Allyl Polymerization Beyond the Gel Point H O W A R D W. S T A R K W E A T H E R , J R . ' , A N D F R E D E R I C K R . ElRlCH INSTITUTE O F POLYMER RESEARCH, POLYTECHNIC INSTITUTE OF BROOKLYN. BROOKLYN N . Y .
T h e u n i t refraction was used t o follow t h e course of t h e polymerization of diethylene glycol bis(ally1 carbonate) beyond t h e gel point. T h e u l t i m a t e conversion was estimated t o be about 70% and independent of t h e a m o u n t of initiator used. T h e polymerization consists of a rapid stage lasting about 10 hours followed by a much slower stage which may continue for several days. Both t h e ultimate conversion and t h e rate of polymerization are greater t h a n was expected f r o m experience w i t h allyl monomers i n t h e liquid state. These effects point t o a decline in t h e importance of degradative chain transfer as a kinetic-chain terminating step beyond t h e gel point.
T
HE polymerization of allyl acetate a t 80' C. initiated by ben-
zoyl peroxide was studied by Bartlett and Altschul (1). This work was extended to a series of monoallyl compounds by Gaylord and Eirich ( 2 ) . Kamath and Eirich (3) studied the polymerization of several diallyl monomers to the gel point. In the polymerization of diallyl compounds at 80" C., gelation occurs when 10 to 15% of the double bonds have reacted. In the fabrication of solid resins from diallyl monomers, the part of the polymerization beyond the gel point is particularly consequential. As the polymerization proceeds, the structure is increasingly tightly cross linked and strained and becomes resistant t o chemical attack. Therefore, a physical method to follow the polymerization within the gel becomes desirable. Experimental Methods
Polymerization Technique. Samples for unit refraction measurements were polymerized in small glass tubes sealed with nitrogen a t l mm. mercury. The average weight of these samples was one gram. During polymerization, the samples were immersed in an oil bath a t 80 i 0.1' C. At intervals, samples were removed from the oil bath and plunged into a dry iceacetone bath to stop polymerization. The tubes were then opened, and the samples were allowed to come to room temperature. Density Measurements. The densities of the samples were determined by the displacement of water in a pycnometer a t 25' C. Refractive Index Measurements. The cylindrical samples were allowed to float on mixtures of organic liquids at 25' C. The composition of the mixture was adjusted to the point at which light passed through the cylindrical sample without being bent, indicating that the sample and the liquid had the same refractive index. The refractive index of the liquid was then measured in an Abbe refractometer. The samples having estimated conversions below 50y0 had some tendency to swell in the liquid mixture. Any uncertainty 1 Present addresa, Polychemioals Department, E. I. du Pont de Nemours & Co., Wilmington, Del.
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could be minimized by making the measurement as quickly as possible. The samples having high conversions seemed to be unaffected by the liquid mixtures. For these samples, the measurements were entirely reproducible by approaching from either higher or lower refractive indices. Mixtures of carbon tetrachloride and bromobenzene were used for most of this work. The refractive index of the diallyl phthalate resins was higher than that of bromobenzene, and mixtures of bromobenzene and carbon disulfide were used in these cases Chemicals. Diethylene glycol bis(ally1 carbonate) was obtained from the Columbia-Southern Chemical Corp. It was distilled a t a pressure of 1 mm. mercury using 2,5-di-tert-hutyl hydroquinone as an inhibitor. Diallyl phthalate from the Shell Chemical Corp. was distilled with the same inhibitor. Methallyl lactate allyl carbonate, ethylene glycol bis( allyl carbonate), and triethylene glycol bis( allyl carbonate) were prepared according to procedures given by Kamath ( 3 ) . Triallyl cyanurate was obtained through the courtesy of the American Cyanamid Co. It was recrystallized twice from its own melt. Benzoyl peroxide was obtained from the Matheson Co.; it was recrystallized from mixtures of chloroform and methanol. Experimental Results
The unit refraction is defined by the Lorenz-Lorentz equation. n a - 1 .M [R] = na+2
P
n = refractive index = molecular weight of monomer p = density
M
The unit refraction may be predicted from the sum of the refraction equivalents for the various atoms and structural factors such as double bonds. Wiley (7) surveyed the unit refractions of 26 polymers. I n most cases the experimental and predicted unit refractions differed by not more than a few 0.1%. A few polymers containing
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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HIGH POLYMER ENGINEERING 65
~~
Table I . Unit Refraction of Partially Polymerized Diethylene Glycol Bis(Ally1 Carbonate) (Polymerized with 3% benzoyl peroxide) 64
-
Density 1 153 (initial value) 1.157 1,169 1,195 1.233 1.274 1.305 1.315 1.318 1.320 1.319
63
.-I: v
I P
61
60
PO
0
40
60
Conversion,
Figure 1.
80
100
%
U n i t refraction of diethylene glycol bis(ally1 carbonate) versus per cent conversion
----0
Predicted from refraction equivalents I n i t i a l value w i t h o u t initiator
polar groups capable of producing strong interchain forces had unit refractions from 1 to 4% below the predicted values. Diethylene Glycol Bis(Ally1 Carbonate). Strain (6) has reported measurements of the density and refractive index of diethylene glycol bis(ally1 carbonate), known commercially as CR-39, polymerized to various conversions. These conversions were determined by hydrolyzing the partially polymerized composition and titrating the unsaturation of the allyl alcohol liberated. The Strain measurements which are unpublished are given in Table I, together with the unit refractions calculated from them. The unit refraction is plotted against the percentage conversion in Figure 1. The predicted unit refractions for monomer and polymer are indicated on the graph. The points below 70% conversion lie on a strajght line starting a t the predicted unit refraction for monomer and extrapolating to a value 1.8% below the unit refraction for polymer predicted from the 64 atomic refraction equivalents. The e x p e r i m e n t a l unit refraction for poly(methy1 methacrylate) is 2.1 % below the predicted value. Since these two polymers have about the 8ame heat distortion temperature, and since apparently homogeneous copolymers can be formed in all proportions, it may be concluded that the interchain forces are comparable. The samples above 70% conversion on the @aph were probably incompletely hydrolyzed. I n that case, not all the remaining unsaturation would be titrated, and the indicated conversion would be too high. I n the high conversion range, the unit refraction method December 1955
Refractive Index 1.453
Unit Refraction 64.23
Conrersion,
1.458 1.460 1.467 1.477 1.489 1,498 1, 5 0 2 1.502 1 ,503 1.503
64.62 64.20 63.62 62.79 62.07 61.54 61.49 61.35 61.36 61.41
10 15 28 42 62 79 88 91 92 93
% 6
for estimating the conversion should, therefore, be more reliable than the direct chemical approach. The predicted unit refraction for monomer-free polymer corresponds to a point of conversion of 74% on the experimental curve. Several samples had unit refractions below this value (61.52), but no sample had a unit refraction below the value obtained by extrapolating the experimental line to 100% conversion (60.38). The estimated conversions reported in this work were taken from this line. Loshaek and Fox ( 4 ) have reported that for a large number of linear vinyl ester polymers, the shrinkage during polymerization is 23.0 cc. for each mole of double bonds reacted. According to this empirical rule, an increase in density from an initial value of 1.145 grams per cc. to 1.320 grams per cc. (Tables I and 11) when the polymerization has stopped corresponds to the disappearance of 69% of the double bonds, in good agreement with the ultimate conversions estimated from the unit refraction. Freshly distilled diethylene glycol bis(ally1 carbonate) was polymerized a t 80" C. under a nitrogen atmosphere a t a reduced pressure of 1 mm. mercury with 2, 3, and 4% benzoyl peroxide to simulate practical casting in an enclosed space. The results are shown in Table 11, and the unit refraction is plotted as a function of time in Figure 2. I n each case, the polymerization consisted of a rapid stage lasting for about 10 hours, followed by a much slower stage which continued for several days. The initial rate of polymerization
INDUSTRIAL AND ENGINEERING CHEMISTRY
0
PO
n 60
c'
8.
$
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ENGINEERING, DESIGN, AND EQUIPMENT carbonate) with triallyl cyanurate possess outstanding strength a t all stages of the polymerization. A 9O:lO mixture by weight of these 64 two monomers was polymerized with 3% benzoyl peroxide a t 80" C. I n calculating the unit refraction of these samples, an average molecular weight, based on the mole fractions of the two monomers was used. The same procedure wa8 used to calculate the predicted unit refractions for pure monomer mixture and pure copolymer. It wa8 assumed that the unit refraction of the polymer would be 1.8% below the predicted J value. 'The results of this experi1000 PO00 3000 4000 ment, which are given in Table IV, Polymerization Time, Min. show that an ultimate conversion of about 76% was attained in about Figure 3. Polymerization of diethylene glycol bis(ally1 carbonate) after 18 hours under I m m . nitrogen a t 80' C. 8 hours. Several other diallyl monomers 0 2% Benzoyl peroxlde 0 4% Benzoyl peroxide were polymerized a t 80" C. for 72 hours. The results of these experiments are given in Table V. The ultimate conversion in the polymerization of diallyl phthalate Table I I. Polymerization of Diethylene Glycol Bis(Allyl was about 60% and was roughly independent of -the amount Carbonate) with Benzoyl Peroxide a t 80' C. of initiator used. Diethvlene dvcol bidallvl carbonate) and its -" Estimated two homologs all had about the same ultimate conversion. ConverJion, Refractive Unit Density, Time,
I
. "
Min.
0
105 150 195 445 845 1310 2265 4205
Grams/&. 1.143 1.171 1.195 1.201 1.257 1.274 1.289 1.300 1.315
Index
2 % Benzoyl Peroxide 1.452 1 ,463 1.468 1 ,469 1.491 1.497 1.501 1.503 1.504
Refraction
%
64.63 64.42 63.76 63.52 63.13 62.95 62.64 62.31 61.68
6
10 24 30 39 42 50 57 71
64,48 64,76 63.59 62.96 62.32 62.09 61.95
0 3 28 42 57 62 64
Discussion
3% Benzoyl Peroxide
i.302
0 100
210 385 505 1575 2265
1.312
1 50: 1 50:
1.145 1.202 1.250 1.286 1.304 1.309 1.312
4 % Benzoyl Peroxide 1.454 1.480 1.493 1.503 1.505 1.506 1.505
was slower with 2% benzoyl peroxide, but after 3 days the conversion was independent of the amount of initiator used. When the monomer was heated for 18 hours a t 80" C. under 1 mm. nitrogen before the addition of the initiator, a much more rapid polymerization was observed. Within the limits of experimental uncertainty, the course of the reaction was the same with 2 and 4% benzoyl peroxide. After about 10 hours, the conversion assumed an approximately constant value of 70% (Table I11 and Figure 3). Since the viscosity of the monomer increased only 3% during this preliminary heat treatment, not more than 1% polymer could have been formed. Moreover, the 'time required to obtain gelation a t 80' C. was unchanged, even when the oxygen was bubbled through the system. This was 52 minutes with 2% benzoyl peroxide and 32 t o 33 minutes with 4Q/, benzoyl peroxide. Polymerization of Other Polyallyl Monomers. It has been reported earlier (6) that copolymers of diethylene glycol bis(ally1
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The polymerization of diethylene glycol bis(ally1 carbonate) a t 80' C. initiated by benzoyl peroxide proceeds rapidly for several hours and then more slowly until a maxamum of about 70% of the double bonds have reacted. Such a limit was expected a t which virtually all the reactive groups are attached to the network so as to be unable to diffuse and to come in contact with each other. In the polymerization of monoallyl compounds (1, a), the ratio between the rate of disappearance of double bonds and the rate of initiator decomposition, dM/dC, is a constant throughout the reaction, and the amount of polymer formed is proportional t o the amount of initiator used. For a diallyl monomer, the ultimate conversion should be
where COand M Oare the initial concentrations of benzoyl peroxide and double bonds. For diethylene glycol bis( allyl carbonate), where dM/dC = 29.9 before the gel point, this indicates that maximum conversions of 34% with 2% initiator, 52% with 3% initiator, and 7001, with 4% initiator would have been expected. Thus the experimentally observed ultimate conversion is higher than would be predicted from the experience with monoallyl compounds. It indicates that degradative chain transfer, the termination of both the physical and the kinetic chain by the abstraction of a hydrogen a- t o a double bond to form a stable allylic radical, is less prevalent in the polymerization of diallyl compounds beyond the gel point than in the pol3.merization of monoallyl compounds. This is equal to assuming that an allylic radical is more apt t o add t o a double bond when both are attached to a cross-linked network and are thereby constrained to same locality. It follows that there is a progressive shift from degradative chain transfer t o effective chain transfer throughout the course of polymerization. The work of Kamath and Eirich ( 3 )shows that in the polymerization of a diallyl compound, unlike
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
HIGH POLYMER ENGINEERING
Table I I I .
Polymerization of Monomer Preheated for 18 Hours a t 80" C. Under 1 Mm. Nitrogen
Time, Min.
Density, Grams/Cc.
Refraotive Index
Unit Refraotion
Estimated Conversion,
%
2% Benzoyl Peroxide
throughout the system. Thus with 2y0 benzoyl peroxide, the initial rate of polymerization is slower and the final structure is weaker than when 3 to 4y0initiator is used. Heating the monomer prior to the addition of the initiator increases the rate of polymerization but does not change the time required for gelation to occur. I t may be that this process renioves subst a w e s which initially compete with monomer iii reacting with allylic free radicals. Literature Cited (1) Bartlett, P. D., and Altschul, R., J . Am. Chem. SOC.,67, 812,816
4% Benzoyl Peroxide
(1945). (2) Gaylord, N. G . , and Eirich, F. R., Ibid., 74,334,337 (1952).
Table IV. Copolymerization of Diethylene Glycol Bis(Allyl Carbonate) with 10% Triallyl Cyanurate Using 3y0 Benzoyl Peroxide Time, Min.
Table V.
Density, Grams/Cc.
Refractive Index
Unit Refraction
Estimated Conversion,
%
(3) Kamath, P.XI., and Eirich, F. R., to be published. (4) Loshaek, S., and Fox, T. G., Jr., J . Am. Chem. SOC.,7 5 , 3544 (1953). ( 5 ) Starkweather, H. W., Jr., Adicoff, A , , and Eirich, F. R., IND. ENG.CHEM.,47, 302 (1955). (6) Strain, F., Columbia-Southern Chemical Gorp., private communication, 1954. (7) Wiley, R. H., IND.ENG.CHEM.,38, 959 (1946). RECEIVED for review June 11, 1955. ACCEPTED October 15, 1955. Part of a research program carried out with the support of the Bureau of Aeronautics, Department of the Navy. Abstracted from the dissertation presented by Howard W. Starkweather, Jr., to the Graduate Faculty of the Polytechnic Institute of Brooklyn in partial fulfillment of the requirements lor the degree of doctor of philosophy.
Polymerization of Alloy Monomers a t 80" C. for 72 Hours
Monomer Diallyl phthalate Diallyl phthalate Methallyl lactate allyl carbonate Ethylene glycol bii(ally1 carbonate) Triethylene glycol bia (allylbonate)
Benaoyl Peroxide, 1 3
Density, Grams/Cc. 1.269 1.275
3
1.209
1.496
54.18
59
3
1.347
1.505
50.65
73
3
1.294
1.501
72.42
70
%
Refractive Unit Index Refraction 1.570 63.58 1.572 63.48
Estimated Copversum,
% 59 64
most monoallyl compounds, there is more than one physical chain per kinetic chain. Therefore, effective chain transfer plays a part in diallyl polymerization even before the microscopic gel point. Beyond the gel point, a polyallyl molecule of a much higher order is being dealt with and this effect may be even more pronounced. It is safe to assume that the allylic radicals which are formed in degradative chain transfer combine more rapidly than they add to double bonds. If, however, the allylic radicals are attached to a network, the probability that they will combine is severely reduced and addition to double bonds becomes relatively more favorable. If degradative chain transfer, which is the principal kinetic chain terminating mechanism in the polymerization of monoallyl compounds, decreases as the molecular complexity of the polymer increases, the rate of polymerization will increase locally where cross linking has gone the fartherest. This will lead to uneven contraction during the polymerization and an uneven distribution of bonds in the final resin, thereby weakening it. The relative importance of these effects would be greatest when the amount of initiator is small. With increased amount of initiator, the polymerization would proceed more uniformly December 1955
COURTESY
B. F.
OOODRICH CO
Some of t h e many control instruments needed for pro. duction of vinyl chloride monomer
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