POLYMER LETTERS EDITION VOL. 13, PP. 279-282 (1975)

SELECTIVE DIMERIZATION OF STYRENE TO

TRIFLUOROMETHANESULFONIC ACID OR ACETY L PERCHLORATE

1,3-DIPHENYL-l-BUTENE CATALYZED BY

This report deals with the selective dimerization of styrene to 1,3-diphenyl- 1-butene (I), a linear unsaturated dimer of styrene, at 5OoC in benzene cata- lyzed by trifluoromethanesulfonic acid (CF3 S03H) or acetyl perchlorate (AcC104 ) .

Although the dimerization of styrene proceeds under various conditions, it usually gives a mixture of linear and cyclic dimers such as diphenylbutenes and 1-phenyltetralin (1-4). In the dimerization of styrene by use of acid cata- lyst (e.g., aqueous sulfuric acid (3,4)), the reaction conditions are very different from those in the cationic polymerization in nonaqueous solution, and the se- lectivity for I is not high. We have recently found in the cationic polymeriza- tion of styrene by superacids and their derivatives that the nature of solvent has a particularly great effect on the molecular weight and the molecular weight distribution of the polymer obtained (5,6). These findings interested us to examine optimum conditions for the dimerization of styrene.

Results and Discussion

Figure 1 and Table I show the results of the reactions in benzene with sev- eral cationic catalysts. Reaction rates considerably depended on the kind of catalyst as shown in Figure 1. Total conversions were determined by measur- ing the residual styrene concentration by gas chromatography. Molecular weight distribution of the products was measured by gel permeation chroma- tography (GPC) in methyl ethyl ketone solution by use of a Shimazu GPC-700 chromatograph. GPC curves of the products obtained in benzene are shown in Figure 2. The peaks at elution volume count 42.1 correspond to the styrene dimer. Selectivity for the dimer was determined from the GPC curve and is also shown in Table I.

The dimer obtained in benzene showed the identical 'H NMR spectrum with that of I previously reported (7,8) (6 1.37 (d, 3H, J = 7 Hz), 3.46 (m, lH), 6.27 (m, 2H), 7.14 (s, 10H)). IR spectrum of the dimer was also the same as that of I found in the literature (1,7). No absorption indicative of the presence of other isomers was found in both NMR and IR spectra. We con- cluded from the above observations that the dimer produced in benzene is ex- clusively I . NMR spectrum of the dimer obtained was the same as that of a trans isomer of I (7). Absorption maxima in UV spectrum also agreed very closely with those of the trans isomer previously reported (2,7) (A,,, (nm) = 253, 284, 293; in cyclohexane). Furthermore a strong absorption due to the

219

0 1975 by John Wiley & Sons, Inc.

280 POLYMER LETTERS EDITION

100 I

l I I

50 100 150 Time, min

Fig. 1. Time-conversion curves for the dimerization of styrene in benzene. Reaction conditions are shown in Table I.

TABLE I

Dimerization of Styrene to 1 ,3-Diphenyl-1-butenea

Time, Total conv., Selectivity) Solvent Catalyst [Clo 9

mmole/l min % wt%

CF,SO,H 0.20 Benzene 120 95 94 AcCIO, 2.0 Benzene 150 96 ca. 100 CH,SO,H 30 Benzene 120 88 75 BF, O(C2 H, )2 ' 8.0 Benzene 180 75 0 SnCl, . T C A ~ 10 Benzene 120 35 0

CF, SO,H 0.40 cc1, 107 99 62

CF,SO,H 0.10 (CHz C1)z 20 99 52 CF, SO, H 0.40 n-C,H,, 120 97 49

a [Styrene] bWeight fraction of 1,3-diphenyl-l-butene in the total product. Approximate values ob-

= 0.50 mole/l; 5OoC.

tained from GPC curve assuming the symmetry of the'dimer peak at elution volume count 42.1.

'Using water as cocatalyst. [BF,O(C,H,),] /[H,O] = 2.0. dTCA: trichloroacetic acid. [ SnCl, ] / [TCA] = 1 .O.

trans structure was found at 965 cm-' in IR spectrum. These results indicate that the dimer obtained consists of trans-l,3-diphenyl-l -butene.

Table I shows that CF3S03H and AcC104 form I with very high selectivity in benzene. It should be noted that the linear dimer I was exclusively obtain- ed with AcC104, although a cyclic dimer (1-methyl-3-phenylindane) has been found as a by-product in the cationic polymerization of styrene by perchloric acid (9,lO). CH3S03H also showed fairly high selectivity for I in benzene as shown in Table I . On the other hand, the reactions catalyzed b y boron tri- fluoride etherate (BF30(CzH5)z) or stannic chloride (SnC14), typical metal

POLYMER LETTERS EDITION 28 1

Mw lo& 103 lo2 Elution 30 35 Lo 25

I I

A Volume

Fig. 2. Molecular weight distribution of the reaction products in benzene (GPC curves). Reaction conditions are shown in Table I .

MW IOL 103 I?* I I 1 I

Elution 30 35 LO L5 Volume A Benzene

CCII,

n-%HlL

Fig. 3. Molecular weight distribution of the reaction products obtained by CF3S03H in several solvents (GPC curves). Reaction conditions are shown in Table I .

halide catalysts for cationic polymerization, did not give the linear dimer I at all but produced polystyrene.

Reactions catalyzed by CF3S03 H in several solvents were investigated to clarify the solvent effect on the selective formation of the dimer I . In 1,2-di- chloroethane, a polar solvent, cationic oligomerization proceeded in parallel with dimerization as seen in Figure 3. Carbon tetrachloride and n-hexane were

282 POLYMER LETTERS EDITION

used as nonpolar solvents. Styrene oligomers up to hexamer were obtained as well as dimer in these two solvents (selectivities for I are shown in Table I). Thus, benzene was found to be particularly effective among nonpolar solvents for the selective formation of I . This high selectivity in benzene should be at- tributed to some specific property of benzene as well as its low solvent polar- ity. The dimer obtained in the above solvents was identified as I by means of IR analysis. At 0°C the reaction in benzene by CF3S03H produces only poly- styrene (M, 'L lo3) and no dimer is obtained (5). Proton elimination from the growing cation is facilitated at higher temperature. This will lead to the predominant dimerization of styrene at 50°C.

The results just described show that the systems CF3S03H or AcClO,/ben- zene/SO"C possess very high selectivity for the formation of I , a linear unsatu- rated dimer of styrene. They also suggest that, in general, I is produced pre- dominantly when one, strong proton acids and their derivatives are used as catalyst, two, a solvent is nonpolar, and three, reaction temperature is rela- tively high. Cationic polymerization can take place if one of these conditions is not fulfilled. The present work has revealed that the linear dimer of styrene (I) can be selectively prepared under the conditions similar to those in cationic polymerization. Detailed investigations on the effect of reaction conditions on the selective dimerization seem to be fruitful for understanding and control of the transfer reaction in the cationic polymerization of styrene.

References

(1) F. R. Mayo, J. Amer. Chem. SOC., 9CJ 1289 (1968). (2) F. Dawans, Tetrahedron Lett., 1943 (1971). (3) M. J. Rosen, J. Org. Chem., Is, 1701 (1953). (4) I. C. Calder, W. Y. Lee, and F. E. Trelor, Aust. J. Chem., 22, 2689

(5) T. Masuda, M. Sawamoto, and T. Higashimura, to be published. (6) T. Higashimura and 0. Kishiro, J. Polym. Sci. Polym. Chem. Ed., 12,

(7) S. W. Ela and D. J. Cram, J. Amer. Chem. SOC., 88, 5777 (1966). (8) S. D. Hamann, A. J. Murphy, D. H. Solomon, and R. I. Willing, J.

(9) A. Gandini and P. H. Plesch, Europ. Polym. J., 4, 55 (1968); V. Ber-

(1969); Chem. Abstr., 9, 42985s (1970).

967 (1974).

Macromol. Sci.-Chem., A d , 77 1 (1972).

toli and P. H. Plesch, J. Chem. SOC., B, 1500 (1968). (10) S. Bywater and D. J. Worsfold, Can. J. Chem., 44, 1671 (1966).

Department of Polymer Chemistry Kyoto University Yoshida, Sakyo-ku, Kyoto 606, Japan

Received December 12, 1974 Revised January 25, 1975

Mitsuo Sawamoto Toshio Masuda Hideo Nishii Toshinobu Higashimura