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Suspension Polymerization of Styrene

B y following th e pH in the reaction flask during the polymerization of vinplmono-

mers in suspension it was found that the pH decreases continuously throughout

the reaction and that the rate of this pH change is related t o th e rate of polymeriza-

tio n. Activation energies for the over-all reaction ha d an average value of 20,600

calories/gram-mole, and the initial reaction rate was directly proportional to thesquare root of initial catalyst concentration. The relationship between polymer

average molecular weight and catalyst concentration was M. W . = KC- . In

these experiments, n was 0.655 for polymer prepared at 60 C. and 0.825 for polymer

prepared at 80' C. There was evidence that this reaction becomes autocatalytic

after polymerization has progressed to between 50 and 70% of completion.

WALTER S. KAGHAN1

Rose Polytechnic Instit ute, Terre Haute, Ind.

R . N O R R I S S H R E V E

Purdue Univer sity, Lafayette, Ind.

SHE addition polymerization of vinyl typ e monomers, two

I methods in which the process is carried out in heterogeneous

dispersion have achieved wide prominence in recent years.

Emulsion polymerization techniques are not only used extensively

in the commercial production of such materials as synthetic

rubber a nd other butadiene-styrene copolymers, polyvinyl

acet ate emulsions, and similar products b ut h ave also been widely

exploited for man y fundamen tal studies of polymerization

mechanisms and kinetics. On th e other hand, studies involving

polymerization of vinyl monomers in suspension have appeared

infrequently in the literature despite the prominent role this

method plays in the commercial production of polystyrene,

polyvinyl chloride, polyvinyl acetate, and other vinyl polymers.

The purpose of this s tudy was threefold: A general investiga-

tion of such process variables as catalyst concentra tion, sus-

pension stabilizer composition and co ncentratio n, pH of the aque-

ous medium, th e presence of oxygen in the system, and reactiontemperature, and a qualitative estimate of th e influence of these

variables on the polymerization process and the polymer product

constituted the first part of the project. The quantitative deter-

mination of reaction rate data and the calculation therefrom of

certain kinetic constants for this reaction made up the second

phase of this work. Finally, an attem pt was made to observe

and correlate the relationship of the pH of the reaction medium

r d h he course of th e polymerization reaction.

Th e process of suspension polymerization has been described

by Hohenstein, Vingiello, and Mark (8) as a special case of bulk

or mass polymerization in which the extensive cooling supplied

by the aqueous medium minimizes temperature variation in the

system. Th e process has been considered to occur in three more-

or-less distinct stages 7 , 17).

Several catalysts of both the water-soluble and hydrocarbon-

soluble type have been tested 8),nd the relative m erits of in-

organic and organic suspension stabilizers have been discussed

quite extensively 7 , l T ) .

Winslow and Mat reyek 17)checked the suspension pH at the

end of each polymerization an d found with benzoyl peroxide

catalyst th at t he p H varied between 4.8 and 5.0. More recently,

an attempt was reported by Yang and Guile 18) o follow the

change in p H during the course of a polymerization reaction.

Their experiments were based on the potassium persulfate

catalyzed polymerization of styr ene in emulsion.

Several autho rs have stud ied th e decomposition of benzoyl

peroxide in various solvents 8, 3,14) and examined the decom-

1 Present address, Olin Industries, Inc. New Haven, Conn

position products. In general, these consist of carbon dioxide,benzene, aldehydes, and solid acids containing mostly benzoic

acid.

At o ne time i t was believed 6) hat the over-all polymerization

rate of pur e liquid styrene was a first-order reaction. However,

the experimental d ata have been reinterpreted 10)n terms of P

sccond ord er reaction

Hohenstein and Mar k ('7) stat e tha t the initial rate of t he ben-

zoyl peroxide-catalyzed polymerization of styrene in suspension

is proportional to the square root of the initial catalyst concentra-

tion an d th at the activation energy for the over-all reaction

was calculated to be approximately 23,000 caloriea/gram-mole.

Redington 16),n a study of st3 rene polymerization, found th e

activation energy to be 21,000 calorieslgram-mole for fourdifferent organic peroxide catalysts rrhich he tested.

A recent paper by Bengough and Norrish (6) discusses the

mechanism an d kinetics of t he polymerization of vinyl monomers

with particular reference to the benzoyl peroxidecatalyzed

polymerization of vinyl chloride. They found that the poly-

merization of vinyl chloride is autocatalytic over a range of per-

oxide concentration from 0.025 to 1.0 mole per cent and a range

of temperatures from 33 to 75 C., but this autocatalytic effect

is not evident in t he presence of a solvent for the polymer.

DESIGN OF EXPERIMENT

The research project was initiated with an extensive series of

preliminary experiments designed to narrow down the large

number of variables involved in the suspension polymerization

of styrene. Several suspension stabilizers, both inorganic andorganic, were tested, and on the basis of these preliminary

results a s well as t he results of TVinslow and M atrey ek I ? ) ,

polyvinyl alcohol in a concentration of 0.1 to 0.2% by weight

of monomer w as chosen as t he suspending agent in fu rther ex-

periments. Early attem pts to measure the p H of samples mt h-

drawn from th e reactor gave convincing evidence of the desira-

bility of measuring the p H of the reactor contents in situ. Other

variables which were considered qualitatively in preliminary

experiments and fixed for subsequent work were the monomer-

water ratio, agitation speed, catalys t composition and concen-

tration, nitrogen atmosphere, and operating temperatures.

In addition t o the somewhat qua litative conclusions drawn from

the preliminary experiments, it seemed desirable to develop quan-

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fer of th e sample to the receiving flask. This tub e was in-serted through one of the openings in the top of the reactorand a sample sucked into the tube. The sample was quicklytransferred to a glass-stoppered 125-ml. Erlenmeyer flaskcontaining 20, 25, or 50 ml. of toluene, a small quantity oftert-butyl catechol polymerization inhibitor, and approximately40 grams of white Drieri te granules. By sucking some of th eDrierite and toluene from the flask into the sampling tubeand agitat ing them up and down, it was possible t o scour the tub ealmost completely a nd, thu s, obtain essentially complete transfer

of the sample taken. The inhibited toluene used in the receivingflask was technical grade (1 ) toluene containing 0.1 gram of tert-butyl catechol per 1000 ml. of toluene.

Th e per cent polymer in a given sample was determined gravi-metrically by precipitating the polymer in an aliquot of thesample solution with methanol.

All the pH measurements made in this study were made with

the Model M pH meter manufactured by Beckman Instruments,

Inc. I n an attem pt to reduce the possibility of polystyrene

coating the electrode bulb, it was given a coating of Desicote;

it was found by checking the electrode against standard buffer

solutions before and aft er coating tha t this procedure did n ot

affect the accuracy or sensi tivity of the electrode.

Because of the space limitations of the reactor and the con-

struction of the calomel reference electrode, it was not feasible

to mount the calomel electrode directly in the reactor as was

done with the glass electrode. Electrical contact between theelectrodes was maintained by means of a sal t bridge. Th e p H

of the polyvinyl alcohol solution was followed throu gh t he heating-

up period, the drop in pH as the styrene was added was noted,

and the change in pH as the polymerization proceeded was

measured until the reaction was stopped.

I n order to characterize the samples of polystyr ene produced

under the various experimental conditions, the viscosity average

molecular weight of each

sample was determined by

standa rd procedures. I n a

further attempt to charac-

terize the polystyrene ob-

tained in a given run, the

per cent polymer in sam-

ples taken from each batch

of dried beads was deter-mined.

An est imate of th e effect

of the process variables on

polystyrene particle size

and particle size distribu-

tion was also attempted by

means of a screen analysis

on each batc h of poly-

styrene obtained. I n sub-

sequent determinations of

per cent polymer and in-

trinsic viscosities, only the

through 60-mesh fraction

from the screen analysis of

e a c h s a m p l e w a s u s e d .This tended to eliminate

minor entrained impurities

an d made for easier solution

of t h e p o l y s t y r e n e i n

toluene or dioxan.

titative data concerning the effect of two principal variables

in the polymerization reaction, namely catalyst concentration

and reaction temperature.

EXPERIMENTAL METHODS

I n organizing equipment and procedures for this investigation,

four more-or-less disti nct problems had t o be accounted for.

The first of these was the polymerization of styr ene in sus-

pension. Associated with this operation were th e problems ofsampling and testing to obtain kinetic data and measuring the

pH of the reaction medium throughou t the reaction. Finally,

molecular weight determinations on samples of polymer from

each run had to be made.

The basic unit in the polymerization reactor was a 3-liter split

resin reaction flask. A photo graph of this reactor is shown in

Figure 1, and a schematic drawing of this reaction assembly is

shown in Figure 2.

The reactor was heated by an electric mantle connected to the

power supply through a variable transformer. By means of a

Chromel-Alumel thermocouple immersed in the flask and by

controlling th e power inp ut to th e jacket i n an on-off cycle, the

reaction temperature was maintained a t thecontrolpoint 50 .5 C.

The charge to the reactor consisted of a solution of 0.500

gram of polyvinyl alcohol in 2500 ml. of distilled water, 250 ml. ofpurified styrene monomer, and from 0.33 to 1 by weight of

benzoyl peroxide based on th e styr ene monomer. Th e com-

mercial styre ne monomer, containing tert-butyl catechol as an in-

hibitor, was purified before use in these experiment s in accordance

with procedures suggested in the literature 21-13,16). The

polyvinyl alcohol used as a suspending agent was all from the

same single sample of technical gr ade Elvanol (high viscosity,

partially hydrolyzed grade

No. 52-40). The benzoyl

p e r o x i d e c a t a l y s t used

t h r o u g h o u t t h e p roje ct

came from a single bottle

of reagent grade material,

s u p pl i e d b y E a s t m a n

Kodak.

In oraer to investigatethe kinetics of the poly-

merization reaction under

study , it mas necessary t o

develop a satisfactory tech-

nique for determining the

p e r cen t co n v e r s io n of

m o n o m er t o p o l y m e r

throughout the reaction.

This involved, first, the re-

moval of re pr es en ta t i ve

samples from the reactor

d u r i n g t h e r u n , a n d ,

secondly, the determination

of per cent polymer in a

given sample.

After considerable experi-

mentation, the f o11ow n g

technique for sampling was

evolved.

A length of straight glasstubing was used as a thief.This tubing was coated in-side and out with Desicote,a silicone compound soldby Beckman Instruments,Inc. The coating reducedthe tendency of t he sampleof suspension to stick tothe walls of the tub e andfacilitated complete trans- Figure 1. Polyme rization Reactor

TREATMENT OF DATA

The primary set of da ta

collected in each polymeri-

zation run consisted of a

series of measurements of

the per cent of styrene

poly mer ized a t various

February 1953 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 293

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times from th e beginning of the ru n until th e reaction was

determined to be substant ially complete. Samples were re-

moved from the reactor at measured intervals and treated in

the manner described in the preceding section. A plot of

per cent polymer against reaction time on arithmetic co-

ordinate paper for a typical run is shown in Figure 3. This plot

is represen tative of th e conversion curves obtained un der all

the conditions investigated in this project,

LEAD TO

THERMOMETE

SALT BRlDQE

Figure 2. Schematic Drawing ofPolymerization Reactor

In order to obtain th e slope of the conversion curve at zero

time, it seemed desirable to rectify the d at a into some straight-line form and thus o btain an equation for the best line through th e

data. I t was found that a straight line could be fitted to the

data when plotted on log-log paper and a logarithmic plot of the

data for the same illustrative run is shown in Figure 4. On the

basis of these considerations, all t he conversion da ta were con-

verted into logarithmic form. Th e best straight line of log

per cent polymer versus log reaction time was then calculated by

the method of least squares over the entire range from zero time

to t he end of t he run using the lumped da ta from replicate runs.

Fro m these calculations, equations of t he form log per cent

polymer = log ’ + log reaction time were obtained. Sett ing

y = per cent polymer and x = reaction time, this is equivalent

to the normal exponential form

y = ’xb

Since th e slope of this curve a t zero time is of prim ary inte rest for

further calculations, this equation is differentiated to o btain

The actua l slopeat zero time is seen to b e infinite, and, therefore,

the slope at x = 1 (1 minute reaction time) was taken instead.

At z = 1,dy dx = b ’which gives a very close approximation of

the slope at th e origin an d thus of the initial reaction rate.

As previously indicated, i t has been postulated t ha t th e kinetic

equation for the benzoyl peroxide initiated polymerization ofstyrene may be given as

Since d M / d t = - d P / d t = d y / d z , where P = per cent polymer,

and th e amount of monomer, -11, charged to the system was the

same for every ru n

should represent the relationship between initial slope and

catalyst concentration at any given reaction temperature. In

order to test this hypothesis, the initial slope for each treat-

ment was plotted Pgainst the corresponding catalyst concen-

tration on log-log paper and this plot is shown in Figure 8.

Starting with th e same kinetic equation and with M = 1 at the

st art of th e reaction

From t he Arrhenius equation

ka = A e - A E / R T

Solving these equations simultaneously

Since for each catalyst concentra tion used th e initial slope was

determin ed for two temperatures , values of th e activation energy

may be calculated from this last equation. The calculated values

of activation energy are shown in Tabl e I.

TABLE. CALCULATED ACTIVATION E N E R G I E S

Activation energy, calories/gram-mole 20,2 22 20,3 08 21,166Catalyst concentration, 0 3 3 0.67 1.00

Rfolecular weights were calculated from viscosity data by theapplication of Hu ggins relationship (9)

= 171 + IC’[7]2C 1 )

and the modified Staudinger equation

[a] = K(M.W.)= 2)

Th e plot of specific viscosity/concent ration r atio shown in Figure

5 was used to determine the value of k’ in Equation 1. The

REACTION TIME MINUTES)

Figure 3

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Run No.Catalyst concn..Temperature, ‘ .

Screen Mesh

f14-14 4 25-25 40-40 4 60-60 + 80-80 + 120-120 + 170- 70

TABLE1. PARTICLEIZEDISTRIBUTIONROM SCRE EN NALYSISF POLYMERRODUCT

115-46 115-49 116-2 116-9 116-13 116-16 116-19 116-22 116-25 116-29 116-32 116-38 116-410.67 0.33 1.00 1.00 0.78 0.33 0.67 0.33 0.67 0.33 1.00 1.00 0.6760 80 80 60 80 60 80 60 60 80 80 60 80

AverageDiam.,Inches

0 0460.03680.02070.01170.00830.00590.0042

- 0.0035

-0.108 0.028 0.047 0.0620.575 0.426 0.187 0.4470.170 0 325 0.238 0.2980.090 0.114 0.298 0.1310.025 0.057 0.191 0.0260.018 0.037 0.033 0.0170.009 0.011 0.005 0.0120.005 0.002 0.001 0.007

0.0340.1770.4120.2550.0940.0220.0040.002

Particle0.6200.2450.0390.0530.0260.0130.0040.001

Size DistributionD0.062 0.207 0.141 0.1190.414 0.486 0.500 0.3730.244 0.152 0.229 0.4470.175 0.109 0.085 0.0510.074 0.024 0.027 0.0050.023 0.012 0.013 0.0030.006 0.006 0.004. 0.0010.002 0.003 0.001 0.001

0.0660.0680.0770.4830.2660.0360.0040

.1530.1990.1550.1010.2540.1630.0150 001

--0.0690.0440.0340.1860.5150.1310.0190.002

Total 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

Mass fraction of “pearls” of indicated average diameter.

intrinsic viscosities of all other samples were then ca lculated by

solution of t he quadratic equation obtained by substitutin g one

measured value in Equat ion 1. Suitable values of the constants

K and LY in Equation 2 were obtained from the literature 1, 6 ,

and the molecular weights calculated are also shown in Table 111.

I I I IIO 20 30 50 100 ZOO 300 400

REACTION TIME lWlNUTESl

Figure 4

Th e pH readings obtained a t intervals throughout each run

were converted to hydrogen ion concentrations, and the particle

size data were converted into a mass fraction analysis for each

sample of polymer prod uct.

RESULTS

The conversion dat a for a typical set of duplicate runs are pre-

sented in graphical form in Figure 6, which shows the sets of

points for duplicate runs of a given treatment plotted together.

Th e solid line curve drawn thro ugh th e points is calculated from

the least squares equation for th at treatment.

Curves of hydrogen ion concentration plot ted against reaction

times for a typical se t of du plicate runs a re given in Figure 7.

Table I1 contains th e summarized d ata obtained in th e screen

analysis of the product from each run. The distribution of par-ticle size obtained in each run is shown as th e mass fraction of

“pearls” of the indicated average particl e diameter.

A general summarized presentation of all the other dat a ob-

tained in the planned set of experiments is shown in Table 111.The dat a in thi s table are grouped for easy comparison of the re-

sults of replicate runs. The over-all reaction time an d final per

cent polymer ar e an indication of t he reproducibility of replicate

runs. Th e p H readings, over-all yield an d average molecular

weights are all measured quan titie s or calculated as indicated

in the preceding section. Th e initial slope refers to th e reaction

rate or per ce nt conversion curve and was also calculated by t he

method previously outlined.

Figure 8 is a plot of the initial slope against catalyst concen-

tration on log-log paper. The stra ight lines drawn through

the two sets of points represent the best lines with a slope of ‘/z.

The viscosity average molecular weights calculated and sum-

marized in Table I11 were plotted against catalyst concentration

on log-log paper. Th e slopes of th ese lines were calculated

graphically and found t o be -0.655 for polymer prepared at 60”C.

and -0.825 for polymer prepared at 80”C. The’straight lines

obtained in t his plot indicate th e following relationship between

molecular weight and catalyst concentration

M.W. = KC-”

where -n has th e values indicated above.

DISCUSSION AND CONCLUSIONS

An extensive preliminary series of runs was made prior to the

sta rt of the planned experiments from which th e results presented

in this paper were obtained. These preliminary experiments

explored th e limits of some of th e imp ort ant process variables.

Th e relative effectiveness of tricalcium phosphate , a n inorganic

suspending agent, and polyvinyl alcohol, an organic stabilizer,

as the influence of pH modification on stabilizer effective-

“tI I0 0.3 IO

CONCENTRATION [ G R AM S SQLU TE / 1 0 0 ML SOLVEHTI

Figure 5

ness were investigated. It was found t ha t t he organic materials

typified by polyvinyl alcohol are much more effective since the

inorganic stabilizers tend to produce large polymer beads and

are very sensitive to pH modification, whereas th e organic stabi-

lizers tend to pr6duce small beads and can function effectively

as protective colloids over a wide range of pH in the aqueous

medium. Furthermore] over-all reaction time, per cent con-

version] and average molecular weight are apparently unaffected

by the nature of the suspending agent or modification of pH.

Suit able ranges of stabilizer concen tration, reaction tempera-

ture] and catalyst concentration were also determined in these


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this work involving p H measuremen t. By refining these tech-

niques to the point where more accurate measurement of the ab-

solute hydrogen ion concentr ation is possible an evalua tion of

the change in peroxide concentration with time during the re-

action should be feasible. This in turn should permit th e fuller

evaluation of the postul ated kinetic equatio n for the over-all

reaction.

0 . 2 0 c

0.10

s41 0 0 8

o I 2 a3 0.4 0.6 1

CATALYST CCNCEHTRATION */.

Figure 8

The plot of initial slope against catalyst concentration on log-

log paper shown in Figure 8 demonstrates the relationship be-

tween initial reaction rate an d initial ca talyst concentration stated

in the kinetic equation for th e over-all reaction. Figure 8 shows

that the points in each set fall very closely about the straight

line of slope drawn through them. The conclusion th at in

the benzoyl peroxide-catalyzed suspension polymerizat ion of

styrene, the initial reaction rate is directly proportional to the

square root of initial catal yst concentra tion is also tak en to indi-

cat e th at t he polymerization does proceed by a free radical mech-

anism.

In the calculation of activation energies for this reaction anaverage value of appro ximately 20,600 calories/gram-mole was

obtained. Hohenstein and Mar k (7) reported a value of approxi-

mately 23,000 calories/gram-mole for the act ivat ion energy in

th e suspension polymerizat ion of styrene, b ut i n a more recent

paper, Redington 16) reported that the activation energy for

peroxide-initiated polymeri zation of styrene (polymerized in

bulk) was found to be 21,000 calories/gram-mole for a whole

group of different peroxides including benzoyl peroxide.

Since in a plot of molecular weight again st initial catalys t

concentration the dat a are represented by s traight lines of nega-

tive slope on logarithmic coordinate paper, the relationship

between polymer molecular weight and catalyst concentration can

be given as M.W. = KC-n, where n is the neg ative slope of t he

stra ight line. In these experiments, n was found to be 0.655

for polymer prepared at 60°C. and 0.825 for polymer prepared

a t 80” C. The literature reports for previous investigations of

catalyst-initiated free radical polymeri zations tha t the molecular

weight of the product is inversely proportional t o the square root

of th e catalys t concentration corresponding t o a value for Tof 0.500. The reason for this discrepancy is not appare nt.

b

ACKNOWLEDGMENT

The assistance of t he Research Depart ment of Commercial

Solvents Corp. with certain critical items of material an d equip-

men t and of Rose Polytechnic Inst itu te, in whose laboratories

these experiments were carried ou t, is gratefully acknowledged.

NOMENCLATURE

A = Arrhenius equation frequency factorC = cata lyst concentration; polymer concentra tionko = kinetic consta ntk ’ , K , K’ = constantsM = monomer concentrationM.W. = molecular weightn = an exponential numberR = gas constantt = timeT = absolute temperature, O K.01 = an exponential constant

v ] = intrinsic viscosityv p = specific viscosity

LITERATURE CITED

1)Abere, J. Goldfinger, G., Mark, H. , and Naidus, H ., Ann.

2)Barnett, B., and Vaughan, W . E., J . Phys. Collo id Chem., 51,

3)Bartlett, P., and Nozaki, K., J . Am. Ch em. Soc . , 69, 2299

4)Bawn, C. E. H., “The Chemistry of High Polymers,” New

5) Bengough, W. I., and Norrish, R. G. W., P r o c. R o y . S O C . Lon-

6)Goldberg, A. I. Hohenstein, W. P., and Mark, H., J . P o l y m e r

7)Hohenstein, W. ., and Mark, H., I b i d . , 1, 127 1946).

(8) Hohenstein, W. P., Vingiello, F., and Mark, H., I n d i a R u b b e r

9)Huggins, M. . . Am. Ch em. Soc., 64, 716 1942).

N . Y . A c a d . Sci., 44 293 1943).

926 1947).

1947).

York, Interscience Publishers, 1948.

d o n ) , A200, 301 1950).

Sci., 2,503 1947).

W o r l d , 110, 91 1944).

H., Ann. N . Y . Aca d . S c i . , 44,371 1943).

1946).

10)Hurlburt, H. M., arman, R. A., Tobolsky, A. V., and Eyring,

11)Koningsberger, C., and Salomon, G., J . Po lg mer S e i . 1 200

12)Koppers Co.,nc., Pittsburgh, Pa., T e c h . B u l l . C-1-119, 1951).

13)Marvel, C.S., ailey, W. J., and Inskeep, G. E., J . Po lymer

14)Nozaki,K.,nd Bartlett, P., J . Am. Chem. Soc., 68,1686 1946).15)Polstein, S., Monomers,” Blout, E. R., Hohenstein, W. P., and

16)Redington, L. E., . Po lymer Sci., 3, 503 1948).17)Winslow, F.H.,nd Matreyek, W., IND. NG.CHEM.,3,1108

18)Yang, P. T.,nd Guile, R. L., J . P o l y m e r Sci., 6, 681 1951).

RECEIVEDor review September 13, 1952. CCEPTEDDecember 2 1962.

From he Ph.D. thesis of Walter S. Kaghrtn, Purdue University, June 1952

Sci., 1 275 1946).

Mark, H. , eds., New York, Interscience Publishers, 1949.

1951).


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