The Synthesis of Sulfonated Polymers by Free Radical
Copolymerization. Poly(butadiene-co-Sodium
Styrene Sulfonate)
R. A. WEISS and R. D. LUNDBERG, Corporate Research Science
Laboratories, Exxon Research and Engineering Company, P.O. Box 45,
Linden, New Jersey 07036, and A. WERNER, Princeton Polymer
Laboratories, 501 Princeton Road, Plainsboro, New Jersey 08536
Synopsis
The copolymerization of butadiene with sodium styrene sulfonate was studied and the copolymer
products characterized. In general these copolymers contain 0.5-4 mole % of sulfonated monomer.
The effects of the following reaction variables are described: emulsifier type and concentration,
monomers feed ratio, chain transfer agent concentration, and reaction conversion. The products
were heterogeneous with regard to composition, molecular weight, and solubility behavior. Copolymers prepared under certain conditions exhibited strong intermolecular interactions derived
from associations of the ionic species as observed in other ionomers.
INTRODUCTION
The modification of hydrocarbon polymers by the inclusion of salt groups has
received considerable attention in recent years. Specifically, when a modest
number, for example, less than 10 mole %, of ionic groups is attached to a polymer
backbone, the intermolecular associations of the ionic species that result develop
properties analogous to those of a crosslinked po1ymer.l An important distinction, however, exists between an “ionic crosslink” and a covalent crosslink
the ionic crosslink is reversible; that is, the ionic associations can be so weakened
by the application of heat or the introduction of specific low molecular weight
polar “plasticizers”2 that under suitable conditions an ionomer can behave as
if it were not crosslinked and under other conditions as if it were.
There are basically two methods of preparing ionomers: (1) by grafting an
ionic substituent onto a preformed polymer and (2) by copolymerization of a
hydrocarbon monomer and an unsaturated acid or salt. The second method
has been used extensively in the preparation of carboxylate ionomers? whereas
postreactions have largely been used to prepare sulf~nate-ionomers.~~~
A
comprehensive discussion of these materials can be found in several monog r a p h ~ ’and
? ~ review articles6 and is not treated here.
The purpose of this article is to describe the preparation of a sulfonate-ionomer
by the copolymerization of butadiene and sodium styrene sulfonate ( S S S ) . In
particular, the effects of several copolymerization parameters on the polymer
products are discussed, with special emphasis on the “ionic” or “nonionic”
character of these products.
Journal of Polymer Science: Polymer Chemistry Edition, Vol. 18,3427-3439 (1980)
0 1980 John Wiley & Sons, Inc.
0360-6376/80/0018-3427$01.30
3428
WEISS, LUNDBERG, AND WERNER
PRIOR ART
Although copolymerization reactions of vinyl and diene hydrocarbon monomers with an unsaturated sulfonic acid derivative have been reported in the
literature, relatively few studies have been concerned with the preparation of
copolymers that contain low concentrations of metal sulfonate groups. Most
of the prior art in this area considered the incorporation of metal-sulfonatecontaining monomers into polymers in order to stabilize latices derived from free
radical polymerizations or to improve the dyeability of polymer fibers.7 No
significant efforts have been directed, however, at the isolation and characterization of the resulting ion-containing polymer, nor have any concerted efforts
been made to control the degree of sulfonate group incorporation in these copolymers.
For the specific case of butadiene its copolymerization with various vinyl
sulfonate esters has been described by Marvel et a1.: although these materials
were not converted to the salt. A patent assigned to Teijin Ltd. in 19729 described the preparation of polybutadiene latices of large particle sizes by polymerizing butadiene in the presence of sodium methallyl sulfonate, but these
latices were used as intermediates in the preparation of ABS resins and not
characterized as ionomers.
EXPERIMENTAL
Materials
The reagents used in these experiments are listed in Table I. With the exception of the butadiene all materials were used as received; butadiene was
evaporated and condensed in a trap cooled with dry ice and acetone. The water
used in the polymerizations was distilled and boiled before use.
TABLE I
Reagents
Reagent
Monomers
Butadiene
Sodium Styrene Sulfonate
Surfactants
Pluronic L62-LF (polyol)
Pluronic F-68 (polyol)
Tween 80 (sorbitan monooleate)
Clearate (soya-lecithin derivative)
WAQE (sodium lauryl sulfate)
Redox Initiators
Triethylenetetramine (TETA)
Diisopropylbenzene hydroperoxide (DIBHT)
Chain Transfer Agent
Dodecanethiol
Buffer
Na4P20~10H20
SuDDlier
Matheson Gas Products
Columbia Organic Chemicals
BASF Wyandotte
BASF Wyandotte
ICI
W. A. Cleary
Du Pont
Union Carbide
Pfaltz and Bauer
Matheson, Coleman, and Bell
Mallinckrodt
SYNTHESIS OF SULFONATED POLYMERS
3429
Polymerizations
The reaction vessel consisted of a beverage bottle sealed with a crown-type
screw cap that contained a 3/32-in. hole in the center and a nitrile rubber gasket
1/32 in. thick. An excess of condensed butadiene monomer was poured into a
reaction bottle that contained the other reagents (SSS,emulsifier, redox initiator,
chain transfer agent, buffer, and water). Butadiene was then allowed to evaporate until the desired charge was reached, at which point the bottle was capped
and placed in a constant-temperature bath equipped with a mechanical bottle
shaker.
At the end of the reaction a methanolic solution of hydroquinone (shortstop)
and 2,2’-methylene-bis(4-methyl-6-t-butyl
phenol) (antioxidant) was injected
with a hypodermic needle through the septum in the bottle cap. After the bottle
was shaken for an additional 5-10 min to disperse the shortstop and antioxidant
effectively, it was cooled in ice water and the cap was removed. The polymer
was precipitated in methanol, which contained 0.05% antioxidant, washed until
the supernatant liquid was clear, and dried under vacuum at 4OOC. The polymers were subsequently stored in glass jars covered with aluminum foil to minimize any ultraviolet (UV) degradation.
The sulfur content of the copolymers were determined by Dietert sulfur
analysis (ASTM D1552). Reduced viscosities a t 25OC were measured with a
Ubbelohde capillary viscometer at a polymer concentration of 0.2 g/dl in a mixed
solvent of 90%toluene and 10%methanol.
RESULTS AND DISCUSSION
Based on the substantial difference in the polarity of the two monomers, it
is unlikely that the copolymerization of butadiene and SSS will behave as a
conventional emulsion polymerization. Whereas in a conventional system the
two comonomers reside within a single phase or have significant mutual solubility, in the butadiene-SSS system the SSS is contained predominantly in the
aqueous phase. Because copolymerization does occur, it has been postulated
that the reaction locus is the surface of the butadiene micelles.l0 This hypothesis
suggests that SSS is a surfactant in this system; this is made clear in the discussion that follows.
The mechanism of this copolymerization is complicated by the fact that, as
SSS is consumed, not only does the comonomer concentration change but so does
the surfactant concentration. Similarly, the solubility of the sulfonate monomer
in the hydrocarbon phase is expected to be dependent on the composition of the
swollen polymer particles and changes that occur during copolymerization may
alter the particle morphology and the locus of polymerization. For these reasons
the effects of conversion on the copolymer composition and molecular weight
are expected to be more complex than is normal for an emulsion polymerization.
The stability of the emulsion latex is also expected to influence the copolymer
characteristics. In many of the reactions described here the latex became unstable a t moderate to high conversions and the presence of large agglomerated
particles was observed. What effect this has on the product of these copolymerizations is not clear. This also raises the question whether these reactions
actually proceed by an emulsion polymerization mechanism or are perhaps more
WEISS, LUNDBERG, AND WERNER
3430
analogous to suspension polymerizations. These questions are beyond the scope
of this initial program but are currently being considered in our laboratory.
The incorporation of SSS in butadiene was confirmed by elemental analysis
of the resulting polymers and by infrared (IR) spectroscopy. A typical IR
spectrum of a butadiene-SSS copolymer is given in Figure 1and the absorption
bands present at 1200,1130,1045, and 835 cm-' confirm the existence of SSS.
The polybutadiene microstructure determined by IR corresponded to approximately 57% trans-l,4,18% cis-l,4, and 24% 1,2.
Molecular weight is characterized in this article solely by the polymer-reduced
viscosity in a mixed solvent system. Although it would be desirable to have a
more complete description of the polymer molecular weight and molecular weight
distribution, the presence of the small amount of ionic substituents in these
materials render them unamenable to other molecular weight techniques; for
example, gel permeation chromatography is complicated by adsorption of the
sulfonated polymer on the gel p h a e and light scattering or osmometry analyses
are complicated by the need to use mixed solvents to dissolve the ionomers.
In the discussion that follows, the effects of various reaction variables on the
copolymer composition, molecular weight, and solubility characteristics are
described.
Emulsifier Type and Concentration
Both ionic and nonionic emulsifiers were evaluated in this study; their effect
on the copolymer product is demonstrated in Table 11. For the nonionic emulsifiers studied, an optimum reaction rate was achieved when the HLB number*
was 29. When the HLB of the surfactant was lower than 29, the conversion after
w
0
z
a
l-
I-
5z
s
I-
s
3800 3400 3000 26002200
18003600140012001000 800 600
1
50
WAVENUMBERS
Fig. 1. Infrared spectrum of butadiene-sodium styrene sulfonate copolymer (83-37), 0.61%
sulfur.
* HLB is an empirical measure of the hydrophilic-lipophilicbalance of a surface-active substance.
A low number indicates lipophilic character and a high value, hydrophilic character.
SYNTHESIS O F SULFONATED POLYMERS
120" I 0 0 0
r - m m m o
3 m m r -
3431
3432
WEISS, LUNDBERG, AND WERNER
20 hr decreased and for an emulsifier HLB of 7 no product was formed. Similarly, for an emulsifier HLB of 70 no polymerization occurred.
A modest increase in SSS incorporation in the copolymer results when the
HLB of the nonionic emulsifier is increased. A more substantial increase in the
polymer molecular weight and ionic fraction of the copolymer product occurs
when the HLB is increased from 15 to 29. The ionic fraction is defined here as
a material that is insoluble in the hydrocarbon solvent, yet soluble in a more polar
mixed solvent of hydrocarbon and alcohol. It has been demonstrated for sulfonated ethylene-propylene-diene terpolymers that insolubility in hydrocarbon
solvents is due to the relatively strong intermolecular associations of the ionic
species which can be disrupted by the addition of a polar cosolvent; for example,
alcoh01.~ In sample 83-32 (HLB = 15) the ionic fraction is 4% of the nongel
material, whereas for samples 83-37 and 97-7 (HLB = 29) the corresponding
numbers are 41 and 68%. The disparity between the last two numbers demonstrates the complexities in the solubility behavior of these materials. Runs 83-37
and 97-7 were duplicate experiments, and except for the difference in the solubility characteristics, the reproducibility was excellent (e.g., conversion, molecular
weight, and sulfur concentration). The amounts of ionic fraction, then, suggest
that sample 83-32 is relatively nonionic, whereas samples 83-37 and 97-7 exhibit
substantial ionic character.
In ionic emulsifier, WAQE, the reaction rate was considerably less than for
HLB-15 (Tween 80) or HLB-29 (Pluronic F-68) nonionic surfactants. The
molecular weight was comparable to that achieved with Tween 80 and the SSS
incorporation to that with Pluronic F-68. The solubility behavior of the products
of duplicate experiments with ionic surfactant (experiments 97-5 and 97-11) were
again substantially different; the reproducibility of all the other copolymer
characteristics was good, however. In general, the ionic fraction appeared to
be comparable to that achieved with Pluronic F-68.
An interesting result occurred when SSS was used for both comonomer and
surfactant (experiment 97-1). In this case the charge of SSS was doubled to
conform more closely to the formulations used in the other experiments described
in Table 11. No product was observed after 2 hr; after 7 hr the reaction mixture
appeared to be a stable latex and after 33 hr the mixture was a homogeneous,
smooth gel. The data in Table I1 indicate that the reaction rate was relatively
slow and the molecular weight, relatively low. On the other hand, the SSS
incorporation was twice that achieved with the other surfactants and the product
was gel-free (100%soluble) with a substantial ionic fraction (34%). The fact that
polymerization even occurred has a greater significance in light of the attempts
to polymerize butadiene homopolymer with Tween 80 or Pluronic F-68 as the
emulsifier. In these experiments no product was formed after 20 hr. In every
case in which SSS was present, however, some polymerization took place. These
results strongly suggest that SSS is present at the butadiene micelle.
SSS Concentration
Because SSS functions as an emulsifier in addition to being the comonomer,
one might expect the concentration of the SSS in the charge to have an important
effect on the product of the reaction; for example, an increase in SSS charge might
be expected to increase the reaction rate by increasing the surfactant concen-
SYNTHESIS OF SULFONATED POLYMERS
3433
tration and to increase the sulfonate incorporation by increasing the SSS/butadiene ratio. The effect of SSS concentration on the copolymerization was
determined for three different surfactant systems: Tween 80 (nonionic, HLB
= 15), Pluronic F-68 (nonionic, HLB = 29), and WAQE (ionic); the results are
summarized in Table 111.
With the exception of experiment 83-45,an increase in the SSS charge resulted
in an increase in the reaction conversion. As explained above, this is most likely
a consequence of the emulsifying capability of the SSS. The effect of increasing
the SSS charge on the SSS incorporation is not, however, straightforward. In
some instances an increase in the SSS feed concentration led to an increase in
SSS incorporation, whereas in others a decrease resulted. Similarly, the copolymer molecular weight did not appear to be a simple function of the amount
of SSS used. For the Tween 80 system molecular weight decreased with increasing SSS charge, and for the WAQE the molecular weight did not appear
to be sensitive to the SSS charge.
The polymer ionic fraction increased with SSS charge for the systems using
Tween 80 and WAQE; when Pluronic F-68 was used the toluene-soluble fraction
(nonionic fraction) decreased and the gel fraction increased. Because of the
relatively high SSS content in the latter polymers, especially samples 83-48 and
97-16, it is not clear whether the gel fractions were covalently or physically
crosslinked by intermolecular associations of the ionic groups. It is shown later
in this article that the gel fractions of the butadiene-SSS copolymers were generally higher in sulfonate content than the soluble fractions, and in many instances the sulfonate content of the gel fraction exceeded 2 mole % (1.12%s).
Chain Transfer Agent Concentration
Chain transfer agents are often used in free-radical polymerizations to reduce
the molecular weight of the polymer produced and to minimize crosslinking reactions. The latter was especially important in this study because of the difficulty of distinguishing between covalent crosslinking and a high level of ionic
crosslinking. Similarly, very high molecular weight polymers were not desirable
because under certain conditions these materials exhibit behavior analogous to
a crosslinked polymer.
The effects of dodecanethiol (chain transfer agent) concentration on the copolymerization products of this investigation are given in Table IV. The most
important effect of increasing the dodecanethiol concentration was the expected
one, a lowering of the apparent polymer molecular weight. The prefix, apparent,
is stressed here in that the molecular weights inferred from the reduced viscosities
of these polymers are influenced by inter- and intramolecular interactions of the
ionic species which can in some instances affect the solution viscosities to a
greater degree than the backbone molecular weight." It should also be emphasized that the reduced viscosities may reflect a fractionation of the copolymer
by molecular weight or ionic interactions. As mentioned in the preceding section,
the latter possibility is especially important for copolymers that contain a relatively high concentration of SSS; for example, more than 2 mole %. In general,
the chain transfer agent concentration did not appear to affect the copolymer
compositions significantly.
a
20
44
44
-
-
87
95
>lo0
>loo
85
39
66
100
100
100
92
38
29
62
78
64
75
30
67
86
59
29
15
27
26
99
74
-
-
0
Mixed solvent
Toluene
0
(9%)
0
0
8
62
71
38
0
22
36
-
Gel
?red
2.1
2.3
0.79
0.46
1.0
1.0
1.1
-
1.6
0.8
-
0.61
0.57
1.44
1.92
0.80
0.64
0.46
-
0.44
0.99
-
%S
Reaction formulation: 38.8g butadiene, variable SSS, 4.6g emulsifier, 1.0g TETA, 1.0g DIBHT, 0.28g dodecanethiol, 0.39g Na~P20~10H20,lOO
g water.
Based on butadiene charge.
WAQE
WAQE
20
8.0
20
20
20
20
20
20
20
time (hr)
12.0
4.0
8.0
4.0
4.0
8.0
0.0
12.0
0.0
4.0
Tween 80
Tween 80
Tween 80
83-44b
83-32
83-39
83-47b
83-37
97-7
83-48
97-16
83-45
97-12
97-14
F-68
F-68
F-68
F-68
F-68
F-68
g NaSS
Emulsifier
Experiment*
TABLE I11
Effect of SSS Concentration in the Reaction Charge on the Copolymer Product
Solubility (%)
Reaction
Conversionb
?l
3
B
8
*
3
0
m
2z
B
8
W
rp
W
rp
0.00
0.72
0.72
0.72
0.72
1.45
0.72
1.45
83-34'
83-32'
83-31ad
83-37'
97-7c
97-9=
83-48e
97-3e
87
67
68
93
95
87
>lo0
>100
5
75
58
59
29
67
15
36
Toluene
33
78
100
100
92
100
38
55
Solubility (%)
Mixed solvent
67
22
0
0
8
0
62
45
Gel
?red
1.7
1.6
1.7
2.1
2.3
1.4
0.79
0.59
%S
0.29
0.44
0.46
0.61
0.57
0.64
1.44
1.06
a Reaction formulation:
38.8 g butadiene, variable SSS, 4.6 g emulsifier, 1.0 g TETA, 1.0 g DIBHT, variable dodecanethiol, 0.39 g NarP207.10H20,lOO g water.
Based on butadiene charge.
4.0 g SSS in charge.
Double the reaction formulation of 83-32.
8.0 g SSS in charge.
(%) Thiolb
Emulsifier
Tween 80
Tween 80
Tween 80
Pluronic F-68
Pluronic F-68
Pluronic F-68
Pluronic F-68
Pluronic F-68
Experiments
Conversionb
(% 6 20 hr)
TABLE IV
Effect of Chain Transfer Agent on the Copolymer Product
rn
2
cd
!
c3
z
G0
E
E
3
-4
rn
WEISS, LUNDBERG, AND WERNER
3436
I
I
I
I
1.4
.o
1.2
.6
1 .o
.2
0.8
L S
‘red
0.6
.8
0.4
.4
0.2
0
0
I
I
20
40
I
I
60
80
100
(X)
Fig. 2. Copolymer composition and molecular weight versus conversion.
CONVERSION
Conversion
The composition, molecular weight, and solubility behavior of the butadiene-SSS copolymers prepared in this investigation were strongly dependent on
the extent of the reaction demonstrated in Figure 2 and Table V. The SSS
content of the copolymer decreased and the molecular weight increased with
increasing conversion. Some question exists, however, of the validity of the
higher sulfonate concentrations for the samples a t the three lowest conversions
TABLE V
Characteristicsof Butadiene-NaSS Copolymers as a Function of Conversion
(Experiment 83-19)
Solubility (%)
Mixed
Xylene
solventa
Conversion
Sample
(%)
%S
q,,d
83-19-4
83-19-5
83-19-6
83-19-3
83-19-1
83-19-2
4
13
29
64
67
1.34
0.95
0.90
0.70
0.56
0.46
0.39
0.36
0.37
0.48
1.8
1.9
100
100
100
100
56
44
98
99
99
99
100
95
Ionic
fractionb
Gel
(%)
0
0
0
0
0
0
5
0
0
0
44
51
* Reaction formulation: 29.0 g butadiene, 3.0 g SSS, 2.3 g Tween 80,0.5g TETA, 0.5 g DIBHT,
0.29 g Na4PzOylOH20,50g water.
95/5 Toluene/methanol.
% Ionic fraction = (mixed solvent solubility) (xylene solubility).
-
SYNTHESIS OF SULFONATED POLYMERS
3437
(83-19-4,5,and 6). These polymers were extremely tacky greases, and it is not
certain whether all of the unreacted SSS monomer was removed during the
workup of the polymer. The solubility data in Table V indicate that the ionic
character of the copolymer increased at the higher conversions, even though the
measured sulfonate content decreased.
The solubilities in mixed solvent of the copolymer products prepared with the
three nonionic surfactants and three concentrations of chain transfer agent are
plotted against conversion in Figure 3. Although other variables such as the
reaction temperature, SSS charge, and latex stability may affect the copolymer
products, it appears that insoluble polymer, that is,'gel, was formed preferentially
a t conversions greater than 60%. This suggests that crosslinking reactions become important a t higher conversions, a result that is not uncommon in the
polymerization of diene monomers.
Compositional Heterogeneities
Fractionations of the butadiene-SSS copolymers with the different solvents
used in this study suggest that these materials were heterogeneous with regard
to composition. The sulfur analyses (SSS content) and reduced viscosities
(molecular weight) for various fractionated polymers are given in Tables VI and
VII. The data in Table VI indicate that the nonionic toluene-soluble materials
were, in general, lower in SSS concentration and lower in molecular weight than
the ionic, toluene-insoluble-mixed-solvent solution fractions. These results
Soluble
Fraction
KEY
(9)
SYMBOL
40
-
EMULSIFIER
TWEEN 80
0
0
F 68
A
WAQE
01
0
4
0
' I
[CTA]/[C4H61
SYMBOL
2o -
I
0
0
Filled
Empty
0.005
HalfFilled
0.007
I
20
I
I
I
J
40
60
80
100
CONVERSION ( X )
Fig. 3. Copolymer solubility versus conversion.
WEISS, LUNDBERG, AND WERNER
3438
TABLE VI
Characteristicsof Toluene-Soluble and -InsolubleFractions of Butadiene-SSS Copolymers
~~
Experiment
Sulfur analysis (% S)
Unfractionated Toluene- Toluenepolymer
soluble
insoluble
83-37
83-39
83-45
83-48
97-1
97-3
97-5
97-7
97-26-2
a
0.61
0.99
0.80
1.44
1.31
1.06
0.61
0.57
0.94
0.49
0.66
0.32
0.56
0.93
0.72
0.41
0.47
0.56
0.64
1.09
1.03
1.71
2.20
1.44
0.71
0.64
1.23
Reduced viscosity (dl/g)a
Toluene- Toluene- Combined
soluble
insoluble
sample
0.98
0.48
-
2.5
1.1
1.5
1.2
-
0.49
1.2
2.1
0.77
1.o
0.79
0.65
0.59
1.31
2.3
1.1
Measured in a mixed solvent of toluene and methanol (95/5) at 25OC.
TABLE VII
Composition of Mixed-Solvent-Solubleand -InsolubleFractions of Butadiene-SSS Copolymers
Experiment
83-39
83-45
83-48
97-3
Sulfur analysis (a S)
Mixed solvent soluble
0.94
0.55
0.65
0.67
Gel
1.21
1.39
1.98
1.44
are consistent with the observation that the toluene-soluble materials were
usually fairly tacky and had little tensile strength. On the other hand, the toluene-insoluble-mixed-solvent-soluble fractions were nontacky, elastic solids
that exhibited superior mechanical characteristics.
It was pointed out earlier in this article that the distinction between covalent
and ionic crosslinking in ionomers is not always obvious. In this regard the data
in Table VII are especially noteworthy. These data show that the insoluble, or
gel, material contained a higher concentration of the ionic moiety SSS than the
soluble materials, which suggests that the insolubility of these polymers may
result from strong ionic associations rather than covalent crosslinking, although
this conclusion has not been confirmed.
CONCLU$IONS AND FUTURE WORK
The overall objective of this study was to explore the viability of copolymerization as a route to sulfonate ionomers. The copolymerization of butadiene
with sodium styrene sulfonate has been studied and the copolymer products have
been characterized. The effects of the various reaction variables on the copolymer products have been described, and under suitable conditions copolymers
that exhibit behavior typical of previously evaluated ionomers can be prepared.
Additional work is needed, however, to characterize further the mechanism of
this particular copolymerization and the copolymer products.
SYNTHESIS OF SULFONATED POLYMERS
3439
References
1. A. Eisenberg and M. King, Ion-ContainingPolymers, Academic, New York, 1977.
2. H. S. Makowski and R. D. Lundberg, Polym. Prepr., 19(2), 304 (1978).
3. L. Holliday, Ed., Ionic Polymers, Applied Science, London, 1975.
4. H. S. Makowski, R. D. Lundberg, L. Westerman, and J. Bock, Polym. Prepr., 19(2), 292
(1978).
5. D. Rahrig and W. J. MacKnight, Polym. Prepr., 19(2),314 (1978).
6. W. J. MacKnight and T. R. Earnest, Jr., J. Polym. Sci., in press.
7. D. A. Kangas, in Functional Monomers, Vol. 1,R. H. Yocum and E. B. Nyquist, Eds. 1973,
Chapter 4.
8. C. S. Marvel, V. C. Menikheim, H. K. Inskip, W. K. Taft, and B. G. Labbe, J.Polym. Sci., 10,
39 (1955).
9. Japanese Pat. 72/51954 (1972).
10. B. Oster and R. W. Lenz, Research Report to Exxon Research and Engineering, December
1975.
11. R. D. Lundberg and H. S. Makowski, Polym. Prepr., 19(2),287 (1978).
Received March, 28,1980
Accepted May 30,1980