April, 1951
CHAINTRANSFER WITH STYRENE AND BENZOYL PEROXIDE
[CONTRIBUTION No. 107 FROM
THE
I691
GENERALLABORATORIES OF THE UNITEDSTATES RUBBERco.]
Chain Transfer in the Polymerization of Styrene. VI. Chain Transfer with- Styrene and Benzoyl Peroxide; the Efficiency of Initiation and the Mechanism of Chain Termination BY FRANK R. MA YO,^^ R. A. GREGGAND MAXS. MAT HE SON^^ The rates and degrees of polymerization of styrene have been measured over a wide range of concentrations of benzoyl peroxide a t 60'. The contributions to the average degree of polymerization of chain transfer of the polymer radical with the monomer and with the peroxide have been determined. The transfer constant of the monomer is about 6 X lo-&,comparable to that of ethylbenzene, while the transfer constant of the peroxide, about 0.055, is about six times that of carbon tetrachloride. About 90% of the radicals formed by the spontaneous decomposition of the peroxide start polymer chains. From the rate of decomposition of benzoyl peroxide and the rate of formation of polymer molecules, and from other evidence, it is concluded that essentially all chains end by coupling rather than by disproportionation. The thermal polymerization of styrene leads to about half the rate and degree of polymerization that would be expected from data a t low peroxide concentrations. These results may mean that biradicals formed in thermal initiation are unable to attain a statistical distribution and undergo an accelerated termination reaction, possibly leading t o very large cyclic molecules. Additional d a t a on the relation between intrinsic viscosity and number average molecular weight of polystyrene lead to the relation: M , = 167000. [q11.87.
.
Preceding papers from this Laboratory on the ef- and because a few Dolvmers had molecular weights too high for satisfaciory osmotic pressure deterfects of carbon t e t r a ~ h l o r i d eand ~ ~ hydrocarbon ~~~~ solventsga~c on the polymerization of styrene have minations. From osmotic pressure measurement not attempted to separate the effects of chain trans- on polystyrenes prepared by us (a) in the absence fer with the monomer and of disproportionation and of catalysts and presence of carbon tetrachloride or coupling of radicals. Isolation of these effects is other solvents or (b) in the absence of solvents and essential to the study of absolute rate constants in presence of benzoyl peroxide, Ewart, Tingey and polymerization which is presented simultaneously. Wales5 obtained the relation6 The present work was originally undertaken to dean= 184000[q']'.4'7 (1) termine, from the rate and degree of polymerization in the presence of benzoyl peroxide, the tendency of Using only the osmotic data on carbon tetrachloride styrene monomer to undergo chain transfer with polymers, together with number average molecular the substituted benzyl radical in polymerizing weights from 10,000 to 200,000 by chlorine deterstyrene, but the problems of chain transfer with minations, the relation the initiator, the efficiency of benzoyl peroxide in = 184000 [q']'" (11) initiating chains in styrene, and the relative impor- was reported,abexperiments with other solvents and tance of disproportionation and coupling in the benzoyl peroxide being omitted for the sake of simtermination reaction have proved to be closely re- plicity. When four experiments with conversions lated. above 12% are'omitted, because of possible broadExperimental.-The purification of styrene and ening of molecular weight distribution, then' benzoyl peroxide, preparation of runs in the aban= 182000[q']'~88 (111) sence of air, isolation of polymer samples, deterTable I gives additional osmotic molecular mination of intrinsic viscosities, and determination of molecular weights by osmotic pressure weightintrinsic viscosity data on polystyrenes have been described previously. 3b Runs 1-10, made a t 60' in the presence of either benzoyl perox11-20, 31-33 and 41-46 were run with different lots ide, butyl bromide, isobutyl chloride or isopropylof styrene over an interval of 9 months. All poly- benzene a t 60 or 100'. In Fig. 1, these data can merizations were carried out a t 60' but the concen- be compared with the previous osmotic data with tration of styrene in the pure monomer has been carbon tetrachloride polymersab(at conversions less taken as 8.63 moles/l. (the 25' value) instead of than 12%) which lead to equation (111). Although 8.35 (60' value). The units used for expressing all chain initiation by monoradicals (from peroxides) rates and rate constants are moles, liters and hours. and termination by coupling (established in the The Relation between Intrinsic Viscosity and present paper) lead to one molecular weight distriNumber Average Molecular Weight for Unfrac- bution, while termination by chain transfer leads to tionated Polystyrenes.-A relation between num- another, experimentally there is no significant difber average molecular weight and intrinsic vis- ference in the intrinsic viscosity-molecular weight cosity for unfractionated polystyrenes was de- relations between polymers initiated thermally in a sirable because of the greater convenience of the solvent or by benzoyl peroxide without a solvent. viscosity method for determining molecular weights Apparently there is enough transfer in the benzoyl (1) Part of the experimental work in this paper was carried out in 1943-1945. The remainder and t h e preparation for publication were assisted by the Office of Naval Research, under Contract N8onr-544. (2) (a) General Electric Research Laboratory, Schenectady, N. Y.; (b) Argonne National Laboratory, Chicago 80, Ill. (3) (a) F. R. Mayo, THISJOURNAL, 65, 2324 (1943); (b) R. A. Gregg and F. R. Mayo, ibid.. 70, 2373 (1948); (c) R. A. Gregg and F. R. Mayo, Discussions Faraday Soc., 2, 328 (1947); (d) F. R. Mayo, THISJOURNAL, 70, 3689 (1948). (4) M. S. Matheson, E. E. Auer, E. B. Bevilacqua and E . J. Hart, ibid.. 73, 1700 (1951).
(5) R. H. Ewart and H. C. Tingey, Abstracts of Papers, 111th Meeting American Chemical Society, April, 1947, p. 4Q. (6) The intrinsic viscosities in this paper (all in benzene solution) are based on concentrations in grams/100 cc. and are therefore equal to intrinsic viscosities based on basal moles/liter divided by 10.4. [?'I indicates intrinsic viscosities which have not been corrected for kinetic energy. (7) The effect of omitting the higher conversion experiments is insignificant, particularly since an error has been found in the least squares calculation of Equation (II), which should actually have read: -
M n = 182000 [?'l'.'B5.
FRANK K. MAYO,I
