Cinetica de Polimerização_PAA Ok

Acrylic Acid Polymerization Kinetics

S. S. CUTIE,1 P. B. SMITH,1 D. E. HENTON,2 T. L. STAPLES,2 C. POWELL2

1 Analytical Sciences

2 Superabsorbent Products R&D, The Dow Chemical Company, Midland, Michigan 48674

Received 10 May 1996; revised 27 March 1997; accepted 31 March 1997

ABSTRACT: The kinetics of the isothermal polymerization of acrylic acid were deter-mined utilizing 1H-NMR spectroscopy. The polymerization rate was observed to dependapproximately on the 32 power of monomer and the

12 power of sodium persulfate concen-

tration. This is consistent with a model in which the rate of initiation is itself dependenton the monomer concentration. The polymerization rate was also observed to havea strong dependence on percent neutralization, decreasing with increasing level ofneutralization up to 75 to 100% neutralization, and then increasing again. The activa-tion energy for the rate of polymerization was between 9 and 13 kcal/mol except for100% neutralized acrylic acid, which had an activation energy of 18 kcal/mol. Thesedata suggest that a transition in mechanism occurred at 100% neutralization. Increas-ing the ionic strength by the addition of sodium chloride also increased the rate. Thedependence of the molecular weight on the above variables was also quantified for usein the model. It decreased with increasing conversion, decreasing ionic strength andincreasing initiator. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 20292047, 1997Keywords: poly(acrylic acid) gel; polymerization kinetics; nuclear magnetic resonance;crosslinking; superabsorbent

INTRODUCTION then increased again until pH 12.2,3 Above pH 12they noted that the rate again decreased. The au-thors proposed that the polymerization rate wasImproving the performance of superabsorbentenhanced by the presence of ion pairs whose con-polymers depends, to a considerable extent, on

controlling the polymerization. Obtaining high centration increased with increasing pH. Thesebackbone molecular weight, uniform crosslinker authors also noted that the stereochemistry of theincorporation, low residual monomer levels, and polymer seemed to be controlled by this ion pairhigh production rates, all at the same time, re- formation and that the addition of high concentra-quires a detailed knowledge of the polymerization tions of salts such as NaCl and CH3COONa in-kinetics. creased both the polymerization rate and the mo-

Several groups previously investigated the lecular weight of the linear polymer.polymerization of acrylic acid. Ito et al. reported a Manickam et al. also observed an enhancementlarge pH dependence of the rate of polymerization of the polymerization rate of acrylic acid with thewith a minimum near pH 7.1 Kabanov et al. also addition of salt, but only in alkaline medium.4,5observed that the rate of polymerization of acrylic The salt effect was not observed at low pH values.acid decreased as the pH was raised from 1 to 6, Laborie, Chapiro, and Dulieu, and Galperina

et al. suggested that the polymerization in aque-ous medium was accelerated by a matrix effect

Correspondence to: P. B. Smithin which monomers associate prior to polymeriza-

Journal of Polymer Science: Part B: Polymer Physics, Vol. 35, 20292047 (1997)q 1997 John Wiley & Sons, Inc. CCC 0887-6266/97/132029-19 tion into H-bonded aggregates.69 In contrast,

2029

9605007/ 8Q31$$5007 07-28-97 20:31:22 polpa W: Poly Physics

2030 CUTIE ET AL.

hydrocarbon and chlorinated solvents, which donot support these H-bonded aggregates, lead to amarkedly reduced rate of polymerization and re-sult in lower molecular weight. This matrix ef-fect was found to persist to fairly high dilutionsand was proposed as the origin of stereochemicalcontrol of the resulting polymer.

Acrylamide is another water-soluble monomerthat yields very high molecular weight polymers.The rate of polymerization of neutral acrylamidewas shown by Riggs and Rodriguez to have astrong dependence on the cation of added salts,potassium causing about twice the increase inrate as sodium.10 Lenka et al. modeled the poly-merization of acrylamide by peroxydiphosphateand showed that its polymerization did not re-

Figure 1. The 1H-NMR spectra of the polymerizationquire a matrix effect to fit the data.11 Prabha andof acrylic acid as a function of time.Nandi interpreted the selectivity of the initiator,

ferric dipivaloylmethide, toward a given vinylmonomer in its polymerization and spectroscopic

tion, with stirring, of 13.456 g of granular sodiumdata, as evidence of complex formation betweencarbonate (Fisher Certified A.C.S.) .the initiator and the monomer.12

This article describes the determination of theisothermal polymerization rate constants for NMR Kinetic Analysisacrylic acid as a function of pH, monomer concen-tration, and temperature. A model, which incorpo- The appropriate diameter NMR tube (depending

on heat transfer requirements) was loaded via arates these effects, is presented.glass pipette and deoxygenated with nitrogen ina microdevice described elsewhere.13 The tubeswere deoxygenated for a minimum of 30 min.EXPERIMENTALAfter the deoxygenation, the cap was sealed andthe sample was sonicated (2030 s) to removeA 1H-NMR technique was used to monitor the po-the nitrogen gas from the saturated liquid so thatlymerization rates of acrylic acid in situ.13 Thisbubbles did not form in the sample as it polymer-method made it possible to obtain isothermal dataized. (Bubbles give rise to severe line broadeningduring exothermic conditions of up to 10% conver-in the 1H-NMR spectra, dramatically degradingsion/minute. Small feed batches of 50100 g weretheir quality.) The sample was then placed in theprepared in glass bottles, as described below, withtemperature-equilibrated, nitrogen-filled probe ofthe appropriate initiator level, monomer content,the NMR spectrometer and data were acquired.percent neutralization, and additives.The time of data accumulation for each spectrumwas determined by the number of scans and thedelay time between scans. A typical series of 1H-Typical Feed Batch Preparation Procedure forNMR spectra, from low to high conversion are31.9% Solids, 65% Neutralization, 1600 ppmgiven in Figure 1. Of the different NMR tech-Sodium Persulfate Based on Acrylic Acid (BOAA)niques, 1H-was preferred over 13C because of the

To 28.18 g of glacial acrylic acid (monomer grade, greater sensitivity, faster acquisition rates, and200 ppm MEHQ) was added 69.925 g of the per- the ability to use smaller diameter tubes, whichsulfate master batch solution. helped maintain isothermal conditions. The con-

centration of polymer was determined from theareas of the peaks at 1.52.5 ppm and that of theMaster Batch of Sodium Persulfatemonomer from the area of the peaks at 5.56.5ppm. The fractional conversion was calculated as698.8 g DI water; 0.450 g sodium persulfate

(98/%, Aldrich). the level of polymer area divided by the sum ofthe areas of monomer and polymer. The rate ofThis mixture was neutralized by the slow addi-


ACRYLIC ACID POLYMERIZATION KINETICS 2031

conversion was typically calculated from the ini- and G6000 PWXL resins. The eluent used was0.3N NaCl with 0.03M Na2HPO4. The pH wastial slopes of these curves. The rate of polymeriza-

tion (Rp ) was obtained by multiplying the rate of maintained at 6.8 with 3% phosphoric acid. Thesolvent was filtered through a Micron Separationsconversion by the initial monomer concentration.

The temperature of the probe was set to within Inc. 0.1 mm Nylon filter available from Fisher Sci-entific, Midland, MI. Sodium azide (400 ppm) was{17C using ethylene glycol as an internal refer-

ence; the 1H-NMR chemical shift difference be- added to inhibit bacteria formation. The pumpwas a Waters 590 with excellent flow reproducibil-tween the methylene and hydroxyl protons of eth-

ylene glycol gives an accurate measure of temper- ity. The flow was kept at 1 mL/min. The detectorwas a Waters refractive index model 410. The col-ature. The polymerization is very exothermic, and

the sample temperature was found to increase umns and detector cells were kept at 357C. Theinjection volume was 200 mL. Samples were fil-above the NMR set point when the rate of reaction

exceeded about 2% per minute for samples at 30% tered through a 0.45 mm Nylon Acrodiscy 13 mm,available from Fisher Scientific, Midland, MI.solids or higher in 5 mm NMR tubes. This was

partly due to the limitations of heat transfer from The apparent molecular weight of the sampleswas determined by comparison of the chromato-within the sample to the NMR tube but mostly to

the ability to transfer the heat from the NMR tube graphic behavior of the sample to that of stan-dards with known molecular weights. Polyethyl-within the NMR probe. The temperature within

the NMR probe was maintained by temperature ene oxide narrow distribution standards rangingin molecular weight from 1470 to 1,390,000 wereequilibrated nitrogen gas that was forced around

the sample. The heat transfer characteristics of used for calibration. The molecular weight dataabove about 1,400,000 Daltons should be usedthe gas were not sufficient to manage the large

exotherms in the sample. with caution due to the lack of standards.To measure the molecular weight distributionThese observations indicated that, under some

conditions, even the NMR tube reactor did not of the crosslinked polymer, the crosslinks neededto be hydrolyzed to produce a soluble polymer.allow adequate removal of the heat generated by

the polymerization to maintain isothermal condi- The crosslinked polymer (0.04 g) was added to0.1N NaOH (10 mL) in water (1/250 ratio bytions. A special NMR tube configuration was used

for samples that were expected to polymerize at weight), MEHQ (0.04 g) was added, and placedin an oven at 757C for 72 h to undergo hydrolysis.rates faster than about 2% per minute. This appa-

ratus was composed of a 2 mm i.d. NMR tube After completion of the hydrolysis the sample wasdiluted 110 with SEC eluent and injected into thecoaxially inserted into a 5 mm o.d. NMR tube. The

sample was placed into the 2 mm tube and the system. This procedure led to the hydrolysis ofvolume between the 2 and 5 mm tubes was filled crosslinks composed of ester bonds but not thewith D2O. The D2O acted both as a heat transfer carboncarbon crosslinks formed by chainfluid and as an internal lock for the NMR. This transfer.apparatus provided better heat transfer and alsominimized the sample size so that less heat wasgenerated. RESULTSThe 1H-NMR spectra were obtained at 299MHz using a Bruker AC-300 NMR spectrometer,

Polymerization Ratemodel number HO2129-ECL-24, S/N 0898. Thedata acquisition parameters utilized were as fol- Effect of Monomer and Initiator Concentrationslows: pulse width 107, delay time 10 s, size 16 K, accumulation time 1.36 s, sweep width The isothermal polymerization of acrylic acid in

water was monitored as a function of concentra- 6 KHz, apodisation exponential, 0.5 Hzbroadening. tion, temperature, degree of neutralization and

initiator level to define a kinetic model and obtainaccurate values for the rate constants. The depen-

Molecular Weight Analysis dence of the polymerization rate on initial mono-mer concentration was determined at 557C byThe size-exclusion chromatography (SEC) system

consisted of two TosoHaas TSK columns GPWXL, measuring the rates of polymerization for feedbatches containing acrylic acid concentrationsand one TSK column G 1000 PW.15 TSK gel

GPWXL columns are packed with G 2500, G3000 ranging from 37.2% (4.56M ) to 9.3% (1.17M ) at


2032 CUTIE ET AL.

Figure 2. The rate of polymerization of 65% neutralized acrylic acid at 557C as afunction of acrylic acid concentration.

constant total persulfate concentration. The will depend on the monomer concentration [M].Because bimolecular termination typically con-monomer concentration in wt % was based on the

weight of the 65% neutralized monomer. A plot of trols the steady state concentration of radicalssuch that [R] depends on the square root of thethe conversion as a function of time for three such

runs is given in Figure 2. It shows that the rate rate of initiation, it follows that the overall depen-dence of propagation would show a 32 dependence,of conversion increases with increasing initial

monomer concentration. Radical polymerizations one power comes from the normal monomer de-generally possess a propagation rate that is pro- pendence, and the additional half power from theportional to kp[M][R], where kp is the propaga- rate of initiation.tion rate constant. This step is almost always firstorder in both the monomer [M] and the growing

[R]

fkd[I][M] (1)radical [R]. Over any significant time period, rad-ical concentrations will generally be constant oronly slowly changing; this gives rise to the so- where f has been introduced as an efficiency fac-

tor and kd[I] is the normal first-order rate of de-called steady state, and in such cases the conver-sion with time curve will be independent of mono- composition of initiator. It should be noted that

this behavior may be particularly dependent onmer concentration.More complex rate expressions arise when the the specific initiator used.

The logarithm of the initial polymerization ratesteady-state radical concentration itself dependson the monomer concentration. This leads to other for these experiments is plotted as a function of

initial monomer concentration in Figure 3. Thethan first order kinetics, which has been found byothers to be the case for acrylic acid. Ito et al. slope of the curve gives a 1.73 dependence on

monomer concentration that is approximatelycited a rate law with second-order monomer de-pendence.1 Manikam et al.5 observed a 32 power consistent with the

32 dependence discussed above.

A line with a slope of 1.5 is shown in Figure 3 fordependence on monomer, as did Kabanov et al.3

Interestingly, the latter paper showed a simple comparison. The discussion of the polymerizationmodel will elaborate on this topic further.first order dependence on monomer for metha-

crylic acid. The dependence of the polymerization rate onthe persulfate initiator concentration was deter-If the undissociated initiator interacts with the

monomer, either due to a redox couple, induced mined at 557C, 65% neutralization, and 33.8% sol-ids, as shown in Figure 4. The rate of polymeriza-decomposition, or because the initiator fragments

are trapped in a solvent cage, the rate of initiation tion increases, as expected, as the persulfate initi-



the O2 is consumed by the radicals, no polymeriza-tion takes place.13 This side reaction is believedto be the cause of the behavior of the plot.

Degree of Neutralization and Temperature

The effect of neutralization on the polymerizationrate was determined at three temperatures55,70, and 857C. Both neutralization and tempera-ture had a marked effect on the polymerizationrate, as shown in Figures 68. The rate of poly-merization at 557C decreased with the increasinglevel of neutralization up to 100% neutralization,above which the rate increased again. At 125%neutralization, the rate of polymerization wasFigure 3. Effect of monomer concentration on the ini-about the same as that at 0% neutralization.tial rate of acrylic acid polymerization at 557C and 65%There are several potential explanations for theneutralization.decreasing rate with neutralization. These in-clude a disruption of the template effect discussed

ator level increases because more chains are earlier, a lower inherent reactivity of the sodiuminitiated. A plot of the initial rate of polymeriza- acrylate relative to acrylic acid, or an increasetion vs. persulfate concentration to the 12 power in the ionic repulsion of the monomer from the

growing polymer due to the increased fraction of(Fig. 5) gives a straight line, verifying the halfpower dependence for this system proposed in the ionized monomer. The latter explanation seems

most likely.equation above.The plot of the initial rate of polymerization vs. One very interesting observation was that the

more highly neutralized (e.g., 125%) feed batchespersulfate concentration given in Figure 5, doesnot go through the origin. There are components had virtually no inhibition times. Polymerization

started almost immediately, frequently at roomin the acrylic acid that scavenge the radicals ini-tially. Commercial acrylic acid has approximately temperature during deoxygenation. However, the

reactions at 125% neutralization reached a pla-200 ppm MEHQ, which works synergisticallywith oxygen to inhibit its polymerization. Until teau at lower conversion at elevated tempera-

Figure 4. Effect of persulfate concentration on the polymerization rate of 65% neu-tralized acrylic acid at 557C.


2034 CUTIE ET AL.

Figure 5. The dependence of the initial polymerization rate on the square root of thesodium persulfate concentration.

tures, possibly due to restricted mobility at this and 75% neutralization, respectively. The rates ofpolymerization are given in Table I and Figurehigh degree of neutralization.

Figure 9 shows the NMR spectra of polymers 10. For these experiments, initial monomer andinitiator concentrations were kept constant;at about 50% conversion, polymerized at different

levels of neutralization. The lines broaden dra- therefore, we have simply used the initial poly-merization rates for our analyses. Figure 11 givesmatically at higher % neutralization, the source

of the broadening being due to restricted mobility an activation plot for these rate data. The activa-tion energies, from the initial slopes of the plotsof the polymer. These findings support the idea

that restricted motion of the polymer backbone in Figure 11, are given in Table II. These plotsare fairly linear for all the levels of neutralization.at the higher levels of neutralization cause the

reactions to reach a plateau at lower conversions. The activation energies for the initial rates of po-lymerization increased as the level of neutraliza-The initial rate of polymerization showed simi-

lar, but not identical, pH dependences at all tem- tion increased up to 100% neutralization. At 125%neutralization, the activation energy decreasedperatures. The rate of polymerization at 557C

showed a minimum at 100% neutralization, again. These trends in activation energy suggest atransition in mechanism at 100% neutralization.whereas at 70 and 857C the minima were at 50

Figure 6. The effect of % neutralization on the rate of polymerization of acrylic acidat 557C.




The increase in activation energy can be rational- [M]3/2 r [I]1/2 determined in a similar range ofconditions from references previously cited alongized using the same arguments as for the pH de-

pendence of the polymerization rates. As neutral- with values determined in this work. No previousstudies were run at the 4M concentrations usedization increases, the ionic repulsion between the

growing polymer in the ionized state and the ion- commercially. The dramatic effect of pH on therate, ranging from a factor of 50 (Kabanov) toized monomers also increases. This repulsion

would be maximized at 100% neutralization and 20 (Ito) for maximum/minimum, appeared to begreater in the more dilute systems; our workthen be mitigated as the ionic strength of the me-

dium increased due to continued neutralization. showed a range of about 5 at 557C (Table III) .Overall, activation energy values from previousThe NMR kinetic data support an expression

for the rate of polymerization, Rp , as a product of literature for the polymerization of sodium acry-late were 16.7 kcal /mol reported by Ito et al.[M]3/2 and [I]1/2 . Table III shows values of Rp /



2036 CUTIE ET AL.

Molecular Weight of Polyacrylic Acid

Temperature Effects

The effect of temperature on the molecular weightof 65% neutralized polyacrylic acid was determinedat three temperatures (55, 70, and 857C). A mix-ture was prepared at 65% neutralization, as before,in a glass vessel from sodium carbonate, acrylicacid, sodium persulfate (1600 ppm based on acrylicacid), and water to a solids content of 32% (4.1M).Small microampoules (1.8 mm o.d.), described pre-viously, were loaded with the feed mixture, deoxy-genated for 45 min, sealed to exclude oxygen, andplaced in a water bath. At 557C, samples were with-drawn over a 90-min period, at 707C samples weretaken over 20- and 60-min periods, and at 857Csamples were taken over a 10-min period. The con-version at each time and temperature was deter-mined in parallel experiments by polymerizing thefeed mixture while monitoring conversion by protonNMR. The samples were hydrolyzed in dilute caus-tic and the molecular weight determined by SEC(Fig. 13). Very high molecular weight polymer (5million Daltons) was formed early in the polymer-ization, while the number average MW decreasedwith conversion (Table IV, Fig. 14). Because of the

Figure 9. The effect of % neutralization on the 1H- lack of molecular weight standards above 1,400,000NMR line width of the Resonances of acrylic acid at Daltons, the absolute values for polymers of greater557C. molecular weight should be used with caution and

only for relative comparison within a given study.

Effect of Initiator Concentrationand 23.5 kcal /mol by Manickam. These may beon Molecular Weightcompared to the values of this work, given in

Table II. A 65% neutralized acrylic acid mixture wasspiked with increasing amounts of sodium persul-fate and samples polymerized at 557C while moni-Counterion Dependencetoring conversion by proton NMR. Each samplewas polymerized for a set period of time to obtainThe dependence of the polymerization rate onkinetic data and then the sample was analyzedcounterion was determined by comparing the ratefor molecular weight by SEC. Concentrations offor acrylic acid neutralized with Na2CO3 to that

neutralized with K2CO3. The samples were 75%neutralized and the polymerization was run at Table I. The Effect of Temperature and %557C. These data, shown in Figure 12, indicate no Neutralization on the Initial Rates of Polymerizationdetectable difference in rate for these counterions. of Acrylic AcidThese results are quite different from those dis-

% Neutralizationcussed earlier for acrylamide polymerization.12

Even though no counterion dependence was ob-Temperature 0% 25% 50% 75% 100% 125%served for the polymerization rate, ionic strength

is known to have a marked effect on the rate.4,5557C 4.0 2.8 1.5 1.2 0.83 3.0The addition of sodium chloride caused a notable 707C 7.2 3.8 2.4 2.4 2.9 6.5

enhancement of the polymerization rate with a 857C 14. 10. 6.2 6.8 9.2 10.concomitant increase in the molecular weight of

The units for the rates are % conversion per minute.the polymer.



Figure 10. The effect of temperature and % neutralization on the initial rate of acrylicacid polymerization, circles557C, diamonds707C, and squares857C.

sodium persulfate ranging from 2.73 1 1004M low conversion is normally used for determiningthis relationship.(218 ppm based on acrylic acid, 32% solids) to

8.72 1 1003M (6963 ppm based on acrylic acid)were used (Table V). The conversion of the sam- Effect of Neutralization on the Molecular

Weight of Polyacrylic Acidples was not constant for each sample, which willaffect the final MW; however, a plot of the number The final polymer samples that resulted from theaverage molecular weight vs. 1/[sodium persul- NMR kinetic study of percent neutralization (pH)fate]0.5 results in a reasonably good straight line effects on polymerization rate were hydrolyzedrelationship (Fig. 14) similar to the rate of poly- with dilute caustic and analyzed by SEC to deter-merization data presented in Figure 5. (This plot mine molecular weight.15 Samples prepared atdoes not go through the origin for the same reason three temperatures (55, 70, and 857C) were char-as that described for the plot of Figure 5.) An over- acterized (Table VI). The neutralization of thelay of the SEC traces for the polymers presented acrylic acid feed mixtures varied from 0 to 125%.in Table VI is shown in Figure 15. The significant In contrast to the data at 65% neutralization pre-influence of the initiator content on the polymer sented in Table IV, differences in the final MW atmolecular weight can be seen from these data. Of the three temperatures were observed at eachcourse, the molecular weight of the polymer at level of neutralization. The conflicting results may

have been the result of residual head space oxygenin the samples prepared in the water bath poly-merizations at 557C (Table IV), which could have

Table II. The Effect of Temperature and %Neutralization on the Activation Energy forAcrylic Acid Polymerization

% Neutralization Ea (Kcal/mol)

0 9.625 9.850 10.675 13.6

100 18.4Figure 11. Arrhenius plot for the polymerization of 125 9.2acrylic acid at various degrees of neutralization.


2038 CUTIE ET AL.

Table III. Comparison of Polymerization Rates with Literature Values

liter/mol minRp

[M]3/2 [I]1/2Reference Temp. pH [M] Initiatora [I]

Manickam et al.5 50 1 0.138 KPS 0.01 0.4910.586Manickam et al.5 50 4.2 0.172 KPS 0.01 0.236Manickam et al.5 50 11 0.258 KPS 0.01 0.0910.444b

Kabanov et al.2,3 60 1 1.2 AIBN 0.005 0.14Kabanov et al.2,3 60 10 1.2 AIBN 0.005 0.19Kabanov et al.2,3 60 7 1.2 AIBN 0.005 0.00278Ito et al.1 50 2.4 0.5 APS 0.00285 0.90Ito et al.1 50 4.7 0.5 APS 0.00285 0.14Ito et al.1 50 7.2 0.5 APS 0.00285 0.045

This work 55 4.5 1.144.56 NPS 0.0023 0.1260.239

a KPS: potassium persulfate; NPS: sodium persulfate; APS: ammonium persulfate; AIBN: azo bis-isobutyronitrile.b Increasing ionic strength.

reduced the molecular weight of the samples or same trend as the rate of polymerization vs. %neutralization (Fig. 10).simply be the result of the greater influence of

temperature on molecular weight at the lower %solids (2.8M in this study vs. 4.1M in the study Effect of Monomer Concentrationreported in Table IV). The data in Table V were on Molecular Weightdetermined at 65% neutralization, where the mo-

Two experiments were conducted to determinelecular weight differences at the three tempera-the effect of monomer concentration on the molec-tures in (Table VI) are minimized. The overallular weight of 65% neutralized acrylic acid. TheMW of the polymer samples in Table VI, preparedfirst study involved measuring the molecularat the lower % solids, are significantly less thanweight of the polymers by SEC that were preparedthose in Table IV prepared at the higher % solids.during the NMR rate studies. The molecularThe highest molecular weight at all temperaturesweight data are shown in Table VII. The molecu-is prepared at 0% neutralization, while the lowest

MW occurs when polymerizing at 100% neutral-ization. The molecular weight increases againwhen polymerizing in the presence of excess so-dium carbonate (125% neutralization). The MWresults shown graphically in Figure 16 follow the

Figure 13. Number-average molecular weight of po-lyacrylic acid prepared at 65% neutralization andFigure 12. The dependence of the polymerization

rate for 75% neutralized acrylic acid on counterion, so- 31.9% solids (4.1 M ) vs. conversion at 557C (squares),707C (filled triangles), and 857C (circles).dium (triangles) vs. potassium (filled circles) at 557C.



Table IV. Molecular Weight of 65% Neutralized Polyacrylic Acid as a Function of Polymerization Time andTemperature

Temp (7C) Time (minutes) Mw 106 Mn 106 Mp 106 % Conv.a

55 5 8.135 2.100 10.967 255 8 6.604 2.033 6.147 755 10 6.898 1.639 6.880 1155 12 5.964 1.636 5.264 1455 16 6.963 1.819 6.382 2255 20 4.215 1.058 3.418 2555 30 3.276 0872 2.290 4355 40 3.527 0.917 2.290 6255 60 1.930 0.480 1.314 8055 90 1.316 0.359 0.845 9270 4 5.896 1.912 5.571 870 6 4.393 1.210 2.310 1170 8 4.257 1.355 2.495 1970 10 4.934 1.109 3.143 2670 20 4.591 1.327 2.355 5970 4 10.690 1.536 7.557 870 6 11.104 1.354 8.730 1170 8 9.858 1.713 5.540 1970 10 1.1392 1.510 7.557 2670 10 9.711 1.349 5.696 5970 40 6.269 0.708 2.195 8070 60 6.373 0.627 2.833 9085 1 6.142 1.779 5.786 385 2 6.238 2.091 6.123 1585 3 4.612 1.219 2.855 3085 4 4.817 1.245 2.967 4485 6 4.105 0.863 2.447 6485 10 3.208 0.555 1.663 81

a Estimated from time of polymerization using separate NMR experiments.

lar weight drops significantly with decreasingmonomer concentration, but with significant scat-ter. Ampoules containing the same feed mixtureswere also polymerized in a water bath for varioustimes and the samples analyzed for molecularweight by SEC. These data are presented in TableVIII and plotted in Figure 17. Based on the slopeof the data in Figure 17, the number-average mo-lecular weight is proportional to approximatelythe 1.5 power of the monomer concentration, con-sistent with theoretical prediction.

Kinetic Model

The simplest polymerization model, which has afirst-order dependence in monomer and half orderin initiator, does not adequately fit the data pre-sented here, as noted earlier. Using the argu-Figure 14. The effect of sodium persulfate concentra-ments developed so far the following kinetiction on the number-average molecular weight of 65%

neutralized polyacrylic acid prepared at 557C. scheme is proposed:


2040 CUTIE ET AL.

Table V. Molecular Weight of 65% Neutralized Polyacrylic Acidd Samples Prepared at 31.9% Solids in NMRTubes at 557C

[Na2S2O8] mol/liter Mwc 106 Mnc 106 Mpc 106 Final % Conversion

0.000273a 12.019 1.824 17.126 440.0016a 3.897 0.605 2.129 780.00433b 0.208 0.065 0.164 820.00872b 0.087 0.033 0.0613 92

a Used 5 mm NMR tube.b Used 2 mm NMR tube.c Molecular weight affected by the conversion of the samples.d Acrylic acid contained 177 ppm MEHQ.

I r (2Rc ) kd (2) 0 d[M]dt

kp[RM][M]. (7)(2Rc ) / M r RM / R k * (3)

R / M r RM k 9 (4)The concentration of the caged radicals [(2Rc )]

RM / M r RM kp (5)RM / RM r polymer kt . (6) d[ (2Rc )]

dt kd[I] 0 k * [ (2Rc )][M]. (8)

In this scheme (2Rc ) represents two radicalspecies trapped in a solvent cage. Only when thesecaged radicals are intercepted by a monomer does Now if these rates were comparable (the caged

radicals are in steady state) and this differentialpolymerization ensue. This is essentially the con-cept proposed by Ishige and Hamieliec14 for acryl- were equal to zero, then the expression for the

overall rate of polymerization would collapse intoamide.The reaction scheme set forth above results in its normal form, and there would be a first-power

dependence on monomer. In other words, if thethe following kinetic expressions:

Table VI. Molecular Weight of Polymers Made at Varied % Neutralization and Temperatures

Temp (7C) % Neut % Conver. Mw 106 Mn 106 Mp 106

55 0 84 3.449 0.563 1.46955 25 82 2.048 0.458 1.25255 50 84 1.637 0.328 1.02655 75 78 1.402 0.303 0.85855 100 62 1.947 0.278 0.98655 125 85 1.600 0.352 0.96770 0 89 2.084 0.264 1.17970 25 86 1.113 0.227 0.69370 50 79 0.975 0.228 0.55570 75 77 1.469 0.205 0.64070 100 85 0.871 0.151 0.38070 125 65 1.162 0.264 0.69385 0 82 1.270 0.207 0.62185 25 90 0.805 0.154 0.46585 50 83 0.869 0.168 0.36385 75 84 1.490 0.163 0.53785 100 74 0.643 0.101 0.29185 125 90 1.340 0.201 0.505



Figure 15. Molecular weight distribution curves of polyacrylic acid samples preparedat 557C with varied sodium persulfate levels as listed in Table V.

rate controlling step is the decomposition of initia- d[RM]dt

2k * [M] fkd[I] 0 2kt[RM]2. (10)tor instead of the interception of the caged radi-cals by monomer, then no unusual dependenceon monomer concentration would be discernible. Also assuming steady state for [RM] and [R],Because this explanation is not consistent with then the growing radical concentration isthe data (Fig. 2), it is concluded that kd[I] mustbe considerably larger than k * [ ( (2Rc )][M].

[RM]

k * f kd[I][M]kt

(11)One approach is to assume there is a relativelyconstant fraction of caged radicals proportional tothe decomposed initiator. Manickam5 proposed a which is proportional to [I]1/2 and [M]1/2 anddirect reaction between monomer and persulfate. leads to a propagation expressionUsing the relation

Rp kp[M][RM] (12)[(2Rc )] fkd[I] (9) kp[k * f kd /kt ]1/2[M]3/2[I]1/2 . (13)where f is an efficiency factor encompassing a con-

stant fraction of caged radicals, then This can explain the concentration dependence

Figure 17. Effect of acrylic acid concentration on thenumber-average molecular weight of 65% neutralizedFigure 16. Effect of temperature and neutralization

on the molecular weight of polyacrylic acid, 557C polyacrylic acid polymerized at 557C and containing1600 PPM sodium persulfate (B.O.A.A.)(squares), 707C (diamonds), and 857C (circles) (a,b).


2042 CUTIE ET AL.

Table VII. Effect of Monomer Concentration on the Molecular Weighta of 65% Neutralized Polyacrylic Acid

[Monomer] mol/liter % Conv. Mw 106 Mn 106 Mp 106 Tube Diameter (mm)

4.56 65 6.802 1.193 4.212 22.28 60 2.576 0.549 1.755 51.71 70 0.943 0.226 0.709 51.14 80 1.170 0.274 0.741 5

a Analysis of samples polymerized in the NMR.

of the rate curves in a general way. The effect of tions, varied by a factor of 10. Values obtainedin this work at higher monomer concentrationspH, ionic strength, and temperature can be incor-

porated as empirical values for an overall coeffi- were within this range. A minimum rate of po-lymerization was observed near neutral pH, ascient. It is also important to take into account

the variation in kd , the decomposition constant was recognized previously. This minimum ap-peared be somewhat temperature dependent,for sodium persulfate, as a function of these same

parameters. The temperature dependence for the resulting in a variation in overall activation en-ergies with pH. In addition, the rate differenceskd has been determined under these conditions,

and this dependence is reflected in the plots in observed for unneutralized polymerizations vs.the minimum rates were smaller than those re-Figure 11.ported in the literature. This was perhaps dueto the higher monomer concentrations of thisstudy.

The initial rate of polymerization of 125% neu-CONCLUSIONStralized samples was very high, but reached a pla-teau at 5060% conversion. This appeared to beThe polymerization rate, as well as the molecu-

lar weight, showed a dependence on the 32 power due to a mobility problem at the higher monomer(polyelectrolyte) concentrations in this study. Theof monomer concentration for [M]0 values upactivation energies for acrylic acid polymerizationto 4.5 molar. This dependence was accountedbetween 0 and 125% neutralization were found tofor by interaction of the monomer with eitherrange from 9.2 to 18.4 kcal/mol, with a maximumthe initiator itself or caged radicals resultingat 100% neutralization. This large change in acti-from it. In either case, the steady-state concen-vation energy at 100% neutralization probably re-tration of radicals appeared to depend on thesulted from a change in the polymerization mech-square root of monomer concentration, leadinganism.to the overall [M] 3/2 dependence for Rp . Both

No significant effect of K/ vs. Na/ counterionsRp and molecular weight possessed a 12 powerwas observed on the rate of polymerization of neu-dependence on the initiator ( sodium persul-tralized polyacrylic acid. Molecular weights var-fate ) concentration. Values for Rp / [M] 3/2[ I ] 1 /

ied with percent neutralization showing a mini-2 obtained from the literature for acrylic acidmum at 100% neutralization and higher molecu-polymerization, all of which were from experi-lar weights at 0 and 125% neutralization.ments at somewhat lower monomer concentra-

Table VIII. Effect of Monomer Concentration on the Molecular Weighta of 65% Neutralized Polyacrylic Acid

[Monomer] mol/liter Approx. % Conv. Reaction Time (min) Mw 106 Mn 106 Mp 106

4.56 65 45 8.550 1.107 3.7833.80 60 55 5.896 0.734 2.3082.28 70 130 5.102 0.563 2.0361.14 63 390 0.916 0.122 0.724

a Analysis of samples polymerized in a water bath at 557C.



APPENDIX

Raw Data for the Rates of Polymerization

Raw Data for Figure 2, the Rate of Polymerization of 65% Neutralized Acrylic Acid at557C as a Function of Acrylic Acid Concentration (Fractional Conversion vs. Time)

Time (min) 4.56 Molar 2.29 Molar 1.17 Molar

2 0.05 0 04 0.07 06 0.13 08 0.17 0.02 0

10 0.2112 0.2514 0.3016 0.34 0.14 0.0518 0.3920 0.4524 0.53 0.24 0.1428 0.6132 0.66 0.32 0.2136 0.7240 0.77 0.42 0.2844 0.8148 0.46 0.3256 0.51 0.3764 0.56 0.472 0.6 0.4380 0.64 0.4688 0.66 0.4996 0.7 0.5

104 0.72112 0.74 0.55120 0.75128 0.76 0.58136 0.77144 0.79 0.61152 0.79160 0.8 0.64168 0.81 0.65192 0.83 0.66224 0.7256 0.72320 0.75

The Initial Monomer Concentration and Initial Rate Data for the Samples of Figure 3

Rp (%/min) [Monomer] Rp (m/l/min) log[monomer] log Rp (m/l/min)

2.2 4.56 0.1 0.659 012 3.08 0.0616 0.4886 01.211.2 2.3 0.0276 0.3617 01.5591.9 3.8 0.0722 0.5798 01.1410.65 1.17 0.007605 0.0682 02.1191.12 2.29 0.0256 0.3598 01.5921.83 3.41 0.0624 0.5328 01.2051.91 4.56 0.0871 0.659 01.061.26 2.28 0.0287 0.3579 01.542


2044 CUTIE ET AL.

APPENDIX Continued

The Initial Monomer Concentration and Initial Rate Data for the Samples of Figure 3

Rp (%/min) [Monomer] Rp (m/l/min) log[monomer] log Rp (m/l/min)

1.22 1.71 0.02086 0.233 01.6810.92 1.14 0.0105 0.0569 01.979

Raw Data for Figure 4, Effect of Persulfate Concentration on the PolymerizationRate of 65% Neutralized Acrylic Acid at 557C (Fractional Conversion vs. Time)

Time (min) 0.273 mmolar 1.60 mmolar 4.33 mmolar 8.72 mmolar

2 0 0 03 0 0 0 0.044 0 0 0.03 0.065 0 0 0.04 0.096 0 0.02 0.07 0.167 0 0.09 0.218 0 0.03 0.15 0.289 0 0.18 0.36

10 0 0.08 0.23 0.4112 0 0.12 0.31 0.4914 0 0.16 0.39 0.6116 0 0.22 0.46 0.6918 0 0.27 0.52 0.7220 0 0.32 0.57 0.7624 0.04 0.4 0.65 0.7928 0.04 0.49 0.74 0.8532 0.05 0.55 0.76 0.8736 0.05 0.62 0.940 0.07 0.65 0.82 0.924448 0.09 0.715256 0.13 0.786064 0.1472 0.1580 0.1988 0.2396 0.24

112 0.31128 0.38144 0.44

Raw Data for Figure 6, the Effect of % Neutralization on the Rate of Polymerizationof Acrylic Acid at 557C (Fractional Conversion vs. Time)

Time (min) 0% 25% 50% 75% 100% 125%

2 0 0 0.074 0.03 0 0.116 0.1 0 0.138 0.18 0 0 0 0 0.22

10 0.26 0.03 0.2812 0.32 0.1 0.06 0.05 0.34



APPENDIX Continued


Time (min) 0% 25% 50% 75% 100% 125%

14 0.42 0.13 0.4116 0.47 0.22 0.13 0.09 0 0.4618 0.52 0.25 0.5120 0.57 0.3 0.18 0.14 0.5622 0.61 0.36 0.5824 0.68 0.43 0.24 0.19 0.07 0.6228 0.74 0.55 0.31 0.24 0.6732 0.76 0.61 0.37 0.29 0.08 0.7236 0.79 0.66 0.41 0.35 0.7440 0.81 0.7 0.47 0.38 0.7644 0.84 0.73 0.5 0.44 0.7848 0.84 0.56 0.47 0.1 0.850 0.75 0.6252 0.7856 0.79 0.62 0.52 0.836064 0.82 0.68 0.58 0.16 0.866872 0.71 0.62 0.8580 0.74 0.66 0.24 0.8584 0.8696 0.8 0.73 0.3

112 0.83 0.76 0.37120 0.84 0.78124 0.78128 0.43144 0.54160 0.62176 0.7192 0.78200 0.79


Time (min) 0% 25% 50% 75% 100% 125%

2 0.12 0 0 0 0 0.134 0.35 0.06 0.04 0.05 0 0.326 0.41 0.14 0.09 0.09 0.08 0.398 0.54 0.2 0.15 0.13 0.14 0.52

10 0.64 0.29 0.21 0.1712 0.7 0.37 0.22 0.22 0.24 0.5714 0.46 0.28 0.5716 0.79 0.51 0.32 0.3 0.37 0.5920 0.84 0.61 0.43 0.39 0.5 0.6124 0.87 0.68 0.47 0.47 0.6 0.6528 0.89 0.73 0.54 0.51 0.67 0.6432 0.77 0.57 0.57 0.73 0.6436 0.81 0.61 0.6 0.7540 0.83 0.65 0.65 0.81 0.6444 0.86 0.68 0.67 0.81


2046 CUTIE ET AL.

APPENDIX Continued


Time (min) 0% 25% 50% 75% 100% 125%

48 0.71 0.71 0.84 0.6552 0.73 0.71 0.8856 0.77 0.74 0.8560 0.7764 0.7768 0.797280


Time (min) 0% 25% 50% 75% 100% 125%

1 0 02 0.05 0.04 0 0 0.07 03 0.164 0.31 0.15 0.05 0.04 0.28 0.075 0.316 0.54 0.35 0.18 0.18 0.49 0.27 0.568 0.68 0.52 0.31 0.34 0.62 0.369 0.63

10 0.75 0.67 0.43 0.43 0.65 0.5212 0.85 0.74 0.53 0.52 0.68 0.6114 0.82 0.78 0.62 0.58 0.72 0.7116 0.81 0.65 0.64 0.7 0.7418 0.83 0.69 0.68 0.74 0.820 0.87 0.7 0.76 0.8422 0.88 0.77 0.75 0.7224 0.9 0.79 0.79 0.73 0.8626 0.79 0.82 0.7428 0.83 0.82 0.72 0.8732 0.84 0.74 0.8836 0.74 0.8840 0.74 0.9

Raw Data for Figure 12, the Dependence of the Polymerization Rate for 75%Neutralized Acrylic Acid on Counterion, Sodium vs. Potassium at 557C (FractionalConversion vs. Time)

Time (min) Potassium Sodium

4 0 06 0.018 0.04 0.08

10 0.1212 0.12 0.1816 0.23 0.2620 0.31 0.3524 0.38 0.4228 0.45 0.46



APPENDIX Continued

Raw Data for Figure 12, the Dependence of the Polymerization Rate for 75%Neutralized Acrylic Acid on Counterion, Sodium vs. Potassium at 557C (FractionalConversion vs. Time)

Time (min) Potassium Sodium

32 0.48 0.5436 0.5540 0.56 0.648 0.63 0.6556 0.676064 0.7 0.7472 0.7580 0.78 0.7996 0.8

104112 0.83120128 0.86144 0.87160 0.88

8. A. Chapiro and J. Dulieu, Eur. Polym. J., 13, 563REFERENCES AND NOTES(1977).

9. N. I. Galperina, T. A. Gugunava, V. F. Gromov,1. H. Ito, S. Shimizu, and S. Suzuki, J. Chem. Soc.,P. M. Khomikovskii, and A. D. Abkin, A. D. Vysoko-Jpn., Ind. Chem. Sect., 58, 194 (1955).mol. Soyed., 7, 1455 (1975).2. V. A. Kabanov, D. A. Topchiev, T. M. Karaputadze,

10. J. P. Riggs and F. Rodriguez, J. Polym. Sci., Partand L. A. Mkrtchian, Eur. Polym. J., 11, 153A-1, 5, 3167 (1967).(1975).

11. S. Lenka, P. L. Nayak, S. B. Dash, and S. Ray, Coll.3. V. A. Kabanov, D. A. Topchiev, and T. M. Karapu-Polym. Sci., 261, 40 (1983).tadze, J. Polym. Sci., 42, 173 (1973).

12. R. Praba and U. S. Nandi, J. Polym. Sci., Polym.4. S. P. Manickam, K. Venkatarao, U. Chandra Singh,Chem. Ed., 15, 1973 (1977).and N. R. Subbaratnam, J. Polym. Sci., Polym.

13. S. S. Cutie, D. E. Henton, C. Powell, R. E. Reim,Chem. Ed., 16, 2701 (1978).P. B. Smith, and T. L. Staples, J. Appl. Polym. Sci.,5. S. P. Manickam, K. Venkatarao, and N. R. Subbar-64, 577 (1997).atnam, Eur. Polym. J., 16, 483 (1979).

14. T. Ishige and A. E. Hamielec, J. Appl. Polym. Sci.,6. F. Laborie, J Polym. Sci., Polym. Chem. Ed., 15,17, 1479 (1973).1255 (1977).

15. S. S. Cutie and S. J. Martin, J. Appl. Polym. Sci.,7. F. Laborie, J Polym. Sci., Polym. Chem. Ed., 15,55, 605 (1995).1275 (1977).


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