Compositional Heterogeneity of Copolymers EFFECT ON VISCOSITY AND FRACTIONATION STUDIES D. B. MACLEANI, MAURICE MORTOY, AND R. V. V. NJCHOLLS .McGill b'niversity, Montreal, Canada
A method is
described for the preparation of copolymers having a relatively narrow range of molecular weight and being relatively homogeneous as to comonomer composition of the chains. By means of this method a series of GR-S-type copolymers was prepared in w-hich the molecular weight was kept constant, but the butadienestyrene ratio was varied. It was found that the effect of a greater styrene content in the polymer chain is to increase the solubility of such chains in benzene-methanol systems, and to decrease the intrinsic viscosity of benzene solutions of the copolymer.
where X K is the kinetic molecular weight, IT' is the weight oi thrpolymer formed, and T is the numbrr of moles of chain transfrr agent consumed. It is obvious that M K coincidrs with thcnumber average molecular weight, in the absence of side reactions. I n emulsion pol) merizations, as exemplified by the GR-S system, thc owl-all rate of polymerization is constant over the greater part of the rpaction (8),while the rate of mercaptan disappearance is first, order. This leads to the following expression, relating the conwmption of mercaptan to the rate of formation of polymer. dm/dP
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N ADDITIOPr' to the usual characteristics of macromolecules,
copolymers present one unique variable-viz., the comonomer composition of the chain. The work of Wall ( I I ) , Mayo and Lewis (6),Alfrey and Goldfinger (I), and others has made i t possible to calculate the variation of such compositional changes during the course of a batch polymerization of two comonomers. Such determinations have been made for several different copolymer systems. I n the case of the GR-S system as in most others i t has been found that the composition of the polymer chains varies continuously during the polymerization reaction, inevitably leading t o a heterogeneity in the comonomer content of the macromolecules, especially at higher conversions. These compositional heterogeneities present a serious problem in the evaluation of the molecular characteristics of copolymers. Such widespread methods as solubility fractionation or dilute solution viscosity, which are generally accepted as a means of evaluating chain length, may be radically affected by simultaneous changes in polymer composition. This difficulty was foreseen by Baker and Heiss (8) in the fractionation studies of GR-S polymers, and i t has been shown that such separations are sensitive to compositional differences. I n view of these limitations imposed on analytical methods of distinguishing between molecular size and molecular composition, a synthetic method is proposed whereby one variable, that of molecular size, can be minimized so that the effect of compositional changes may be studied independently. This method involves the following steps:
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Figure 1.
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(1)
Present address, Dominion Rubber Company, Guelph, Ontario, Can. address, University of Akron, Akron, Ohio.
* Present
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The role of mercaptans as chain transfer agents in polymerization is well known (9, 10, I d ) . Where such chain transfer is the . predominant mechanism of chain termination i t is possible to calculate the usual kinetic chain length b y means of the simple relation =
-km
IO1
1. Polymerization in the presence of a suitable regulator of chain length, specifically a tertiary hexadecyl mercaptan. 2. Production of polymers a t low conversions, specifically, less than 10%. 3. Variation of initial charging ratio of comonomers.
MK
=
where m is meicaptan concentlation; P , weight of polymri formed; and k , rate constant for mercaptan disappearance relative to polymer formation. It is obvious that, for these systems, the degree of polymerization must undergo an increase during the progress of the reaction, the magnitude of this increase being a function of IC. By using a mercaptan of low reactivity, the change in molecular weight with conversion may be kept at a minimum, leading to t h e formation of a polymer having a higher degree of homogeneity with regard to chain length than usually encountered. T h e higher mercaptans, such as tertiary hexadecyl mercaptan, have been found suitable for this purpose (8). This mercaptan is also known to be relatively free of side reactions, due to its low reactivity, and hence may be assumed to behave purely as a chain tiansfer agent-i.e., in such a way that one molecule of regulator will be consumed for every molecule of polymer formed. Harris and Kolthoff (3) have demonstrated remarkably good agreement between molecular weights calculated from mercaptan consump-
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Rate of Emulsion Polymerizationof ButadieneStyrene Systems
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August 1949
tion data and those obtained osmometrically, when tertiary hexadecyl mercaptan was used in the GR-S system. By restricting the polymerization t o low conversions, any changes in molecular weight are kept a t a minimum, while the composition heterogeneity can be reduced t o a negligible value. I n order to prepare copolymers of the desired molecular weight or degree of polymerization, it is necessary first to have the following data available:
1. Rate of polymerization with the various charge ratios of comonomers which are to be used. 2. Rate of mercaptan consumption with the various charge ratios of the comonomers. 3. Combining ratios of the comonomers. The above data are required sincethe ratioof comonomersaffects both the rate of polymerization and the k value of the mercaptan. By limiting all polymerizations t o a fixed low conversion of, say lo%, and varying the charge ratios, i t becomes possible to calculate the initial concentration of mercaptan required t o produce polymer of the same average molecular weight or degree of polymerization for all cases. As for the monomer composition of the chain, this will be a function both of the charge ratio and the conversion, and can be calculated by means of the well-known equation of Mayo and Lewis (6)
S US
dS
a3
=
B' p
+B B
(3)
S
This is the equation which relates the instantaneous composition of polymer formed with the instantanedus concentrations of the two monomers, S and B,and the monomer reactivity ratios, u and M.
with the soap and mercaptan used. The soap was obtained from the Rubber Reserve Company and was specially prepared for purposes of emulsion polymerization. It consisted of a mixture of saturated fatty acids. The mercaptan used was the main fraction obtained from a sample of tertiary hexadecyl mercaptan (Sharples Chemicals Inc., Philadelphia, Pa.) and had the following constants: Roiling point (1 mm.)
ng
105-115" C. 1.4749-1.4765
Mercaptan content (as CiaHsaSH)
97.0%
For mercaptan analysis by the amperometric method of Kolthoff and Harris (6),the contents of each bottle were first coagulated by directing a thin stream of unvented latex into 400 ml. of rapidly stirred absolute ethanol. For this purpose a special bottle cap, fitted with a capillary stopcock (7), was used. To ensure the complete removal of mercaptan from the bottle, the empty bottles were rinsed with a small amount of alcohol. Determinations of residual mercaptan were carried out on duplicate bototles, immediately after the bottles were removed from the 50 C. water bath and chilled a t 0" C. for one half hour. No shortstop was added to the latex, to avoid interfering effects. The data on rate of polymerization were obtained by coagulating the total bottle contents in 1-butanol and drying in a vacuum oven'at 400 C. for 24 hours. Separate bottles were used for these determinations, because the polymer samples from the mercaptan determination were contaminated, and could not be expected to yield accurate conversion data. However, the same polymer samples from the conversion determination were used for viscosity measurements and fractionation. A complete description of the techniques used for the latter determinations can be found elsewhere (8). I n the polymerization of styrene, it was found necessary to remove oxygen, because it gave rise to an induction period of several hours. This removal was accomplished by sweeping each bottle with nitrogen, after loading, using a hypodermic needle inserted through a hole in the metal screw cap and butyl rubber gasket. I n this way a reproducible rate curve was obtained.
EXPERIMENTAL
The techniques used in this investigation were the same as previously described (8),except that the polymerization was carried out in 1-ounce bottles instead of the usual Counce bottles. In this way, the complete contents of the bottle, amounting to 22.5 grams, were used for analytical determinations, thus avoiding sampling difficulties which are encountered especially at low conversions. The polymerization recipe is reproduced below. The bottles were rotated end-over-end a t 35 r.p.m. in a water bath a t 50 * 0.1 C. O
Polymerization Recipe (Parts by weight) Water Butadiene (yariable) Styrene (variable) Soap flakes Potassium persulfate Mercaptan
1
180 100 6 0: 3 Variable
Details concerning type and purity of materials may be found elsewhere (8). The only additional description required deals O"
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RESULTS
EFFECT OF MONOMER CHARGERATIOON RATEOF POLYMERIMERCAPTAN DISAPPEARANCE. Figures 1 and 2 show
ZATION AND
the effect of changhg butadiene-styrene charge ratios on the rate of polymerization. It is interesting to note the linearity of the curves in Figure 1 up t o conversions of about 70%, a fact which seems to be characteristic of this emulsion system and is independent of charge ratio of comonomers. Figure 2 offers good evidence of true copolymerization, since it is not linear, as would be the case for independent polymerization of the two monomers. The effect of varying charge ratios on the rate of mercaptan consumption is illustrated in Figures 3 and 4. As might be ex-02 pected, t h e amount of mercaptan consumed per gram u. of polymer deP O L Y BU T A D IE N E U creases with in551 0 I t I 5 creasing styrene W content, owing t o t h e higher molecular weight of the styrene. Hence it can be found that the actual rate of mercapB U T A D I E N E /S r Y R E N C 7 5 / 2 5 tan disappearance per mole of 20 40 60 80 100 PO L Y M E R 1 Z A T I ON monomer does not vary greatly Figure 3. Rate of Mercaptan Diswith charge appearance in Butadiene-Styrene ratio. These Systems L
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Figure 2.
20
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40 60 WEIGHT %STYRENE
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Effect of Charge Ratio on Rate Polymerization
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