4780
LEIGHTON H. PEEBLES, JR.,
[COXTRIBUrION FROM THE
JOHN
DEPARTMEXO F
THACHER CLARKEA N D WALTERH. STOCKMAYER Vol. 82
CHEMISTRY, MASSACHUSETTS INSTITUTE O F M ASSACHVSETTS]
TECHNOLOGY, CAMBRIDGE,
The Copolymerization of Benzene with Vinyl Acetate’,’ 13Y
LEICIITON€1. PEEBLES,
JR.,3 JOHN
TIIACHER CLARKE‘ AND WALTER H. STOCICMAYER
RECEIVED JANUARY 15, 1060 The rate of polymerization of vinyl acetate in the undiluted state and in benzene solution was studied dilatometrically a t 59.6’ with a,a’-azobiscyclohexanecarbonitrileas the initiator. The rate of polymerization of undiluted monomer was in satisfactory agreement with the rates observed by other workers. I n order to correlate the rate of polymerization in benzene solution with the molecular weight of the polymer, it was necessary t o depart from the conventional kinetic scheme and to assume that a vinyl acetate radical can add to the benzene nucleus and the chain can then be continued by further addition of vinyl acetate monomer, leaving a benzene residue copolymerized into the vinyl acetate chain In order t o verify whether benzene copolymerizes in this fashion, vinyl acetate was polymerized in the presence of ring labeled benzene-C14, and the radioactivity of the polymer obtained was in satisfactory agreement with the amount calculated from the kinetic analysis.
Introduction This paper describes in detail the results reported in our earlier communication5 regarding the copolymerization of benzene with vinyl acetate. The free-radical polymerization of vinyl acetate in benzene has been studied by many workers ,6-12 The results are characterized by three features which cannot all be simultaneously accommodated by the conventional kinetic involving degradative chain transfer to solvent : (i) The rate of pdymerization is markedly reduced below that found in many aliphatic solvents, such as dioxane and methylcyclohexane, but similar to those found in so-called “degradative” solvents, including aromatic or allylic compounds. l 4 (ii) The order with respect to initiator remains close to one-half over a wide range of initiator concentration. (iii) The reduction in molecular weight of the polymer is slight as compared to that in many other solvents. As m-ill be shown below, the magnitude of the transfer constant to benzene required to explain the observed polymerization rates (i.e., the first two features enumerated above) is about twenty times larger than the value derived from the observed molecular weights by the conventional chain transfer scheme. The hypothesis of copolymerizatiori is required to reconcile these observations. Kinetic Scheme.-IVe employ the conventional kinetic scheme12,13 ( 1 ) Based o n t h e thesis submitted by L. H. Peebles, J r , i n partial fulfillment of t h e requirements for t h e Ph D. degree a t t h e Massachusetts I n s t i t u t e of Technology, 1953. ( 2 ) Supported b y B grant-in-aid from t h e American C h i d e Company. (3) T h e Chemstrand Corporation, Decatur, Alahama. (4) Research Associate, 1948-1950. ( 5 ) W. H. Stockmayer and L. H. Peebles. J r , , T H I S J O U R N A L , 76, 2278 (1953). (6) A. C. Cuthbertson, G. Gee a n d E . K. Rideal, Proc. Roy. Soc. ( L o n d o n ) ,A170, 300 (1939). (7) S . Kamenskaja and S. Medvedev, Acta Pkysicocheinica U.R.S.S., 13, 565 (1940). ( 8 ) G. M. B u r n e t t and 11. W. Melville, Discussions Faraday Soc., 2, 322 (1947). (9) A . Conix a n d G. Smets, J . Polymer Sci., 10, 5 2 5 (1953). (10) G. hI. Burnett and I,. D Loan. Trans. Faraday Soc., 61, 214, 219, 226 (1955). (11) A . D . Jenkins, J . Polymer S i . , 29, 245 (19%), a n d disciission following. ( 1 2 ) A. D. Jenkins, Trans. Farodoy Soc., 64, 1895 (1958). (13) J. L . Kice, THIS J O U R N A L , 76, 6274 (3954). (14) J. 1’.Clarke, unimblished result.- i n this Laboratory.
initiation propagation termination transfer t o solvent reinitiation solvent termination solvent dimerization transfer t o monomer
where M is monomer; I, the initiator; growing polymer chains; s, solvent; Sa,radical formed from a solvent molecule; P, inert product. The efficiency of initiation as seen by Table I is unity. If, as suggested by Arnett15 and others, the eficiency of initiation is less than unity, a correction factor may be included in the kinetic derivation. The constant ka is defined by a term 2 k3[Rf.]2 in the rate equation for variation of [&I.]with time and is the same as that used by hlatheson, Auer, Bevilacqua and Hart16 and by Kwart, Broadbent and Bartlett.l7 Except in the most dilute solutions, we may neglect solvent dimerization (k7 N 0 ) and the usual steady state condition then gives the expression M
e
,
in which the concentrations of non-radical species are represented by unbracketed symbols. With the neglect of small quantities -d In M/dt
E
Q
=
kn[hI.]
(10)
Using a subscript (0) or (s) to denote absence or presence of solvent, a t the same concentration of initiator, we may then rewrite equation 9 in a form identical with Kice’s13equation 223
with a = k3k6/k4ka,P = k3/kZk4,Qo = k2(klI/k3)’/1. The Initiator and Rate of Initiation.-The initiator used in this work was CY, d-azobiscyclohexanecarbonitrile, hereafter referred to as ABC. It was chosen for its relatively low rate of decomposition as compared to other azobisnitriles. The rate of initiation of vinyl acetate polymerization by ABC a t 59.6’ was measured by the inhibitor method (15) L. M. Arnett, TRISJOURNAL, 74, 2027 (1952). (16) M. S Matheson, E. K. Auer, E. B. Revilacqua and E. J. H a r t , ibid., 71, 2610 (1949). (17) H. ICwart, 11 S Broadbent and P. D. D n r t l r t t , ibid , 74, 1060 ( I rr50).
COPOLYMERIZATION OF BENZENEWITH VISYL ACETATE
Sept. 20, 1960
of Bartlett and Kwart.'* Duplicate runs, with duroquinone concentrations of about 4 X 10-4M and ABC concentrations of about 4.5 X 10-2M, gave a value for kl of (2.06 0.10) X min.-'. We also measured the rate of nitrogen evolution from ABC in ethyl acetate a t 59.6', obtaining kl = 2.13 X min.-', in excellent agreement with the results of the inhibitor method in vinyl acetate. A similar measurement in n-propyl acetate a t 98.5'gave kl = 4.75 X min. If the effect of solvent is neglected, the activation energy for ABC decomposition is 34 kcal. mole-'; and the interpolated rates of decomposition a t 77 and SOo are in satisfactory accord with published results.l9 Rate of Polymerization of Undiluted Vinyl Acetate.-Experimental values of Qo a t 59.6' are given in Table I, for several initiator concentrations. These are well correlated by Qo = 1.77 X lo-* 1'13 (12) as shown in the last column. With kl = 2.10 X 10-6 min.-l, this corresponds to a value of 2.72 f 0.2 l.'/* mole-'/' min.-'/a for the combination k2(2 k3)-'/3, while the expression of Matheson and co-workers16leads to 2.36. If we combine our figure a t 59.6' with that of Kwart, Broadbent and Bartlettl' a t 2 5 O , m e derive an activation energy (E2 - '/2 E X )of 5.G kcal. mole-' as compared to Matheson's 4.7 kcal. mole-'.
4751
*
Io 0
2
1
3
1 0 4 /M. ~~ Pig. 1.-Rate
of polymerization of vinyl acetate in benzene, plotted according to equation 11.
In order to compare the rate of polymerization with that calculated from equation 11 for large benzene-to-monomer ratios where dilatometric measurements are insensitive, eight ampoules of high benzene-to-monomer ratio were made up, TABLE I polymerized and the polymer isolated, dried and RATE OF POLYMERIZATION OF UNDILUTEDVINYL ACETATE weighed. From the time of reaction and the yield AT 59.6' of polymer, Qswas calculated and is also given in ABC X 109 -Qa X 10' (min.-')Table 11. (mole 1.- ) 0bsd.o Calcd. 0.196 0.727 0.781 1.50 2.26 2.16 1.58 2.36 2.22 2.88 2.85 2.99 8. 70b 5.0 f 0 . 6 5.2 These values are corrected for temperature differences as described in the Experimental section. *This run was so fast that the correction to 59.6" was much more uncertain than in the other experiments.
Rates of Polymerization of Vinyl Acetate in Benzene.-The values of Q. are given in Table 11, together with the concentrations of initiator, monomer and solvent. The values of Qs in the fifth column are those calculated from equation 11 with CY = 0.033 and /3 = 34.6. These constants were obtained from the plot of Fig. I , which corresponds to equation 11. Together with the figure quoted earlier for k2/(2k3)lIy, the value of ,B thus found leads to a kinetic transfer constant of vinyl acetate to benzene equal to C = kd(k2 = 2.0 X The plot of Fig. 1 is quite sensitive; if the point nearest the origin were neglected, the value of C would be changed to 2.7 X without appreciable change In the over-all fit of the rate data. The agreement between observed and calculated rates can be seen from the fourth and fifth columns of Table 11. From the value of a! we find that the ratio k6/k6 is about which seems reasonable. (18) P. D. B a r t l e t t a n d H. K w a r t , THISJOURNAL, 7 2 , 1031 (1950). (19) (a) F. M . Lewis a n d M. S. Matheson, i b i d . , 71, 747 (1949); (b) C. G. Overberger, M. T. O'Shaughnessy a n d H. Shalit, i b i d . , 71, 2603 (1940): ( c ) L. M. Arnett. ibid.. 74. 2027 11952).
TABLE I1 RATESOF POLYMERIZATION OF VINYLACETATEIN BENZENE AND RECIPROCAL DECREES OF POLYMERIZATION ABC X 101
Monomer (moles
1.67 9.64 0.245 8.54 0.239
1,-1)
Benzeiie
8.18 8.05 8.24 6.49 6.26
2.12 2.22 2.06 3.89 4.14
Qa
X 10' obsd.
0s
X 10' calcd.
Dilatometric 0.807 0.798 2.00 1.99 0.309 0.302 1.28 1.29 0.185 0.190
10'
l/Fn X 10'
3.80
3.88 2.50 4.46 3.63
'c"$ 2.93 3.66 2.63 2.56 2.84
Gravimetric 1.770 9 . 2 8 0.173 0.146 8.13 9.26 0.874 9.82 .114 .102 15.9 23.9 0.90 1.575 9 . 2 5 .145 ,130 10.0 10.5 11.3 0.98 1.315 9.32 ,180 .125 10.5 1.00 0.906 9 . 7 6 ,139 .lo3 14.5 19.1 0.94 .848 9 . 8 2 .148 ,097 1 5 . 5 18.2 1.00 .YO6 9 . 7 4 .135 .lo3 14.5 19.5 1.OOn .898 9.77 ,138 ,103 ... 10.3 a $ C , defined by equation 22. bContained 6 x 10-6 mc./mg. of ring-labelled benzene-CI4.
1.00 1.02
+
For the ampoule runs of Table 11, the observed rates are faster than the rates calculated from equation 11. This is to be expected, since the assumption was made that a growing chain with an active solvent radical end does not react with a similar solvent radical but only with a growing polymer chain, i.e., the mutual solvent termination constant k7 is zero. I n the ampoule experiments of Table 11, the ratio of benzene to monomer is so lame t h a t
4782
LEIGHTON H.
0
PEEBLES, JR., JOHN
0.2
THACHER C L A R K E AND
WALTER
H. S~oc1thr.41’~~ v01. 8%
0.5
S’Z, Fig. 2.-Chain
transfer t o monomer, see eq. 14 with S/M = 0.
this approximation is no longer completely justified. Inclusion of a non-zero value of k7 would not appreciably alter the rate constants deduced from the dilatometric experiments by means of (11). Degrees of Polymerization.-The reciprocal degree of polymerization is given by the conventional analysis as
4
0
A plot of l/Pn against I‘/Z for solvent-free runs is shown in Fig. 2. The intercept a t zero initiator concentration when no solvent is present results in the transfer constant to monomer, C, = (2.4 f 0.2) X Clarke14 obtained 2.5 X for C, a t 60’ with benzoyl peroxide as the initiator. The slope of Fig. 2 corresponds to the value g = 1.5 in the first term in the right-hand member of equation 14, and this figure will be used subsequently. Transfer to initiator, which would produce upward curvature in Fig. 2 , has been neglected. Equation 13 niay be rewritten in the forni
The conventional plot corresponding to the first of these equations is shown in Fig. 3, the data being found in Table 11. Fortunately, the value of last term in the second of equations 15 is only about so that the value taken for g is unimportant. The slope of Fig. 3 gives C = 1.2 X or much smaller than that earlier derived from the rates. This figure may be compared with 2.4 X measured by Chadha and Misra20and 2.96 X ( 2 0 ) R. N. C h a d h a and G S hZisra, T r n i z s F n r n d n y Soc , 54. 1227 (1958).
12
transfer t o benzene, see eq. 15
Fig. 3.-Chain
where pn is the number average degree of polymerization, C, the transfer constant to monomer, C the transfer constant to solvent and g a factor whose value is 2 3 g 3 1, depending upon the mode of termination, L e . , whether by combination of radicals (g = 1) or by disproportionation (g = 2). Substitution of (k11/k3)’/%for [M.] in (15) yields
8
S/M.
by Palit and Das.al Presumably the lowest value is the most trustworthy. Addition of Benzene to the Vinyl Acetate Chain.-The chain transfer constant computed from the molecular weights with equation 15 may now be called the “apparent” transfer constant C‘ and that determined from the constants CY and 3 from the equation 11 may be called the “kinetic” transfer constant C. The latter is greater than the former by a factor of about eighteen. To reconcile this discrepancy, we postulate addition of the growing chain to the benzene ring to forni a radical which can be resonance stabilized Fi
--(:H>--k.
+ [-\
; oxc
--f
J
HtI -rH2-\
