SGME STUDSES OF THE EMULSION
{PGWMERIZAHON OF STYRENE

3351336 POTASSIUM PERSULFATE
AS A CATALYST AND SODIUM
BiSULFiTE AS A PROMOTER.

Thesis for the Dogma of M. S.
MICHIGAN STATE COLLEGE
Themes Millard Luring

1948

 

This is to certifg that the

, thesis entitled

SOME STUDIES OF EMULSION POLYMERIZATI 0N 0F STYRENE
USING POTASSIUM PERSULFATE AS A CATALYST AND SODIUM

BISULFITE AS A PRCMOTER

 

presented by

Thmas Me Loring ‘ j :2."-

has been accepted towards fulfillment j‘_ .l . ' ;
of the requirements for .535? _" j a

11.8 . .1] Chemistry _} i: - T

”fie-5

ajor professor

 

December 10, 191:8

Date

   

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'SOME 'STUDIES OF THE EMUL'SION POLYMERIZATIION
OF 'STYRENE USING POTASSIUM PERSULFATE :AS lA
CATALYST IAND 'SOUIUM BI'SULFITE AS A PROMOTER

BY

THOMAS Ml LLARD LORI NG

A THESIS
SUBMITTED TO THE SCHOOL OF GRADUATE STUDIES OF MICHIGAN
STATE COLLEGE OF AGRICULTURE AND APPLIED SCIENCE
‘IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE 0F

MASTER OF SCIENCE
DEPARTMENT OF CHEMISTRY
I948

.9

z .‘

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r\ l- - ;.
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f L': 4’ 3L

GIEMISTRY mew

ACKNOWLEDGMENT

The writer wishes to express his
appreciation to Doctor Ralph L. Guile, for
his counsel and guidance, and to Doctor

Kenneth G. Stone for his assistance with

inorganic redox systems.

‘9 17605

 

TABLE OF CONTENTS

Page
'Introduction ___________________________ 1
Historical ____________________________ 2
Experimental --------------------------- 9
Discussion ____________________________ 19
References ———————————————————————————— .25

 

 

I I NTRODUCTI I ON

Research.in emulsion polymerization in this laboratory began.in
the year 1946 with preliminary work by Morgan.1 In 1947 Yang,2 using
potassium persulfate as a catalyst for styrene polymerization, observed
a minimum limit of catalyst concentration for obtaining high molecular
weight polymer. Early in 1948 Hallenbeck3 found an effect of stirring
on the polymerization rate of styrene catalyzed by hydrogen peroxide.
Later in this same year, Mihina4 presented his thesis work on alcohols
as deactivation agents for the emulsion polymerization of styrene.

This paper reports the results of the first attempt within this
laboratory to closely examine the effect of one redox system on

polymerization. The redox system employed was that of meta sodium

bisulfite with potassium persulfate.

 

 

 

H I'STOR I CAL

Polymerization of olefinic molecules such as styrene has long been
recognized as a chain reaction. The first mechanism for chain initiation

, 5
was prOposed by Staudinger who suggested that the double bond.in the

monomer molecule could be opened by thermal activation to form a diradical.

C6115CH : CH2 a. C6H5CH - CH2
The diradical was then believed to react with monomer molecules without

loss of free radical activity.

C6H5

More recently this diradical state has been assumed by Melville6 to be
the chain.initiator for the photo-activated polymerization of vinyl
acetate.

Another type of chain initiation is that of catalytic activation of
monomer. The compounds successful as initiators may be classified into
two catagories:

1) Compounds which produce free radical fragments and induce
polymerization by activation of the double bond.

(C6H5)3C° + CEHSCH : CH2 -—>(C6H5)SC;C H — CH

(36%
,2) Reagents which produce an.ionic state.in the monomer molecule.

+ T _ __ +T
BF 3 + canscn — CH23 --v-BF 3 ('JH — (3.12
CsHs

Confirmation that these radicals and ions actually.initiate polymerization

has been in the fact that they occur as end fragments.in many different
7 8 ’ ' ' ' ' ’ h l t referred to
polymers. Free radlcal.1n1t1atlon W111 be t e on y ype
throughout this paper as the catalyst used.in this work.is of a type

that has been considered to give free radicals.

III-J

The propagation process by a free radical mechanism.is expressed

by the following equations:

N1. + M -~a- M2.
MB. 1’ hi ~> Mg‘

Mh--+ ibaQN6'q.1)

Termination may occur by the following methods:

1) Addition of the.initiator radical at the active end of a chain.

’2) Reaction of the free radical end of one chain with a similar
end of another molecule.

3) Transfer of activation to another molecule.

4) Termination by impurities.
Extensive kinetic studies have shown that the usual termination reaction
.in the absence of a chain transfer agent is bimolecular, and probably
similar to type‘B.

(C6HSIBCIMIn + '(M)mC(CeH5)5 ---o--»(C€Hf>):5 CIan C(06H5)3

All of these mechanisms for.initiating, propagating and terminating
the polymerization reaction were prOposed when the only polymerization
known was in bulk and solution. The overall velocity constant for bulk
or solution polymerization has been found most satisfactorily expressed
.in the form of a second order reaction equation. For example, the rate
of benzoyl peroxide catalyzed styrene polymerization in toluene solution

.is expressed as follows:

"3% I rate of disappearance of monomer
3 concentration of catalyst
pg : RD 31/2 M3/2
dt M 2 concentration of monomer
k 2 overall rate constant

The use of other monomers.indicated that the rate of polymerization
always varied with the square root of the catalyst concentration and
some root of the monomer concentration; the exact root of monomer
concentration varied with the monomer.

Emulsion polymerization was first studied in connection with
synthetic rubber. LIt was found that a monomer could be polymerized.in
an emulsion system to produce a latex from which the polymer could be
coagulated. .Instead of the usual rubbery product, a powdery product was
produced on coagulation. The emulsion consisted of an aqueous phase,
monomer, emulsifying agent and a water soluble catalyst.

iIn an extensive review of emulsion polymerizations, Hohenstein and
Mark10 described some simple experiments in which monomer vapor was
permitted to come in contact with an aqueous solution of an.initiator
(persulfate). These experiments clearly.indicated that polymerization
was initiated in the aqueous phase. Further evidence supporting the
belief that emulsion polymerization.is.initiated in the aqueous phase
was contributed by Fryling,ll whose work showed that the monomer was
solubilized by soap to give micelles.in the aqueous phase before polymer
formation. Harkinslg’ls postulated that polymerization began in the
aqueous phase and that beyond 20%, polymerization occurred in the polymer—
monomer particles. The kinetics and mechanisms of emulsion polymerization
have been studied by many different workerslll’ls’l6 and the general con—
clusions reached are as follows:

1) Monomer is solubilized into soap micelles.

2) Activation occurs by the same methods as.in bulk polymerization,

.i.e. catalytic salts or heat.

3) The reaction is fed.in these micelles by more solubilized monomer.

4) As these micelles enlarge they become polymer particles and

"J

because monomer is soluble in polymer, the reaction continues.in these
latex particles.

fIn the case of the emulsion polymerization of styrene, the reaction
.is characterized by a well defined induction period after which the rate
becomes a linear function with time. The apparent.inhihiting agent
present which must be overcome is oxygen.17 ,It has been shown that if
oxygen contamination.is eliminated and the reagents are very pure, the
.induction period approaches zero.18 A possible explanation of the
.inhibiting role of oxygen was offered by Kolthoff,19 who stated that
oxygen reacts with activated monomer to form peroxides and with acti—
vated polymer to form polyperoxides.

Although the mechanism of persulfate polymerization is still
unsolved,.it has been shown that the concentration of persulfate de-
20;82

creases very slightly during the course of polymerization. Kolt—

20,121

hoff Ehas shown that the rate of polymerization is prOportional to

the square root of the catalyst concentration. This relationship.is in
agreement with bulk or solution polymerization by catalytic activation.

Extensive.investigations have shown that:

1) After the induction period the rate of reaction is.independent
of the concentration of emulsified styrene.

'2) The activation energy.is approximately the same as that
necessary for polymerization.in solution.

3) The average molecular weight.is constant during polymerization.
These facts indicate that the reaction proceeds by a free radical
mechanism which.is similar to the mechanism of polymerization.in solution.
The zero order dependence on monomer concentration may be explained by
the fact that the loci of reaction is continually being replenished with

monomer at a rate fast enough to maintain the natural propagation rate.

Other possible methods of termination in emulsion polymerization.in
addition to those already listed for bulk and solution polymerization
.include local exhaustion of monomer and dissipation of activation

=25,24
energy.

, *25,26,27 ,
In the year 1946 several English workers described a new
’27
method of activating the polymerization reaction. Morgan termed the
mechanism "reduction activation polymerization". The outstanding feature
of this redox type activation was the.increased rate of polymerization.
This increased rate permitted polymerization at much lower temperatures
, . , *26,28,29
than were p0881b1e by conventional methods. Evans and hlS coworkers
have conducted a thorough examination of the effect of a redox system.in
polymerization. They measured the polymerization rate of several vinyl

monomers using hydrogen peroxide and ferrous ions. .In the absence of the

monomer, the peroxide decomposed according to the Haber—Weiss mechanism

with the evolution of oxygen.

H202+ Fe" —-» HO" + HO‘ + Few

Ho- + Fe“ —- HO” + Few

HQ' + H202 ~.. HBO + H02'

H02'+ H 02-»- HO' + H20 + 02

2

.In the presence of monomer the oxygen production was negligible because

of the reaction of the hydroxyl radical with monomer to produce the

propagating chain.

so + (3635 CH : CH2 -- HO'CH — CH2
CeHs

26‘2 29
Baxendale and Evans ’ 8’ showed.in their work that the rate of pro-

duction of ferric ion.in presence or absence of monomer was the same.

Their experimental data also indicated that the termination was as

follows:

Hod/1);1 + - (M)mOH -—- HO(M)mnOH

rather than:
HO(M)fi'+ °OH ~a—-HO(M)nOH

Bacon25 and Morgana7 found.in the persulfate catalyzed emulsion poly- I
merization of acrylonitrile and styrene, that many reducing agents hasten
the polymerization similar to the action of the ferrous.ion with hydrogen
peroxide. Baun51 prOposed, by analogy to the above system, the following
reactions for the persulfate and thiosulfate catalysis:

8208:4- 3205: .. so4:+ SO47 . 82057

8047 + H20 _... H30; + H0-

HO°-+ M ——-HOM°

5047+ M +143 M'

8203' + M ~c~ 0332 M'
r)

' 7
Morgana also pointed out the fact that addition of cupric.ion.in the

persulfate-thiosulfate system increased the rate of the catalysis.

The molecular weights of polymers are not only.important.in the
study of mechanism of termination and rate studies, but are also
significant.in the industrial use of the polymers. The various methods
thus far devised for determining molecular weight include methods based
on the determination of certain physical properties of polymer solutions,

. 32 . . 56 . . 33 .
namely, osmotic pressure, Visc031ty, sedimation (ultracentrifuge)

'84
and light scattering. All of the methods listed give average molecular
35
weight values rather than absolute values. Schulz has pointed out the
fact that the average molecular weight of the polymer at any stage of

the reaction.is fairly constant, due to a constant ratio of chain propaga-

tions to chain terminations throughout the course of polymerization. The

viscosimetric method is generally used for determining molecular weight

I
for reasons of simplicity of equipment and operation. This method will

 

be explained.in detail in the experimental part of this paper.

 

EXPERIMENTAL

REAGENTS:

Styrene —— The styrene used in this experimental work was obtained
from the Dow Chemical Company. Before use, the styrene was vacuum dis—
tilled (20 mm.) and the fraction having an.index of refraction of 1.544
was collected and used as the monomer in the following studies.

flateg — — Conductance water distilled under nitrogen was used for
the aqueous phase of the emulsion. The procedure for producing the
.impurity-free water was as follows: Fifty ml. of alkaline potassium
permanganate (300 gm. potassium hydroxide, 8 gm. potassium permanganate
per liter of solution) and 2000 ml.distilled water were refluxed under
a nitrogen atmosphere for 30 minutes. The water was then distilled, the
first 200 ml. discarded, and the receiver flushed with steam, before
distillate was again collected, to insure against possible contamination
by ammonia. The distilled water was then kept under a positive pressure
of nitrogen until ready for use.

Potassium Persulfate —— Merck's reagent grade was dissolved in

 

conductance water at 30°C, recrystallized at 5°C, filtered and dried at
room temperature for 48 hours.

Duponol-G -— A Du Pont emulsifying agent reported as being lauryl
amine sulfate. The Duponol G was stored in the liquid phase at 40°C
to facilitate ease.in weighing the product. A test for.inorganic
.impurities in the emulsifying agent by decomposing a weighed amount in

a silica crucible was made. No residue was present.

 

Sodium Bisulfite (meta) —~ Merck's reagent grade.
'Sodium Bisulfite --Merck's reagent grade (95% active ingredients).

 

Ethanol -- USP 957.413)? 76 - 77°C).

10

Aluminum Chloride -- Baker and Adamson's CP;(A1C13‘6 H20).

Nitrggen —— water pumped. The nitrogen was passed through alkaline
pyrogallol solution before use. This solution was prepared according to
the following recipe: Fifty gms of potassium hydroxide were added to 100
mls of water, to which were added five gms of pyrogallic acid.

Toluene —— J. T. Baker's CP and Purified.

PROCEDURE:

The technique of emulsion polymerization employed in this work was
adapted from co—workers in this 1aboratory.l’2’3’4 The polymerization
was carried out.in a four necked, one liter, round bottomed flask with
standard taper ground glass joints. The flask was fitted with a mercury—
sealed, swivel stirrer, nitrogen intake tube that permitted the flow of
nitrogen over the emulsion, sampling tube that extended to the bottom
of the flask, thermometer and a short bulb type water cooled reflux
condenser. The condenser was attached to a small double water trap to
permit the nitrogen to escape and prevent air from sucking back into
the system.

The reaction temperature of 40°C was maintained by placing the
polymerization vessel in a constant temperature water bath. Polymeri—
zation was.initiated and propagated under a nitrogen atmosphere, the
pressure of which was controlled by keeping the rate of nitrogen escape
at one bubble per second through the water trap. Although the rate of
stirring was not recorded, it was rapid and constant throughout all of
the experiments in this work. 'Sampling of the emulsion was done
periodically throughout the polymerization reaction by means of the
sampling tube, previously mentioned, with the aid of an aspirator.

The emulsion consisted of a ratio of eight parts aqueous phase to

one part monomer, 1% Duponol G based on the aqueous phase, and the water

11
soluble catalyst and promoter.in molarity also based on the aqueous phase
as described in each experiment.

In preparing and making a run, the equipment was first assembled and
flushed with nitrogen. The Duponol G and the oxygen-free conductance
water were added to the flask. The solution was stirred under nitrogen
while permitting the bath to come to Operating temperature. The weighed
amounts of catalyst and cocatalyst were then added to the system and
after allowing five minutes for these to completely go into solution, the
styrene was added. The initial time of the polymerization was recorded
as the time of addition of the monomer, styrene. The monomer was not
preheated before addition because of the activating effect of temperature
and a negligible temperature drOp of less than one degree observed on
the addition of the styrene.

Duplicate samples of approximately 10 gms. were taken at various
time intervals, the intervals depending on the rate of polymerization.
One sample was used for pH determination by a Beckman pH meter, and the
other sampled directly into a tared flask containing 40 gm. of chilled
ethanol and .1 gm. aluminum chloride. The alcohol was measured volumet—
rically by means of a pipette and the density for each batch of ethanol
was determined. The purpose of the ethanol and aluminum chloride was to
quench the reaction and coagulate the polymer.

The weight of the sample was obtained by weight difference and the
contents were centrifuged to separate the solid polymer from the liquid.
The method of titrating unreacted styrene in the decanted liquid, reported
by ifxihinal‘i and Hallenbeck:5 of this laboratory, was tried unsuccessfully.
The solid polymer was washed with ethanol.into an Erlenmeyer flask and
allowed to stand overnight. The following day the polymer samples were

filtered, washed with distilled water and finally with ethanol. They

12
were allowed to dry for 24 hours at 400C and weighed.
The percent solid polymer was calculated as follows:
% solid polymer : polymer weight X 100

theoretical weight of styrene
.in sample

 

The theoretical weight of styrene.in the sample was calculated by
dividing the sample weight by 9 (total parts aqueous plus monomer).

The relative average molecular weight was determined on polymer
samples using the Staudinger viscosity method.5a A weighed amount of
polystyrene (approximately .1 gm.) was placed in a 100 ml. volumetric
flask and toluene added to bring solution up to mark. It was necessary
to keep solution at 40°C for 48 hours to permit solution of high mole—
cular weight polymers. The time of efflux of polystyrene solutions and
pure toluene at ZOO‘t .IOC were determined to 0.1 second by the use of a
Cannon-Fenske-Ostwald viscosity pipette. The specific viscosity was
calculated from the following relationship:

NSp : time of efflux of solution at 200C _ 1
time of efflux of solvent at hCOC

The average molecular weight was calculated by means of Staudinger's
equation:

M = average molecular weight

r—a

VIZ
,IE;
0

specific viscosity

SP

C - moles of polystyrene/liter of
solution
(mole of polystyrene is equivalent
to weight of recurring unit)

II

x 1.8 x 10‘4
Because the value K varies slightly from polymer to polymer sample, the

molecular weights are considered to be relative values only.

All experiments had the following.ingredients and factors.in common:

Styrene 80 grams
Water 655.6 grams
Duponol G 6.4 grams (1 percent of aqueous phase)

Nitrogen Atmosphere

Temperature 40°C

DATA

Indication is made.in each experiment of the molarity (based on

13

aqueous phase) of potassium persulfate and meta sodium bisulfite present.

EXPERIMENT #l

.001 Molar Potassium Persulfate (.1728 grams)

Sample #

[\3

Time
(minutes)

90
150
219

270

Percent
801 Id Polymer

8.5
82.4
44.4
57.0
76.8

86.5

Average Mole-
cular Helght

797,000
910,000
782,000
722,600
674,000
672,000

651,000

8.1

8.5

8.4

8.5

8.4

8.5

8.2

EXPERIMENT #2
.001 Molar Potassium Persulfate (.1728 grams)

.00014 Molar meta Sodium Bisulfite (.0167 grams)

‘Selple # Tlne Percent Average Mole-
Mlnutee Solld Polymer cular Height

1 -20 1.7 —--

*2 40 14.6 545,000
'5 6O '26.? 842,000
4 80 41.5 854,000
5 100 55.0 817,500
6 120 58.5 827,000
7 . 140 67.7 754,000
8 160 75.6 664,000

EXPERIMENT #3
.001 Molar Potassium Persulfate (.1728 grams)

.00027 Molar meta Sodium Bisulfite (.0555 grams)

Senple # Tlne Percent Average Mole-
Mlnutee Solld Polymer cular Weight

1 18 5.5 ___

.2 ' 56 120.0 -597,500
.3 54 56.5 604,000
4 72 56.1 —-—

5 90 69.8 680,500
6 108 78.2 657,000
7 126 81.0 "‘

8 144 82.0 575,000

7.8
7.9
7.9
8.0
8.0
8.1
8.1

892

EXPERIMENT #H
.001 Molar Potassium Persulfate (.1728 grams)

.00055 Molar meta Sodium Bisulfite (.06655 grams)

'Senple # Tlne Percent Average Mole-
Mlnutee Solld Polyeer cular Weight
1 12 1.8 ---
‘2 '24 12.4 ---
5 56 55.2 652,000
4 48 49.2 716,000 .
5 60 62.5 659,500
6 72 75.2 652,500
7 85 85.2 657,000
8 110 90.8 598,000

6.5
6.5
6.5
6.5
6.5

6.4

0)
()3

6.7

EXPERIMENT #5

.001 Molar Potassium Persulfate (.1728 grams)

.0008 Molar meta Sodium Bisulfite (.0998 grams)

SOHPIO # Tlne Percent Average Mole-
Mlnutee Solid Polymer cular Weight
1 10 5.4 -—-
.2 .20 20,9 445,000
5 »50 45.5 452,000
4 40 60.8 420,000
5 50 70.5 414,500
6 60 81.7 401,000
7 70 87.2 "‘
92 89.5 375,000

6:2

6.8

6.9

6.8

6.8

6.8

6.8

EXPERIMENT #6

.001 Molar Potassium Persulfate (.1728 grams)

.0011 Molar meta Sodium Bisulfite (.1550 grams)

"Sample # Tlee

Minutes
1 10
2 51
.3 40
4 50
5 6O
6 7O
7 81
8 90
9 100

Percent
Solid Polymer

1.2

45.2

62.0

74.5

82.8

86.0

86.5

88.5

88.6

EXPERIMENT #7

Average Hole-
cular Nelght

452,000
458,000
-585,500
577,000
~569,000

576,000

.001 Molar Potassium Persulfate (.1728 grams)

.001 Molar Sodium Acid Sulfate (.0852 grams NaHSO4°H20)

Sample # Tine
Minutes

0 00

1 51

2 119

5 181

Percent
Sol Id Polymer

0.0
0.0
12.1
-54.9

47.5

Average Mole-
cular Helght

552,000
788,000

742,000

PH

pH

5.7

'2.6

2.6

‘2.4

‘2.4

16

EXPERIMENT #8

.001 M Potassium Persulfate (.1728 grams)

.0011 M Sodium Bisulfite (.0767 grams - 95% efficient)

Salple #

Tlle
Mlnutee

10
‘20
5O
40
5O
60
70
90

101

Percent
Sol Id Poly-er

1.4
=20.4
55.8
51.6
67.5
75.8
79.5
84.5

86.4

EXPERIMENT #9

.0005 M Potassium Persulfate (.0864 grams)

.00055 M meta Sodium Bisulfite (.0665 grams)

'Salple #

Tlne
MInutea

22
42
57
77
119
159

186

Percent
Solid Polymer

7.9
51.0
45.9
59.7
78.7
82.8

84.5

Average Mole-
cular Helght

425,000
496,500
426,000
425,000
457,500
422,500

421,000

Average Mole-
cular Welght

865,000
1,550,000
982,500
741,000
686,000

660,000

pH

7.2
7.2
7.5
7.4
7.5
7.5
7.2

7.2

pH

5.6
5.6
5.7
5.7
5.6
5.6

5.6

17

EXPERIMENT #IO
.00014 M Potassium Persulfate (.0242 grams)

.00014 meta Sodium Bisulfite (.0167 grams)

Sample # Tine Percent Average Mole-
Minutea Solid Polymer cular Weight
1 20 1.2 --—
2 40 4.6 --—
5 60 10.6 659,000
4 80 16.8 657,000
5 100 -25.2 —--
6 120 50.7 722,000
7 140 '33.0 ---
8 160 255.5 729,000
9 257 48.2 590,000

7.5

7.6

7.7

7.6

7.5

7.5

7.5

7.5

7.7

 

 

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19

DISCUSSION

A series of reaction rate curves were obtained for the redox catalyzed
polymerization by plotting percent solid polymer versus time of reaction
for each of the experiments. (See Graphs 1-10.) The portion of the
curve that.is a straight line function represents the true rate of poly—
merization. .In all cases the straight line function.is preceded by a
curve representing an.induction period caused by inhibiting oxygen and
other.impurities. All polymerization rates in this work were obtained
by evaluating the lepes of the straight line portion of these curves.

The polymerization rate of styrene, catalyzed by persulfate (.001 M)
in the absence of bisulfite and polymerized under the conditions specified
.in the experimental part of this paper, was .EUfi per minute. (Graph #1.)
Adding meta bisulfite to the above system reduced the.induction period
and increased the rate of polymerization. Varying the meta bisulfite
concentration from .00014 M to .0008 M.increased the polymerization rate
from .64% per minute to 1.80% per minute. (See Graph #7.) Further in-
creases above .0008 M meta bisulfite had no effect on the polymerization
rate. (See Graph #6.)

To determine the reaction order, certain functions of meta bisulfite
concentration were plotted against time of 50% polymerization, thus
drawing an analogy to plotting the time of half-life of a reaction to
determine reaction order.36 The plot, log concentration meta bisulfite
versus time of 50% polymerization produced a straight line indicating a
first order reaction. (See Graph fill.)

The reaction being of first order, the graph of the rate of poly-
merization versus concentration of meta bisulfite is a straight line.in

accord with the equation

— QM rate of disappearance of monomer
dt after induction period

— g5 : (c) + C k I slope of line (constant)
dt
c 2 concentration of meta bisulfite
(See Graph #12.) C = Y intercept (variable constant)

From the graph the value of 1760 determined by the slope of the line.is
equal to k, and .4 determined by the Y intercept of the line is equal to
C. The value of C should equal the rate of polymerization initiated by
.001 M potassium persulfate in the absence of meta bisulfite. This value
determined experimentally was .30% per minute. (See Graph #1.) The dif—
ference may be attributed to either one or both of two factors - experi—
mental error or a minimum limit of meta bisulfite that must be necessarily
present to produce a point on the line. The fact that the .001 M persul—
fate catalyzed polymerization reaction has been duplicated by this writer
and the fact that all of the other rates when plotted against the meta
bisulfite concentration (Graph #12) lie very close to the straight line,
would.indicate a minimum limit of the order of .000055 M meta bisulfite.
However, since slight experimental variations between individual poly-
merizations cannot be completely eliminated, it is impossible to definitely
determine an absolute minimum limit of meta bisulfite concentration based
on this work.

The expression develOped for this series of curves may be written

as follows:

(911g) -_- 1760(Cb) + (Q11) + F
dt 1. dt p

~QM : rate of disappearance of monomer after induction period
dt
r 2 redox catalyzed

*U
I

" persulfate catalyzed

21

Cb : concentration of meta bisulfite. Cb cannot be in excess of the
molar ratio eight to ten with persulfate.
F : constant that must be added resulting from the minimum limit of

bisulfite necessary for equation to hold.

A check may be made on the above expression by changing the initial
persulfate concentration. Using experiment #9 as an.illustration, the
rate of polymerization may be calculated.in the following manner:

From Kolthoff's relationship

.* 1
(.001 we : . .0005 M a
.SOk/min. -gg
dt p

.*(See Graph #1.) dt p

Assuming the proportion of persulfate to meta bisulfite to yield a
maximum rate remains the same

1991 M KBSBOB : .0005 M §ZSZOa

.oooe M NaBSZOS T7 waged-05

 

X 2 .0004M Nagogos
The difference between .0004m and .00055 M meta bisulfite.is in excess

and has no effect on the rate.

("it“)
dt I,
dt 1.

Adding .1% as was necessary in the original set of curves, the calculated

1760(.ooc4) '1“ "Egg/minute 1" F

.‘93fi/minute fi'F

rate becomes 1.02% per minute. The rate determined experimentally for
this particular reaction was 1.06% per minute. (See Graph #9.)
To observe whether or not the persulfate.in presence of excess meta bisul—

fite catalytic system still produces a polymerization rate proportional to

322
the square root of persulfate concentration, graph #13 was made. .It
would appear that the polymerization rate does follow this relationship
after a minimum limit of .000064 M potassium persulfate when excess meta
bisulfite.is present. However, owing to the availability of only three
points for the line, the writer suggests more research for confirmation.

Upon examination of the relative molecular weights of polymer obtained
from the redox catalyzed polymerizations in which the persulfate concen—
tration is held constant,.it should be noted that increasing meta bisulfite
concentration lowers the molecular weight. (See Graph #14.) A small
excess of meta bisulfite above the concentration needed to produce a
maximum rate of polymerization with a specified concentration of persulfate
has no significant effect on the average molecular weight.

As would be expected, holding the meta bisulfite concentration con—
stant and lowering the persulfate concentration,.increases the molecular
weight until a minimum limit of persulfate concentration is reached. (See
Graph #15.) The reaction catalyzed by .00014 M persulfate with .00014 M
meta bisulfite produces a lower molecular weight than that of .001 M
persulfate with .00014 M meta bisulfite and .0005 M persulfate with
.00055 M meta bisulfite catalyzed polymerizations.

The effect of temperature on rate of polymerization and average
molecular weight of polymer can be observed by comparing this data
obtained at 400C and that obtained under the same conditions at 6000
by Yang.2 In the .001 M persulfate catalyzed emulsion polymerization
of styrene at 40°C, the rate of polymerization was .30% per minute and
the average molecular weight approximately 800,000. Under the same
'conditions at 60°C the rate of polymerization was .88% per minute and
the average molecular weight was approximately 300,000.

.In the experimental procedure the writer mentioned that the method

 

23
of titrating unreacted styrene reported by Mihina4 and Hallenbeck:5 was
tried unsuccessfully. The method.involved mercuric acetate reacting
with the double bond in styrene and liberating one mole of acetic acid
per double bond. The acetic acid was then titrated with sodium hydroxide
to determine the number of moles of styrene present. The difficulty in
titrating the redox catalyzed polymerization reaction for unreacted
styrene was that a constant blank could not be obtained for any one re—
action. The redox apparently interfere with the titration.

Any attempt to explain the observed effect on polymerization of the
redox system, persulfate and meta bisulfite, must be preceded by consid—
eration of catalysis by persulfate. Yost and Russell:57 show that hydrogen
peroxide can be manufactured from the breakdown of persulfate and that the
.intermediate product is Caro's acid (per monosulfuric acid). This break—
down may be expressed in equation form as follows:

sgodZ-r HOH as» so; + sob: + 2H+
805: + HOH ~+~ 304: + 2 Ho-
110° 2w liOhr
The 805: ion may go through an intermediate such as follows:

805: + H0}: —-- 8047 + 310- + H0—

804: + HOH ——-r so: + H0° + H+

 

305: + HOH ~+ 804‘ + 2310-
.If it is felt that the breakdown through the Caro's acid intermediate.is

too slow, the reaction may be expressed.in this manner:

: . - : - +
8,20d + hOh —-> 804 + SO"; + H + HO°

5047 + HOB -> s04: + Ho- 1» H"

"' 2 1' H
8208‘ + anon -> 2504 + 2H + 2:10.

HO° + M --> HOM'

 

24
01‘

SO47 + Hon» so: + H*+ HO-

HO- + M ->HOLE'

In any case the initiation is assumed to be primarily by the hydroxyl
free radical and the remainder by the negative sulfate radicals liberated
.in the above equations.

Having outlined the possible breakdown of persulfate, consideration
.is now in order of the active ions or molecules in the persulfate~meta
bisulfite redox system. Bisulfite is acid and a possibility existed
that the observed increase in rate of polymerization was due to a pH
effect. This possibility was ruled out by experiment #7 (persulfate—
sodium bisulfate as catalyst and pseudo promoter), since the rate of
polymerization by experimental determination was neither increased nor
decreased from the rate of neutral persulfate initiated polymerization.
Bacong5 has shown in his work that sodium sulfite and persulfate together
will not increase the polymerization rate in acrylonitrile over persulfate
.initiated polymerization unless the concentration of sodium sulfite is
low enough to permit an acid medium to exist. Bacon's and my work.indi—
cate that the active reducing agent.is either the bisulfite ions, pyro-
sulfite ions or sulfurous acid molecules. To test the possibility of
pyrosulfite ions being the active ingredient, a run was made using
persulfate and sodium bisulfite to provide the initiator, the concen—
trations of which were equivalent to those in experiment 54. The rate
of the persulfate-bisulfite catalyzed polymerization was 1.57% per
minute. The rate in the case of persulfate-meta bisulfite catalysis

was 1.57% per minute. .It should also be noted that the induction period

was noticeably longer and molecular weight was slightly higher (See

 

25
Graph #16) for the persulfate—meta bisulfite system. These facts further
.indicate that the bisulfite ions or sulfurous acid molecules were the
active reducing particles.in the redox as both bisulfite and pyrosulfite
.ions are available from meta bisulfite. The pyrosulfite.ions are available
from bisulfite.in only an extremely acid medium. The small difference.in
rate, the noticeable difference.in induction period, and the.increased
molecular weight may be attributed to the hydrolysis rate of the pyrosul-
fite ions to bisulfite ions or sulfurous acid molecules.

The writer favors sulfurous acid molecules as being the active
reducing agent because an.increasing pH value is noted.in most cases of
'the persulfate—meta bisulfite redox catalysis. The decreased acidity.is
probably caused by the hydrolysis of the sodium bisulfite to sulfurous

acid and sodium hydroxide.

From the conclusions drawn thus far, the writer postulates the

following possible mechanisms:

: - : : h +-
1) 32305 + Hos » 305 + 804 + aH
805: 4.11.2305 ——a» (5206: + ano-

HO' + lfl-—c-iHOMf

The intermediate steps for the reaction between 805: and sulfurous acid

are probably

. - 3047 + so- + HSO3’

SO 3

-- o
5 4' H200

.. _ Z . +
sop + H2803 1., 5206 + no + Hg

 

2) 3208- —-- 2304-
0 : . Z . 1-

H0°«+ M-a> HOM'

 

26

Mechanisms l and-2 would require one and two moles of sulfurous
acid respectively to react with one mole of persulfate. Actually three—
quarters of a mole of meta bisulfite, capable.in theory of producing one
and one—half moles of sulfurous acid, was required to produce the fastest
rate of polymerization with one mole of persulfate catalyst. The fact
that some of the pyrosulfite.ions may not completely dissociate to bi-
sulfite.ions and the bisulfite ions do not completely associate.into
sulfurous acid molecules would indicate that mechanism l.is the probable
reaction. 0n the other hand, it is impossible to tell how many 804:
radicals.initiate polymerization by reaction with monomer directly.
Therefore it.is a matter of.individual preference at to which mechanism
.is the more logical. The important fact.is that the persulfate and meta
bisulfite redox system yields dithionate 8206:.ions and hydroxyl radi-
cals,38 the latter of which initiates polymerization.

Bacong5 also found that thiosulfate—persulfate catalyzed polymeri—
zation took place.in only an acid medium and accompanied by an increasing
pH value. This may be accounted for by the fact that thiosulfate.is quite

37a
readily decomposed in a dilute acid medium to the bisulfite.ion.

_ +
820:3 + H ——-r— H.303 + S

Another reducing agent used by Bacon in his redox catalysis with
57b
persulfate was hyposulfite. Yost and Russell show that hyposulfite
is quite readily hydrolyzed to bisulfite ions.
1253204: + HOH ~9- ansos' + 8203:
8021-3414» H“0—+ S
8 5 O 3
The writer has presented these last two examples to bring more

bearing on the fact that the probable active reducing particle.is the

sulfurous acid molecule. Potassium persulfate, possibly through the

 

27

Caro's acid intermediate, reacts much faster with sulfurous acid to

produce hydroxyl free radicals than it will hydrolyze with water to

produce them. These hydroxyl free radicals.in turn initiate the poly-

merization reaction; however, it should be understood that some of the

initiation can be attributed to the 804: radical attacking the monomer.

 

28
REFERENCES

1) Morgan, D.H., m.s. Thesis, Michigan State College (1946).

Yang, P.T., ibid, (1947).

Hallenbeck, V.C., ihid, (194a).

Mihina, J.s., ibid, (1948).

Staudinger, H., "Die Hoch Molekularen Organischen Verbindungen",
J. Springer, Berlin, 1932, p. 151.

Staudinger, H., ibid, p. 179.

Melville, H. w., J. Chem. Soc., 274 (1947).

Price, C.C., Kell, R.W., and Krebs, E., J. Am. Chem. Soc., 64, 1103
(1942).

Price, C.C., and Tate, 9.3., ibid, pg, 517 (1945).

Schulz, G.v., and Huseman, E., z. Physik Chem., @gg, 595 (1959).
Hohenstein, w.P., and Mark, H., J. Polymer Sci., 1, 549 (1946).
Fryling, C.F., and Harrington, E.W.,.Ind. & Eng. Chem., §;, 114
(1944).

Harkins, J., J. Chem. Phys., lg, 381 (1945).

Harkins, J., ibid, g3, 47,-215 (1946).

Montrol, E.W.,.ibid, 15, 557 (1945).

Flory, P.V., J. Am. Chem. Soc., pg, 572 (1943).

Staudinger, H., Trans. Faraday 80., 5g, 97 (1936).

Barnes, C.E., J. Am. Chem. Soc., QZ,'217 (1945).

Kolthoff, I.M., and Dale, w.J., ibid, pg, 441 (1947).
Kolthoff,.I.M., and Bovey, F.A.,.ibid, 62, 2145 (1947).
Kolthoff, I.M., and Dale, W.J;,.ibid, g1, 1672 (1945).
Kolthoff,.I.M., and Dale, w.J., ibid, pg, 441 (1947).

Kolthoff, Guss, May, and Medulia, J. Polymer Sci., 1, 540 (1946).

 

.In-x -—--—

-51)
-52)

35)

37)

-37a)
57b)

38)

29
Price, C.C., and Durham, D.A., J. Am. Chem. Soc., 64, 2506 (i942).

Abere, Goldfinger, Naidus, and Mark, J. Chem. Phys., ll, 379 (1943).
Bacon, R.G.K., Trans. Faraday Soc., 4;, 140 (1946).

Baxendale, J.H., Evans, M.G., and Park, G.S., ibid, 4g, 155 (1946).
iorgan, L.B., ibid, 43, 169 (1946).

Baxendale, J.H., Bywaters, S., and Evans, M.G.,.ibid, $2, 675 (1946).
Evans, M.G., J. Chem. Soc., 266 (1947).

Haber, F., and Weiss, J., Proc. Roy. Soc. A., 142,-352 (1939).

Baun, C.E.H;, "The Chemistry of High Polymers", Interscience Publishers,
Inc., New York, 1948, p. 92.

iBaun, C.E.H;,,ibid, p. 95.

Schulz, c.v., z. Physik Che., g116, 517 (1956).

Signer, H., and Gross, H., Helv. Chem. Acta. 13, 555 (1954) C.A;,'§§,
.5059 (1950).

Debye, P., J. Phys. Colloid Chem., _1, 18 (1947).

Schulz, G.V., z. Physik Chem., Egg, 579 (i955).

Getman, F.H., and Daniels, F., "Outlines of Physical Chemistry",
Johnfifliley and Sons, Inc., New York, 1943, p. 345.

Yost, D.M., and Russell, H., "Systematic Inorganic Chemistry",
Prentice Hall, Inc., New York, 1944, p. 361.

Yost, D.M., and Russell, H.,.ibid, p. 389.

Yost, D.M., and Russell, H.,.ibid, p. 554.

. . . . . .I ,
Ephriam, F., "Inorganic Chemistry", Nordeman Publishing Co , nc ,

New York, 1948, p. 570.

 

 

 

SUMMARY

1) The polymerization rate of styrene in emulsion catalyzed by potassium
persulfate is Sharply.increased by the addition of meta sodium bisulfite.
2) The molar ratio of meta sodium bisulfite to potassium persulfate

that produces the maximum rate of polymerization was found to be eight

to ten.

-3) A theoretical expression was derived for the polymerization rate

of styrene in emulsion, catalyzed by the potassium persulfate-meta

sodium bisulfite redox system, under the conditions specified.in the

experimental part of this paper.

(111) ; 1760(Cb, + (@111) + F
dt I. cit p

4) The rate of the emulsion polymerization of styrene catalyzed by
potassium persulfate in presence of excess meta sodium bisulfite,.is
proportional to the square root of the persulfate concentration. There

appears to be a minimum limit of persulfate concentration for this

expression to hold.

5) The average molecular weight of the polystyrene formed decreases

with increasing rate of polymerization.if the persulfate concentration

.is held constant.in the meta sodium bisulfite—potassium persulfate redox

catalyzed polymerization of styrene.

6) Probably mechanisms for the redox, persulfate—meta bisulfite,

.initiation of polymerization have been cited.

 

 

 

 

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