Equilibrium Monomer Concentration for the Anionic Polymerization of m-Isopropyl-a-Methylstyrene in Cyclohexane

INTRODUCTION

The polymerization of a-substituted styrenes generally exhibits a low ceiling temperature, i.e., they undergo equilibrium polymerizations except a t very low temperatures. The anionic equilibrium polymerization of a-methylstyrene has been studied extensively, and its ceiling temperature has been well The polymerization, both anionic and cationic, of some ring-substituted wmethylstyrenes has been studied, but the ceiling temperature of only p -isopropyl-0-methylstyrene appears to have been determined.9 There is scant information in the literature on the polymerization of rn-isopropyl-a-methylstyrene, and no value of its ceiling temperature is given. The T, has been determined in this laboratory, together with [MIe values between 5 and 4OoC, for its anionic poly- merization in cyclohexane.

EXPERIMENTAL

Materials

Sources and purifications of cyclohexane and tetrahydrofuran were described previously, as were preparations of solutions of tetrahydrofuran and sec- butyllithium.8 The m-isopropyl-a-methyl- styrene was obtained from Chemical Samples Co. and was 99% pure. Solvent and monomer were dried by passage through a water-cooled silica gel column under nitrogen pressure.

Polymerization

Appropriate solutions of rn-isopropyl-a-methylstyrene in cyclohexane were prepared, and 20-ml portions were transferred to 1-02, screw-capped bottles. They were initiated with equal volumes of 3.0M tetrahydrofuran and 1.5M sec- butyllithium solutions. I t was shown in previous experiments with a-methylstyrene that, with tetrahydrofuradli = 2, equilibrium was attained in 24 hr in the temperature range being used.* All polymerizations were carried out for a t least 48 hr, however, to insure that equilibration was achieved. Longer reaction times (from 72 to 90 hr) produced no marked change in polymer yields. All operations were carried out under a nitrogen blanket.

Reactions were run at least in duplicate a t each temperature, by using two concentrations of ini- tiator (Li = 0.01 and 0.02M). Methods of carrying out the polymerizations, isolation of the polymers, and determination of [MIe were described previously.8 Initial monomer concentrations [MIo were determined by gas chromatography. In calculating the moles of monomer that polymerized, [MIp, allowance was made for a 10% volume contraction that occurs during conversion of monomer to polymer.

RESULTS

The equilibrium monomer concentrations found a t the respective temperatures are given in Table I. The relation between temperature and equilibrium monomer concentration has been defined by Dainton and Ivin according to the equation

AHa ASo + R In[M],

T, =

where T, is the ceiling temperature corresponding to a given [MI,, the equilibrium monomer con- centration in mole/liter.1° Thus 1n[Mle is a linear function of 1/Tc; the data of Table I were so plotted in Figure 1, verifying a linear relation of these parameters for anionic polymerization of this monomer in cyclohexane.

The data for a-methylstyrene polymerization, determined previously; are also plotted in Figure 1 for comparison. Below about 25'C, a-methylstyrene has slightly lower [MIe values; above 25OC, it has slightly higher [MIe values. The different slopes of the linear plots show that these monomers have unequal enthalpies and entropies of polymerization. The respective quantities were calculated

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 19, 1287-1290 (1981) 0 1981 John Wiley & Sons, Inc. CCC 0360-6376/81/051287-05$01.00

1288 J. POLYM. SCI.: POLYM. CHEM. ED., VOL. 19 (1981)

TABLE I Equilibrium Monomer Concentrationsa

T IMlo N I P [Mle (“0 (mole/liter) (molehiter) (molebiter)

5 10 20 30 40

1.88 1.88 2.37 3.26 4.53

1.36 1.16 1.09 1.38 1.58

0.52 (f0.04) 0.72 (f0.02) 1.28 (f0.04) 1.88 (f0.11) 2.95 (f0.03)

a All values are averages of a t least duplicate determinations; average errors are included with [MIe values to show their reproducibility.

from eq. (1). For rn-isopropyl-a-methylstyrene, AH:$ = -8.4 kcal/mole and ASis = - 28.9 cal/mole OC; for a-methylstyrene, they were -11.4 kcal/mole and -39 cal/mole OC, respectively. I t has been shown that for anionic polymerizations in tetrahydrofuran, both enthalpy [AHL] and entropy [ASc] of p-isopropyl-a-methylstyrene decreased from the respective values for a-methyl~tyrene.~ These differences were attributed to the para substitution in the phenyl ring. Direct comparison of those results with the ones reported herein may not he strictly feasible, since, depending on heat of mixing of the monomers in various solvents, the trends in the two systems may not be the same. The present system using cyclohexane solvent, however, indicates that meta substitution increases both enthalpy and entropy of polymerization for an a-substituted styrene.

The graphs in Figure 1 were extrapolated to the concentrations of the respective neat monomers, represented by the short horizontal lines extending from the ordinate. This gives T, = 55OC for neat rn-isopropyl-a-methylstyrene. This may be regarded as its “absolute” ceiling temperature, above which it cannot be polymerized to high-molecular-weight polymer. The absolute T, for rr-methylstyrene had been found to be 54°C.s Within the margin of experimental error, it may be identical for these monomers. It should be noted that the absolute T, for p-isopropyl-a-methyl- styrene has been found to be 37OC, determined from its anionic polymerization in tetrahydro- furan.9

2.0

1.0 1 \

\

1. 5 3 . 0 3 . 1 3 . 2 3 . 3 3 . 4 3 . 5 3 . 6

Fig. 1. Logarithm of equilibrium monomer concentration (molebiter) as a function of reciprocal temperature; (--) rn-isopropyl-a-methylstyrene, ( - - - -) a-methylstyrene.

NOTES 1289

The author wishes to thank the Gotdyear Tire & Rubber Co. for permission to publish these results. The m-isopropyl-a-methylstyrene was furnished by Dr. J. J. Tazuma, who obtained it from Chemical Samples Co. Gas chromatographic analyses were carried out by J. D. Bennett. This is contribution No. 624 from the Research Laboratories of the Goodyear Tire & Rubber Co.

References

1. D. J. Worsfold and S. J. Bywater, J . Polym. Sci., 26,299 (1957). 2. H. W. McCormick, J . Polym. Sci., 25,488 (1957). 3. A. Vrancken, J. Smid, and M. Szwarc, Trans. Faraday Soc., 58,2036 (1962). 4. K. J. Ivin and J. Leonard, Eur. Polym. J . , 6,331 (1970). 5. J. Leonard and S. L. Malhotra, J . Polym. Sci. A-l ,9 , 1983 (1971). 6. I. Mita and H. Okuyama, J. Polym. Sci. A-1,9,3437 (1971). 7. J. Leonard and S. L. Malhotra, J. Mucromol. Sci. Chem., 10,1279 (1976). 8. R. E. Cunningham, Polymer, 19,729 (1978). 9. S. L. Malhotra, J. Leonard, and P. E. Harvey, J . Macromol. Sci. Chem., 11,2199 (1977).

10. F. S. Dainton and K. J. Ivin, Nature, 162,705 (1948).

ROBERT CUNNINGHAM

Research Division The Goodyear Tire & Rubber Company Akron, Ohio 44316

Received July 1, 1980 Accepted September 16,1980