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Kinetics of the reactions of polystyryl sodium with alkyl chlorides in dioxane
Letter K i n e t i c s of t h e r e a c t i o n s of p o l y s t y r y l s o d i u m w i t h a l k y l c h l o r i d e s in d i o x a n e Introduction
The propagation reaction in those anionic polymerizations which give rise to 'living' polymers is easily isolated and many studies of the effect of solvent and counter-ion on the kinetics of such reactions have been reported 1. It is equally easy to isolate termination reactions by deliberately adding a terminating agent after polymerization is complete. We wish to report the preliminary results of a kinetic study of the reactions between alkyl chlorides and 'living' polystyrene in dioxane. Experimental
All reagents were rigorously purified and handled under high vacuum conditions using break-seals. 'Living' polymer solutions were prepared by contacting 1 - 2 ~ solutions of styrene in dioxane with the appropriate alkali metal mirror. Concentrations of 'living ends' in the range 8-20 x 10 .-4 M were readily obtained, n-Butyl, s-butyl and t-butyl chlorides were dried over calcium hydride and then treated with freeze-dried 'living' polystyrene before use. The reaction was followed spectrophotometrically using a reaction vessel consisting of a I cm silica cell containing the 'living' polymer solution and a break-seal ampoule containing the alkyl halide. After mixing the reagents, the cell was placed in a thermostated bath in the cell compartment of a Unicam SP500 spectrophotometer and the disappearance of the characteristic colour of the 'living ends' was monitored. Concordant results were obtained whether this was done at 340 nm (Amax) or at 530nm, the latter being more convenient at high concentrations of 'living ends'. Results
Preliminary studies of the reaction between polystyryl sodium and t-butyl chloride in dioxane indicated that the reaction was relatively slow at room temperature. Using a large excess of t-butyl chloride, the disappearance of the 'living ends' was precisely first order over a period of at least 3 half-lives. The observed first order rate constants obtained at various initial Table I
Rates of reaction of polystyryl sodium with
alkyl chlorides in dioxane at 20°C Alkyl chloride t-butyl
s-butyl n-butyl
Run No.
[RCI]o (M)
kl x 103 (sec -1)
ktx 103 (M-lsec -1)
138 142 140 136 133 128 129
0.134 0.237 0.391 0- 674 0-06 0-027 0.071
0.81 1.37 2.27 3.93 3.53 6-07 14.67
6.04 5-78 5.80 5.83 59 225 207
Order of reactivity: n-butyl, 37; s-butyl, 10; t-butyl, 1
t-butyl chloride concentrations at 20°C are given in Table 1 and indicate a linear dependence of the first order rate constant on t-butyl chloride concentration. Thus the reaction appears to be a simple bimolecular process with a rate constant (kt) of 5.9 x 10-a M -1 sec-1 at 20°C. The validity of the data is supported by the fact that different preparations of both polystyryl sodium and purified t-butyl chloride gave reproducible results. Similar results, not reported here, were obtained at other temperatures, although at higher temperatures a slow spontaneous decomposition of the 'living ends' (isomerization 2) complicates the interpretation. It should be noted, however, that the rate of isomerization at 20°C was negligible in comparison with the rates of termination. To estimate the variation in reactivity of the polystyryl sodium with other alkyl chlorides, one run was performed with a sample of s-butyl chloride and two with n-butyl chloride. Both reactions were first order with respect to 'living ends' and the values of k, quoted in Table 1 assume a first order dependence on the alkyl chloride concentration and should therefore be regarded as estimates only at this stage. Preliminary studies of the reaction of t-butyl chloride with polystyryl potassium and polystyryl caesium indicated little change in the overall rate of reaction. However, the disappearance of the 'living ends' showed deviations from first order behaviour beyond approximately 7 5 ~ reaction so that firm conclusions must await a more detailed study. Discussion
The simple kinetic behaviour observed here with polystyryl sodium is consistent with what is known of 'living' polymers in dioxane. Studies of the propagation reaction with styrene indicate that the active centres are all present as ion-pairs, there being no evidence for significant concentrations of free ions or ion-pair aggregates. We would conclude, therefore, that the rate constants obtained refer to attack of the ion-pair on the alkyl chlorides. The products of the reactions have not yet been identified though by analogy with similar work in tetrahydrofuran 3 it seems probable that substitution predominates with n-butyl chloride while elimination is the major process with t-butyl chloride. The variation in reactivity n-butyl > s-butyl > t-butyl would support this view. A . R. B a k e r a n d G. C. East Department of Textile Industries, University of Leeds, Leeds LS2 9JT, UK (Received 10 October 1973) References
1 Szwarc, M. 'Carbanions, Living Polymers and Electron Transfer Processes', Interscience, New York, 1968, Ch VII 2 Ibid. p 407 and p 647 et seq. 3 Davis, A., Richards, D. H. and Scilly, N. F. Makromol. Chem. 1972, 152, 133
P O L Y M E R , 1973, V o l 14, D e c e m b e r
649
The propagation reaction in those anionic polymerizations which give rise to 'living' polymers is easily isolated and many studies of the effect of solvent and counter-ion on the kinetics of such reactions have been reported 1. It is equally easy to isolate termination reactions by deliberately adding a terminating agent after polymerization is complete. We wish to report the preliminary results of a kinetic study of the reactions between alkyl chlorides and 'living' polystyrene in dioxane. Experimental
All reagents were rigorously purified and handled under high vacuum conditions using break-seals. 'Living' polymer solutions were prepared by contacting 1 - 2 ~ solutions of styrene in dioxane with the appropriate alkali metal mirror. Concentrations of 'living ends' in the range 8-20 x 10 .-4 M were readily obtained, n-Butyl, s-butyl and t-butyl chlorides were dried over calcium hydride and then treated with freeze-dried 'living' polystyrene before use. The reaction was followed spectrophotometrically using a reaction vessel consisting of a I cm silica cell containing the 'living' polymer solution and a break-seal ampoule containing the alkyl halide. After mixing the reagents, the cell was placed in a thermostated bath in the cell compartment of a Unicam SP500 spectrophotometer and the disappearance of the characteristic colour of the 'living ends' was monitored. Concordant results were obtained whether this was done at 340 nm (Amax) or at 530nm, the latter being more convenient at high concentrations of 'living ends'. Results
Preliminary studies of the reaction between polystyryl sodium and t-butyl chloride in dioxane indicated that the reaction was relatively slow at room temperature. Using a large excess of t-butyl chloride, the disappearance of the 'living ends' was precisely first order over a period of at least 3 half-lives. The observed first order rate constants obtained at various initial Table I
Rates of reaction of polystyryl sodium with
alkyl chlorides in dioxane at 20°C Alkyl chloride t-butyl
s-butyl n-butyl
Run No.
[RCI]o (M)
kl x 103 (sec -1)
ktx 103 (M-lsec -1)
138 142 140 136 133 128 129
0.134 0.237 0.391 0- 674 0-06 0-027 0.071
0.81 1.37 2.27 3.93 3.53 6-07 14.67
6.04 5-78 5.80 5.83 59 225 207
Order of reactivity: n-butyl, 37; s-butyl, 10; t-butyl, 1
t-butyl chloride concentrations at 20°C are given in Table 1 and indicate a linear dependence of the first order rate constant on t-butyl chloride concentration. Thus the reaction appears to be a simple bimolecular process with a rate constant (kt) of 5.9 x 10-a M -1 sec-1 at 20°C. The validity of the data is supported by the fact that different preparations of both polystyryl sodium and purified t-butyl chloride gave reproducible results. Similar results, not reported here, were obtained at other temperatures, although at higher temperatures a slow spontaneous decomposition of the 'living ends' (isomerization 2) complicates the interpretation. It should be noted, however, that the rate of isomerization at 20°C was negligible in comparison with the rates of termination. To estimate the variation in reactivity of the polystyryl sodium with other alkyl chlorides, one run was performed with a sample of s-butyl chloride and two with n-butyl chloride. Both reactions were first order with respect to 'living ends' and the values of k, quoted in Table 1 assume a first order dependence on the alkyl chloride concentration and should therefore be regarded as estimates only at this stage. Preliminary studies of the reaction of t-butyl chloride with polystyryl potassium and polystyryl caesium indicated little change in the overall rate of reaction. However, the disappearance of the 'living ends' showed deviations from first order behaviour beyond approximately 7 5 ~ reaction so that firm conclusions must await a more detailed study. Discussion
The simple kinetic behaviour observed here with polystyryl sodium is consistent with what is known of 'living' polymers in dioxane. Studies of the propagation reaction with styrene indicate that the active centres are all present as ion-pairs, there being no evidence for significant concentrations of free ions or ion-pair aggregates. We would conclude, therefore, that the rate constants obtained refer to attack of the ion-pair on the alkyl chlorides. The products of the reactions have not yet been identified though by analogy with similar work in tetrahydrofuran 3 it seems probable that substitution predominates with n-butyl chloride while elimination is the major process with t-butyl chloride. The variation in reactivity n-butyl > s-butyl > t-butyl would support this view. A . R. B a k e r a n d G. C. East Department of Textile Industries, University of Leeds, Leeds LS2 9JT, UK (Received 10 October 1973) References
1 Szwarc, M. 'Carbanions, Living Polymers and Electron Transfer Processes', Interscience, New York, 1968, Ch VII 2 Ibid. p 407 and p 647 et seq. 3 Davis, A., Richards, D. H. and Scilly, N. F. Makromol. Chem. 1972, 152, 133
P O L Y M E R , 1973, V o l 14, D e c e m b e r
649