The Emulsion Copolymerization of Styrene and
Sodium Styrene Sulfonate
S. RICHARD TURNER,* R. A. WEISS,' and ROBERT D. LUNDBERG,
Corporate Research Science Laboratories, Exxon Research and Engineering
Company, Route 22 East, Annandale, New Jersey 08801
Synopsis
The emulsion copolymerization of styrene and sodium styrene sulfonate has been shown to
be a feasible preparative route to ionomeric sulfonated polystyrene. The properties of these
copolymers are reported elsewhere. The copolymerization rate was found to be dramatically
enhanced when compared to that for the emulsion copolymerization of styrene under identical
conditions. This copolymerization was studied in detail and two mechanisms were proposed
to account for these rate differences. An increase in the number of polymerizing particles in
the copolymerization with consequent rate enhancement was substantiated by electron microscopy. However, the data indicate that the rate differences cannot be fully accounted for
by this effect. In addition, a gel effect is proposed as a second contributor to the enhanced
rate. This gel effect is believed to result from the intermolecular association of the incorporated
metal sulfonate units in the growing polymer particles. When a third monomer that plasticizes
the ionic interactions is used the polymerization rate decreases. This supports the gel effect
hypothesis.
INTRODUCTION
Lightly sulfonated polystyrene (S-PS)has received considerable attention
recently because of the scientifically interesting and potential technologically important properties manifested by the ionic associations in this glassy
hydrocarbon polymer. For example, plasticization,' solution behavior,2 viscoelastic proper tie^,^ EXAFS analyses of the aggregation aspect^,^ and fluorescence measurements on S-PS5 have all been recently reported. The
technological interest in S-PS arises from the ionomer characteristics. Because of these properties S-PS has been shown to have outstanding potential
as a polymer for forming rigid foams.6
Sulfonation of preformed polystyrene has been the principal synthetic
route for preparation of S-PS.7 Sulfonation has several key attributes: (1)
sulfonation of the preformed polymer probably yields a random distribution
of sulfonate groups along the polymer chain; (2) a minimum of chain-tochain heterogeneity is expected from this process; (3) since the reaction has
been shown to proceed without polymer degradation, the molecular weight
of the functionalized polymer is known; and (4) preformed polystyrene can
be obtained with narrow distributions, so that the interpretation of data
from studies on the ionomers is simplified.
* Present address: Eastman Kodak Company, Research Laboratories Rochester, NY 14650.
Present address: Institute of Materials Science, University of Connecticut, Storrs, CT 06268.
+
Journal of Polymer Science: Polymer Chemistry Edition, Vol. 23, 535-548 (1985)
@ 1985 John Wiley & Sons, Inc.
CCC 0360-6376/85/020535-14$04.00
536
TURNER, WEISS, AND LUNDBERG
In concurrent publication^^.^ we communicate the preparation and properties of S-PS prepared by emulsion copolymerization, and describe the
differences in properties manifested by S-PS from emulsion vis-a-vis S-PS
from sulfonation. This article describes the unusual characteristics of the
copolymerization reaction.
EXPERIMENTAL
Sodium Styrene Sulfonate (NaSS)
NaSS was obtained from Air Products and used as received. HPLC analysis indicated only a very small amount of organic impurities, and a check
of the salt content was consistent with the 4 4 . 5 % NaBr and Na2S0, reported by Air Products.
Styrene (St)
All copolymerizations were carried out with freshly distilled St.
Water
Distilled water was deareated before use by boiling and then cooling in
a nitrogen atmosphere. Sodium dodecyl sulfate (SDS), potassium persulfate
(K2S20s),sodium thiosulfate (Na2S205),and n-dodecylthiol (C12H25SH)
were
used as received.
Copolymerization Procedures
Capped Bottles
A small glass pressure bottle was charged with a,small magnetic stir bar
to enhance agitation, 25 g of St (0.24 moll, 1.0 g of NaSS (0.0048 mol), 60
mL of H20, 0.1 g of C12SH,1.6 g of SDS, and 0.1 g of K2S208.The flask was
purged with nitrogen and capped with a two-hole crown cap containing a
rubber septum. The bottle was placed into a safety screen in a thermostatted
Erbach water shaker bath at 50°C and was agitated for a 6 h polymerization
time. The bottle was removed and ca. 3 mL of a hydroquinone %hortstop”
solution was added via a syringe. The bottle was shaken for a n additional
10 min, cooled, and opened. The polymer was coagulated by pouring the
reaction solution into about 250 mL of methanol with some NaCl added.
The very fine precipitate was difficult to filter, thus the slurry was centrifuged at ca. 2000 rpm for about 45 min. The solid polymer was washed two
to three times with distilled water and brought down by centrifugation each
time. About 15 g (after drying under vacuum at 40°C) of white powder were
obtained by this procedure. This represented about 58% conversion. Evaporation of the washes yielded the remainder of the expected product. These
washes, of course, were contaminated with the surfactant and NaCl used
to assist in coagulating the emulsion. Elemental analysis of the coagulated
portion was 0.71% S. A control without NaSS present gave a S analysis of
0.21%. Therefore, the polymer actually contained 0.50% S, which is equiv-
EMULSION COPOLYMERIZATION
537
alent to 1.57 mol % NaSS incorporation. The evaporated fraction was found
to contain 1.21% S.
2-L Flasks
The-reaction was scaled from the pressure bottles to a 2-L flask, and the
components were charged in the same proportion as in the pressure bottles.
This amounted to a lox scale-up, i.e., 16 g of SDS, 10 g of NaSS, 600 mL
of H20, 250 g of St, 1.0 g of C12H25SH,
and 1.0 g of K2S208.Agitation was
provided by an overhead stirrer, and the system was carefully purged with
N, and kept under a N, purge during the course of the reaction. The shorter
induction periods were found when the reaction solution was sparged with
N2 for ca. 15 min before bringing the reaction to 50°C. Heat was provided
via a thermostatically controlled oil bath. For kinetic studies the initiator
was added when a constant temperature of 50°C had been reached. Reactions
were stopped after about 6 h and quenched with a hydroquinone solution.
Yields and compositions were identical to those from the bottle reactions.
It was discovered that these reactions develop a vigorous exotherm and a
rapid temperature rise to 89-90°C.
1-L Adiabatic Flask
The copolymerization was done near adiabatic conditions in a l-L doublewalled, silvered, three-necked flask. The reaction contents, without initiator, were heated externally to about 55°C with nitrogen purge and then
added to the flask which was also under a nitrogen purge. This procedure
resulted in a temperature of 53°C for the reactants. The initiator was added
at this time, and stirring was maintained along with a slow nitrogen purge.
It was necessary to use the two-component, K2S208 Na2S205,initiator system in order to minimize the induction period caused by 0, picked up during
the transfer of the heated reaction emulsion. In addition, in order to avoid
the potential of boiling the reaction was intentionally diluted from that of
the original charge in capped bottles and 2-L flasks above. In this configuration temperature was monitored or small samples were taken periodically. The reactant charges for these reactions were 500 mL of H20, 125 g
of St, 5.0 g of NaSS, 8.0 g of SDS, 0.5 g of C12H25SH,
0.25 g of K2Sz08,and
0.25 g of Na2S205.
RESULTS AND DISCUSSION
The copolymerization of St and NaSS has been the subject of previous
studies, but only for preparing lattices for various purposes.lOJ1The NaSS
was added to the styrene emulsion either to form a more stable latexlo or
in the second case, which was an emulsifier free system, to control surface
charge density and size of the polystyrene particles." In this study higher
levels of NaSS were utilized, up to about 40 mol % NaSS, and the copolymerizations were done with the surfactant sodium dodecyl sulfate (SDS)
as the emulsifier.
As has been reported earlier the copolymerization could be effected with
either water soluble initiators such as potassium persulfate or a potassium
TURNER, WEISS, AND LUNDBERG
538
persulfate and sodium thiosulfate mixture or oil soluble/water soluble redox
couples such as diisopropyl benzene hydroperoxide/ trimethylenetetramine.
Similar copolymer compositions were obtained in both cases.8 All copolymerizations in this study were done with the water-soluble systems.
Composition Versus Comonomer Charge
The NaSS incorporation into the S-PS copolymer increases as the amount
of NaSS in the comonomer charge is increased (Figs. 1and 2.) This increase
in NaSS content is reflected in the polymer properties. For example, as the
NaSS charge is increased, the copolymer becomes more dispersable in water
and more difficult to work up. The dispersed phase (Fig. 2) can be separated
from a precipitated phase (Fig. 1).As expected, the dispersed phase contains
a higher level of NaSS than the precipitated phase. This heterogeneity may
proceed to such a degree that significant amounts of water-soluble polymer
product are formed. This is suggested by the asymptotic behavior shown in
Figure 2. The properties of these copolymers are discussed elsewhere.8
Copolymerization Chemistry
It was expected that with a high St charge ratio (96 wt % St) in the
emulsion copolymerization of NaSS and St that the water soluble NaSS
comonomer would not be located in sufficient concentration at the locus of
polymerization of the hydrophobic particle to influence significantly the
reaction kinetics, even though, as shown in this study, NaSS can readily
10.0
Y
U
I
t
-1
0,
8
5.0
I
I
I
I
I
10
20
30
40
MOL % NaSS IN FEED
Fig. 1. Plot of mol % NaSS in copolymer from precipitated phase versus mol % NaSS in
copolymerization feed for 21 reaction, standard condition from Experimental section.
539
EMULSION COPOLYMERIZATION
1
I
I
I
I
10
20
30
40
60
MOL % N.SS IN FEED
Fig. 2. Plot of mol % NaSS in copolymer from dispersed phase versus mol % NaSS in
copolymerization feed for 21 reaction, standard conditions from Experimental section.
be incorporated into the backbone. Our observations indicate otherwise,
namely that inclusion of these small amounts of NaSS into the copolymer
result in a dramatic alteration of the polymerization kinetics. To understand
the origin of this effect, we have studied this reaction in some detail, and
the results are reported in the following discussion.
When NaSS and St are copolymerized under emulsion conditions, as
outlined in the Experimental section, a vigorous exotherm is noted that is
significantly faster and reaches a higher temperature than the exotherm
observed for the corresponding styrene polymerization control. These observations are plotted in Figure 3. The temperature peaks and then drops
because of the fast heat dissipation from the 2-L glass flask.
This exothermic behavior suggests a very fast copolymerization reaction,
and this has been verified by following the rate of disappearance of NaSS
by HPLC and by gravimetric determination of reaction conversion. These
data are plotted in Figures 4 and 5. Figure 5 also shows a comparison of
the copolymerization versus homopolymerization. From these data overall
rates of polymerization can be calculated. For the copolymerization a value
mol/L s was obtained compared with 2.0 x
mol/L s
of 16.7 x
540
TURNER, WEISS, AND LUNDBERG
Time (hn.)
Fig. 3. Plot of reaction temperature versus time for (-) St/NaSS copolymerization
(a)and St (-Cl-) homopolymerization(b) in 21 reaction standard conditions from Experimental
section.
for the homopolymerization.
We have considered two possible mechanisms that could contribute to
this enhanced rate of the copolymerization of NaSS and St over the homopolymerization of styrene: gel effect and particle size.
Gel Effect
One possible mechanism for this unique kinetic effect would involve an
early onset of a “gel effect” in the growing polymer particles. The large
100
50
0
Time (minutes)
Fig. 4. Plot of NaSS as determined by HPLC in reaction mixture versus reaction time for
21 reaction, standard conditions, from Experimental section.
541
EMULSION COPOLYMERIZATION
Time (minuter)
Fig. 5. Plot of conversion (gravimetric determination) versus time for St/NaSS copolymerization (+)
run 1, (-C-) run 2, and for St homopolymerization ( 0 ).
... ...
rate difference in the copolymerization vis-a-vis the homopolymerization
would originate from the associations of the metal sulfonate groups in the
growing polymer particles. As the apparent molecular weight of the copolymer increased due to these associations, the termination rate would be
lowered, and thus there would be an increase in the rate of polymerization
as compared with the homopolymerization of styrene where these associations are not present. The rate of polymerization is given by
where k, is the rate constant for propagation, k, is the rate constant for
termination, f is initiator efficiency, kd is the rate constant for initiator
decomposition, and [I] and [MI are concentrations of initiator and monomer,
respectively. Inspection of this equation clearly shows that a decrease in k,
increases Rp’
The occurrence of “gel effects” in emulsion polymerizations is well documented. Early reports include the work of Gerrens12 on styrene, van der
H0ffl3 on styrene, and ZirnmtI4on methyl methacrylate. Styrene was shown
to exhibit this effect at about 60% conversion. All styrene emulsion polymerizations have been said to exhibit gel effects noted by a change in slope
of the conversion versus time curve at 5 5 4 0 % conversion. Recent work
has been extended in developing models for understanding these effects for
glassy polymers in terms of a critical molecular weight for chain entanglements that lead to reduction in diffusion controlled terminati~n.’~.’~
The idea of ionic association enhancing the gel effect in the copolymerization system appears to be consistent with these models. A low critical
molecular weight, because of the associations, would lead to an earlier onset
542
TURNER, WEISS, AND LUNDBERG
of the gel effect and thus a higher exothermicity and a faster reaction rate.
An inspection of Figure 6 shows the effect of the charge of NaSS on the
copolymerization kinetics as measured by exothermic temperature. The
copolymerizations were done with the standard surfactant concentration.
As the NaSS concentration is increased, small but significant rate enhancements are noted. It is interesting to note that even at a NaSS concentration of 1.0 g or 99.5% molar ratio of styrene, a considerably faster
reaction is observed than that for styrene. These data are consistent with
the “gel effect” explanation in that higher ionic content should correspondingly lead to an increase in intermolecular association and an earlier entry
of the polymer particle into a “gel state.”
Also consistent with the gel effect explanation is the observation of. an
increase in molecular weight, as measured by viscosity, as the NaSS level
was i n c r e a ~ e dAttempts
.~
have been made to plasticize the growing polymer
chain in a styrene emulsion with known plasticizers for styrene, e.g., octane,
Nujol, and dioctylphthalate, and with various low-molecular-weight comp o ~ n d s . ’ ~InJ ~
both cases the authors observed rate decreases and attributed
these to phase separation in one
and to moderation of the gel effect
in the other study.18 However, it is noted that these conclusions have recently been questioned by Azod, Fitch, and Haynes.lg
In recent years it has been shown that it is possible to plasticize the ionic
associations that are present in lightly sulfonated S-PS2 For example, these
polymers can be swollen in an aromatic solvent and then addition of an
alcohol can break the association and bring about solution. Alcohols, acids,
amides, etc., have been shown to be effective in diminishing the rubbery
0
I
I
10
20
1
30
Time (minutes)
Fig. 6. Plot of reaction temperature versus reaction time for St/NaSS copolymerization at
various NaSS feeds as measured in adiabatic reactor as described in the Experimental section:
(-04 10 g of NaSS, (-0-45.0 g of NaSS, ( - - 0.
- ) 2.5 g of NaSS, (
) 1.0 g of
NaSS.
-
-
..-
- ..
EMULSION COPOLYMERIZATION
543
plateau exhibited by S-PS. Recent work by LundbergZ0has established the
concept of internal plasticization by incorporating a termonomer such as
acrylamide. We have thus investigated the copolymerization in the presence
of known plasticizers to see if the reaction rate is moderated. In addition,
we have attempted terpolymerizations to see if internal plasticizations occurs in the growing polymer particles.
We have observed that addition of hexanol or diluted phthalate, both
known ionic plasticizers, show no effect on the copolymerization. We believe
that these compounds never really are dispersed in the polymer particles
and are either in the monomer droplets or emulsified separately, and hence
they have no effect. On the other hand, incorporation of acrylamide or
acrylic acid as termonomers into the polymerization leads to a significant
decrease in the reaction exotherm in the nonadiabatic reaction. Under
adiabatic conditions this is shown as a decrease in the copolymerization
rate as measured by rate of temperature increase in Figure 7. This result
is consistent with a picture of ionic associations governing the reaction
kinetics. The plasticization of the ionic associations thus could raise the
critical molecular weight for onset of the gel effect, and, therefore, slower
overall rate is observed.
Particle Size
However, before we can say with certainty that the “gel effect” contributes to all or to even part of the rate enhancement, we must consider the
effect of particle size and number of the copolymerization kinetics. In the
classical Smith-Ewart scheme of emulsion polymerization, the rate of polymerization is proportional to the number of polymer particles formed,
I
I
.&..O.O...o..O
8E
I
,,,
......o......
.o
-0
i
I
’
StyINaSSIAA
StylNaSSlAm
75
o
70
.f
1
65
I0
-
00
55
50
I
I
I
10
20
30
I
Time (minutes)
Fig. 7. Plot of reaction temperature versus reaction time for St/NaSS copolymerization,
St/NaSS/Am, and St/NaSS/AA terpolymerization and St homopolymerizationsas measured
in adiabatic reactor as described in the Experimental section.
544
TURNER, WEISS, AND LUNDBERG
i.e., the greater the number of polymerizing sites the faster the polymerization. This is represented by
R,
=
Nkp[M]/2
(2)
where R, is the rate of polymerization, Nis the number of polymer particles,
k, is the rate constant of polymerization, [MI is the monomer concentration,
and the one-half comes from the theory that one-half the particles are
polymerizing at any one time. Since we have shown that the copolymerization rate is about eight times faster than the rate of homopolymerization,
it should require eight times the number of particles present in the copolymerization reaction to account for the rate enhancement.
Electron micrographs of emulsions from the homopolymerization and a
corresponding copolymerization, done under identical conditions with the
addition of NaSS, are shown in Figures 8 and 9. Analysis of these pictures
clearly indicates the larger size of the homopolymer emulsion, however,
actual analyses show the copolymer particles to have a mean size of 39 nm
(+ 22 nm) and the homopolymer to have a mean size of 54 nm (+ 53 nm).
Although we do not have a direct measure of the number of particles, the
size differences do not appear to be sufficient to account for the entire rate
enhancement. These data also indicate that the copolymerization yields a
much narrower particle size distribution than the homopolymerization.
These distributions are plotted in Figure 10.
Since the number of polymer particles in an emulsion polymerization is
dependent on the surfactant concentration, we have increased the surfactant level in the styrene homopolymerization so that the total molar concentration of SDS is equivalent to the molar concentration of SDS and NaSS
in the copolymerization. As expected, the rate of homopolymerization in-
Fig. 8. Electron micrograph of St/NaSS copolymer emulsion after dialysis.
EMULSION COPOLYMERIZATION
545
Fig. 9. Electron micrograph of St homopolymer emulsion after dialysis.
creased (Fig. 11).However, it remained significantly below that observed
for the copolymerization. Substitution of sodium-p-toluene sulfonate
(NaTos) for NaSS also led to some rate enhancement for styrene, but this
rate was also significantly below the copolymerization rate (Fig. 11).
The experiments with NaTos show that two other potential rate-controlling events are not operative in the copolymerization. The first of these
is a rate increase due to a n enhanced initiator decomposition due to the
presence of a n aromatic sulfonate group. It would be expected that this
effect would be the same for NaSS or NaTos, and this clearly is not the
case. A second consideration is that the comonomer-NaSS-somehow
changes the activity of styrene in the monomer droplets in a manner described by Azod, Fitch, and Haynes et al.19Here again the same effect would
be expected for NaTos, and this was not observed.
The level of SDS also has a n effect on the rate of the copolymerization.
Particle Size Distribution
70
Diameter (nm)
Fig. 10. Particle size distributions for (0)
St/NaSS copolymer (Latex 94) and (0)St homopolymer (Latex 97).
546
TURNER, WEISS, AND LUNDBERG
0
I
I
10
20
J
30
Time (minuter)
Fig. 11. Plot of reaction temperature versus reaction time for St homopolymerization with
various surfactant levels and with sodium tosylate as measured in adiabatic reactor as dewith NaTos + NaLS,
scribed in the Experimental section: (-O-) with extra NaLS, (+)
(+I
standard NaLS.
Lowering the surfactant level at constant monomer ratio and concentration
led to a depressed rate and also a lower exothermic maximum temperature.
This was reflected in a lower copolymer conversion (Fig. 12).
In our studies copolymerization occured in the absence of surfactant,
however, at a much slower rate. Flocculation also was observed during the
reaction.
Initiation Mechanism
In obtaining conversion data on these fast copolymerization, we have
obtained samples from the early reaction stages. The sulfur content was
observed to be high and to decrease on conversion. The observation of smaller polymer particles vis-6-uis the homopolymerization (and these composition data) suggest a mechanism for initiation that we envision as follows.
The water-soluble persulfate initiates some NaSS in the aqueous phase
which yields NaSS oligomers. These begin to capture styrene in the early
stages of the polymerization and very quickly aggregate to stabilized particles, similar to the mechanism proposed by Krieger” in a n emulsifier free
copolymerization. This would be a type of “homogeneous nucleation” as
proposed by Fitch.21It is possible that such particles could be much smaller
than particles without the NaSS present due to the hydrophilic nature of
the S03-Na+ groups which, if located at the H,O/polymer interface, could
lead to a stable particle with the aggregation of a relatively small number
of chains. This argument has been proposed in a recent paper on seeded
emulsion copolymerization of water soluble/nonsoluble vinyl monomers.22
547
EMULSION COPOLYMERIZATION
q..o-o
50
0
’
20
10
30
limo (minutor)
Fig. 12. Plot of reaction temperature versus reaction time for St/NaSS copolymerization
with various surfactant levels: ( -0 ) 8.0 g of NaLS, (+)
4.0 g of NaLS, (- -A-) 2.0
g of NaLS.
.
...
CONCLUSIONS
The copolymerization rate of St and NaSS in emulsion is dramatically
enhanced over that observed for the homopolymerization of styrene. Two
mechanisms, completely different in nature, are proposed to account for
these rate differences. An increase in the number of polymerizing particles
in the copolymerization with thus subsequent rate enhancement, was substantiated by electron microscopy. However, available data indicate that
the rate differences are too large to be fully accounted for by this effect.
Therefore, it seems reasonable to propose a “gel effect” over that for the
styrene homopolymerization as a significant contributor to the rate enhancements observed. This “gel effect” would be manifested by intermolecular association of incorporated metal sulfonate units in the growing
polymer particles. Terpolymerization data with acrylamide and acrylic acid,
where internal plasticization of the ionic association depresses the reaction
rate, are consistent with the “gel effect” occurring in the styreneINaSS
copolymerization.
The authors acknowledge the contribution of Dr. E. B. Prestridge for electron microscopy,
Dr. W. Schulz for HPLC, and by N. Brown for technical assistance.
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TURNER, WEISS, AND LUNDBERG
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Received June 13, 1984
Accepted July 26, 1984