shown10 that the selection of the SPR composite-model parameters does not significantly affect the optimized rate coefficients kbb, ktp, and ks,s t (1,1), once the numbers for αs, αl, and ic are sensibly chosen, e.g., adopted from a monomer with similar chain flexibility. The termination rate coefficient ks,t t (i) was implemented into the model as a coupled s,s parameter ks,t t (1) = akt (1,1) with the factor a being varied. The composite-model parameters αs, αl, and ic for SPR homotermination were used for cross-termination with the chain-length dependence being restricted to the SPR species.

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single laser pulse initiation propagation of SPRs propagation of MCRs

i ≤ ic

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RESULTS AND DISCUSSION Shown in Figure 1 is an EPR spectrum recorded during polymerization of 20 wt % sodium acrylate in aqueous solution at 10 °C. The reaction is induced by laser pulses which are applied at a repetition rate of 20 Hz. The spectrum consists of spectral contributions of about 5% SPRs, which are easily identified by the oscillating line structure that reflects the rapid termination of SPRs in between two successive pulses,16,17 and of about 95% MCRs. The MCR lines do not show oscillations, as these radicals are less reactive. The line positions of SPRs are indicated by the dotted line. Fitting overall EPR spectra such as the one in Figure 1 by simulated (pure) SPR and MCR signals yields fractions of stationary MCRs between 94 and 98% for NaA at temperatures between 0 and 80 °C. The EPR signals of MCRs may not be fitted by assuming a single radical conformation. They are indicative of restricted rotation around the carbon−carbon bond next to the radical functionality; for details see refs 7 and 18−21. Detailed interpretation of full EPR spectra measured under stationary conditions is however no core part of the present study, which aims at the determination of individual rate coefficients from time-resolved measurements

turns out to be negligible; see text turns out to be negligible; see further below

No β-scission of MCRs has been included into the kinetic scheme. This reaction, which is known to occur in acrylate polymerization at temperatures well above 100 °C, is considered to play no major role at the mild temperatures of the present study. The initial concentration of secondary propagating radicals, SPR1, is identified with the maximum concentration of SPRs, c0(SPR), measured immediately after firing a laser pulse at t = 0. The initial increase of c0(SPR) can not be resolved with the presently available time resolution of the experiment. Via PREDICI, c0(SPR) in the very initial time period is fitted via the concentration of photoinitiator-derived radicals and by adopting a very high value of ki. 4153

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rate which accompanies ionization at modest AA concentrations. Another characteristic feature of the SP-PLP-EPR traces for sodium acrylate is the pronounced variation of the MCR concentration vs t traces with conversion (Figure 3). The SPPLP-EPR studies into BA and into nonionized AA exhibit no such change.10,11

Figure 1. EPR spectrum measured during aqueous-phase polymerization of 20 wt % sodium acrylate at 10 °C. Excimer laser pulses (351 nm) applied at a repetition rate of 20 Hz were used to initiate polymerization. The oscillating lines are due to SPRs (see text). The positions of constant magnetic field strength, which have been employed for SP-PLP-EPR detection of SPRs and MCRs, are indicated by the arrows.

at the characteristic field positions indicated by the arrows in Figure 1. SPR and MCR concentration vs time traces from SP-PLPEPR experiments measured during an aqueous-solution polymerization of 20 wt % NaA at 0 °C are shown on the left-hand side (lhs) of Figure 2. For comparison, values for 10 wt % nonionized AA at 5 °C are given on the right-hand side (rhs) of Figure 2. In the latter experiment, 15 wt % poly(AA) have been added to the reaction mixture to enhance signal quality.22 Approximately the same amount of SPRs is produced by a single laser pulse in both systems (see the maximum values of cSPR in Figure 2). The lifetime of SPRs and MCRs increases significantly upon ionization. Whereas the initial SPR concentration in NaA decreases from 2.0 × 10−5 to 1.0 × 10−6 mol L−1 in 0.05 s, this decay takes only 0.003 s in nonionized AA. As a consequence of enhanced SPR lifetime, the maximum concentration of MCRs formed by backbiting of SPRs is increased from about 5 × 10−7 mol L−1 in nonionized AA (10 wt %) to about 5 × 10−6 mol L−1 in 20 wt % NaA. The decrease in cMCR from its maximum value by a factor of 5 occurs within 0.5 s with 10 wt % nonionized AA, whereas the same decay takes 12 s in aqueous solution of 20 wt % NaA. These changes demonstrate the significant reduction of termination

Figure 3. SPR and MCR concentration vs time traces from SP-PLPEPR experiments on 20 wt % NaA at 60 °C and different degrees of monomer conversion. The MCR traces refer to conversion levels separated by about 20% up to a maximum of around 75%. The arrow indicates the direction of increasing monomer conversion, i.e., decreasing AA content.

Depicted in Figure 3 are SPR and MCR traces from SP-PLPEPR experiments on NaA at 60 °C and different degrees of monomer-to-polymer conversion. AA conversion increases in steps of ∼20% up to a maximum of 75%. The arrow on the rhs of Figure 3 indicates the direction of increasing conversion. Whereas the decay of MCR concentration (rhs of Figure 3) is strongly reduced toward higher monomer conversion, thus indicating enhanced MCR lifetime, the associated SPR traces (lhs in Figure 3) for the same extended range of monomer conversion are more or less insensitive toward AA conversion. The dependence of the MCR concentration vs t traces on AA conversion is indicative of MCR termination being controlled by propagation of MCRs to produce SPRs which terminate far more readily. If lower radical mobility at higher conversion (and thus at higher polymer content) would be the reason for the reduced decay of MCR concentration, SPRs should exhibit the same type of concentration vs time behavior. That the decay of

Figure 2. Comparison of SPR and MCR concentration vs time traces from SP-PLP-EPR experiments on 20 wt % sodium acrylate (left) and on 10 wt % nonionized AA (right). Within the polymerization experiment on nonionized AA, 15 wt % poly(AA) has been added. 4154

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cMCR with time t is controlled by propagation suggests relatively slow termination of MCRs. The data in Figure 3 (lhs) further indicate that termination of SPRs is not controlled by reaction diffusion13,23 as otherwise the SPR concentration vs t traces should clearly depend on AA conversion. It should be noted that prior to termination an SPR may run several times through the cycle of backbiting, to form an MCR, and propagation of the so-obtained MCR to produce an SPR again. The visual observations of SPR and MCR concentration vs time traces may be quantified by PREDICI fitting. On the basis of the reaction scheme in Table 1, PREDICI fitting of the experimental cSPR and cMCR vs t traces yields rate coefficients ks,t t and kt,t t which are by at least a factor of 100 below the associated s,s ks,t t and kt values of nonionized AA under otherwise identical conditions. The variation of cMCR(t) with conversion may be adequately described by a model which just uses the rate coefficients kp, ktp, kbb, and ks,s t (i,i) and ignores cross-termination as well as homotermination of MCRs. Monomer concentration is deduced from quantitative NIR spectroscopy and is implemented into PREDICI fitting in the manner described above. The minor deviations between experimental and simulated MCR traces in Figure 3 are probably due to averaging cM over the range of each SP-PLP-EPR experiment during which up to 15 individual scans were coadded and monomer conversion is enhanced by about 10%, e.g., for the experiment illustrated in Figure 3. Backbiting rate coefficients, kbb, deduced from SP-PLP-EPR experiments on 20 wt % NaA are depicted in Figure 4 (symbols

state structure for backbiting is not accurately known, nor is the action of counterions. Charges may retard backbiting because of decreasing chain flexibility and increasing repulsion. The major effect probably comes from reduced chain mobility, which would be consistent with the observation that the chainend propagation rate coefficient is affected to similar extents by high (meth)acrylic acid concentration and by ionization.4,5 Depicted in Figure 5 are the rate coefficients for propagation of MCRs, ktp, for aqueous-solution polymerizations of 20 wt %

Figure 5. Propagation rate coefficients of tertiary midchain radicals, ktp, in aqueous-solution polymerization of 20 wt % sodium acrylate (diamonds). For comparison, ktp for polymerization of nonionized acrylic acid, at 10 and 50 wt %, is included in the figure.

NaA (diamonds) and of 10 and 50 wt % nonionized AA. The reported ktp data are assumed to be accurate within a factor of 2 (see Supporting Information). The Arrhenius expression for the propagation rate coefficient of MCRs in 20 wt % NaA reads kpt = 6.4 × 104 exp(−2712/(T/ K)) L mol−1 s−1. As compared to the situation with 10 wt % nonionized AA, ktp for 20 wt % NaA is lower by ∼1 order of magnitude. More or less the same difference has been measured upon full ionization for kp of SPRs in dilute aqueous solution of (nonionized) AA.4 This large effect has been assigned to a lowering of internal rotational degrees of freedom in the transition-state structure for propagation due to enhanced friction by the ionized environment.5 The same genuine kinetic explanation should hold for ktp of NaA. Further support for this argument comes from ktp for 50 wt % nonionized AA (see Figure 5). The close agreement of this data with ktp for 20 wt % NaA suggests that passing from dilute solution of nonionized AA to a concentrated solution, e.g., of 50 wt % AA, has a similar effect on the propagation kinetics as has full ionization. It should be noted that the viscosities of aqueous solutions containing 50 wt % nonionized AA and 20 wt % NaA are more or less identical (see Supporting Information). Plotted in Figure 6 are measured termination rate coefficients for SPRs of chain length unity, ks,s t (1,1), in aqueous solutions of 20 wt % NaA (triangles) as well as for 10 wt % (open circles) and 50 wt % (full circles) nonionized AA polymerization. The reported ks,s t (1,1) data should be accurate within ±20% (see Supporting Information). Termination of SPRs is significantly slowed down by ionization: k1,1 t decreases by more than 1 order of magnitude in passing from 10 wt % AA to 20 wt % NaA. The k1,1 t data for 20 wt % NaA may be represented by the Arrhenius relation 10 −1 −1 ks,s s . The data t (1,1) = 8.9 × 10 exp(−1782/(T/K)) L mol for 50 wt % nonionized AA demonstrate that this large effect is primarily due to ionization, as k1,1 t for 50 wt % AA is by about a

Figure 4. Arrhenius plots of backbiting rate coefficient, kbb, for 20 wt % sodium acrylate (triangles) as well as for 10 and 50 wt % nonionized AA (dashed and dotted lines, respectively) in aqueous solution (all data are from SP-PLP-EPR).

and full line) and compared with kbb data for 10 and 50 wt % nonionized AA (dashed line).10 The reported kbb data are assumed to be accurate within ±20% (see Supporting Information). Literature data for kbb of further nonionized monomers have been reported and discussed in ref 10. Arrhenius fitting of kbb for 20 wt % NaA yields kbb = 2.2 × 106 exp(−3090/(T/K)) s−1, as represented by the full line in Figure 4. Depending on temperature, kbb of 20 wt % NaA is by a factor of 2−4 below the numbers for 10 wt % nonionized AA but is relatively close to kbb for 50 wt % nonionized AA. The question whether and to which extent increased stiffness and ionic repulsion contribute to this modest effect on kbb is not easily answered, as the degree of ionization of the (polymeric) carboxylic acid groups engaged in the six-membered transition4155

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ks,s t (1,1), were derived. Cross-termination and homotermination of MCRs were found to have no significant impact on the kinetics. As compared to the situation with nonionized AA, the termination of chain-end radicals is significantly slowed down, which is assigned to equally charged radicals in case of NaA. The rate coefficients kbb and ktp were also found to be smaller than in nonionized AA. These effects are of similar size as the ones found upon increasing the concentration of (nonionized) AA, e.g., from 10 to 50 wt %. Within subsequent work, the kinetics of partially ionized AA should be investigated.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 6. Rate coefficients for termination of SPRs of chain length unity, ks,s t (1,1), in aqueous solution of 20 wt % sodium acrylate (triangles) and of 10 wt % (open circles) and 50 wt % (full circles) nonionized AA.

Figures S1−S4 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

factor of 4 above k1,1 of 20 wt % NaA, although solution t viscosity at these two conditions is more or less identical (see Supporting Information). The viscosity at 10 wt % nonionized AA is lower by a factor of 2, which partially accounts for the large k1,1 t value. On the other hand, the effect of ionization is not that all invasive and is smaller than one might have expected. This is probably due to the fact that the negative charge is not sitting next to the radical functionality, that the charges are screened by counterions, and that the carboxylic acid groups of polymerized AA segments are not fully ionized. As has been reported in ref 10, k1,1 t of 10 wt % nonionized AA is close to the maximum value predicted for diffusion-controlled rate. As the hydrodynamic radius of the diffusing species and the viscosity of the monomer−solvent mixture for 10 wt % nonionized AA and 20 wt % sodium acrylate should be close to each other, the discrepancy between the k1,1 t values is assigned to a significant difference in capture radius. One may conclude that the Smoluchowski picture should not be applied to special situations, e.g., to the one of equally charged radicals. A situation similar to the one induced by ionic repulsion may occur under conditions of severe steric hindrance, as with itaconates.25 The shielding effect formally translates into a significantly reduced capture radius. In the opposite case of poor shielding and high chain mobility, as with nonionized AA in dilute aqueous solution, the capture radius should be close to the one estimated from structural information about the terminating moiety, as has been demonstrated in ref 12. With shielding being adequately accounted for by a constant factor, which is independent of temperature, the variation of k1,1 t may be adequately represented by an activation energy that is close to the temperature dependence of solution fluidity (inverse viscosity). This assumption is supported by the finding in Figure 6 of an activation energy of about EA(k1,1 t ) = 15 ± 2 kJ mol−1, which holds for 20 wt % NaA, 10 wt % AA, and 50 wt % AA and which is close to “EA”(η−1) ≈ 16 kJ mol−1 of pure water.26 The measured activation energies “EA”(η−1) of aqueous solutions containing 20 wt % NaA, 10 wt % nonionized AA, and 50 wt % nonionized AA (see Supporting Information) are 19, 18, and 19 kJ mol−1, respectively.

*E-mail: [email protected]; Fax: +49 551 393144. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.B. is grateful to the Fonds der Chemischen Industrie for financial support. The authors acknowledge funding and stimulating discussions within the collaboration with BASF SE, Ludwigshafen, as well as the scientific interaction with the groups of Professor Robin Hutchinson (Kingston, Canada) and Professor Igor Lacik (Bratislava, Slovakia).

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CONCLUSION Radical polymerization of sodium acrylate in aqueous solution has been studied via the SP-PLP-EPR technique. Rate coefficients for backbiting, propagation of midchain radicals (MCRs), and termination of radicals of chain length unity, 4156

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