Aug., I946
MODIFIERACTIONIN GR-S POLYMERIZATION [CONTRIBUTION FROM
THE
1429
KOYESLABORATORY, UNIVERSITY OF ILLINOIS]
The Mechanism of Modifier Action in GR-S Polymerization. 111 BY F. T. WALI,, F. 'CV. BANESAND G. D. SANDS In the preceding paper2 there has been discussed the theory of modifier action in GR-S polymerization. Briefly the theory assumes that certain compounds, such as mercaptans, can act as transfer agents3 for growing free radical chains. The net effect of this action is to reduce the molecular weight of the polymer and to keep i t more soluble. Assuming that chain transfer occurs much more frequently than initiation and termination, i t follows that practically every polymer molecule will have incorporated in i t one modifier fragment. Experimental support for the theory was obtained by studies on various different polymer systems,2 which, however, did not include direct measurements o n commercial GR-S. To provide further evidence for the chain transfer-free rS.dical mechanism of polymerizationI3 a number of careful molecular weight measurements and sulfur determinations were carried out on $certain portions of GR-S samples. The purpose of these measurements was to find the average number of sulfur atoms per molecule of polymer. If the number found was close t o unity, then that would provide further support for the above-mentioned theory. Several difficulties were encountered in the determination of molecular weights and sulfur contents of GI.:-S. In the first place, GR-S is highly heterogeneous so it was found desirable to fractionate the material before making the measurements. Secondly, the very low sulfur content in the fractions of interest necessitated the development of special techniques for the quantitative determination of those small amounts of sulfur. Fina.ily, the molecular weights had to be determined osmotically since only number average molecular weights would serve the purpose in mind. Since an appreciable fraction of each of the GRS samples investigated was insoluble in benzene and similar solvents, the gel fraction had to be .removed if nieaiiingful molecular weights were to he obtained. In addition the very low molecular bveight materia:, present in GR-S also had to be removed to make possible significant osmotic pressure measurements on the bulk of polymer. If the low molecular weight material were not removed, the membranes used would have to be so impermeable that osmotic equilibrium could not be attained in :a reasonable length of time. Accordingly, the fraction investigated was taken as that portion which was soluble in benzene but in(1) This investigation was carried out under t h e sponsorship of the Office of Rubber Reserve, Reconstruction Finance Corporation in connection with t h e C:overnment Synthetic Rubber Program. ('21 Snyder, Stenarl., Allen and Dearborn, THISJOUKBAI., 67, 1,122 (1945). 1.31 I'Iory. ; b i d , 159, 241 (lC137).
soluble in a mixture consisting of equal volumes of benzene and methanol.
Experimental Procedure Considerable care had t o be exercised in preparing the polymer f o r osmotic measurements. In particular the effects of heat, light, and long drying had t o be avoided. The best results were obtained when the following procedure was adopted. About 10 g. of GR-S was extracted in the dark a t room temperature by cycling acetone through the sample. The extracted sample was dried for nine hours in a vacuum desiccator and then placed in a large Erlenmeyer flask and covered with two liters of C.P. benzene. The polymer was permitted to dissolve by allowing the system t o stand for three and one-half days a t room temperature. The gel was removed by filtering through an 80-mesh Muriel metal screen covered with glass wool. After removal of the gel, the polymer was subjected to partial fractional precipitation by adding the solution to an equal volume of methyl alcohol. The resulting polymer suspension was then coagulated with a small amount of sodium chloride. Following coagulation the sample was washed with water and with alcohol, after which i t was dried in the dark in a vacuum desiccator for nineteen hours. About 1 g. of this material was dissolved in 200 nil. of C.P. distilled benzene, and portions of this solution were diluted for osmotic pressure measurements. The osmotic pressures were measured by means of glass osmometers4 immersed in a thermostatically controlled water-bath. The membranes employed were made of no. 300 non-moisture proof cellophane which was treated with GOc;', zinc chloride solution aecordirig to the method of McBain and Stevens5 for the purpose of swelling the material to the proper permeability. The nature of the membrane depended a great deal on the concentration of the solution in which it was swollen, and many of the membranes had t o be discarded because of the improper permeability characteristics. The membranes were sealed to the osmometers by means of polyvinyl alcohol and were kept soft during storage by having them immersed in benzene containing a trace of triethylerie glycol and water. The height of the column of solution above t h a t of the solvent was measured by means of a cathetotcerer. To make sure that osmotic pressures were really being mcasured, the level of the solution in the capillary tube was initially set a t a point below the equilibrium level, adjustments being made if necessary after observing the early effects. (The system was, of course, allowed t o reach temperature equilibrium before any osmotic tendencies were observed.) The level of the sdution was ther. ohserved as a function of time by means of periodjr cathetometer readings. The level would always reach a niasimum in the course of several hours, after which it mould fall off slowly as a result of diffusion of solute through the membrane. The time required t o reach the maximum clepended upon the initial difference in heights and upon the porosity of the membrane. When approximate information was available on certain samples, the measuremtints could be expedited by starting the system near equilibrium. If the falling off of height after the maximum was reached proved to be very rapid, the measurement was discarded. n'hen the rate of diffusion of solute through the membrane was slow, a correction for that diffusion could be applied by extrapolating t o zero time, but usually that correction was small enough t o be neglected. (4) T h e osmometers were similar t o those used by E w a r t and French. The design was obtained from t h e m through private conimunication (5) hlcBnin and S t e l e n s , J Phgs. Cheni., 40, 1157 (1936).
F.T. W A L L , F. w. BANESAND G. D. S A N D S Measurements were made at several different concentrations for each solution investigated. The equilibrium solution rise was corrected for the capillary rise of the solvent in each case. I n analyzing the results, the best quadratic function of the form 11 = u c
+
bL2
(1)
was determined for each sample by the method of least squares. I n equation (l),h represents the corrected osmotic rise in centimeters a n d c the concentration in grams per 100 ml. of solution. With benzene as a solvent at 30.4’ (the temperature of the experiments) , the molecular weight can be calculated by means of the equation
VOl. 68
were computed by the method of least squares from a t least eleven and sometimes as many as sixteen osmotic pressure measurcments. Of the results given in Table I, the values lor Polymer C are the most accurate, since the greatest precautions were taken in preparing that sample and ixi carrying out the sulfur analyses.
Discussion
In discussing these results, it should be made clear that the commercial GR-S samples used were among the first ones made in the government program, and accordingly must not be regarded as representing the current product. The particular samples investigated had considerable gel, so the average molecular weight of the soluble portion was low. This was true because gelation preferwhere (h,’c)o = lim h’c = Q entially removes large iiiolecules. However, this (3) c-0 should not seriously alter the theoretical interSulfur was determined in the prepared samples pretation to be attached to the results. of GR-Sby a combination of the methods of Luke6 It should be recognized that several effects can and of Cheyney.’ The procedure involved the arise which will bring about deviations from the complete oxidation of the sample by means of one-to-one correspondence between sulfur atoms fuming nitric acid, bromine, and potassium and polymer molecules. First of all, i t is possible chlorate. Excess oxidizing agents were removed that radical chains can be terminated by disproby prolonged heating of the solid materials, fol- portionation or combination, which would have lowed by successive additions of hydrochloric acid the effect of modification without mercaptan conand further evaporations to dryness. sumption. This process would tend to lower the The sulfate resulting from the oxidation was sulfur content per molecule. Secondly, crossreduced by adding some hydrogen iodide reagent, linking reactions doubtless take place which and the. hydrogen sulfide generated was distilled would tend to increase the number of sulfur into a cadmium chloride solution. An excess of atoms per molecule. If cross-linking goes far standard 0.01 N potassium iodate solution was enough, the molecules so affected will become inadded to the latter solution, which was then back corporated into a gel structure which is insoluble. titrated with standard 0.01 N sodium thiosulfate Since the experiments were carried out only on the solution, using starch as an indicator. soluble portion, it is clear that gelation should not Because of the small amounts of sulfur involved, affect the interpretation advanced. Only the blank determinations had to be carried out to cor- moderately cross-linked molecules which remain rect for the sulfur present in the reagents used. soluble will affect the sulfur-polymer ratio. Using the best grade of reagents available, blanks The amount of sulfur in the fractions of interest amourited to as much as 0.006 milliequivalent of was found to be in the neighborhood of 0.07y0 sulfur, which in extreme cases was equivalent to with a probable error amounting to about 5 or 40y0 of the total sulfur found by the final titra- even 1OVc of that figure. The precautions which tion. This source of error is the most serious in had to be taken for the accurate determination of the whole procedure and is the principal cause of such small amounts of sulfur were discussed in the uncertainty iil the results. experimental section. The osmotic molecular weights varied from 33,000 to 50,000 for the Experimental Results different samples with probable errors equal to A summary of osmotic results, molecular about 5%. weights, sulfur contents and atoms of sulfur per From some of the earlier determinations i t was molecule is given in Table I. The values of (h/c)o calculated that an average of about 0.7 atom of sulfur was present per molecule. The way the TABLE I early experiments were carried out suggested that Atoms of Molecular sulfur per the figure 0.7 was experimentally too low, and so (hjc)o weight Sulfur, 7 0 molecule Polymer more accurate experiments were conducted on a A1 8.870 331300 0.071 0.73 small number of samples. The best experiments -42 9,100 32,600 indicated 1.1 atoms of sulfur per molecule. AlB, 8.510 34,800 1 o.064 though the figures 0.7 and 1.1 have large probable 0.69 B2 8 540 34,600 1 errors, they are suAiciently close to unity to lend C 5 959 49,740 0.0728 1.13 support to the free radical-chain transfer mechanism. Considering the difficulties in\-olved, the (G) Luke, I i i d E n g . Chem., A n a l . E d . , 15, 602 (1843) figure 1.1 in particular (which was the result of (7) Cheyney, i b t d , 15, 164 (1043).
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THENITRATION OF BICYCLO [2,2,1]HEPTANE
Aug., 19116
the most accurate measurements) must bc regarrid 3 s lwin;: compatible with theory.
Summary The transfer theory of nlodifier action has been tested on commercial GR-S by means of molecular weight and sulfur content deterniina-
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tions. The number of sulfur atoms per molecule was foiintl to range between 0.7 and 1 . 1 , to be compared to the theoretical. value of unity. T i l view of the experimental dificultie.; iiivolvcd, thc results arc considered to in qc)otf :igrcertim t with the theory. ~ R B A N A TI,I.IVC)IS ,
[CONTRIBUTIOS PROM PCRDUE r N I V E R S I T T AND P U R D r E
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