4510
1'01, 77
T. E. FERINGTON AND A. V. TOBOLSKY [CONTRIBUTION O F
THE
FRICK CHEMICAL LABORATORY, P R I N C E T O S UNIVERSITY]
Organic Disulfides as Initiators of Polymerization : Tetramethylthiuram Disulfide BY T. E. FERINGTON AND A. V. TOBOLSKY RECEIVED FEBRUARY 25, 1955 The compound tctramethylthiuram disulfide ( T M T D ) has been used as a polymerizatioti iiiitiator with methyl methacrylate and styrene monomers a t 70". T M T D is surprisingly only somewhat less reactive as an initiator than benzoyl peroxide a t 70". Ri',/[cat] is 5.45 X.10-6 set.-' for T M T D in methyl methacrylate and 3.34 X 10-6 set.-' in styrene a t low catalyst concentrations. The chain transfer to catalyst constant, Ct,, ,,atr has been evaluated as 0.0115 in methyl methacrylate and 0.0136 in styrene. T M T D appears to have the interesting property that it acts as a retarder as well as an initiator, this effect being very marked a t high concentrations.
Introduction The compound tetramethylthiuram disulfide (TMTD) S
cH3)N-c-s-s-c-N /I
s /I
CH3
was studied as the initial member of a series to investigate the properties of organic disulfides as initiators of polymerization. It is used extensively in the rubber industry as an accelerator and has also been found capable of vulcanizing rubber without the presence of sulfur. It was originally thought t h a t this compound would be most useful as an initiator a t temperatures above 100" since i t is used in vulcanization a t temperatures in the neighborhood of 125'. It was soon found t h a t TMTD as an initiator for styrene or methyl methacrylate was much too reactive to use a t such elevated temperatures and thus it has been studied a t 70' in these familiar monomers. Experimental The T M T D used was commercial "Thiurad" obtained from the Monsanto Chemical Company and recrystallized as follows: three times from boiling CHCI8; once from boiling CHC13, initiating the precipitation of T M T D by dropwise addition of ethanol and then allowing to cool, and finally once from cold CHCl,, precipitating with ethanol. The purpose of the final conditions was twofold: (1) to prevent the formation of decomposition products on heating the material, ( 2 ) ethanol is listed as a solvent for tetramethylthiuram monosulfide which has been mentioned as an inhibitor in vulcanization by TMTD. An experiment in which yoconversion was studied as a function of time gave an induction period of 10 minutes with a catalyst concentration of 0.0415 g./100 ml. after the third recrystallization using methyl methacrylate as the monomer. After the fifth recrystallization the apparent induction period was cut to two minutes. This was felt to represent the amount of time taken to warm the 10-ml. sample from room temperature to 70" in an oil-bath. The methyl methacrylate was purified by extracting twice with 5Oj, NaOH solution, washing four times with distilled water, drying for a t least four hours over anhydrous iKazCOs and finally distilling a t 100 mm. under nitrogen, taking roughly the middle 60%. The styrene was purified by distillation from copper pellets a t 20 mm. under nitrogen and taking the middle 60%. The sample solutions of monomer and TMTD were prepared using normal precautions as to accuracy and cleanliness and 10-ml. aliquots transferred to thin walled glass tubes, where they were degassed using three cycles of freezing, evacuation and thawing. The evacuated tubes were finally sealed off and polymerized for the desired length of time a t 70" (zkO.5') in an oil-bath. The rate of polymerization was calculated from the time of polymerization and the per cent. conversion to polymer obtained by precipitating the contents of the tube in a large excess of methanol, collecting it o n , I iintrred g l , t > b filtrr m d weighing after dry-
ing for 2 days a t 60" in a vacuum oven. The per cent. conversion was about 2% in most cases and below 7y0 in a11 cases. I n those cases in which molecular weights were required, they were obtained from the intrinsic viscosities of the polymers in benzene a t 30" using the relationship of Mayo, Gregg and Matheson' for unfractionated styrene.
TI,= le05 X [ q ] ' . 3 ' Since some of the data used in arriving at this equation were obtained from polymers prepared in the presence of CC14 the transfer activity of TMTD should not affect the accuracy of the P , > ' sobtained by use of the above equation. For methyl methacrylate the equation of Tobolsky and Baysa12for unfractionated polymer was used. -
P, = 315 X
[q]'.z5
The intrinsic viscosities here were run in c1Icls a t 25".
Results and Discussion A. Rate Measurements.-The equ- t'ion (1) Ri = R L &[cat1 where R, is the total rate of polymerization, R p , t h is the rate of thermal polymerization, B I is an
+
experimental constant and [cat] is catalyst concentration, has been verified for a large number of cases, although exceptions to this rule are not unknown.3 Polymerization data from Table I are given in Fig. 1 as a plot of R; LIS. [cat] in accordTABLE I VINYLPOLYMERIZATION INITIATED BY TMTD EXPERIMENTAL RESULTS A . Methyl methacrylate, 70" B. Styrene, 70" RP RP, [cat] x 103, mole/l.
0.082 ,149 ,248 ,411 1.23 1.24 1.47 2.06 2.46 3.69 3.72 6.14 9.28 23.2 38.7
X 105,
moie/l. sec.
2.21 2.87 3.50 4.15 5.63 6.09 5.97 6.44 6.59 6.84 6.88 7.01 6.70 ,552 4.18
1/Pn X 105
5.83 6.46 8.59 10.90
,.. ,
..
24.60
x 105, [cat] x 102, moles/I. moles/]. sec. 0.0982 ,238 .59,5 1.32 1.49 3.30 8.25
1.87 2.51 3.63 5.29 5.10 6.92 8.50
1/Pn
X 103
0.401 0.570 1.07 2.02 1.81 3.94 9.90
... 32.2 54.0
... 69.9
...
... ...
(1) F. R . Mayo, R. A. Gregg and M. S. Matheson, THISJ O U R N A L , 73, 1691 (1951). (2) B. Baysal and A. V. Tobolsky, J . Polymev Sci., 9, 171 (1952). (3) (a) K . S . Haward and W . Simpson, Trans. Faraday Soc.. 47, 212 (1(Jt11;, (b) C . i'. Schiilz, %. Elckiruchcm., 47, 26.5 (lY41).
TETRAMETHYLTHIURAM DISULFIDES AS INITIATORS OF POLYMERIZATION 451 1
Sept. 5 , 1955
ance with this equation for methyl methacrylate monomer and T M T D catalyst at 70". Figure 2 gives the experimental results from Table I in the same type of plot for styrene monomer. In both cases the deviation of R; vs. [cat] from linearity becomes quite marked a t rather low catalyst concentrations. In the case of methyl methacrymole/l. sec. is oblate a maximum R p of 7 X served a t a catalyst concentration of 5.5 X mole/l. A maximum in the R i vs. [cat] curve was not observed with styrene, but this may be due to the fact t h a t sufficiently high T M T D concentrations could not be used because of the limitation imposed by solubility in the monomer. For polymerizations which obey the rate equation l the rate of initiation of polymer radicals R[ can be obtained from equation 24
R:
=
2'4'
[M? [RE - R L l
'g 0
!
I
I
20
30
40
cat^
Fig. 1.-Rate
1
1 10
m / ~x 103.
of polymerization of methyl methacrylate with T M T D a t 70'.
r----
7
(2)
where f 2ktd k; kt, = rate constant for termination of polymer radicals by
A'
ktd
k,
= ktc ~
combination = rate constant for termination of polymer radicals by disproportionation = rate constant for the propagation step in the polymerization
Equations 1 and 2 are valid for the case in which the only ways in which polymer radicals are deactivated are by combination and disproportionation. This condition is very probably true for the T M T D data in the nearly linear portions of the R i vs. [cat] curves in Figs. 1 and 2 which obtain a t very low concentrations. At higher T M T D concentration neither equation 1 nor 2 is valid, since an additional mode of termination is no doubt operative. Presumably this involves some interaction between the growing polymer radicals and the T M T D molecules. I n view of these considerations a straight line was drawn tangent to the initial portion of the R i versus [cat] curve in Fig. 1. This represents an extension of the linear region of the R i versus [cat] curve. It is presumed t h a t equation 2 is valid for this fictitious line and yields the true rate of initiation of polymer radicals. The value of R[/[cat] obtained in this manner has been tabulated in Table 11,column 3.
Fig. 2.-Rate
of polymerization of styrene with T M T D a t
70".
Further information on the cleavage of T M T D was obtained by the use of the stable free radical diphenylpicrylhydrazyl (DPPH) . One can follow the production of radicals in solutions of T M T D in methyl methacrylate by observing the rate of fadinq of the characteristic violet color of the D P P H with a p h ~ t o m e t e r . ~This was done for the same concentrations as used in the polymerization experiments. This data is tabulated as Rdpph/[Cat] in column 2 of Table 11. It shows good agreement with column 3 of Table 11. In contrast, the extent of deviation of equation 2 from the true rate of initiation of polymer radicals may be seen by comparing columns 2 and 3 with column 4 of Table 11. I n column 4 the value of R, used in equation 2 was the actual experimental value. The fact that columns 2 and 3 in Table I1 are nearly identical indicates quite well that the curves TABLE I1 shown in Figs. 1 and 2 are not due to irregularities COMPARISON OF RATESOF FORMATION OF RADICALS BY in the production of radicals by the catalyst. TMTD FROM POLYMERIZATIOX DATAAND AS DETERMINED For comparison it may be noted that the value WITH DPPH of Rr/[cat] for benzoyl peroxide in methyl methA . Methyl methacrylate, 70" acrylate a t 70' is 2.91 X lo-+. B. TMTD as a Retarder with Azo-I Catalyst.The behavior of T M T D as a retarder was examined by working a t 30" and using 2-azobisiso3.33 x 10-4 5.44 5.4 3 . 9 2 X 10-6 butyronitrile (Azo-I) as a catalyst. It was as1.66 x 10-3 5.21 5.4 2.62 X 10-8 sumed that a t this temperature the rate of initiation 4.16 X lo-* 5.94 5.4 1.33 X 10-6 due to T M T D was negligible and that the rate of 1.04 X 10-2 6.51" 5.4 4.73 x 10-7 production of radicals by the Azo-I was not afThe rapid disappearance of the DPPH color prevented fected by the presence of the TMTD. The monoan accurate determination a t this catalyst concentration. (4) A . V . Tobolsky and B . Baysal, J . Pulymcv S c i . , 11, 471 (19.53).
( 5 ) C . E. H. Bawn and S. F. Mellish, (1951).
Trqns. Forodor
Soc., 47, 121G
4512
1-01. TT
T. E. FERINGTON A N D A . V. TOBOLSKY
mer used was methyl methacrylate. -4 plot of yc 49il.l0 I n both cases marked curvature of the plot conversion against time of polymerization was was obtained indicating that Ctr,,at was appreciconstructed by using several tubes a t a given able. Accordingly equation 3 was transformed t o Azo-I concentration, From these experiments it was found that T M T D did not act as an inhibitor but rather as a retarder. A straight line was obtained up to 670 conversion a t an Xzo-I concentration of 2.12 X lo-? molefl. and a T M T D and the graphs of A vs. [cat] constructed. As may mole 1. From this be seen in Figs. 1 and 5 , the plot was nicely linear concentration of 2.19 x line the observed R, was 5.82 X mole 1. sec. while a similar run with no T l L T D present gave an R,of 25.3 X Retardation may be considered as due to the action of the T M T D molecule itself or as due to some decomposition product of the T M T D . T M T D is known to break down on heating to CS?, Sg and tetramethylthiourea.6 Of these SS ;t-Ax80 has been found to be an inhibitor of polymerization in styrene by Schulz' and CSl has been shown by us to have practically no effect. If the retard2ing action is due to the decomposition products the 30 / / 20 40 , 60 80 plot of conversion against time using T h I T D as the initiator should be curved downward indicating greater retardation as conversion increases. This (cat) rn/l I I O 9 was not found to be the case (Fig. 3), the conversion Fig. 4.-l/Z',L - A ' R , , / ' [ M ] Z= A z'.x catalyst cniicciiz'er\iic time curve beinq quite linear. ~
1 1 -
tratioii for methyl methacrylate polymerized with T l l T r ) a t 70".
i d 50
I00
0
50
200
Time (minutes)
Fig. 3.--Methyl
I
2
3
250
methacrylate polymerized with TRITD. % conversion z'c time a t io".
C. Chain Transfer Activity of TMTD.-The chain transfer activity of T J I T D was studied with the help of equation 2
4
5
Fig. %?.-A
ZJS
p,, Ct,,,
= no. av. degree of polymn. of polymer obtained by =
viscosity measurements chain transfer t o S constant
=
ktr.x
k,
, k t , , , = rate constant for the transfer step
-1plot of 1 :pn us. R, was constructed for both methyl methacrylate and styrene using polymer representing the entire range of catalyst concentrations given in Figs. 1 and 2. A ' for methyl methacrylate was taken as 448,g and for styrene as (6) J . v. Braun and K . Weissbach, B e l . . 83, 2836 (1930). (7) G. 1. Schulz, J l n k ~ o m u i .Chem., 1 , 94 (1947). 18) G . V. Schulz and G. Haborth, i b i d . , 1, 106 (1947). (9) R I . S. Matheson. E. E . Auer, R R Revilacqua and I
