Cross-linking reaction in butadiene polymerization

JOURNAL OF POLYMER SCIENCE VOL. VI, NO. 2, PAGES 225-237

Cross-Linking Reaction In Butadiene Polymerization*

MAURICE MORTON and PETER P. SALATIELLO, University of Akron, Ohio

INTRODUCTION

The recent introduction of low-temperature polymerization (5 "C. and lower) has aroused renewed interest in the effect of polymerization temperature on chain structure of polydienes. In view of the improved perform- ance of synthetic rubber produced at low temperatures, the effect of polymerization temperature has been the subject of several recent investiga- tions.' The general consensus appears to be that the reduction in temperature resuIts in a greater degree of regularity in the chains, as evidenced by x-ray diffra~tion~-~ and dilatometer data.6 In an interesting review of the effect of polymerization temperature upon chain structure, Meyera showed that, although the proportion of 1,Zaddition suffers only a slight drop with decreased temperature of polymerization, the proportion of trans-1,4-addition increases at the expense of the cis-1,4-addition (from 80/20 trans-cis at 50°C. to 90/10 at OOC.). He also showed some qualitative data, from a study of gel points a t several temperatures, to indicate a decreased amount of branching and cross-linking at reduced temperatures. In this connection, it is of interest to note the work of Johnson and Wolfangel? on the viscosity-molecular weight relation of polybutadiene prepared a t vari- ous temperatures. They found that the viscosity exponent a increased with lower polymerization temperatures, and ascribed this to an increase in the linearity of the chains as a result of reduced branching and cross- linking during low-temperature polymerization.

It is obvious that a quantitative measure of the effect of polymerization temperature upon cross-linking and branching of chains would be of con- siderable interest. The theoretical basis for such a determination has become available only recently.

Theoretical Aspects of Cross-Linking

The theory of gelation in high polymers has been developed from statistical considerations mainly by Flory*-lO and Stockmayer11#l2 for polycon- densates with a functionality greater than two. Floryl* has recently dis cussed possible mechanisms for the cross-linking reaction in diene poly-

* This investigation was carried out under the sponsorship of the Office of Rubber Reserve, Reconstruction Finance Corporation, in connection with the U. S. Government Synthetic Rubber Program.

225

226 M. MORTON AND P. P. SALATIELLO

merization. He made a careful distinction between the two types of reactions that occur between growing free radicals and polymer chains:

(a) The cross-linking reaction, consisting of attack by the free radical a t the unsaturated double bond in the diene chain and leading to network formation.

(b) The branching reaction, consisting of chain transfer between a growing free radical and a polymer chain, probably at a methylenic carbon adjacent to a double bond.

These reactions have been postulated before and have been used inter- changeably to account for the phenomenon of gelation of polydienes, but i t is obviously important to distinguish between them in assigning a mechanism for the cross-linking reaction. Furthermore, the above cross-linking reaction would involve two possibilities related to attack on the two types of unsaturation present in polydienes : internal and external (vinyl) double bonds. Fortunately, the ratio of these two types of bonds seems to be a function of the type of polymerization system and is largely independent of the conversion or temperature. Hence it is not necessary to distinguish between these two types of unsaturation in treating cross-linking during a polymerization reaction of any given type.

On the basis of the above assumptions, Flory13 developed a simple kinetic treatment of the relative rates of cross-linking vs. propagation, leading to the following relationships:

dv/da = 2Kc~/(l - a)

and the integrated form:

where Y = proportion of cross-linked units (expressed as moles of cross- linked units per mole of initial monomer), a = fractional extent of conversion of monomer to polymer, K = k3/kz, k2 = velocity constant for normal chain propagation (attack upon monomer double bond), k3 = velocity constant for cross-linking (attack upon polymer double bond), and p = by definition, the “density” of cross-linked units in the polymerized por- tion of the mixture. Hence K can be regarded as the “reactivity ratio” of an unsaturated polymer unit as compared with a monomer molecule.

It is obvious that p increases with increase in conversion (a), i.e., the frequency of cross-links in the polymer becomes greater as conversion pro- ceeds, and gelation becomes difficult to avoid at higher conversion. Since, according to theory,12 gelation occurs when p becomes equal to the recipro- cal of the weight-average degree of polymerization (yw) of the primary chains,* it becomes possible to calculate K from a knowledge of the value of jiu at the gel point.

* This statement is valid only for a random distribution of cross-linked units13 but is actually applicable up to 60-70% conversion. The term “primary molecule”10 is used to designate the original polymer chains formed before any cross-linking reactions occurred.

CROSS-LINKING IN BUTADIENE POLYMERIZATION 227

CROSS-LINKING IN EMULSION SYSTEMS

To evaluate the relative cross-linking constant, K , for a monomer such as butadiene the emulsion polymerization system has obvious advantages. Not only is this type of system in practical use, but the mechanism has been greatly elucidated during the past five years. Thus the required knowledge about the primary molecular weight does not have to be obtained from tedious and sometimes questionable physical measurements. Instead, the chain length can be controlled and determined by using chain- transfer agents (“modifiers”) such as the mercaptans14-17 whereby simple analytical techniques yield reliable values of the degree of polymerization. Very recently Bardwell and Winkler l8 applied the statistical treatment of molecular size di~tribution’~~ 2o to systems involving chain-transfer agents. They derived an expression for the weight-average degree of polymerization at any given conversion as follows:

(era - 1) 2 jiw = -

ar2Ro

where a and ym are the same as previously defined, Ro = initial concentration of modifier (mercaptan) expressed as moles per mole of initial monomer, and r = “regulating index”14 of the modifier, i.e., the velocity constant relating modifier disappearance to conversion, which usually follows a first-order rate up to about 50% yieldl4t2‘ and can be expressed by -dR/da = rR. Hence it is possible to calculate values of j i , from conversion and mercaptan disappearance data, within the region where the modifier disappears a t a first-order rate.

In attempting to apply Flory’s equation to an emulsion system, it is apparent a t once that due regard must be given to the locus of the polymerization as it affects the relative concentrations of monomer and polymer. The comprehensive theory developed by Harkins and co-workers22 concerning the loci of emulsion polymerization in soap systems is of great importance here. It has been shown that in these systems there are two main loci for the polymerization reaction: the soap micelles, where polymer particles are initially formed up to about 2001, conversion (normal soap charge of 2.8%) ; and the polymer-monomer particles during the remainder of the reaction. In the GR-S type system it has been found that a free monomer phase still exists up to about 5 0 4 0 % conversion, during which time the monomer diffuses constantly into the polymer-monomer particles. After the free monomer phase disappears, the system becomes essentially a bulk polymerization within the polymer-monomer particles. Only at this stage, therefore, does Flory’s equation become applicable. Since, according to hark in^,^^ 99% of the polymer is formed within the polymer-monomer particles, it is the polymer-monomer ratio within these particles which must be known for the proper values of the concentration. This ratio will be

228 M. MORTON AND P. P. SAJATIELLO

governed by the relative rates of diffusion and polymerization, as long as a free monomer phase remains. In the case of styrene and isoprene, it has been found by C o ~ ~ i n , ~ ~ Harkins and H e r ~ f e l d , ~ ~ and others that the monomer-polymer ratio shows a continuous decrease, indicating a rate of diffusion which is slower than the rate of polymerization within the particles. Recent work by Meehan2K on butadiene, however, has indicated that the rate of diffusion for this monomer is rapid enough to maintain the polymer particles saturated with butadiene in the presence of free monomer. Although the “solubility” of butadiene in the polybutadiene particles was found to vary somewhat with degree of conversion and with the initial mercaptan charge, the monomer-polymer ratio lies in the close vicinity of 0.85, under the conditions of the work reported herein (see Ap- pendix). This relation will hold only up to conversions somewhat higher than 50%, at which point the free monomer phase disappears.

Hence, up to about 50% yield, in the case of butadiene, Flory’s equation may be modified as follows:

a 2 K a d a 2 K d a or d p = - = - dv d a 1 - a 1 - a 0.85 - = 2 K -

Hence :

p = v / a = 2.35K (2)

The significance of equation (2) lies in the fact that p, the density of cross-links, is independent of the conversion up to yields of about 50%; beyond that an increase can be expected. For purposes of evaluating K , the relative rate of cross-linking, it should be s a c i e n t to use this equation and restrict the conversion to this value.

The results contained in this paper show how the above equation can be used to obtain values of K from data on gel point and modifier concentration, and how this cross-linking constant varies with temperature.

EXPERIMENTAL PROCEDURES

Polymerization

All polymerizations were carried out in 4-0~ . bottles having cap liners of cloth-reinforced butyl. The recipe used was as follows:

Water ..................... 180 Butadiene ................... 100 Soap. ...................... 5 Potassium persulfate. ....... 0 . 3 Mercaptan. ................ Variable

The materials used were as follows : Bufadiene. Petroleum butadiene obtained from the Government Laboratories, Akron.

Ohio. It was freed from inhibitor by bubbling through 2 N sodium hydroxide, dried over calcium chloride, and condensed in a dry ice-acetone bath.


Soap. Office of Rubber Reserve sodium soap flakes. Persulfate. Baker's A. R. Potassium Persulfate. Mercaptan. Sulfole B-8 mercaptan from Phillips Petroleum Co. This material is

stated to be a mixture of isomeric tertiary dodecyl mercaptans, but its molecular weight was found" to be 193, which indicates an undecyl mercaptan. It was chosen because it is known to be insensitive to polymerization variables and follows a first-order disappearance rate reasonably well up to 50% yield.27

Bottles were loaded with a charge of 85.7 g. in the usual manner, the aqueous solution of soap and persulfate being charged fist, the bottles cooled in an ice bath, the mercaptan added, followed by liquid butadiene in excess, so that some of it was allowed to boil off and flush out oxygen from the bottles. End-over-end rotation a t 35 r.p.m. in a water bath was used. Conversions were determined by the total solids method as de- scribed by Kolthoff and Harris28 and Medalia.29

Mercaptan Analysis

The residual mercaptan in the latex was determined by amperometric titration with silver nitrate, according to the method of Kolthoff and Harris,28 except that a modified mercuric iodide cell was used as a source of e.m.f. This modification was suggested by the Government Laboratories, Akron, Ohio, and consisted of a Leeds and Northrup calomel electrode in which the solutions were replaced by the required iodide solutions. This type of cell had the disadvantage of having a high resistance (ca. 15,000 ohms) but it had the advantage of eliminating the salt bridge and of showing a negligible loss of iodide by diffusion. It was capable of maintaining its e.m.f. for several months, and could always be easily recharged with fresh solution.

Vistex Determination

The Vistex technique was used to obtain viscosity-conversion curves for accurate gel-point determinations. An 80-20 benzene-isopropanol solvent was used (thiophene-free benzene and reagent isopropanol), and all solutions made up by dissolving 1 ml. of the latex in 100 ml. of total solution. The procedure of Henderson and Leggem was used, wherein the intrinsic viscosity was obtained by successive dilution of the original solution with benzene instead of with the vistex solvent. In this way it is possible to obtain the true intrinsic viscosity of the polymer in benzene with a high degree of accuracy. It is, of course, necessary to calculate the relative viscosity at each dilution by using the correct solvent flow time a t that dilution. This involves the preparation of a calibration chart of the solvent flow time for each dilution, with varying water content.

The viscometer used was an ASTM type S50, for which the kinetic energy correction is negligible. Hence the intrinsic flow time [ t ] , can be taken equal to the intrinsic viscosity [ v ] , when In t,/c is plotted against c and extrapolated to zero concentration. A typical example of such a plot is shown in Figure 1, in which it may be seen how the slope of the In t,/c


us. c lines changes markedly when the gel point is passed (73.6% conversion line).

FIGURE I

VISTEX-CONC. P L O T F O R 50'C. POLYBUTADIENE

3.0

In t, - C

2 .o

CONVERSION

I .o

I 1 I I I .O 5 .I0 .I5 .20

C. I N GM. P E R 100ML. S O L U T I O N

RESULTS AND DISCUSSION

The effect of temperature upon the polymerization rates is shown in Figure 2, while the mercaptan disappearance rates are shown in Figure 3.

FIGURE 2

RATE OF POLYMERIZATION OF BUTADIENE

I I I I I I

12 24 36 T I M E IN HOURS

CROSSLINKING IN BUTADIENE POLYMERIZATION 23 1

2.5

2.0

z t a L I a 0 K W I P W + I 0 a W K z =I

8 W ' 0 -1

-

- 0 0.20% M E R C A P T A N

A 0.26% MERCAPTAN

F I G U R E 3 E F F E C T OF TEMP. ON R S H

DISAPPEARANCE

60'C. 0.2% RSH 0

50'C. 0.2% RSH 1

50 'C . 0.14% R S H 4O'Ca 0.2% R S H 0

I I I I

20 40 60 80 CONVERSION (%I

It can be seen that the mercaptan disappears at a first-order rate up to about 50% conversion. The regulating index, r, at 50°C. has a value of 3.83 as compared with 3.07 found by Kolthoff and Harris27 for this mercaptan in the GR-S system at 50°C. It can be expected that with buta-

FIGURE 4

INTRINSIC V I S C O S I T Y VS. CONVERSION

POLYBUTADIENE AT 60.C-

25 50 C O N V E R S I O N (%)

7 5


4.0

3.5 t

FIGURE 6

INTRINSIC VISCOSITY VS. CONVERSION - POLYBUTADIENE AT 40° C.

3.5

3.0

0.2% MERCAPTAN A 2.5

0.14% MERCAPTAN 0

FIGURE 5

2.0 INTRINSIC VISCOSITY VS. CONVERSION

DIENE AT 50'C.

25 5 0 7 5 CONVERSION (%)

I POLYBUTADIENE AT 50'C. I I I

25 5 0 7 5 CONVERSION (%)

diene alone the regulating index would be somewhat higher than for butadiene-styrene." The temperature also has an effect on mercaptan disappearance, higher temperatures leading to slower disappearance rates. This is in agreement with the data of Kolthoff and Harris28 on similar mercaptans.

3.5

3.0

0 0.12% MERCAPTAN

I A 0.2 0% -ME R C APTA N

20 40 60 CONVERSION (%l


The Vistex conversion curves for all these temperatures are shown in Figures 4, 5, and 6. It can be observed that the gel point (Vistex peak) can be determined with a precision of * 1% conversion units as a maximum deviation. It is also a commentary on the technique used that the points for each curve represent at least two separate bottle runs, yet show good reproducibility. With the available data on mercaptan disappearance and gel points, it is possible to calculate values for K , the relative cross- linking rate constant, by means of equation (2). These are shown in Table I below. The maximum error in the K values can be expected to be about * 3%, and is shown in appropriate units.

TABLE I CALCULATION OF RELATIVE CROSS-LINKING RATE CONSTANT ( K )

6OOC. 500 400 r = 3.26 3.83 4.32

RSH charge, RO (moles/mole monomer x 104) ........................... 5.80 7.22 3.70 5.80 3.30 5.60

Gel-point conversion (q). . . . . . . . . . . . 0.40 0.50 0.38 0.57 0.46 0.60 YuI at gel point ( X . . . . . . . . . . . 2.17 2.13 3.03 3.22 4.43 3.93 patgelpoht (X 10') . . . . . . . . . . . . . . . 4.60 4.69 3.30 3.10 2.25 2.54 K ( X 10') ......................... 1.96 1.99 1.40 1.32 0.96 1.08 Max. error in K values.. . . . . . . . . . . . . *0.06 *0.045 *0.03

It is apparent a t once that the calculated values for K show satisfactory agreement for two different gel points at each temperature, and that the relative cross-linking rate constant decreases noticeably with decreasing temperatures. It is interesting to illustrate, at this point, how Flory's original equation can lead to serious discrepancies if applied without due regard to the actual concentration of reactants. Thus from equation (1) above, p/2K would have a value of about 0.25 at 38% conversion and 0.5 at 57% conversion, leading to K values of 6.7 X 10-4 and 2.9 X lo-* at the two different conversions at 50°C.

Since the K values represent the ratio of the velocity constants for cross- linking and propagation, it is possible to calculate from them the difference in activation energy between the cross-linking reaction (E,) and the propagation reaction (E,). Calculated values are shown in Table I1 below. The

TABLE I1 CALCULATION OF E, - E, FOR BUTADIENE E ~ S I O N POLYMERIZATION

K valuea used E, - E, Q ad.)

7.9 * 0.6 7 . 1 * 0.6 7 . 1 * 0.6 7 . 7 * 0.6

50" and 60°C. (av. values). ......................... 50" (av.) and40"c. (1.04). ......................... 50' (1.52) and 60°C. (2.12). ........................ 50" (1.52) and 40°C. (1.04). ........................

last two calculations in Table I1 involve K values obtained where the gel point occurred at about the same conversion of all three temperatures


(cu. 40%). In the first two calculations, average values of K at each temperature were used, except for the 40°C. value where the high-conversion run was not considered as too reliable, since the gel point occurred at 60% conversion, where the rate curve had departed from linearity.

The average value for E, - E, is approximately 7500 calories. With this information it is possible to calculate the K value at any temperature and thus obtain a value for yw at the gel point, i.e., the weight-average degree of polymerization required for gelation. In this case the usual Ar- rhenius equation becomes:

7500 log ylcflT, - log TwgT* = ___ - - - 2.303R [il iJ

where ywC represents Tw at the gel point. A plot of log ymg against 1/T is shown in Figure 7. Thus at 50"C., ym has a value of 3.13 X lo3, while a t

0°C. this value rises to 2.63 X lo4, i.e., the chain length can increase by a factor of 10 before networks are formed. This extrapolation is based upon the assumption that the polymer-monomer ratio in the latex particles is unaffected by temperature, and this may be not entirely true for the wider ranges of temperature. Meehan's workz6 has shown that this ratio is unaffected by temperature between 30" and 50°C. which represent the limits of this investigation. In view of the magnitude of the change in K values with temperature, and the subsequent changes in values of yw at the gel point, it would seem that this reduction in rate of cross-linking with temperature must be a very significant factor in the improvement in physical properties observed in elastomers prepared at lower temperatures.

Finally, it is interesting to note that the data obtained permit quantitative predictions relating gel point and mercaptan charge at any tempera-


ture, provided the basic assumption regarding the monomer-polymer ratio is valid. Thus, an examination of the change in the regulating index, r, with temperature, shows that the difference in activation energy between the propagation and chain-transfer processes (E, - E,) is about 2.9 =t

0.3 kcal. Using this value, it is possible to calculate the regulating index of the mercaptan for any given temperature, and thus to calculate the relation between mercaptan charge and gel point. This has been done, within the range of temperatures studied, and predictions have been accurate within 2 units of the per cent conversion scale.

APPENDIX

Monomedolymer Ratio in Emulsion Polybutadiene

As shown by the work of Meehan,25 the solubility of butadiene in polybutadiene latex decreases slowly during polymerization. It also varies somewhat with the initial mercaptan concentration, showing lower values at lower mercaptan charges, but this effect is not very marked unless the mercaptan concentration is varied widely. Apparently the solubility of butadiene seems to depend on the molecular weight of the polybutadiene, being more soluble in polymers of lower molecular weight.

At the mercaptan concentrations used in the work reported here, the relation between polymer-monomer ratio and conversion can be expressed approximately as follows:

P / M = 1 + 0 . 4 4 ~ ~

where a refers to the degree of conversion, as previously noted. This relation will hold up to the point at which the free monomer phase disappears. Using this expression in Flory's equation:

dv CZ - = 2K - = 2K (1 + 0 .44~~) da 1 - a

Hence:

Or :

Or :

Y = 2Ka + 0.44Ka2

p = v/a = K(2 + 0 . 4 4 ~ ~ )

K = p / ( 2 + 0 . 4 4 ~ ~ ) (3) This illustrates the dependence of the relative cross-linking rate on degree

of conversion, due to the slow change in monomer-polymer ratio in the latex particles. Using equation (3) above, the following values of K can be calculated from the values of p shown in Table I:

60T. SOT. 4OOC.

Gel-point conversion (a,). . . . . .. . . . 0.40 0.50 0.38 0.57 0.46 0.60 K(X lo4). . . . . . . . . . . ... ..... .. ... 2.11 2.11 1.52 1.38 1.02 1.12

The values obtained at 60" and 4 O O C . show better agreement than those


shown in Table I, but the values at 50” do not. However, these small dif- ferences do not af€ect the calculation of the E, - E, values sufficiently to warrant a more precise recalculation, so that the value of 7.5 * 0.6 kcal. can still be considered valid.

References

1. “Symposium on Low Temperature Rubber,” Znd. Eng. Chem., 41, 1553-1616

2. E. E. Hanson, and G. Halverson, J. Am. Chem. Soc., 70,779 (1948). 3. E. K. Beu, d al., J. Polymer Sci., 3,465 (1948). 4. P. H. Johnson, and R. L. Bebb, Znd. Eng. Chem., 41,1577 (1949). 5. V. E. Lucas, P. H. Johnson, L. B. Wakefield, and B. L. Johnson. ibid., 41, 1629

6. A. W. Meyer, ibid., 41, 1570 (1949). 7. B. L. Johnson and R. D. Wolfangel, ibid., 41,1580 (1949). 8. P. J. Flory, J. Am. Chem. Soc., 63, 3083, 3091, 3096 (1941). 9. P. J. Flory, J. Phys. Chem., 46, 132 (1942).

10. P. J. Flory, J. Am. Chem. Soc., 69,30 (1947). 11. W. H. Stockmayer, J. Chem. Phys., 11,45 (1943). 12. W. H. Stockmayer, ibid., 12, 125 (1944). 13. P. J. Flory, J. Am. Chem. Soc., 69,2893 (1947). 14. Ewart, Smith, and Hulse, private communication to Office of Rubber Reserve. 15. H. R. Snyder, J. M. Stewart, R. E. Allen, and R. J. Dearborn, J. Am. Chem. Soc.,

16. F. T. Wall, F. W. Banes, and G. D. Sands, ibid., 68, 1429 (1946). 17. W. V. Smith, ibid., 68,2059 (1946). 18. J. Bardwell and C. A. Winkler, Can. J. Research, B27, 116, 128, 139 (1949). 19. P. J. Flory, J. Am. Chem. Soc., 58, 1877 (1936). 20. G. V. Schulz, 2. physik. Chem., B30,379 (1935). 21. M. Morton and R. V. V. Nicholls, Can. J. Research, B25, 159 (1947). 22. W. D. Harkins, J. Am. Chem. Soc., 69,1428 (1947). 23. M. L. Corrin, J. Polymer Sci., 2,257 (1947). 24. S. H. Herzfeld and W. D. Harkins, private communication to Office of Rubber

25. E. J. Meehan, private communication to Office of Rubber Reserve; J. Am. Chem.

26. H. A. Laitinen and J. S. Nelson, private communication to Office of Rubber Re-

27. I. M. Kolthoff and W. E. Harris, private communication to Office of Rubber Re-

28. I. M. Kolthoff and W. E. Harris, J. Polymer Sci., 2,49 (1947). 29. A. I. Medalia, ibid., 1,245 (1946). 30. D. A. Henderson and N. R. Legge, Car. J. Research, B27,666 (1949). 31. D. B. MacLean, M. Morton, and R. V. V. Nicholls, Znd. Eng. Chem., 41, 1622

(1949).

(1949).

68, 1422 (1946).

Reserve.

Soc., 71,628 (1949).

serve.

serve.

(1949).

English Synopsis, see Summaries, page 525, Vol. V, 1950.

R6sumi5

Une €tude a ktk &ectu& sur la polymCisation en emulsion du butadiike en vue d’6valuer les vitesses relatives des rkactions de pontage et de voir leur variation avec la tempkrature.

CROSSLINKING IN BUTADIENE POLYMERIZATION 237

En utilisant une forme modifih de l'6quation de Flory. les valeurs suivanm ont 6t6 obtenues pour la constante de vitesse relative de pontage (c.a.d. le rapport des constantes de vitesse de pontage A la constante de vitesse de propagation) :

40°C.. .......................................... 50°C ............................................ 60°C.. ..........................................

Temperature K - k d k p

1.02 =t 0.03 X 10-4 1.36 * 0.05 x 10-4 1.98 * 0.06 X 10-4

Ces rhultats donnent une valeur calcul& pour la diff6rence d'6nergie d'activation des rhctions de pontage et de propaga$,ion &ale A (Ez - Ep) : 7.5 * 0.6 kcal.

Au d6part des hypothbw admises, on peut calculer le degrb de polym6risation moyen en em) poi& au point de ghlification pour n'importe quel polymhre. On peut ainsi mon- trer que la g6lScation s'effectue A 50" quand j , atteint 3.1 X los, tandis qu'elle n'est rk l i sb A 0" que pour une valeur de 9, &ale A 2.6 X 104. D'oh on voit, qu'il serait possible de dkupler la longueur de chaine moyenne en poids sans qu'il n'y ait de g6lifica- tion, si on obaisse la temphrature de polym6risation de 50" h 0". Un changement de cette importance constitue un facteur important dam les essais des propri6ttb physique des blastom&res. pr6parb A basse tempbrature.

Zusammenfassung

Die Emulsionspolymerisation yon Butadien wurde untersucht, mit der Absicht, die relative Geschwindigkeit der Querbindungsreaktion und ihre hderung mit der Tem- peratur zu ermitteln.

Unter Benutzung einer abgeiinderten Form der Flory'schen Gleichung wurden die folgenden Werte fiir die Konstante der relativen Querbmdungsgeschwindigkeit erhalten (das heisst fiir das Verhdtnis der Geschwindigkeitskonstanten der Querbmdungs- und Propagierungsreaktionen) :

Temperatur K * k d k p

40°C ............................................ 50°C ............................................ 1.36 * 0.05 X 60°C ............................................

1.02 * 0.03 X lo-'

1.98 * 0.06 X 10-4

Aus diesen Daten kann die DifTerenz der Aktivierungsenergie Zwischen der Querbin- dungsreaktion und der Propagierungsreaktion (Ez - ITv) zu 7.5 * 0.6 kcal. hechne t werden.

Auf Grund dieser Voraussetzungen ist es dann maglich, den Gewichts-Mittel-Poly- merisationsgrad em) am Gelierungspunkt fiir jede gegebene Temperatur zu berechnen. So kann gezeigt werden, dass Gelierung bei 50" eintritt, wenn der j , Wert 3.1 X 10' erreicht, wiihrend dieselbe bei einem 9, Wert yon 2.6 X lo4 erst bei 0" einsetzt. Somit sollte es mijglich sein, eine 10-fache Zunahme der Gewichts-Mittel-Kettenhge zu erzielen, ohne dass Gelierung eintritt, indem man die Polymerisationstemperatur von 50' auf 0' herabsetzt. Eine so grosse Veriinderung kann als ein sehr bedeutender Faktor zur Verbesserung der physikalischen Eigenschaften yon bei verminderten Tem- peraturen hergestellten Elastomeren betrachtet werden.

Received December 18, 1949

Cross-linking reaction in butadiene polymerization

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