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The control of molecular size in emulsion polymerized styrene Broadhead, Ronald Leslie 1947

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11*7 8Y THE CONTROL OP MOLECULAR SIZE  EMULSION POLYMERIZED STYRENE. by Ronald Leslie Brbadhead. A Thesis submitted in Partial Fulfilment of The Requirements for the Degree of MASTER OF ARTS in the Department of CHEMISTRY. The University of British Columbia A p r i l 1947. ABSTRACT. The effect of increment addition of monomer, during the emulsion polymerization of styrene, upon the molec-ular homogeneity of polystyrene has been investigated. This procedure results in a polymer containing large amounts (greater than 60%) of benzene insoluble material indicating that there is a considerable degree of cross-linking during the process. A small amount of the pro-duct (less than 20%) exhibits a f a i r degree of homogeneity with respect to the molecular weight. Low molecular weight polystyrene can be obtained by the use of mercaptan modifiers. The mercaptan appears to be carried into the reaction lo c i by the styrene during the early stages of the polymerization. Later, the effectiveness of the modifier is governed by diffus-ion through the aqueous phase. ACKNOWLEDGEMENT. I wish to acknowledge the fact that the keen interest, helpful guidance and many suggestions of the late Dr. Ure were largely instrumental in the completion of the work. I also wish to thank Dr. Clark, whose many suggestions, and discussions of polymerization were of such value in understanding the principles involved. TABLE OF CONTENTS. I. Introduction. II. THEORY. The locus of Reaction in Emulsion Polymeriz-ation. The Nature of the Persulfate Catalysis. Theory of Modifier Action in Emulsion Polymerization. Viscosity Molecular Weights. III. EXPERIMENTAL WORK. The Mutual Formula Materials. General Polymerization Procedure. Determination of Percentage Conversion. Determination of Styrene in GR - S Rubber. Determination of Molecular Weight. Fractionation of the Polymer. Variation ofMolecular Weight with Modifier Concentration. IV. DISCUSSION OF. RESULTS. V. CONCLUSIONS. VI. BIBLIOGRAPHY. 1. INTRODUCTION. It has long been a point of conjecture as to why-natural rubber has, in general such desirable properties compared to those of synthetic rubber. Many rubber chem-ist s are of the opinion that this superiority is due t o a high degree of homogeneity in the natural rubber although this point has not been definitely settled. This at once implies that the synthetic rubbers are heterogeneous or at least less homogeneous than natural rubber. That GR-S rubber, a copolymer of butadiene styrene i s heterogeneous has been well established. Stockmayer (l) states that in copolymerization the compositions as well as the lengths of the individual molecules cannot a l l be identical and even in the copolymer formed during a very small time interval a distribution of chain lengths and composition exists. Most certainly this synthetic rubber can be fractionated to give portions of f a i r l y widely varying molecular weights. In addition to this, this copolymer is not homogeneous with respect to composition or structure. The composition of butadiene and styrene in GR-S varies considerably, during the course of emulsion polymerization, from the ratio of the monomers originally charged (2). Furthermore, the monomers do not necessarily enter the growing chain of polymer in the ratio of the monomers charged. A. report from Germany (3) 2. indicates that there is a considerable bunching of the styrene in the chains of this copolymer. Consequently, there are regions in the chain which are of polybutadiene and what is more significant there are regions which are of polystyrene. With the above details in mind, i t was considered desirable to carry out research in an attempt to produce polymers and copolymers: that would be relatively more homogeneous with respect to their molecular weights. Ho consideration was given at this point to the prepar-ation of copolymers, that would be more homogeneous with respect to composition since several investigators (4) have already shown seme measure of success in overcoming this d i f f i c u l t y . Accordingly, a number of experiments were carried out in which an attempt was made to build the polymer chains in a "stepwise" fashion by the increment addition of monomer during polymerization. At the same time the molecular weights of the polymers were controlled by a mercaptan modifier. In order to reduce the number of variables to a minimum only one monomer, styrene, was used in the work. Ultimately, i t was planned, however, to apply any desirable resulting modifications to the emulsion copolymerization of 1-4 butadiene and styrene. Consequently, only variations were applied to the poly-styrene system which could feasibly be applied to the butadiene-styrene system. 4 THEORY. There are essentially two types of polyreactions leading to the formation of polymers? polycondensation processes and polymerization reactions in a more restric-ted sense. The f i r s t is a step reaction consisting of a stepwise addition, accompanied by the removal, for example of a water molecule, of monomer on polymer. Polymeriz-ation processes,* on the other hand, are processes involv-ing several elementary reactions. As a result of numbr-ous investigations, i t has been established that four elementary acts must be considered in a chain polymeriz-ation: 1. an in i t i a t i o n reaction, leading to the activation of a stable monomer molecule of the polymerizable material. 2. a growth reaction of the activated nuclei creating more or less large molecules. 3. stabilization reaction of the growing particles. 4. polymer may grow in two or more directions thus leading to branched molecules. Styrene may be polymerized in bulk, in solution or in emulsion. Emulsion polymerization, which is becoming increasingly important commercially involves the use of an aqueous solution of emulsifier, a catalyst usually an organic peroxide or an inorganic persulfate, and a modifier to control the chain length of the polymer in addition to the monomer or monomers. The emulsion type is much more d i f f i c u l t to explain by theory than the other two types, but Kolthoff and Dale ( 5 ) have shown that the polymerization of styrene in emulsion can be interpreted by S'tahdinger• s classical theory. In this theory Standinger considers that the double bond of the monomer opens up and in this form the monomer molecules are activated and react with one another through the double bonds to form a chain. In equation form this may be represented as follows GH - GH2 -GH-CHa_ G 6 H 5 C 6 H 5 -CH/CH„-\ : • * • 6 5 n Staudinger has further suggested that the end groups, on a polystyrene molecule are hydrogen and an unsaturated group. It has definitely been shown by Houtz. and Adkins ( 6 ) that large molecules of polystyrene increase in mol-ecular weight when placed in monomeric styrene under suitable conditions. This would seem to indicate that polystyrene is activated under these conditions. If the above theory holds, this would further indicate that there i s an unsaturated double bond in the polystyrene molecule and the only place that this could be, i f the above formula holds, would be in a. terminal group. The long chain polymers thus formed may ttcross-link to form three dimensional molecules. These crosslinked polymers are relatively insoluble in comparison to the straight chain polymers and in addition to this are also comparatively b r i t t l e or hard. It is desirable,;particu-l a r l y in the manufacture of synthetic rubbers, to produce polymers which are free from crosslinkage and branched chains. The Locus of Reactions in Emulsion Polymerization. According to Harkins (7) the principal locus for the initiation of polymer particles nuclei is the soap micelles. During the i n t i a l part of the reaction, at least, monomer diffuses continually from the emulsion into adjacent soap micelles where i t forms monomer layers which polymerize continually. In support of this view this investigator states that "a layer of isoprene-styrene 16 angstroms thick ( as shown by Xray measure-ments) disappeared upon heating in the presence of a catalyst which the polymer appeared outside in the aqueous phase. Upon shaking with more monomer the i n i t i a l thick-ness was restored though the amount of micellar soap had been decreased." Since styrene can be polymerized in homogeneous systems i.e., dissolved in benzene or carbon tetrachlor-ide, to give polystyrene i t is evident that styrene molecules themselves may act as activated centres for addition polymerization. This is also true in emulsion polymerization. In this case the loc i are formed in an emulsion of a monomer in an aqueous solution of the emul-sifying agent even when micelles are absent and with suff-icient agitation slowly even in the absence of any e f f i c -ient emulsifier. The monomer droplets then form a second but lesser locus than the soap micelles for the ini t i a t i o n of polymer particles. The latex particles, made up of polymer swelled by monomer, constitute, after they have been initiated and have subsequently escaped from the l o c i , the location in which nearly a l l of the polymer is formed. By diffusion the monomer of the emulsion droplets moves continually through a thin diffusion layer into the adjacent polymer monomer particles where continuous polymerization takes place. As would be expected, the more of the polymer that is formed the less is the amount of monomer in these latex particles. Both the monomer and the latex particles in an aqueous soap solution are surrounded by an adsorbed monolayer of soap molecules at a soap concentration of about-0.1 molar. In contrast to the emulsion droplets the polymer-monomer particles are much smaller in size and consequently the area on which the soap is adsorbed increased rapidly as the material of the monomer droplets 8. is transformed into monomer-polymer and f i n a l l y polymer particles. With this increase in area the soap from the micelles rapidly becomes adsorbed on the polymer particles, thus leading to the disappearance of the soap micelles. Even after a rather low conversion of monomer to polymer, the soap micelles w i l l have dis-appeared and consequently there is one principal remain-ing locus of polymerization; the monomer polymer particles. The remaining emulsion droplets of monomer may s t i l l serve as a much lesser locus. As the conversion proceeds, the emulsion droplets disappear completely since by this time a l l of the unreacted monomer has diffused into the latex particles. Therefore, at this stage of the polymerization the only locus of polymerization is the latex particles. Obviously the stage of polymerization at which the soap micelles disappear'will depend on the i n t i a l charge of soap in the polymerization'mixture. The less soap originally charged, the sooner the soap micelles w i l l disappear. With an early disappearance of the soap micelles the more predominant becomes the role of the monomer droplets in the ini t i a t i o n of the polymer nuclei. The more soap that is charged into the original mixture the greater is the number of soap micelles formed and consequently, the greater is the number of nuclei for polymer i n i t i a t i o n . Therefore, for a given yield and a given charge of monomer, the higher the soap concentration the smaller is the particle size of the polymer: a greater number of polymer particles have been formed and consequently must have a smaller molecular weight than those formed i f less soap is used. Converse-ly, without soap, polymerization w i l l result in particles which are in general very large. The Nature of the Persulfate Catalysis. According to Kolthoff and Dale (5) the potassium persulfate is actually an in i t i a t o r and not a catalyst. It is assumed that the "-catalyst", dissociated into free radicals which in turn react with the monomer. The unstable reaction products, of the free radical and the monomer initiates the chains, by reacting with more mon-omer molecules. Thus the dissociation of catalyst may be represented by the following equation: o 2 — r s e Then the formation of activated proceeds according to the equation: C+--M i — C M * Chain i n i t i a t i o n and propagation then proceeds by the following reaction: * K 2 CM tnm c^ca^ According to the classical view, termination of the active chains occurs either by reaction with other active chains or with simple free radicals: 1 0 . Rn f- R£ —=>R R where R„ and R represent " n m n m any activated molecules which may be from a growing polymer chain or may be just a simple free radical. Since the free radicals form from the •"catalyst", i t is apparent that the rate of their formation w i l l depend upon the "catalyst 1* concentration. The rate of the reaction is dependent upon the rate of formation of the free radicals and consequently upon the concentrat-ion of the catalyst. Price and Adams (8) have shown quantitatively that the rate•of polymerization of styrene in emulsion is proportional to the square root of the persulfate catalyst concentration. Theory of Modifier Action in Emulsion Polymerization. It is now accepted that the modifier probably serves as a chain transfer agent in the emulsion as well as in o i l phase polymerization ( 9 ) . It is assumed that a growing polymer radical M n acts by either of two prin-cipal reactions: as mentioned above, with monomer M by the chain growth reaction or with regulator R by the chain transfer reaction. M^tR ^3 > Mn f R*(l) giving inactive polymer M and a free radical R* derived n from the modifier. When conditions are such that nearly a l l of the inactivated polymer molecules are formed by this chain transfer, then the number of polymer molecules formed w i l l 1 1 . be just equal to the number of modifier molecules react-ed. In addition to this, the chain length of the polymer formed at a given instant is dependent upon the ratio of the rate of monomer reaction to that of modifier reaction. Expressed mathematically this becomes: M = £LS~ dr' where M is the number average molecular weight of the dm polymer formed at a given instant and dr is the ratio of grams of monomer to moles of modifier reacting. If a homogeneous system,.such as o i l phase polym-erization be considered the rate law governing the disappearance of monomer and modifier is dependent upon the relationship d ln R = kg r G (2) d ln m k g " C has been defined by Mayo as the transfer constant. In emulsion polymerization the system is much more complicated and hence i t would be expected that a more complicated equation than (2) would be required to ex-plain the relative rates of mercaptan and monomer react-ion. As previously stated the emulsion droplets of monomer serve chiefly as reservoirs for supplying the loci of the reaction with monomer and modifier. The rate of diffusion of mercaptan through' the aqueous jlayer plays: a predominant role in determining the rate of mercopton 12. reaction. The ratio of mercaptan. to monomer in the re-action locus must be less than that in the emulsion droplets i f diffusion is to take place. There must be a lack of equilibrium of modifier in the system and consequently appreciable activity gradients of modifier. The effectiveness of wa&er as a barrier to the transport of these high molecular weight mercaptans from emulsion droplet to the reaction locus is determined by the diffusion constant of modifier in the water and by the distribution coefficient of modifier between reservoir and water. Another factor important in determining the rate of transport of mercaptan through the water solution during the polymerization is the pH of the aqueous phase. Being weak acids the aliphatic /njarcaptans ionize in high pH solutions giving mercaptide ions. Since both unionized mercaptans and mercaptide ion w i l l diffuse through water, increase in pH w i l l aid in the transport of mercaptana through the water. This w i l l in turn increase the regu-lating action of the mercaptans even though the mercaptide in i t s e l f does not regulate. Harris and Kolthoff (10) state that in addition to being a modifier in the GR-S recipe, mercaptans perform the function of an activator. Without mercaptan in the mutual recipe no, or very slight, {10%) conversion of the monomers is obtained under the same conditions in 13. which 75-80$ conversion is obtained when mercaptan i s present. These authors further state that the consumpti-on of mercaptan during the polymerization occurs accord-ing to a f i r s t order reaction. Expressed quantitatively this becomes: P s -K l o g 1 0 R/ R 0 - constant in which P is the fraction of monomer converted to polymer R/ is the fraction of modifier l e f t at ^° conversion P K is a constant. The value, of K decreased with increasing modifier action. Constant equals K log R/ at P = 0 (extropo-1 0 K ° ( lated graphically) The decrease in K after 30$ conversion is thought to be due to the reaction of mercaptan with double bonds of the polymer particles, in the latex. Further i t was found that varying the pure dodecyl mercaptan concentration in the mutual recipe between 0.06 and 0.3$ resulted in a constant for the product of the molecular weight, calculated from viscosity measure-ments and the amount of mercaptan charged at 5$ conver-sions:. At higher conversions: there was a definite qualitative relationship between the viscosity molecular weight and the mercaptan concentration* High charges of modifier resulted in low molecular weight copolymer. 14. Viscosity Molecular Weights.. Staudinger ( l l ) has empirically derived a realtion-ship between the molecular weight of a polymer and i t s viscosity in solution in a suitable solvent. Such re-lationships were based on the assumption that the polymer molecules are essentially rodlike in nature. Mathematic-a l l y the relationship is represented'by the following formula M-e I x_2I£ Km Ggm in which M i s the molecular weight, Km is a constant, nsp *^ -e specific viscosity and Ggm the concentration of polymer expressed in terms of the concentration of the monomer, in moles per l i t e r . This relationship holds i f the proper value of Km is obtained experimentally. It should be emphasized thatnthis relationship has l i t t l e i f any theoretical background although Huggins (12) has show mathematically that for randomly kinked molecules n Sp there is a proportionality between Q and n the Staudinger relationship. It should not be assumed, however, that this method is of no practical value in determining molecular weights of polymers. Much experimental work has been done in comparing the molecular weights determined by this method and molecular weights obtained by other available methods such as the ultracentrifuge, osmotic 15. pressure measurements, and eryoscopic methods (13)(14) (15). It has been found that there is a f a i r degree of agreement within limits. EXPERIMENTAL WORK 16 Since i t was desired to build up gradually the molecular weight of polymers of low molecular weight by the increment addition of monomer under appropriate conditipns, some means of obtaining the^ i n i t i a l low molecular weight polymer had to be found. Two experi-mentally feasible methods were suggested from the literatures If a higher concentration of emulsifier than normal is used in the polymerization, then the molecular weight of the polymer is lower than that produced when the normal emulsifier concentration is used. , It was, recognised at once that this method would be somewhat limited by the low solubility of the emulsifier in water. The second method which appeared to be more feasible was the variation of the modifier concentration. As cited above Harris and Kolthoff (10) have clearly demonstrated for butadiene-styrene systems in emulsion polymerization that high concentrations of modifier result in low molecular weight polymers. Although the greater part of the experimentation was, carried out using styrene only as the monomer, the prim-ary purpose of this work was the application of any favorable results to the butadiene-styrene system in emulsion polymerization. Accordingly, a number of samples of butadiene-styrene copolymers was prepared in which 17. the emulsifier or the modifier were varied. The effects of these two factors on the rate of conversion of monomers to copolymer and on the composition of the copolymer were f i r s t investigated. The Mutual Formula. A l l polymerizations and copolymerizations carried out in the course of the work were based on the following mutual.formula with various modifications: D i s t i l l e d water - - 180.0 parts RRC Soap Flakes (emulsifier) 5.0 parts. 1-4 Butadiene 75.0 parts Styrene 25.0 parts Potassium Persulfate (Catalyst) 0.3000 parts DD Mercaptan (modifier) 0.42 parts. Materials. Monomers. 1-4 Butadiene. - This was obtained from the Polymer Corporation and had a purity of 98.18$. It con-tained 150 parts per million of para-tertiary butylcatechol to inhibit any polymerization before i t was used. This monomer was twice d i s t i l l e d before use under its own pressure and condensed in a pyrex gas wash bottle cooled in an acetone dry ice mixture. 18. Styrene. - This monomer was the commercial grade obtained from Dow Chemical Company and i t contained 10 p a r t s per m i l l i o n of p a r a - t e r t i a r y b u t y l c a t e c h o l i n h i b -i t o r . This m a t e r i a l was p u r i f i e d by d i s t i l l i n g under vacuum, 20 m i l l i m e t r e s of mercury a t 46°C. The p u r i f i e d styrene had a r e f r a c t i v e index of 1.5462, a t 20°C. E m u l s i f i e r . - This was al s o obtained from the Polymer Corporation. I t i s a commercial soap and c o n s i s t s of a mixture of sodium o l e a t e , sodium s t e a r a t e , and sodium m y r i a t a t e . M o d i f i e r . - This product, a l s o obtained from S a r n i a , i s a mixture of primary mercaptans (C^Q, GJ_2' ^14' ^16^ w i t h the dodecyl mercopfon being the main c o n s t i t u e n t . C a t a l y s t . - The Potassium P e r s u l f a t e was Baker's C. P. analyzed and contained 99.8$ p e r s u l f a t e . General P o l y m e r i z a t i o n Procedure. P o l y m e r i z a t i o n was c a r r i e d out i n a b o t t l e polym-e r i z e r , which turned the charged b o t t l e s end over end at 10 r e v o l u t i o n s per minute i n an e l e c t r i c a l l y heated water bath. A temperature c o n t r o l of 45.0°CF 0.5° was obtained by means of a thermoswitch i n the heater c i r c u i t . The s i x t e e n ounce b o t t l e s , f i t t e d ' w i t h metal screw caps and b u t y l rubber gaskets, were charged according to the f o l l o w i n g procedure. The soap f l a k e s , weighed 19. on the a n a l y t i c a l balance were d i s s o l v e d i n the required amount of water (.5.00 grams of soap f l a k e s i n 170 grams-of water). This was accomplished by heating the mixture. So that the c a t a l y s t would not be destroyed when i t was added the soap s o l u t i o n was cooled to 45°C before i t was charged i n t o the b o t t l e s . I t was convenient when a l l the charges were to c o n t a i n the same con c e n t r a t i o n of soap and water, to prepare a stock soap s o l u t i o n of the above mentioned p r o p o r t i o n s . 3.000 grams pottassium p e r s u l f a t e was then d i s s o l v e d i n d i s t i l l e d water a t room temperature and d i l u t e d to 100 m i l l i l i t e r s i n a volumetric f l a s k . 10.00 m i l l i l i t e r a l i q u o t s of t h i s s o l u t i o n were then p i p e t t e d i n t o each b o t t l e . While the soap s o l u t i o n and d a t a l y s t were being prepared the p u r i f i c a t i o n of the monomers was a l s o under way. A f t e r the a d d i t i o n of the c a t a l y s t the styrene was charged. The m o d i f i e r , which i s s o l u b l e i n the styrene was then added from a c a l i b r a t e d p i p e t t e . When styrene was the only monomer used the a i r was. f l u s h e d out' of the b o t t l e with" n i t r o g e n from a pressure tank, the b o t t l e was1 corked, the cork was cut o f f l e v e l w i t h the top. of the b o t t l e and the rubber l i n e d cap was screwed on.. In the cases where butadiene was used, i t was added l a s t . The b o t t l e w i t h a l l the contents except the 20. was- ta r e d . The balance, w i t h the b o t t l e s t i l l on i t , was then set f o r the c o r r e c t amount of butadiene to be added. The l i q u i d butadiene was then added u n t i l there was an excess of 2 or 3 grams. This excess was allowed to evaporate, thus f l u s h i n g the a i r out of the b o t t l e . The b o t t l e was then q u i c k l y corked and capped and placed i n the polymerizer before the soap s o l u t i o n c o uld s o l i d i f y . In the runs where butadiene was used, the f o l l o w i n g unloading procedure was adopted. The b o t t l e s were r e -moved from the polymerizer and were weighed to check f o r any l o s s of butadiene. They were opened w i t h c a u t i o n and i n order to stop the r e a d t i o n 5.0 m i l l i l i t e r s of 4.5$ aqueous s o l u t i o n of hydroquinone was added from a p i p e t t e . This reagent destroys the c a t a l y s t . The l a t e x was then t r a n s f e r r e d to a f l a s k where i t was heated under r e f l u x w i t h vigorous s t i r r i n g f o r 15 minutes a t 45°G i n order to remove unreacted butadiene. Unconverted styrene was removed from the l a t e x by steam d i s t i l l a t i o n . During the d i s t i l l a t i o n there i s considerable foaming and consequently f o r a 250 gram charge of l a t e x a 5 - l i t e r f l a s k , equipped w i t h a d i s t i l l i n g head to prevent spraying of the l a t e x i n t o the condenser was r e q u i r e d . When l e s s than 150 grams of l a t e x was being s t r i p p e d a 3 - l i t e r f l a s k was found than to be more/adequate so that no d i s t i l l i n g head was 21. r e q u i r e d . D i s t i l l a t i o n was c a r r i e d out u n t i l the o i l y styrene ceased to come over. This p o i n t i s i n d i c a t e d when the condensed d r o p l e t s no longer have a t u r b i d appearance. About 30 minutes i s re q u i r e d to remove the styrene from a 280 gram charge of butadiene-styrene l a t e x which has been run to 72-73$ conversion. The l a t e x i s u s u a l l y d i l u t e d to about one h a l f of i t s o r i g i n a l c o n c e n t r a t i o n during t h i s process. When styrene alone was used the hydroquinone s o l u t -ion was added a f t e r the b o t t l e s were removed from the polymerizer and the l a t i c e s were s t r i p p e d of styrene as described above. Determination of Percentage Conversion. In those runs where butadiene was" used, t h i s com-ponent had f i r s t to be removed before a sample could be taken. In the styrene runs, the l a t e x was ready f o r sampling a f t e r the a d d i t i o n of the hydroquinone. The t o t a l weight of the l a t e x was c a l c u l a t e d and then a 1.0 m i l l i l i t e r sample of l a t e x was t r a n s f e r r e d to a ' f l a t weighing b o t t l e which had been weighed to 0.1 m i l l i g r a m . The cover was replaced and the b o t t l e plus the l a t e x was weighed a c c u r a t e l y . The l a t e x was then d r i e d under vacuum at 60°C u n t i l a constant weight was obtained. A c o r r e c t i o n was made f o r the soap and hydro-quinone i n the sample and then from the polymer content of the l a t e x and the t o t a l weight of monomer ( or mono-22. mers) used i n making up the charge the percentage conver-s i o n was c a l c u l a t e d . Determination of Styrene i n the Copolymer. The iodine c h l o r i d e method was used and was essent-i a l l y that of Kemp and Peters (16) w i t h m o d i f i c a t i o n s by Maher and G a l l e t l y (17.) To prepare the samples f o r t h i s method, the l a t e x c o n t a i n i n g only hydroquinone shortstop, was coagulated w i t h i s o p r o p y l a l c o h o l and thoroughly washed to remove the e m u l s i f i e r . The coagulum, thus prepared, was then d r i e d under vacuum f o r 18 hours at 60°C. A f t e r d r y i n g the samples were stored i n a vacuum desidcatcDE - u n t i l r e q u i r e d . About 0.1 grams of the p u r i f i e d polymer, weighed to 0.1 m i l l i g r a m , was placed i n a 500 m i l l i l i t e r i o d i n e f l a s k w i t h 30.0 grams of reagent grade para-dichlorobenzene and the f l a s k placed on a hot p l a t e at 175 -185°C to decompose the polymer. When the polymer was completely decomposed ( a f t e r to 3 hrs.) the f l a s k was allowed to c o o l and 25 m i l l i l i t e r s of chloroform was added to l i q u i f y the m a t e r i a l before c r y s t a l l i z a t i o n became complete. To the cooled f l a s k 25.0 m i l l i l i t r e s of 0.2 normal i o d i n e - c h l o r i d e i n carbon t e t r a c h l o r i d e was added. The stopper of the f l a s k was coated w i t h a t h i n f i l m of 15$ potassium iodide s o l u t i o n and then the f l a s k 23. was stoppered. The stoppered f l a s k was stored i n the dark at room temperature f o r one hour and then 25.0 m i l l i -l i t e r s of 15$ potassium iodide was added fo l l o w e d by 50 m i l l i l i t e r s of d i s t i l l e d water. The excess i o d i n e - c h l o r i d e was t i t r a t e d immediately w i t h standard 0.1 normal sodium t h i o s u l f a t e s t a r c h being used as the i n d i c a t o r . T i t r a t i o n was continued u n t i l the s o l u t i o n turned c o l o r l e s s . Towards the end of t i t r a t i o n 25 m i l l i l i t e r s of ethanol was added to break up the emulsion that formed. Blank determinations were c a r r i e d through a l l stages of the a n a l y s i s . In the c a l c u l a t i o n s of the above mentioned method the percentage u n s a t u r a t i o n of the copolymer i s assumed to be equal to the percentage of polybutadiene. A c c o r d i n g l y , Unsaturation of Copolymer s net ml. of sodium t h i o s u l f a t e x n o r m a l i t y of " t h i o M x .1269 weight of sample The i o d i n e number of polybutadiene i s 469.6 but the authors mentioned above s t a t e that t h i s has to be reduced to 45? i n order to b r i n g the r e s u l t s i n t o agreement w i t h those of United S t a t e s Bureau of Standards. Then, Percent Styrene i n Sample r ( i o d i n e No. of polybutadiene - Iodine Ho. of sample) x 100 Iodine No. of polybutadiene In Run Number 1, a number of b o t t l e s of GR-S l a t e x were prepared according to the general p o l y m e r i z a t i o n procedure The c o n c e n t r a t i o n of the e m u l s i f i e r was v a r i e d 24. but a l l other factors including the polymerization time (of 17.5 hours.) was kept constant. As seen from Table I. there was a considerable variation of the rate of polymerization and also there was a marked change in the styrene contBnt of the copolymers produced. Table I . Run Parts Emulsifier Percent Percent Styrene Number Bottle per 100 ,parts Conversion in Copolymer. of Monomer 1 A 2.5 57.5 19.4 B 3.0 65.0 21.8 e 4.0 73.3, 22.1 D 4.5 74.5 22.1 E 5.0 76.0 22.5 P 5.5 80.0 23.0 G 6.0 86.0 23.2 H 7.5 88.9 -I 8.0 88.5 -J 9.0 89.5 23.9 K 10.0 90.0 24.0 In run Number 2" the modifier was varied in a number of different bottles that were prepared. The polymerization time was reduced to 16 hours but was kept constant throughout the series. No other variables were introduced. Prom Table II i t is seen that there was no appreciable v a r i a t i o n i n the r a t e of conversion or i n composition of the polymer when m o d i f i e r was v a r i e d . A c c o r d i n g l y , i t was decided to use the v a r i a -t i o n of mercaptan r a t h e r than the v a r i a t i o n of soap to c o n t r o l the molecular weights of the polymers. Table II. Run P a r t s M o d i f i e r Percent Percent Styrene Number B o t t l e per 100 p a r t s Conversion i n Copolymer. Monomers. A 0.17 73.2 21.9 B 0.34 73.2 22.1 C 0.68 72.0 -D 0.84 73.6 -E 1.01 73.4 -P 1.18 72.4 -Gr 1.34 73.6 -H 1.51 72.4 22.1 I 1.68 73.0 22.2 The Determination of Molecular Weights by the V i s c o s i t y  Method. As p r e v i o u s l y sta&ed the r e l a t i o n s h i p between the v i s c o s i t i e s of s o l u t i o n s of polymers and the molecular weights of polymer has been deri v e d on an e m p i r i c a l b a s i s . I t i s explained, t h e r e f o r e , a t t h i s p o i n t , that a l l of the molecular weights found i n the course of the f o l l o w i n g work are not considered to be absolute 26. values. It is thought, however, that this method did afford, a means of comparison of the relative molecular weights of the polymers produced. A l l calculations were based on the relationship M = log nr x Kdm. c where M is the molecular weight nr is the relative viscosity of a solution of the polymer compared to the viscosity of the solvent C is the concentraion of polymer in base moles per l i t e r . For polystyrene a base molar solution contains 104 grams of polymer per l i t e r of solution. Kcm is a.constant evaluated by Kemp and Peters (IS) as being 0.45 x 10^ for benzene solutions of polystyrene, at 25°C. The samples -of polystyrene were prepared from the latex by coagulation with isopropyl alcohol. The coag-ulum was thoroughly washed to remove any contaminating emulsifier and was then thoroughly dried under vacuum, at 55°C for 24 hours. It was found that if'the polymer was not thoroughly dried i t was impossible to get i t into solution. Weighed portions of the polymer were transferred to flasks andnsufficient CP.Benzene was added to make a l$solution. The a i r was flushed out of the flasks with nitrogen and the stoppered flasks were stored in the dark u n t i l a l l of the solid had dissolved. The resulting solutions were passed through a 100 mesh wire sieve to remove any gel and in this form were ready for viscosity measurements. 27. Viscosity measurements were made in Ostwald viscosimeter which, was suspended in a water bath kept at a temperature of 25 0.1°C. A 10.0 m i l l i t e r sample of liquid of which the relative viscosity was to be measured was pipetted into the viscosimeter. The same pipette was used in a l l measurements. After a sufficient time had elapsed so that the liquid had assumed the temperature of the bath, the liquid was drawn up into the arm containing the capillary and the time of outflow wad measured with a stop-watch reading to 0.1 second. Five such determinations were made and then the viscosi-meter was drained and was: thoroughly rinsed with benzene. The benzene was then drained off and the arm of the viscosimeter containing the capillary was attached to a suction line in order to dry the instrument. In order to prevent dust being drawn into the apparatus during the drying, a scintered glass funnel was fi t t e d into the otherarm of the viscosimeter by means of a rubber stopper. After the drying, a new sample of the same solution was transferred to the viscosimeter and another set of readings was taken. Between determination for different solutions the flow time of benzene was taken in order to ascertain that the instrument had been properly cleaned. 28. Since such d i l u t e s o l u t i o n s were used i t was found that there was no s i g n i f i c a n t d i f f e r e n c e between the d e n s i t y of the s o l u t i o n of the polymer and that of the s o l v e n t . Therefore, the r e l a t i v e v i s c o s i t y was evaluated as the r a t i o of the flow of time of the s o l u t i o n to,the flow of time of pure s o l v e n t . nr - t s o l u t i o n t s o l v e n t To determine the c o n c e n t r a t i o n of the polymer i n the s o l u t i o n a 10.0 m i l l i l i t e r sample was t r a n s f e r r e d by means of a p i p e t t e to a f l a t weighing b o t t l e weighed to 0.1 millgram. The solvent was allowed to evaporate and the b o t t l e was d r i e d to constant weight i n an 80°G oven. From the increase i n weight the c o n c e n t r a t i o n i s base moles per l i t e r could be calculated.' F r a c t i o n a t i o n of the Polymers. In order to determine the various molecular weight f r a c t i o n s making up the polymer, f r a c t i o n a t i o n was c a r r i e d out using a method which was e s s e n t i a l l y that of F l o r y (14). In t h i s procedure 3.0 grams of the d r i e d polymer was d i s s o l v e d i n 100 m i l l i l i t e r s of benzene. Then the p r e c i p i t a n t , d r i e d C P . methanol was added from a bur e t t e w i t h gentle s t i r r i n g u n t i l there was a s l i g h t excess over that required to produce i n c i p i e n t c l o u d i -ness at room temperature. The mixture was then heated 29. slowly to 45°C to d i s s o l v e the p r e c i p i t a t e and then was cooled to 25°C so that r e p r e c i p i t a t i o n occurred. The mixture was kept at t h i s temperature u n t i l the p r e c i p i -t a t e had s e t t l e d out. This sometimes r e q u i r e d as long as two days. The supernatant l i q u o r was decanted o f f w i t h great care so that none of the p r e c i p i t a t e got i n t o the decantate. I f the p r e c i p i t a t e was c o l o r e d i t was r e d i s s o l v e d i n the minimum amount of benzene and was subsequently r e p r e c i p i t a t e d w i t h the r e q u i r e d amount of methanol. This second treatement always r e s u l t e d i n white polymer f r a c t i o n s . Methanol was again added to the decanted l i q u o r so that a f u r t h e r p r e c i p i t a t e was obtained. The process was repeated u n i t l a l l the d i s s o l v e d polymer or a t l e a s t 90$ of i t had been .p r e c i p i t a t e d i n the v arious f r a c t i o n s . These f r a c t i o n s were then d i s s o l v e d i n benzene and the molecular weights were determined. This procedure was long and tedius and as a r e s u l t i t was found d e s i r a b l e to prepare the l a t e x b o t t l e by b o t t l e r a t h e r than as a number of b o t t l e s a l l a t once sin c e polystyrene may. f u r t h e r polymerize i f allowed to stand too long. A l l benzene s o l u t i o n s were stored i n the dark i n an atmos-phere of n i t r o g e n i n order to reduce t h i s source of e r r o r to a minimum.; 30, The V a r i a t i o n s of the Molecular Weight of Polymeric  Styrene w i t h V a r i a t i o n i n M o d i f i e r Coneentrations. I t was next d e s i r a b l e to see j u s t how the mercaptan concentration a f f e c t e d the molecular weight of the polymers. A c c o r d i n g l y a s e r i e s of three b o t t l e s of l a t e x was prepared i n which the mercaptan c o n c e n t r a t i o n was v a r i e d i n each b o t t l e . Styrene was used as the monomer and only 25 grams of i t was used i n place of the usual 100 grams, used o r d i n a r i l y i n the mutual form-u l a . The r e s u l t s i n Table I I I show that DD mercaptan i s very e f f i c i e n t i n c o n t r o l l i n g the molecular weights of polymers prepared i n emulsion p o l y m e r i z a t i o n . Table I I I . Run P a r t s of P a r t s of DD V i s c o s i t y Number B o t t l e Monomer Mercaptan Mo l e c u l a r Weights..  • 3 A 25.0 0.42# 8,000 B 25.0 0.84 5,700 C 25.0 3.36 1,100. # - normal mercaptan co n c e n t r a t i o n f o r 100 parts monomer Since the marcaptan was d i s s o l v e d i n the styrene before the s t a r t of p o l y m e r i z a t i o n , i t was thought that, perhaps, the r e l a t i v e amount of styrene present a l s o played some part upon the e f f e c t i v e c o n c e n t r a t i o n of the m o d i f i e r . Therefore, three b o t t l e s of l a t e x were prepared i n whichnthe styrene content of the charges was v a r i e d widely while the normal amount of m o d i f i e r 31. was added. A f o u r t h b o t t l e , D, was prepared i n the normal manner except that the normal amount of mercaptan was d i s s o l v e d i n 100 grams of styrene but only 25 grams of t h i s s o l u t i o n was charged i n t o the b o t t l e . This meant that the styrene contained the normal amount of m o d i f i e r but that there was a smaller amount of mercaptan than normal i n the emulsion as a whole. A l l f o u r b o t t l e s were kept i n the polymerizer u n t i l 72$ conversion had been obtained. The v i s c o s i t y average molecular weights are given i n Table IV. Table IV. Run Number B o t t l e P a r t s of Monomer Par t s of M o d i f i e r P a r t s of M o d i f i e r per 100 p a r t s of Monomer V i s c o s i t y Average Molecular Weight. 4 A 12.5 0.42 3.36 7,000 B 50.0 o.4a 0.84 16,750 C 100.0 0.42. 0.42 17,000 D 25.0 0.105 0.42 16,600 These r e s u l t s are q u i t e s i g n i f i c a n t and w i l l be discussed at some le n g t h under the heading D i s c u s s i o n of R e s u l t s . I t appeared, however, that some measure of c o n t r o l of the mercaptan concentration could beobtained by varying the monomer c o n c e n t r a t i o n . This was of p r a c t i c a l importance, since i f l a r g e amounts of mercaptan had to be charged i n order to reduce the molecular weights of the i n i t i a l polymer, considerable d i f f i c u l t y 32. would be encountered i n d i l u t i n g the emulsion so that a h i g h molecular weight polymers could form i n the l a t t e r stages of the increment a d d i t i o n of the monomer. I t was decided t h e r e f o r e , to make up a standard l a t e x , except that only 2.0 grams of styrene was charged a t the s t a r t .of.ipolymerization. When s u f f i c i e n t time had. elapsed a f u r t h e r increment of styrene was added and the a i r flushed out of the b o t t l e w i t h n i t r o g e n and p o l y m e r i z a t i o n was continued. S u f f i c i e n t time was allowed a f t e r the a d d i t i o n of the increment f o r the styrene to be completely converted to polymer and a f u r t h e r increment was added. The polymer thus prepared was coagulated, d r i e d and d i s s o l v e d i n benzene and then f r a c t i o n a t e d according to the procedure a l r e a d y described. In run number 5, the 4.0 gram increment was a l l o w -ed to polymerize f o r 2 hours, and then a f u r t h e r 8.0 gram increment was added and p o l y m e r i z a t i o n was c o n t i n u -ed f o r a f u r t h e r three hours. A t o t a l conversion of 100$ was a t t a i n e d . The r e s u l t s of the f r a c t i o n a t i o n are given i n Table V. 33. Table V. Run Fraction Percent of Viscosity Percent of Total No. Number Polymer in Molecular Polymer Insol- Polymer ; the Fraction V/eight. uble in Benzene Accounted For 5 1 5.94 22,900 11.0 90.52 2 4.16 15,600 3 54.0 13,600 4 1.93 11,200 5 5.35 5,140 6 8.14 3,920 In runs 6, 7, 8 the increment additon of styrene was carried out with the styrene being added in increments of 12 grams every two hours un t i l there was a total of 26 grams in run 6, a total of 80 grams in run 7 and a total of 100 grams in run 8. Respectively the conversions were 93.5$ 83.5$, and 90.1$. In runs 5, 6, 7, 8 i t was observed that there was a considerable amount of polymer that was insoluble in benzene. A large portion of the soluble polymer, in each case was obtained as one fraction during the treatment with methanol. These large fractions were dissolved in benzene and then methanol was added slowly but no furthernfraction-ation of these large fractions could be obtained. It was assumed from this that these fractions were relatively homogeneous. 34. Table IV. •Run Percent of Number Polymer In-soluble in Benzene. Fraction Percent of Percent of Viscosity Total Polymer in Soluble Molecular Polymer Fraction Polymer in Weight Accounted Fraction For. 1 18.2 47.0 39,700 2 12.8 .33.0 12,650 97.3$ 3 3.0 7.8 12,650 -4 1.1 2.8 6,700 5 .9 2.8 • 4,500 6 1.0 2.6 3,700 61.1 7# 68.2 l 13.5 43.5 38,200 2 3.2 10.0 • 25,700 3 0.6 1.8 90.9$ 4 0.6 1.8 10,100 5 2.0 6.1 4,040 6 3.0 9.9 3,250 8/ 63.3 1 0.8 2.4 104,000 2 22.3 61.0 60,200 98.2$ 3 9.6 26.2 42,000 4 0.3 0.9 7,200 5 1.3 3.4 5,200 6 0.6 1.7 4,140 # average viscosity molecular weight = / average viscosity.molecular weight = 53, 32, 500 000 In runs 9 and 10, .40 grama and 100 grams respective-of styrene were add ed. Polymerization of the styrene 35. was continued f o r 8'hours i n 9 and f o r 17.5 hours i n run 10. This r e s u l t e d i n a t o t a l conversion of 78$ f o r run 9 and 76$ conversion f o r run 10. The polymers obtained from run 10 were f i r s t f r a c t i o n a t e d using e x a c t l y the same , concentrations of the benzene s o l u t i o n s and the same q u a n t i t i e s of methanol used i n f r a c t i o n a t i n g run 8. The f r a c t i o n s thus obtained could be qui t e r e a d i l y f r a c t i o n -ated by r e d i s s o i v i n g the f r a c t i o n and r e p r e c i p i t a t i n g w i t h methanol. Table VII. Run Percent of Percent of V i s c o s i t y Percent of ¥o. Polymer In- F r a c t i o n Polymer i n Mol e c u l a r T o t a l Poly-s o l u b l e i n F r a c t i o n Weight mer Account-Benzene ed f o r . 9# o. 1 36.9 13,500 2 24.0 8, 760 3 6.1 5,700 97.3$ 4 16.7 5,100 5 10.0 3,780 6 3.6 2,150 10/ 0 1 58.3 18,290 2 10.2 12,720 3 5.3 9,200 96.7$ 4 8.2 7, 340 5 7.9 5,670 6 6.8 4,300 # - average v i s c o s i t y molecular weight = 8,100 / - average v i s c o s i t y molecular weight = 17,100 36. This i n d i c a t e d that there was a considerable d i f f e r e n c e i n the molecular weights of the two samples prepared w i t h the normal amount of i n g r e d i e n t s but vary-ing i n the manner cf a d d i t i o n of the monomer. 3 7 . DISCUSSION OP RESULTS. The r e s u l t s i n Table I . are i n agreement wi t h the theory of Harkins that the soap m i c e l l e s provide the i n i t i a l locus f o r p o l y m e r i z a t i o n . The higher the soap, the more m i c e l l e s that are formed and hence the g r e a t e r i s the number of centres i n i t i a t i n g the r e a c t i o n and con-sequently the greater i s the r a t e of the r e a c t i o n . The change i n styrene content i s a t t r i b u t e d to the change i n the conversion w i t h the change i n e m u l s i f i e r . Meehan(2) s t a t e s that i f the conversion i s kept constant even though there i s a change i n soap, there i s no change i n the com-p o s i t i o n of the copolymer. The r e s u l t s i n Table IV are most s i g n i f i c a n t . I t would appear a t f i r s t that the molecular weight of the polymer i s governend by the amount of styrene i n i t i a l l y charged. Since the m o d i f i e r i s s o l u b l e i n the styrene, however, i t was considered that the e f f e d t was due to an increased d i l u t i o n of t h i s i n g r e d i e n t by the monomer. This appears to be the case f o r small charges of monomer. I t i s suggested that most of the m o d i f i e r i s , when only small amount of monomer are present, c a r r i e d i n t o the a c t i v e centres d i s s o l v e d i n the s t y r e n e . When l a r g e r amounts of styrene are present, however, i n s u f f i c i e n t a c t i v e centres are a v a i l a b l e f o r a l l the styrene to be used up a t once or i n a very short time so that the d i f f u s i o n .38. through the aqueous medium now becomes the prime mode of t r a n s f e r thus r e s u l t i n g i n a. d i l u t i o n of the mercaptan p e r m i t t i n g higher molecular weights to be obtained. To f u r t h e r t e s * t h i s B o t t l e D was prepared. The concentrat-io n of mercaptan was so adjusted that the i n i t i a l mercapt-an c a r r i e d i n t o the a c t i v e centres i n the styrene was of concentration s i m i l a r to that which would be c a r r i e d i n the i n i t i a l phases of a normal p o l y m e r i z a t i o n . The r e s u l t -ing polymer had a molecular weight comparable to that obtained under normal c o n d i t i o n s . Tables V. and VI. show that a large amount of i n s o l u b l e polymer i s produced during the increment p o l y m e r i z a t i o n technique. This i s taken to mean that a d d i t i o n of monomer on to polymer molecules that a l r e a d y have been run to 100$ conversions r e s u l t s i n branched-chain molecules or c r o s s -l i n k e d molecules. A f u r t h e r explanation might be that a l a r g e amount of the mercaptan i s used up i n the i n i t i a l phases.of the p o l y m e r i z a t i o n i n t h i s procedure w i t h a r e s u l t that exceedingly h i g h molecular weight f r a c t i o n s form with the a d d i t i o n of succeeding increments. When there i s l i t t l e mercaptan l e f t to l i m i t the molecular weights. The s o l u b l e f r a c t i o n of the polymer was r e l a t -i v e l y uniform w i t h respect to molecular weight as compared w i t h runs number 9 and 10 but these polymers were t o t a l l y s o l u b l e i n benzene so that o v e r a l l they were a c t u a l l y much 3 9 . more uniform than were the polymers prepared by the increment addition method.. Ac t u a l l y increasing the mercaptan concentration should resu l t in a more homegeneous polymer since by this means the maximum molecular weight i s considerably decreased. This would, therefore, l i m i t the range over which the molecular weights would be spread and consequently there would be larger amounts of the lower molecular weight f r a c t i o n s . Commercially this is,perhaps,not f e a s i b l e since i n many cases the high molecular weight polymers are desirable. This i s why this work was carr i e d out using the monomer i n i t i a l l y to control the mercaptan concentration. It i s suggested, that perhaps, maximum control of mercaptan concentration could be obtained by the increment addition of water ( and ca t a l y s t too, to keep this concentration constant) along with the increment of styrene. This would d e f i n i t e l y control the mercaptan concentration in the aqueous phase and with increasing d i l u t i o n s , the mercaptan which would have been added in concentrations much higher than normally used, would be s u f f i c i e n t l y d iluted to allow high molecular weight polymers to form. CONCLUSIONS. 40. The increment a d d i t i o n of sty r e n e d u r i n g the emulsion p o l y m e r i z a t i o n of t h i s monomer r e s u l t s i n polymers con-t a i n i n g l a r g e amounts of benzene i n s o l u b l e m a t e r i a l . At the s t a r t of the. p o l y m e r i z a t i o n , the m o d i f i e r appears to be c a r r i e d i n t o the r e a c t i o n c e n t r e s i n the monomer but the d i f f u s i o n through the aqueous phase soon becomes the primary means of t r a n s p o r t i n g the mercaptan. BIBLIOGRAPHY 41. Stockmayer, W.H. J o u r n a l Chemical P h y s i o l 13, 199 (1945) Meehan, E. J . - Composition of Butadiene-Styrene Copolymer Prepared by Emulsion P o l y m e r i z a t i o n -Jo u r n a l of Polymer Science, 1, 318 (1946) Rubber Subcommittee M i s s i o n J o i n t I n t e l l i g e n c e , Object-ives Agency - The Status of S y n t h e t i c Rubber and Polymer E v a l u a t i o n i n Germany, Report No. CR - 907, CD- 540. Morton M.-, and N i c h o l l s R.V.V - "Preparation of Polymer Having High Degree, of Molecular and Composition-a l Homogeneity',' Report number QSR-32 PR-1-3 Kolthoss and Dale - The Mechanism of Emulsion Polymer-i z a t i o n s I The E f f e c t of P e r s u l f a t e Concentrat-ion on the Emulsion P o l y m e r i z a t i o n of Styrene -Journal of the American Chemical S o c i e t y , 1672 67, (1945) Houtz and Adkins - J o u r n a l of the American Chemical . S o c i e t y 1609, 55 (1933) Harkins W. D. J o u r n a l of Chemical Physics 381, 13(1945) P r i c e C.C. and Adams C.E. Journal of the American Chemical S o c i e t y , 1674,67(1945) Smith W. V. - J o u r n a l of the American Chemical S o c i e t y 2059, 68 (1946) H a r r i s W. E. and K o l t h o f f I . M. "A Comprehensive Review of M o d i f i c a t i o n , Disappearance Curves and A c t i v a t i o n by Mercaptan i n the GS-S r e c i p e . " Report RRC No. CR-626. •Staudinger "Die hochmolekularen organischen Yerbindugen" J . Springer, B e r l i n (1932). Huggins M. L. Jou r n a l of P h y s i c a l Chemistry 911, 42, (1938). Kraemer E. 0 and Lansing W. D. -Journal of P h y s i c a l Chemistry, 153, 39 (1935). (14) F l o r y F. J . - Jo u r n a l of the American Chemical S o c i e t y 372, 65, (1943). (15) Kemp A. :R. and Peters H. - I n d u s t r i a l and Engineering " Chemistry, 1097,. 34,(1942) (16) Kemp A. R. and Peters H. - I n d u s t r i a l and Engineering Chemistry, A n a l y t i c a l E d i t i o n 15, 453,(1943) (17) Maher E. D. and G a l l e t l y B. - "The Halogen A d d i t i o n Method f o r the Determination of Combined Styrene i n GR-S Report No. SSR-13 CSRL -13 

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