1966
JR. J. T. ATKINSAND F. W. BILLMEYER,
from the results of Lemaire and Livingston.2 The spherical approximation for perfluorocyclobutane is moderately good. Although the Stockmayer and the Kihara equations represent the experimental values of B
Vol. 63
within experimental accuracy in the temperature range of the measurements, the calculated values of B begin to diverge at high temperatures and different values are calculated for the Boyle temperature, as shown in Fig. 3.
LONG CHAIN BRANCHING IN POLYSTYRENE POLYMERIZED WITH STANNIC CHLORIDE1 BY J. T. ATKINSAND F. W. BILLMEYER, JR.~ Contribution from the Department of Chemistry, University of Delaware, Newark, Delaware Received June 86,1969
Long chain branching was observed in pol styrene made by ionic polymerization in carbon tetrachloride-nitrobenzene solution with stannic chloride as initiator. $he weight average number of branch points per molecule was about 2 in polymer made a t high conversion (SO%), while branching was not detected in polymer made at 35% conversion. Branching was determined by the comparison of the intrinsic viscosity of the ionic polymerized polystyrene to that of linear free radical polymegzed polystyrene of the same weight average m_olecular weight ( Mw). The relation between intrinsic viscosity [TI and M , of linear polystyrene in the range 20,000 < M , < 140,000 was determined as [9]t,utsnone,26’ = 3.9 X loe4 MwO*s‘dl./g.
Introduction The formation of long chain branches through chain transfer to polymer in free radical polymerization is now widely a~cepted.8.~Similar mechanisms have been postulated5.6 to occur in ionic polymerization but conclusive evidence for the presence of long chain branches in polymer prepared by an ionic mechanism has not been published. The long chain branching mechanism proposed by Fontana involves the transfer of a hydride ion from a polymer molecule to a growing chain. The resulting tertiary carbonium ion then propagates to give a branched molecule. Endres postulates that the transfer step proceeds by the same mechanism as Friedel-Crafts alkylation. Here the growing chain becomes attached to the polymer molecule a t the point of transfer and the proton removed in the transfer step activates a monomer to initiate a new growing chain. Endres found that aromatic compounds, including cumene, act as chain transfer agents in the cationic polymerization of styrene initiated with stannic chloride. The similarity between the structures of these chain transfer agents and the repeat unit of polystyrene suggested that chain transfer to polymer might also occur. The effects of such a transfer reaction would of course be most pronounced at high conversion. This paper presents evidence for the occurrence (1) Research performed by Joseph T. Atkins in partial fulfillment of the requirements for the degree of Master of Science. Presented at the 131at National Meeting of the American Chemical Society, Miami, Florida, April 10, 1957. (2) Mailing addresa for both authors: Polychemicals Department, E. I. du Pont de Nemoura & Company, Du Pont Experimental Station, Wilmington 3, Delaware. (3) P. J. Flory. J . A m . Chsm. Soc., 69, 241 (1937); 69, 2893 (1947). (4) F. W. Billmeyer, Jr.. tbid., 76, 6118 (1953); J. I(. Beaaley, tbid., 76, 6123 (1953). (5) C. M. Fontrtna, G . A. Kidder and R. J. Herold, Ind. Eno. Chem., 44, 1688 (1952); C. M. Fontana, R. J. Herold, E. J. Kinney and R. C. Miller, i W . , 44, 2955 (1952). (6) C. G . Overberger and G. F. Endres, J . Am. Chsm. Soc., 76, 6349 (1953); 77, 2201 (1955); J . Polymer Sci., 16, 283 (1955).
of long chain branches in polystyrene prepared at high conversion by cationic polymerization initiated with stannic chloride. Branching was detected by comparing the intrinsic viscosity of the ionic polymerized polystyrene with that of linear polystyrene (prepared a t low conversion in a free radical system) having the same weight average molecular
Experimental Ionic Polymerized Samples.-Styrene was polymerized by a carbonium ion mechanism by G. F. Endres.618 The polymerization was in 60/40 carbon tetrachloride/nitrobenzene solution. The initial concentration of styrene was 1.95 M , and that of the initiator, stannic chloride, was 0.023 M . Two groups of samples were studied, consisting of material polymerized to conversions below 35% and above 74%, respectively. Free Radical Polymerized Samples.-Styrene (Matheson) was vacuum distilled through a 10” packed column to remove inhibitor. One hundred ml. of distilled styrene was placed in a test-tube containing initiator (benzoin, recrystallized from ethanol) and chain transfer agent (carbon tetrachloride). The test-tubes were immersed in a constant temperature bath a t 25 1’ and irradiated with ultraviolet light from a bank of BL-360 fluorescent lamps. A stream of Seaford grade nitrogen was bubbled slowly through the tubes during the polymerization to exclude oxygen. After a predetermined time, the contents of the tubes were poured into methanol in a Waring Blendor. The polymers were collected by filtration and twice dissolved in butanone ?nd reprecipitated into methanol. The polymers were dried overnight at 60’ under vacuum. Molecular Weight.-Weight average molecular weight was determined by light scattering with an automatically recording photoelectric dissymmetry meter.@ The instrument was calibrated with solutions of the Cornel! Standard Polystyrene, assuming turbidities in agreement with the socalled “high values” in the literature.10 The solvent was butanone and the refractive index gradient dn/dc was taken aa 0.220 ml./g.ll Mercury green light (5460 A.) was
*
(7) C. D. Thurmond and B. H. Zimm. ibid., 8, 477 (1952). (8) We are indebted to Drs. Endrea and Overberger, Polytechnic
Institute of Brooklyn, for furniahing the samples. (9) F. W. Billmeyer, Jr.. and C. B. de Than, J . A m . Chem. SOC.,77, 4763 (1956).
(IO) D. R. Carpenter and W. R. Krigbaum. J . Chem. Phyr., 24, 1041 (1956).
LONGCHAINBRANCHING IN POLYBTYRENE
Nov., 1959
1967
porcelain filters1*with a maximum pore radius 0.6 p. Intrinsic Viscosity.-Intrinsic viscosities were measured in butanone a t 25" using Ubbelohde suspended level viscometers modified to permit successive dilutions in the vis-
/
0
/
/
TABLE I MOLECULAR STRUCTURE PARAMETERS OF FREERADICAL
Sample
conversion,
I
10.0
I!
.O
R, x 10-4. Fig. 1.-Intrinsic viscosity and weight average molecular weight of polystyrene: 0 free radical polystyrene; 0 ,ionic polystyrene: solid line, tdis work; dashed line, Outer, Carr and Zimm.14
[Iilbutamne~ 9.5'
dl./g.
A B C D"
(I
6.0
3.0
nv
%
I
I
POLYSTYRENES
8 17,000 0.093 6 20,000 .098 7 30,000 .127 ,205
