Kinetics of anionic polymerization of α-methylstyrene in tetrahydropyran

Europmm Polymer Journal, 1969, Vol. 5, pp. 387-394. pergamon Press. Printed in England.

KINETICS OF ANIONIC POLYMERIZATION OF a-METHYLSTYRENE IN TETRAHYDROPYRAN F. S. DMNTON*, K . M . Hm~f a n d K . J. IVIN++ Department of Chemistry, The Queen's University of Belfast, N. Ireland (Received 16 August 1968)

Abstract--The kinetics of anionic polymerization of a-methylstyrene in tetrahydropyran have been studied dilatometrically over the temperature range --20 to 9 °. The concentration of living ends was determined spectrophotometrically. The rate constant for ion-pair propagation decreases a hundredfold when the counter-ion is changed from Li + to Na +, rises a factor of 5 from Na + to Rb + and decreases slightly from Rb + to Cs + ; the activation energy is about zero for Li + and 5"7, 6"5, 7.0 and 7-3 kcal mole -1 (24, 27, 29 and 31 kJ mole -1) for Na +, K +, Rb + and Cs + respectively. The results are discussed in terms of the participation of two types of ion-pair in the propagation process for the smaller cations. The rate constant for the propagation by free anions at -- 1I. 5° is about 2501. mole- 1 sec- t, which is much higher than for propagation by any of the ion-pairs at this temperature, but much smaller than for propagation of the polymerization of styrene by free anions in tetrahydrofuran. IN THE p r e v i o u s p a p e r " ) w e d e s c r i b e d s o m e results o n t h e k i n e t i c s o f a n i o n i c p o l y m e r i z a t i o n o f s t y r e n e in t e t r a h y d r o p y r a n , w i t h N a +, K +, R b + a n d C s + as c a t i o n s . I n t h e p r e s e n t p a p e r w e r e p o r t t h e results o f a p a r a l l e l i n v e s t i g a t i o n o n a - m e t h y l s t y r e n e u s i n g all t h e alkali c a t i o n s .

EXPERIMENTAL The methods were similar to those employed for styrene. (t) However because of the high concentration of a-methylstyrene in equilibrium with dissolved polymer at room temperature, it was possible to premix the initiator and monomer at 20 ° and to start the reaction by rapid transfer to a thermostat at a lower temperature. The procedures for the purification of a-methylstyrene, Na, K, Rb and Cs are described in the previous paper, ct) Ethyl lithium was prepared from lithium and ethyl bromide in n-hexane by a method similar to that described by Morton et al. c~ and purified by several recrystallizations from dry benzene under vacuum. For the cations Na +, K +, Rb + and Cs + dilatometric measurements were carried out in the presence of the appropriate alkali tetraphenylborate to suppress ion-pair dissociation, but with Li + the technique had to be modified for two reasons. First, the rate constant turned out to be much larger than with the other alkali metal cations; this made it necessary to work at much lower living end concentrations. Secondly, although addition of LiBPh~ reduced the rate of polymerization, it also resulted in a marked change in the absorption spectrum, a stronger peak appearing at 420 rim. This made the determination of the living end concentration in the presence of LiBPh, uncertain. It was therefore necessary to carry out measurements in the absence of LiBPh, and at several low living end concentrations in order to allow for the effect of ion-pair dissociation. The dilatometer assembly for the study of the ethyl lithium initiated reaction is shown in Fig. 1. The dilatometer A had a volume of 18 ml and was made from four small tubes, i.d. 8 ram, and a capillary B, i.d. 4 mm. C was a quartz optical cell with a quartz spacer allowing the optical path length to be changed from 10 mm to 1 or 0" 1 mm. The shorter path lengths were calibrated by optical density measurements on potassium chromate solutions. The procedure for filling the dilatometer was as follows. Ampoule F contained a living solution, made by mixing ethyl lithium in THP with a-methylstyrene; this solution was used at a later stage for * University of Nottingham. 1"Department of Chemistry, University of Singapore. **To whom requests for reprints should be addressed. 387

388

F . S . D A I N T O N , K. M. H U I and K..1. IVIN VQCUUIT~

Line

j

i )( )(

A D

J

FIo. 1. Dilatometer assembly for the ethyl lithium initiated reaction. purging the system of impurities. Ampoule G contained 5 ml a-methylstyrene and H contained ethyl lithium. The apparatus was assembled as shown in Fig. 1, 20 ml T H P were distilled into D and the apparatus sealed off at E. Ampoule F was opened, the apparatus washed first with the living end solution, and then, after tipping the solution back into F the apparatus was washed with solvent condensed from F. Finally the solvent was distilled into D and frozen at -- 196*. F was then sealed off, the ampoules G and H opened, their contents transferred to D and after cooling D to -- 196" (to avoid pyrolysis of solvent vapour), the section G H was sealed off at I. The amounts of monomer and initiator were such as to give initial concentrations in D of 1.5 M in monomer and 10 - s to l 0 - s M in living ends. This mixture did not polymerize at 20*. After completion of the initiation reaction at room temperature the optical density of the reaction mixture was measured in C at the wave-length of the absorption maximum (in the region of 337 to 340 nm). The living end concentration was calculated using the extinction coefficient of 1 "5 × 10~ M -1 m -x determined electrolytically by Funt eta/. ~s) for the a-methylstyrene-sodium-THF system. Kinetic measurements were made by filling A and B with part of the solution in D and rapidly transferring the apparatus to a thermostatted hath below room temperature. After allowing sufficient time for the establishment of thermal equilibrium, dilatometric measurements were commenced. In order to study the reaction at another temperature it was only necessary to rewarm the reaction mixture to room temperature, allow depolymerization to go to completion, check the optical density and repeat the procedure. The bulbs J permitted the same initial reaction mixture to be used for measurements at lower living end concentrations. This was achieved by first tipping some of the solution into one of the bulbs J, distilling monomer and solvent back into D and sealing off the appropriate bulb J. A certain amount of monomer was usually left behind in J (as polymer) so that both the initial ion-pair and monomer concentrations were reduced. The procedure could be repeated with the further bulbs J. The procedure in the case of the other alkali metals differed in two respects from that used for lithium. First, the appropriate alkali tetraphenylborate was added to the reaction mixture from a n additional ampoule in order to suppress ion-pair dissociation. Secondly, the bulbs J were not necessary since it was relatively easy to make up solutions at the higher ion-pair concentrations simply by controlling the period of contact with the metal mirror in H. RESULTS The individual wherever possible meter readings at analysis was used

r e a c t i o n - t i m e c u r v e s f o l l o w e d a first o r d e r l a w a n d w e r e a n a l y s e d b y p l o t t i n g l o g Cn-hoo) a g a i n s t t i m e , w h e r e h a n d h ~ w e r e t h e d i l a t o time t and at infinite time. Occasionally Guggenheim's method of instead.

Kinetics of Anionic Polymerization of a-Methylstyrene in Tetrahydropyran

389

7.0

I

7

m 0

E +

Rb .64

+ K

2g

O

o

"~.~ L°

8

g.2

~

I 3"6

4CS

o

I 3.8

I /.'0

Fio. 2. Arrhenius plots for k(+) for the N a +, K +, R b + a n d C s + systems. ( F o r the sodium line the concentrations of sodium tetraphenylborate were: O , 1"4 x 10 - 2 M ; ~ , 2 - 3 ×

10 -2 M; Q, 4"0 x I0 -2 M).

For the N a +, K +, Rb + and Cs + systems the first order rate constant kl was accurately proportional to the concentration o f living ends [LE] when the appropriate tetraphenylborate salt was present. The second order rate constants, k(+~ = kl/[LE] are then the rate constants for ion-pair propagation and are plotted in Fig. 2. T h e y are not likely to be in error by more than 10 per cent as a result o f error in the assumed value for ¢. The Arrhenius parameters are summarized in Table 1. " ,~,

390

F . S . D A I N T O N , K . M. H U I a n d K . J. I V I N

For the Na + system the tetraphenylborate salt was sufliciendy soluble to allow its concentration to be varied; Fig. 2 shows that there was no effect on changing the concentration by a factor of 3 which confirms that ion-pair dissociation was completely suppressed. For the K +, Rb + and Cs + systems the reaction mixture was saturated with alkali tetraphenylborate, which has been shown to provide a sufficient concentration of cations to suppress propagation by free anions in the styrene systems. (*) TAet~ 1. ~ u S

1)ARAMr~RS l~OR ION-PAIR PROPAGATION IN THE ANIONIC POLYI~..R,IZATION OF a= INTHP

k(±) at 25 ° (1. mole - 1 sec= ~)*

E(±) (kcal m o l e - 1)

2"6 0"047±0"003 0-246±0"005 0"351 ± 0"009 0.260±0.006

0 5-6±0-2 6"4±0-1 7 - 0 ± 0"2 7.3±0.1

2"8±0-2 4"1 ± 0 " I 4-7 ± 0-2 4"8±0-1

- - 1 6 - 2 to - - 3 - 0 - - 1 2 " 6 to 9"0 --20-0to 5-0 - - 2 0 " 0 to 5"0 - - 2 0 . 0 to 5-0

7"2

8"2

- - 5 9 - 1 to - - 3 . 0

Counter ion

Li ÷ Na + K+ Rb + Cs + free a n i o n (s)

830

log A(±) (1. mole - 1 sec= 1)

Temperature r a n g e (°C)

* By extrapolation a n d b a s e d o n G ~- 1" 5 × 10 e M - 1 m - 1 o f ion-pairs a t t h e a b s o r p t i o n m a x i m u m in the region o f 337 rim.

For K + as cation some experiments were attempted without the addition of the tetraphenylborate salt but reproducible results could not be obtained: the apparent second order rate constant was higher than with added tetraphenylborate but fell offin successive experiments at the same temperature (after depolymerizing by raising to a higher temperature). This may have been due to a build up in the concentration of free K + by reaction of the living ends with trace impurities. For the Li + system (no added salt) kl was not quite proportional to [LE], indicating a contribution from free anion propagation as in the polymerization of styrene in THF. (*) The results were therefore analysed in terms o f Eqn. (1), ks (apparent) = k(+) -F k(_) K* [LE]-*

(1)

which is valid so long as the concentration of free anions is much less than that o f the ion-pairs. (*) k s (apparent) is the apparent second order rate constant, equal to kl/[LE], k(_) is the rate constant for propagation by free anions and K is the ion-pair dissociation constant. Plots of k s (apparent) against [LE] -~ at three temperatures are shown in Fig, 3. Each set of points was analysed by the method of least squares and gave k(±) -~ 2.4 -1-0.3, 2 - 4 - / - 0 - 2 and 2-7 q - 0 . 1 I. mole-* sec -1 at --3-0, - - l l - 5 and - - 1 6 . 2 ° respectively. These result in an apparent Et± ) of zero (Table 1). The values of k(_~ K ~ at --3-0, - - l l . 5 and --16.2 ° were 0.0252 -4-0.0088, 0-0157 ~ 0.0034 and 0.0075 q- 0.0018 l~ mole -~) sec -1, respectively. The errors quoted are standard deviations. The slight discrepancies between the different series o f Li + experiments are likely to be due to reaction with trace impurities leading to an increase in free lithium ion concentration and hence a variable degree o f suppression

Kinetics of Anionic Polymerization of =-Methylstyrene in Tetrahydropyran

391

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3.0

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I

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- l e 2°c

o I

I

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[ L E ] - ~ (.,.. ~2 mot;V2 ) Flo. 3. Plot of kp (apparen0 against [LE] - t for the Li + system (no added tetraphenylborate) at -- 3.0, -- 11.5 and -- 16.2 °. Points of a given type represent one series of experiments starting from a given reaction mixture (see text). o f i o n - p a i r dissociation. T h e Li + system is p a r t i c u l a r l y p r o n e to these effects b e c a u s e o f the necessity to w o r k a t very low i o n - p a i r c o n c e n t r a t i o n s . T h e values o f k c_) K t were c o m b i n e d with K values d e t e r m i n e d c o n d u c t o m e t r i c a l l y (s) to give k(_ ) -----338 -t- 118, 190 - I - 4 2 a n d 89 ~ 2 1 I. m o l e - 2 sec -1 at - - 3 . 0 , - - 1 1 - 5 a n d - - 1 6 . 2 °. T a k e n in c o n j u n c t i o n with the values d e t e r m i n e d in T H F as solvent, (5)

392

F.S. DAtNTON, K. M. HUI and K. J. IVIN

these lead to the quantities given in the last line of Table 1. k¢_~ at 25 ° for a-methylstyrene is some 80 times smaller than for styrene. ¢6>

DISCUSSION The main points for discussion are first, the large drop in reactivity in going from Li + to Na + and second, the upward trend in E¢i~ in going from Li + to Cs + which contrasts with the downward trend for styrene tl) in going from Na + to Cs +. The values of A<+~ also show a reverse trend. Three possible factors have been considered in interpreting these results: (1) An ion-pair-monomer c o m p l e x m a y be formed before addition of monomer. This has been suggested by several groups of workers <7" a) as the mechanism of anionic polymerization in hydrocarbon solvents. However there is no evidence for the formation of such complexes in ethereal solvents; in fact in mixtures of hydrocarbon and ethereal solvents there is strong evidence for preferential solvation of the ion-pair by the ether. <9> It therefore seems unlikely that the non-polar a-methylstyrene molecule can compete favourably with ether solvents in complex formation with the ion-pairs. (2) The electrical work component of the free energy of activation will decrease as the radius of the cation is increased. Calculations of the magnitude of this effect require assumptions about the interionic distance in both reactant and transition states. Treating the medium as a continuum and using the measured values of the dielectric constant D = 4.82 and d In D/dT = --0-0033 deg - i for a 1.5 M solution of a-methylstyrene in T H P at 25 °, one may show that if this were the only effect k<±; should increase monotonously by a factor of about 1000 in going from Li + to Cs +, but that the activation energy should decrease by only about 0.02 keal mole- t (0.08 kJ mole- x). Thus the very high k~±~ value for Li + as cation compared with Na ÷, and the reverse trend in Et±~ values cannot be accounted for in terms of this effect. (3) Two types of ion-pair (contact and solvent-separated) may be in equilibrium with one another and propagate the polymerization at different rates. Spectroscopic measurements on alkali fluorenyls"°~ and conductometric measurements on alkali salts in T H F tt~> lead one to expect the equilibrium to be displaced in favour of the solvent-separated ion-pairs as the temperature is lowered (their formation being exothermic), as the size of the cation is reduced (the smaller cations having a strong tendency to become solvated and therefore to form ion-pairs while solvated) and as the polarity of the solvent is increased. Furthermore it is reasonable to expect the solvent-separated ion-pair to have a reactivity intermediate between that of the contact ion-pair and the much more reactive free anion (k<_~ = 830 1. mole - I see - t at 25°). The high value of kc± ~ for Li + as cation is thus accounted for if the solvent-separated ion-pair is the dominant propagating species in the Li + system. The trend in E± values can similarly be explained. Suppose there is a transition from propagation entirely via solvent-separated ion-pairs S at low temperature (rate constant ks) to propagation entirely via contact ion-pairs C at high temperature (rate constant kc), as discussed by Barnikol and Schulz. ¢t2~ The behaviour in the intermediat e temperature range will depend on the magnitude of the heat of interconversion of the two types of ion-pair: -

C~----S

~ Ki,

AH~,

~.

.

Kinetics of Anionic Polymerization of a-Methylstyrene in Tetrahydropyran

393

When free anion propagation is suppressed, the apparent activation energy at a given temperature is given by E(±) =

kcEc + k,K, (E, + AHT) k~+k,K!

K, AH~1 + Kl

(2)

Three special cases may be considered: (1) If k,Ki >>k= (propagation entirely via solvent-separated ion-pairs), Eqn. (2) reduces to E(±) = Es q- AH~/(1 q- K,).

(3)

(2) If k,Kl >>kc and Kl is small (contact ion-pairs dominant, but propagation mainly via solvent-separated ion-pairs), Ec+ ) = E, q- AH~"

(4)

(3) For the temperature where the two types of ion-pair make equal contributions to the rate, k,Kl = kc, and Eqn. (5) follows from Eqn. (2): I-1 Klq E,+, -----½(Ec + Es) + ½[ ~ ] AH~. -

-

(5)

E~ may be expected to fall from Li + to Cs +, as in the anionic polymerization of styrene and ¢t-methylstyrene in dioxane. (13" 14) E, will probably vary much less with cation but still in the same direction. Examining Eqns. (3-5), and remembering that AH~ is negative, it may be seen that E(±) can be less than either Ec or E, and may even be negative. The extent to which E(±) is less than E= may be expected to depend on the cation and temperature so that the fall in E(±> in going from Cs + to Li + can be accounted for. The magnitudes of E(±> and A(±) for a-methylstyrene, when compared with those for styrene,~1) suggest that while solvent-separated ion-pairs play relatively little part in the propagation of styrene in THP at 20 ° for Na + or K + as cations, they play some part in the propagation of ¢-methylstyrene in THP at --10 ° particularly for Li + as cation, probably for Na + and perhaps for K +. Although we have obtained spectroscopic evidence for the presence of two types of ion-pair in a-methylstyrene-THF systems at low temperature, (Is) attempts to obtain similar evidence for THP systems have not been successful. This may simply mean that although solvent-separated ion-pairs may make a significant contribution to the rate of polymerization their concentration remains too low for spectroscopic detection, corresponding in the limit to special case (2), Eqn. (4). Finally it should be noted that the drop in k(±) in going from Rb + to Cs + is comparable with that for styrene;") but whereas with styrene it is apparently associated mainly with a fall in A(±>, with a-methylstyrene it is associated mainly with a rise in Ec±). The results will be discussed further in the next paper, c1`) Acknowledgement--K. M. H. thanks the Commonwealth Scholarship Commission for the a w a r d o f a Scholarship. POLVa~m5/$---~

394

F . S . D A I N T O N , K. M. H U I and K. J. IVIN

REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (I0) (I 1) (12) (13) (14) (15)

F. S. Dainton, K. J. Din and R. T. LaFlair, Europ. Polym. J. 5, 379 (1969). M. Morton, A. A. Rembaum and J. L. Hall, J. Polym. Sci. A1, 461 (1963). B. L. Funt, S. N. Bhadani and D. Richardson, J. Polym. $ci. A4, 2871 (1966). D. N. Bhattacharyya, C. L. Lee, J. Smid and M. Szwarc, J. phys. Chem. 69, 612 (1965). J. Comyn, F. S. Dainton, G. A. Harp¢ll, K. M. Hui and K. J. Ivin, Polym. Lett. 5, 965 (1967). T. Shimomura, K. J. T611e, J. Smid and M. Szwarc, J. Am. chem. Soc. 89, 796 (1967). Y. L. Spirin, A. A. Arest-Yakubovich, D. K. Polyakov, A. R. Gantmakher and S. S. Medvedev, J. Polym. ScL 58, 1181 (1962). K. F. O'Driscoll and R. Patsiga, J. Polym. Sci. A3, 1037 (1965). S. Bywater and D. J. Worsfold, Can. J. Chem. 40, 1564 (1962). T. E. Hogen-Esch and J. Smid, J. Am. chem. Soc. 88, 307 (1966). P. Chang, R. V. Slates and Iv[. Szwarc, J. phys. Chem. 70, 3180 (1966). W. K. R. Barnikol and G. V. Schulz, Z. phys. Chem. 47, 89 (1965). F. S. Dainton, G. C. East, G. A. HarpeU, N. R. Hurworth, K. J. Ivin, R. T. LaFlair, IL H. Pallen and K. M. Hui, Makromolek. Chem. 89, 257 (1965). F. S. Dainton, G. A. Harpell and K. J. Ivin, Europ. Polym. J. 5, 395 (1969). J. Comyn and K. J. Ivin, Europ. Polym. J. To b¢ published.

R6sum6--L'6tude cin6tique de la polym~risation anionique de I' a-m6thylstyr6n¢ dans le tetrahydropyrane a 6t6 effcctu6¢ par dilatom6trie pour une zone de temperature s'~tendant de --20/~ 9 °. La conccntration des bouts de chaine vivants est d6termin6¢ par spcctrophotom6trie. La constant¢ de vitess¢ de propagation par les paires d'ions est divis6¢ par cent quand on passe du centre-ion Li + au Na +, multipli6¢ par 5 du Na + au Rb + et d6croit faiblement du Rb + au Cs÷; r6nergi¢ d'activation pratiquement nulle pour Li +, vaut respectivement 5,7; 6,5; 7,0 et 7,3 kcal mole- ~ (24, 27, 29 ¢t 31 k.I mole -~) pour les centre-ions Na +, K +, Rb + et Cs +. On discute ies r~sultats en envisageant la participation de deux sortes de paires d'ions darts le processus de propagation en pr6sence des cations les plus pctits. La constante de vitesse de propagation par anions libres est d'environ 2501. mole- ~ so:- I/L -- 11,5 °; soit une valeur beaucoup plus importante que pour la propagation par n'importe quelle paire d'ions /~ cette temp6rature, mais beaucoup plus petite que pour la propagation par ions libres de la polym6risation du styr6ne dans le tetrahydrofurane. Sommario--La cinetica di polimerizzazione anionica dell' a-metiistirene in tetraidropirano 6 stata studiata dilatometricamente oltre rintervailo di temperatura --20-9 °. La conccntrazione delle terminazioni attive 6 stata determinata spettrofotometricamente. La constante di velocith per ia propagazione a coppie di ioni decresce un centinaio di volte quando il controione 6 cambiato da Li + a Na +, sale di un fattore dI 5 da Na + a Rb + e decresc¢ lentamente da R b + a Cs + ; renergia di attivazione 6 circa zero per il Li + e 5,7, 6,5. 7,0 e 7,3 kCal mole- 1 (24, 27, 29 e 31 kJ mole- 1) per Na +, K +. Rb + ¢ Cs + rispettivamente. I risultati sono discussi nei termini della partecipazione di 2 tipi di coppie di ioni nel processo di propagazione per i cationi minori. La constante di velocit,h per la propagazione da anioni libcria -- 11,5 ° 6 quasi 250 1. mole- 1 sec- 1, maggiore che per ia propagazione da alcune coppie di i o n i a qucsta temperatura, ma minore ch¢ per la propagazione della polimerizzazione dello stirene da anioni liberi in tetraidrofurano. Zusammenfassung--Die Kinetik der anionschen Polymerisation von a-Methylstyrol in Tetrahydrofuran wurde dilatometrisch fiber ¢inen Temperaturbereich yon --20 ° bis 9 ° untersucht. Die Konzcntrationen an lebenden Kettenenden wurde spektrometrisch bestimmt. Die Geschwindigkeitskonstante ffir die Wachstumsreaktion des lonenpaars nimmt um das hundcrtfach¢ ab. beim Obcrgang yon Li + zu Na + als Gegenion, steight um den Faktor 5 von Na + zu Rb ÷ und fiillt leicht ab von R b + zu Cs +. Die Aktivierungsenergie ist etwa Null ftir Li + und 5,7; 6,5; 7,0 und 7,3 kcal.Mol- 1 (24, 27, 29 und 31 kJ. M o l - ~) fiir Na +, K + Rb + und Cs +. Die Ergebnisse werdon diskutiert hinsichtlich dcr Bcteiligung yon zwci Arten yon Ionenpaaron beim Wachstumsprozcss mit kloinercn Kationcn. Die Geschwindigkeitskonstante ffir den Wachstumsschritt der freien Anionen ist bei -- I 1,5 ° ¢ t w a 250 L. M o l - ~. scc- 1 und damit schr vi¢l h6hcr als die Wachstumsgeschwindigkeit der verschicdencn Ionenpaare bei diescr Temperatur, aber sehr vi¢l kleiner als for die Polymerisationsgeschwindigkei yon Styrol in Tetrahydrofuran mit freien Anion©n.