Cationic and pseudocationic polymerization of aromatic olefins—II The reactions following polymerization of styrene by perchloric acid

European Polymer Journal, 1968, Vol. 4, pp. 55-74.

Pergamon P'te~s

Printed ha England.

CATIONIC AND PSEUDOCATIONIC POLYMERIZATION OF AROMATIC OLEFINS--II. THE REACTIONS

FOLLOWING

POLYMERIZATION

BY P E R C H L O R I C

OF STYRENE

ACID

A. GANDINI* a n d P. H. PLF,SCH University of Keele, North Staffordshire, England (Received 22 June 1967)

Abstract--We report spectroscopic and conductimetric studies on the reactions of styrene in the presence of perchloric acid (most in methylene dichloride but some in ethylene dichloride), both during and after the polymerization. During polymerization no ions are detectable, but at the end aralkyl (and subsequently aUylic) ions are formed. Quantitative results show that ion formation sets in when the styrene concentration has fallen to four times the acid concentration; we interpret this finding as showing that the ester (oligo-)styryl perchlorate requires four molecules of styrene for stabilization in solution. The ionogenic reaction which follows polymerization changes its kinetics from second to first order. It is completely inhibited if the solution contains more water than acid, though this does not affect the polymerization itself. At the end of the polymerization, the ionogenic reaction sets up an equilibrium which involves many components and which is slowly shifted by formation of stable species; the position of equilibrium can be driven back towards predominance of non-ionic compounds by addition of fresh monomer. The relation between our work and that of other authors is discussed in detail. INTRODUCTION IN arm first p a p e r o f this series (11 we r e p o r t e d on the kinetics o f the p o l y m e r i z a t i o n o f styrene b y perchloric acid in methylene dichloride. Some o f the other features of this r e a c t i o n a n d the reasons for describing it as " p s e u d o c a t i o n i c " have also been r e p o r t e d briefly(2, 3> a n d have been discussed in a wider context. ~4-61 The present p a p e r deals specifically with the f o r m a t i o n a n d d e s t r u c t i o n o f ions during a n d after the p o l y m e r i z a tion. A l t h o u g h these results have been available for some time, we c o u l d n o t interpret t h e m with confidence until the nature o f the ions concerned h a d been clarified. N o w t h a t the c o m p l i c a t e d ionogenic reactions o f styrene with an excess o f perchloric acid have been unravelled, (7> the ionogenic reactions following its p o l y m e r i z a t i o n have also b e c o m e much m o r e intelligible. EXPERIMENTAL Materials Styrene,(8) perchloric acid,(9) methylene dichloride(lo) and 1,2-dichloroethane(101 were purified, stored and dosed as described. Phials containing known quantities of water were prepared as described.(111 The products of all the polymerizations mentioned in this work were oligomers of DP between 3 and 8. * Present address: Brookhaven National Laboratory, L.I., N.Y., U.S.A. 55

56

A. GANDINI and P. H. PLESCH

Techniques All the spectroscopic experiments were carried out in the all-glass high-vacuum device described,(lz) provided with a 1 cm Pyrex or silica cell; solutions were made up by vacuum distillation of styrene, solvent and, when needed, water, into the device which had been previously charged with a phial of perchloric acid solution. Reactions were started by crushing the phial of acid in the olefin solution; scanning could be started about 20 see after the mixing. Additions of styrene at the end of the first reaction were accomplished through magnetic break-seals mounted on the device. All this work was performed at room temperature (20--23*) with a UNICAM SP700 recording spectrophotometer. Because of the relatively high styrene concentrations, it was impossible to obtain spectroscopic information at wave lengths below about 350 mt~ in most experiments. Conductivity measurements were carried out in a high vacuum cell in the shape of an inverted cone provided with a phial breaking device( TM and with bright platinum electrodes fixed near the apex supported by tungsten leads. The cell contents were stirred magnetically. With a perchloric acid phial in the breaking device, the cell was pumped out for a few hours. The solvent and the styrene solution were then dosed into the cell from dispensing burettes through allmetal taps.(~4) The reactions were started bycrushing the acid phial in the well-stirred, thermostatted (oil bath) olefm solution. The changes in conductivity were followed by readings taken every 5-10 see on a Wayne--Kerr B221 Universal Bridge. The cell constant, computed by Fuoss's method,(ts) was 0.110 cm-L Conductivity of materials The specific conductivity of the styrene solutions in ethylene dichloride was (I to 2) x 10-9 mho/cm; that of a 4.0 x 10-z M solution of perchloric acid was 9 x 10-s mho/crn. The conductivity of the acid solutions increased very slowly, and linearly, with time, but this change was so small that it did not cause any ambiguity in our results. It is due to a slow attack of the acid on the tungsten leads carrying the Pt electrodes. A plot of the spectroscopically computed maximum ionic concentrations [S.D. ions],. (sum of free ions and ion-pairs) at the end of the reactions* against the final values of the specific conductivity x,, for the equivalent runs (Tables 1 and 2) gave a straight line through the origin, showing that virtually only free ions were present in our systems; if there had been important quantities of ion-pairs, this plot would have been markedly curved. Reproducibility and errors In nominally identical experiments the differences between rates, maximum optical densities and conductivities were never greater than could be accounted for by instrument errors or by errors arising from computation of concentrations. No inexplicable irreproducibilities were found.

QUALITATIVE

RESULTS

Like all earlier authors, we assigned (16) the a b s o r p t i o n near 309 a n d 421 m ~ , o b s e r v e d when styrene reacts with a n excess o f p e r c h l o r i c acid a n d also in m a n y related systems, to the 1-phenylethyl ion. W e n o w k n o w t h a t this assignment is incorrect. (v) This a b s o r p t i o n is in fact due to the ions derived f r o m 1-methyl-3-phenylindane (the cyclic dimer o f styrene) a n d its higher h o m o l o g u e s (oligostyrenes with i n d a n y l end groups). There can be no d o u b t t h a t the ions f o r m e d at the end o f the p o l y m e r i z a t i o n o f styrene b e l o n g to the same families o f c o m p o u n d s (indanyl a n d various p h e n y l alkyl c a r b o n i u m ions(V)). O u r evidence showed t h a t the 1-phenylethyl c a t i o n is absent f r o m the ions f o r m e d f r o m styrene b y excess o f a c i d ; its dimeric h o m o l o g u e , the 1,3-diphenyl-n-butyl cation, is a m i n o r c o m p o n e n t o f the i o n mixture. W e refer to this mixture o f ions f o r m e d r a p i d l y f r o m styrene b y excess acid, or at the end o f a styrene p o l y m e r i z a t i o n , as S.D. (styrene-derived) ions. I n o r d e r to study the v a r i a t i o n o f the c o n c e n t r a t i o n o f ions d u r i n g a n d after p o l y merization, we scanned the solutions at 424 m/~ a g a i n s t time. W e f o u n d that, in * The subscript m denotes maximum values throughout this paper, the subscript 0 denotes initial "values.

Cationic and Pseudocationic P o l y m e r i z a t i o n o f A r o m a t i c Olefins--II

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Cationic and Pseudocationic Polymerization of Aromatic Olefins--II

59

addition to the S.D. ions, several other species giving peaks in the visible were formed at the end of the polymerization. Therefore the kinetic study on the formation of the S.D. ions had to be supplemented by repeated scannings of the fuU visible and u.v. spectra. Some of the experiments carried out in the spectroscopic device were repeated in the conductivity cell. In view of the rather complicated behaviour of the system after the end of the polymerization, we shall first give a chronological account of the b

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FIG. 1. A typical sequence of visible spectra for a spectroscopic experiment, a: At the end of the ion formation (only S.D. ions), b: Some 10--20min later (mixture of S.D. and allylic ions), c: About 1 hr later (more allylic ions), d: A few hours later (dienic polymer molecules + allylic ions), e: 20--30hr later (more dienic polymer molecules and less allylic ions), f: Same as " e " after addition of a few drops of ethanol (only dienic polymer molecules). phenomena, and then discuss their quantitative aspects. Figure 1 illustrates the different phases of the reaction from a spectroscopic point of view.

1. The latency period Provided that the ratio [styrene]/[HCIO4] was higher than about 4, D424did not exceed 0.0087 (experimental error for 0-000) for some time after the mixing of the reactants. This indicated that if ions with ~424 equal to about 4 × 103 had been formed at all, their concentration was lower than about 10-6 M. No other absorption band

60

A. GANDINI and P. H. PLESCH

could be detected in the visible spectrum of the solutions. The only change in conductivity during this first phase of the reaction was a slow and linear increase, independent of working conditions, due to attack on the tungsten leads. This period of complete visible transparency and absence of organic ions we name the latency period (t'). It is a well-defined and reproducible feature which comes to an abrupt end with the onset of ion-formation. We ascertained that, at the end of the latencyperiod, the polymerization itself is almost complete; this was done by calculations involving the rate constant determined from kinetic experiments, by "killing" the reaction mixture at this stage and isolating the polymer and, for experiments with very low monomer concentration, by observing disappearance of monomer spectroscopically. Thus the reactions following the latency period cannot involve the monomer.

2. The formation of an unstable intermediate At the end of the latency period, the reacting solutions became pinkish-red and a peak appeared at 510 m/z. The lifetime of this peak lay between 5 and 30 see, according to the working conditions, but a detailed study of its origin could not be made because of its transitory nature.

3. The formation of the S.D. ions The vanishing of the peak at 510 mtz left a yellow solution with a single peak at 424 m/z in the visible region of the spectrum. 9424 and the specific conductivity of the 10-9--

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typical pair of curves for the formation of S.D. ions; SGU14, spectroscopic; SGC9 by conductivity(see Table I); A is the cell constant.

solutions increased with time, as shown in Fig. 2. The curves representing this production of ions were S-shaped, but the accelerating part was always very short compared with the rest of the curve.

4. The formation of indanyl end-groups If the reaction mixture was left under vacuum for several hours after the end of the ionogenic reaction described in section 3, the polymer no longer contained doublebonds (u.v. spectrum), indicating that the formation of indanyl end-groups went to

Cationic and PseudocationicPolymerizationof AromaticOIefins--II

61

completion. At the same time D424 fell to zero and the conductivity decreased to the back-ground level due to impurities.

5. The formation of allylic ions Towards, or after, the end of the reaction leading to the formation of the S.D. ions, the spectra of the solutions slowly developed a rather broad peak around 450 m/z (Fig. 1). We attribute this peak to allylic carbonium ions formed by hydride ion abstraction from an unsaturated polymer molecule by an S.D. ion X+: --CHPh. CHz.CHPh.CH: CHPh + X+ + C102" --XI-I + - - C H P h . CHz--(CPh'"'CH=zCHPh) + +

CI02"

This conclusion is based on a study of allylic ions,(t7) on the recovery of polyunsaturated oligomers with conjugated double bonds (see section 6), on analogous observations in aliphatic systems, (is, 19) and on the occurrence of analogous processes in anionic systems. (z°) Inoue and Mima (zl) have reported that, when gaseous styrene was bubbled through 98 % sulphuric acid, a yellow solution, absorbing at 450 m/~ was obtained. They attributed this rather broad peak to the 1-phenylethyl carbonium ion, but it is clear now that allylic ions were formed in their system following the partial oligomerization of the monomer.

6. The formation of poly-unsaturated oligomers On long standing under vacuum, the reacting solutions became fluorescent and their visible spectra exhibited a pair of peaks at 390 and 410 m/x whilst the 450 mtz peak was reduced: these peaks were not destroyed by neutralization of the solution and were present in spectra of solutions obtained by redissolving the recovered polymer in methylene dichloride. We attribute these twin peaks to oligomers containing at least two conjugated double bonds and formed by proton abstraction from an allylic ion by some more basic species, such as a highly conjugated system: --CHPh. CH2(CPh'"'CH'"'CHPh)+

--H+

~ --CHPh--CH~--------CPh---CH~---CHPh

This attribution is based on the fact that diphenylpolyenes absorb with a strong twin peak, at wavelengths which increase as the number of double bonds is raised (the compound Ph(CH=CH)4Ph absorbs at 385 mtz and 405 m/z). Polystyrenes with conjugated double bonds, having a phenyl group on alternate carbon atoms of the chain, would possess a higher degree of conjugation than the diphenylpolyenes, so that one can estimate the number of conjugated double bonds in our oligostyrenes to be two or three. By using the extinction coefficient of the diphenyl polyenes, we can estimate the concentration of these groups of conjugated double bonds to be between I0 and 25 per million oligostyrene chains of ~ about 5. Despite their very smaU number, these groups impart to the polymer solutions a strong fluorescence which slowly disappears ifthe solutions are left exposed to air and light, because ofoxidation. The concentration of aUylic-type ions was presumably about the same as that of the groups of conjugated double bonds. However, since the extinction coefficient of such ions is probably as high as ~ 105, i.e. about twenty times greater than that of the S.D. ions, the resulting D4s0 was of the same order of magnitude as D424.

62

A. GANDINI and P. H. PLESCH

7. The effect of further monomer additions At any stage after the end of the polymerization of the first portion of styrene (i.e. after the latency period t'), addition of a further quantity of monomer induced complete decoloration of the solution (except for the fluorescence ~ven by the poly-unsaturated TABLE 3. THE EFFECTOF SUCCESSIVE STYRENEADDH'IONS ON THE SPECTROSCOPIC BEHAVIOUROF THE

SYSTEM HCIOa-STYRENE-CH2CIz

Experiment SGU11 Addition N

10z[St]o.N

I 2* 3t

(M)

t'~v (sec)

[D450-480]m

(M)

104[HClO4]o.,v

34.0 37.9 25.5

12.0 11"8 11.4

450 480 470

1"1(452mF) 1.3(462mF) 1.4(477m~)

[D410]m

0.25 0.41 0-64

* 30 hr after beginning. ? 50 hr after beginning.

N.B. D450-480 went to 0 after each addition.

oligomers), with a corresponding reduction in conductivity. After a second latency period, during which the monomer was polymerized, colour and conductivity reappeared. The formation of ions followed a pattern similar to that encountered at the end of the first polymerization: the position of the broad peak, which slowly replaces

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300 Time,

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600

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(See Table 4.) the initial peak, was found to be variable depending on the polymer chain concentration in the reaction mixture. Thus for Exp. SGU11, in which three successive additions of styrene were made (Table 3), the h ~ was 450-453 mF at the end of the first reaction, 462-463 m/z at the end of the second and 475--479 m/~ at the end of the third. This effect is probably due to progressive formation of more highly conjugated systems.

Cationic and Pseudocationic Polymerization of Aromatic Olefins--H

63

Figure 3 illustrates a conductivity experiment with four successive additions of styrene: whenever more styrene was added, the equivalent conductance (referred to the total perchloric acid concentration) fell abruptly, but not quite down to its original value during the first latency period. This is not because some carbonium ions remained after the styrene addition, but because the free acid present after the end of the polymerization reacted with the tungsten leads, as mentioned above. This was confirmed by a blank experiment without styrene which gave the dotted base-line in Fig, 3. Table 4 gives details for the experiment illustrated in Fig. 3. Similar features were obtained in spectroscopic experiments involving further styrene additions. Table 3 refers to one of these: the plot of D424 against time was strictly equivalent to that shown in Fig. 3. TABLE4. THE EFFECTOFSUCCESSIVESTYRENEADDITIONSONTHE CONDUCTIMETRIC BEHAVIOUR OF THE SYSTEMHC104-STYRENE-CH2CI2 Experiment SGC11 Addition N 1 2 3 4

102[St]o..v (M)

104[HC104]o..v (M)

t"N (see)

K,n (tzmho cm-1)

8.0 6.67 5-24 3.75

320 450 700 1000

1.22 2.29 2.57 2.30

6.1 8.3 10-6 13-5

8. The influence of added water The influence of added water upon the ionogenic reaction was studied only spectroscopically: the results obtained are given in Table 5. When sufficient water was added to make [HzO] ~ 5[HC104]0, no carbonium ions at all were formed at the end of the polymerization (run SGU17): this shows once again the strongly poisonous effect of water on these ions. When [HzO] ~ ½[HC104]0, both the rate of ion formation and the final ionic concentration were considerably lower than those obtained for a " d r y " run (compare SGU9 and SGU20, Table 5). These results indicate that, at the end of the polymerization when free acid is regenerated (see below), the water which was TABLE 5.

THE INFLUENCEOF WATERUPONTHESPECTROSCOPICBEHAVIOUROFTHESYSTEM HC104-STYRENE-CH2Clz

Exp. No.

[St]o (10z M)

SGU8 SGUI7 SGU9 SGU20

7"1 7.5 18.0 20-0

[HC104]0

[I'-120]

t'

R"

(104 M)

(104M)

(see)

(105 M-1 rnin-1)*

[S.D. ions],. (105 M)*

6"3 6"3 18.0 17-0

-32"0 -7.4

475 -180 240

0"725 none 11-09 7-17

5"96 none 37-6 29-5

* See the footnote to Table 1. 5

64

A. GANDINI and P. H. PLESCH

present in solution and which did not affect the actual polymerization, reacts with an equivalent amount of perchloric acid to form H30+CIO4- which is too weak to cause ionization. If [I-I20] < [HCIO4]0, the rate of formation of S.D. ions and their maximum concentration, both of which depend on [HC104], are reduced because the active acid concentration is only [I"ICIO4] 0 - [ H 2 0 ]. Our evidence concerning the effect of water on carbonium ions shows convincingly that, since the rate of polymerization is not affected by relatively large quantities of water, [HCIO4].

9. The reactions in 1,2-dichloroethane Five reactions in 1,2-dichloroethane were performed to discover whether the interaction of styrene with perchloric acid in this solvent, used by Pepper and Reilly,<22~ produced a pattern of behaviour similar to that encountered with methylene dichloride. A latency period was noticed in all the reactions, but it was always shorter than the corresponding one in methylene dichloride. The absorption peak growing at the end of this period was centred around 395 m/z; the maximum D395 was always lower than the corresponding D424 in methylene dichloride. The third phase in the reaction produced a broad peak at about 450 m/z, which slowly grew while the 395 m/z peak correspondingly declined. Several hours after the mixing, the usual pair of peaks at 390 m/z and 410 m/z had formed, while the peak around 450 mtz was still present. Ethanol discharged the latter peak but not the former ones. The fact that the latency times were shorter than in methylene dichloride is readily explained because the polymerization in 1,2-dichloroethane is faster, for the same working conditions.~221 The only substantial differences between the behaviour in the two solvents is the absence of the S.D. ions in runs conducted in 1,2-dichloroethane; we were never able to detect a peak at 424 m/z in these spectra. The nature of the ions, produced at the end of the polymerizations and absorbing at 395 m/z, is still obscure. The rate of formation of ions after the polymerization in 1,2-dichloroethane was very similar to that obtained for the equivalent experiments in methylene dichloride. On the basis of these findings and of those reported by Pepper and Reilly, we conclude that the polymerization of styrene, catalysed by perchloric acid in 1,2-dichloroethane, is very similar to the reaction in methylene dichloride, but the reactions following polymerization are different. 10. Attempts to isolate 1-phenylethyl perchlorate The reactions to be described were carried out in an all-glass high vacuum device consisting of a reaction tube connected through a sintered-glass falter plate to the tipping device described, c9~ When a phial of 1-phenylethyl bromide was crushed in a silver perchlorate suspension in methylene dichloride, reaction started immediately; the suspension turned orange-yellow and its colour deepened considerably within a few minutes while silver bromide was precipitated. After about 1 hr, the mixture was filtered off into the tipping device and phials of the reaction product were sealed off. The visible spectrum of the solution had a major peak at ~ 470 m/z. Analysis of the

Cationic and PseudocationicPolymerizationof Aromatic Olefms---II

65

organic part of the reaction product showed it to consist mainly of oligostyrenes with absorption at 390 and 410 m/~. Addition of a large excess of styrene to a solution of the reaction product made the colour disappear and the solution became hot. After about 2 min the colour began to reappear: the spectrum showed a peak at 424 m/z, which in a few minutes was shifted to a broad peak centred at about 475 m/z. The final value of D475 was higher than that obtained from the original solution. The polymer obtained from this run had DP = 6. The situation can be summarized as follows: (a) The reaction of silver perchlorate with 1-phenylethyl bromide gives S.D. ions at first which, with their anions, form perchloric acid and styrene which is then polymerized. The ultimate result of this series of reactions closely resembles the state of affairs at the end of a styrene polymerization catalysed by perchloric acid: a twin peak in the near visible portion of the spectrum and the broad peak at about 470 mp.. (b) Addition of styrene to the ionic solution obtained from the reaction suppresses the ions whilst the added monomer polymerizes in the same way as when a second addition of styrene is made to a polymerized, coloured solution after a " n o r m a l " polymerization experiment. The colour re-emerges at the end of the polymerization and its final intensity is, as in the " n o r m a l " spectroscopic experiments (Table 3), higher than it was before the styrene addition. QUANTITATIVE RESULTS AND DISCUSSION The following discussion is based on our earlier conclusion that the propagating species in these polymerizations is oligostyryl perchlorate ester, which is stabilized in solution by excess styrene. One question arising concerns the number of molecules of styrene per ester molecule required for this stabilization; another concerns the kinetics and mechanism of the ionogenic reactions which ensue once the styrene concentration has been reduced by polymerization to such a low level that the quantity of styrene no longer suffices to stabilize the ester.

The number of styrene molecules required to stabilize the ester Three series of experiments were carried out; one involved constant [HC104]o and variable [St]0 (Table 1), and two involved fixed [St]0 and variable [HC104]0 (Table 2). The values of t' for both conductivity and spectroscopic experiments are given in Tables 1 and 2; the agreement between them was always very close. Since we know the kinetics of the polymerization under the present conditions, (1) we can relate the latency period to the extent to which the polymerization had proceeded during that time. The disappearance of styrene follows the rate equation: -d[St]/dt = kp[St] [HC104]0. It follows that log [St]0-1og [St]t = kp[HC104]0 t/2.3.

(t)

If one assumes that at the end of the latency period, i.e. at t = t', the number of styrene molecules remaining is a multiple n of the number of chain carriers which is in turn equal to the number of acid molecules introduced, (1) i.e. that [St]c=n[I-IC104]0, Eqn. 1 takes the form: log [St]0-1ogn-log [HCIO4]0 = kp[HC104]ot'/2"3.

(2)

66

A. GANDINI and P. H. PLESCH

Equation 2 can be used in two ways to obtain values of n. For runs at constant [St]0, a plot of log [HC104]0 against [HalO4]0 t' should give a straight line with slope -kp/2"3 and intercept log [St]0-1ogn. For runs at constant [HCIO4]0, a plot of log [St]0 against t' should again give a straight line with slope kp[HC104]0/2.3 and intercept log [Stir, = log [HC104]0 + logn.

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FIG. 4. Determination of the quantity of styrene remaining at the end of the latency period t' by means of Eqn. 2. ©--Spectroscopic experiments. @--Conductimetric experiments. Straight lines were in fact obtained (Fig. 4) from the slopes of which we computed values of kp (11.8 and 11.7 1. mole - l see-1 respectively). These agree with the value of 11.7 obtained from the kinetic results (° for 22 ° and thus confirm the validity of the procedure. The values of n obtained from the intercepts are 4.2 and 4.0 respectively. Our results thus show that at least four molecules of styrene are needed for the stabilization of one molecule of ester. Another, necessarily much less precise, method for determining n is available from 6--

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<~

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0

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FIo. 5. Polymerization curve for a reaction carried out in the calorimeter (see Ref. 1. Exp. SGP 14). T0~---19°, [St]0=0-264, [HC104]o=7-3× 10-s M. When the colour appears and the r a p i d r e a c t i o n sets in, [St] = ~ 2"7 × 10"2 M.

Cationic and Pseudocationic Polymerization of Aromatic Olefms---II

67

the kinetic experiments. (1) At the end of these reactions, at the precise instant at which the reaction mixtures turned yellow, a very fast reaction took place (Fig. 5). This represents the polymerization of the residual styrene by a true cationic reaction caused by the ions formed from the ester at the point where the styrene concentration was reduced to a level no longer sufficient to stabilize the ester. From the very small temperature rise during this final fast reaction, the number of styrene molecules polymerized could be calculated; it was always about four times the initial concentration of perchloric acid. This phenomenon was particularly evident in the reactions carried out at - 1 9 ° in which relatively high acid concentrations were used so as to obtain reasonably fast polymerizations. (1) Our interpretation of these phenomena is as follows: the ester styryl perchlorate is not stable alone in solution, but this ester and its oligomeric homologues do exist in the presence of excess styrene, consequently the styrene must stabilize the ester. Presumably it does this by being co-ordinated (probably to the oxygen atoms) and thus reduces the polarity of the ester carbon-oxygen bond. It is not known yet whether any other compounds can exert the same effect.

The ionogenic reaction The ionogenic reaction which sets in abruptly at t = t' (see section 1 above) was followed spectroscopically by scanning at 424 m/z against time and by following the

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L.o

(mole/L}2C

Fro. 6. The dependence of the maximum rate of ion formation, Rm, on the initial acid concentration (Table 2). Spectroscopicexperiments: o--scales A and D, e--scales B and E. Conductimetric experiments: G--scales C and F. change of conductivity. The results from both methods were always in complete agreement. The kinetic information thus obtained cannot yet be interpreted in detail. One consistent feature was that the maximum rate (at t,,, the end of the acceleration) was proportional to [HCIO4] 2 (Fig. 6) and most of the reaction curve thereafter obeyed

68

A. G A N D r N I and P. H. P L E S C H

IZ

O O O

÷ O

i~" 0.4 B --

+ 0

-I-"

0

!

j.t~

o-e

__

+,..+/+

4

0

v.%

I

200

[

I

400

600

Time,

800

sec

FIG. 7. Kinetic analysis of a typical ionogenic reaction (Exp. SGC 9) by means of first and second order plots, p is the ratio [S.D. ions]d[S.D, ions],,,. The change of order occurs at about 300 sec when p : ~ 70 per cent.

second order kinetics (equal initial concentrations, with respect to the final concentration of ions) (Fig. 7). This means that the ions are formed by some species reacting with its own kind; in view of the fact that at t = t' the nominal (i.e. initial) concentration of acid, [HCIO4]0, is in fact the concentration of ester, it means that the ions are formed by reaction of two ester molecules. If, for simplicity, we show only the principal component of the S.D. ion mixture (the 1-aralkyl-3-phenylindanyl cation) the stoichiometry of the reaction is this: R 2 RCH.,CHPh.O.ClO~

>

~ ) ~

ClO]

Fh + RCH2CH,Ph + HClO4

where R = oligostyryl. The mechanism of the reaction is still obscure but our results are probably compatible with those of Pepper and Barton C23)who found that the rate of disappearance of unsaturation, when 1,3-diphenyl butene-1 (distyrene) reacted with perchloric acid, was of second order in olefin and in acid. Unfortunately they did not investigate whether any saturated products other than the cyclized dimer were formed, although they found some higher oligomers. The final part of the ionogenic reaction is of first order with respect to the ionogenic species (Fig. 7) and the rate constant kl is proportional to [HCIO4]0 (Fig. 8). This may indicate that when the concentration of ester becomes small, and that of free acid large, the dominant ionogenic reaction is the acid-catalysed ionization of the ester.

Cationic and PseudocationicPolymerization of Aromatic Olefins---II

69

When the conductivity and optical density had attained their maximum values, the spectra showed that double bonds were still present. The acid released by the decomposition of the ester catalyses the subsequent slow cyclization whereby these terminal double bonds form indanyl end groups. (23) The situation at the end of the ionogenic reaction (maximum conductivity and D424) is complicated, in that the reaction mixture then contains ester, saturated and 50

4O i

30

.

2O

I0

;xl 5

io

IO 4 [HCt 04]o,

I 15 molell.

I 20

FIG. 8. The dependence of kt on the initial acid concentration, kl =2"303St, where St is the slope of the plot for the first-order part of the ionogenic reaction (Fig. 7). O--Spectroscopic experiments. @--Conductimetric experiments.

unsaturated oligomers, and S.D. and allylic ions. That all these are in a quasi-equilibrium (which is slowly shifted by the progessive cyclization and formation of multiple unsaturation) is shown by the following observations: (i) The concentration of S.D. ions is always much lower than the initial concentration of acid. (ii) The conductivity increases considerably when the temperature is reduced from 23 ° to 0 °, and returns to its initial value when the system regains the original temperature (Tables 1 and 2). (iii) When the concentration of double bonds is increased by addition of more monomer the ions disappear completely. The scheme shown in Fig. 9 indicates the reactions involved. The scheme expresses most of what is known about the system; some parts of it are less certain than others and it may need some detailed modification in the light of further work, but it is a useful model whereby this system can be understood. The species IA, IB, IC represent the chain-propagating ester molecules, stabilized by styrene, and they are equivalent from the point of view of the polymerization. At the end of the polymerization, the now unstabilized ester II reacts with its own kind to give the indanyl ion III and a saturated linear polymer IV. It is also in equilibrium with unsaturated polymer V, and it reacts with acid and/or V to give the polymer with indanyl end groups VI. This is equivalent to the transfer reaction with monomer which gives the indanyl end groups. (23) The oligostyryl ion VII can only be present in very small concentration, as it is much less stable than the other S.D. ions which

70

A. GANDINI and P. H. PLESCH

co-exist with ion III(7); these other S.D. ions have been omitted from the scheme so as not to complicate it unnecessarily. The equilibrium V I - V I I requires some comment. We reported originally (16) that the indane dimer of styrene (VI, R = CH3) did not react with strong acids. Subsequent experiments(7, 24) have shown that it does react to give the ions III, VII, and others. (We have been unable to discover the reasons for our erroneous result.) In sulphuric acid the ion VII is formed as a minor constituent of the mixture of

(IA)

(If)

- 4St

(RCH2CHPhA)4st

•

(V)

[RCHzCHPhA]

-~-. . . . . ~- RCH:CHPh+HA

~eh

+ 4St

I

R

~

CH2CHPhA

(IB)

+HA (VI)

R

A+VI

Ph /~

(Ill)

. . . . -._~ +RCHzCHzPh +HA

Allylic ions, muliiply unsatd.

polymer (Vlll)

nSt

) Polymer

R = oligostyryl,A=CIO4 FIG.9.Comprehensive reaction scheme.

-VI

[RCH2CHPh~A-] (VII)

Cationicand PseudocationicPolymerizationof AromaticOlefins--II

71

ions but we were unable to find it in experiments with perchloric acid in methylene dichloride.C7~ None the less, it seems likely that minute amounts of it are formed at the end of the polymerization when the perchloric acid is set free. The main reason for introducing the oligostyryl ion VII into the scheme is that we believe it to be responsible for the very fast polymerization of the last styrene molecules (Fig. 5) and also the fast, abnormal polymerization, especially obvious at low temperatures, which occurs when a high local concentration of acid is introduced into a styrene solution. It seems reasonable, therefore, to suppose that these ions are also important in the ionogenic reactions following the normal polymerization but they do not affect the end-products directly. These consist essentially of polymer of types VI and VIII, whose formation is a slow draining of the system of equilibria. At the end of the polymerization, when species IA has disappeared and ions are present, the addition of styrene makes the ions vanish instantaneously and they remain absent whilst polymerization proceeds. Moreover, this polymerization has the same rate constant as the first. This means that it cannot have been initiated only by the acid that was free at that time and that the acid bound as ions must also have become available. These facts are represented by the reaction paths leading to esters IB and IC, which complete the cycle whereby eventually ions are formed again, and can be destroyed again by addition of more monomer. Of course, reaction of the freshly added monomer with the then free acid leads to formation of ester IA. The maximum concentration of carbonium ions increases after each addition because of the increasing double bond concentration, as the polymer concentration increases. Thus the final value of the equivalent conductance and of D424 are higher after each addition, and they depend upon the actual concentrations of double bonds and of acid. The bathochromic shifts with increasing chain concentration are compatible with the mechanism proposed above, since an increase in the concentration of unsaturated chains will favour hydride abstraction and will therefore give allylic ions with higher degrees of conjugation, which will absorb at wavelengths greater than 450 m/x. The only serious chemical (as opposed to mechanistic) uncertainty in this scheme is whether route III--> IB or III--+ IC, or both, or perhaps some other process, adequately represent the removal of the ions by addition of monomer. Some reaction path of this kind seems to exist since there is no evidence that either route II --->III or route II --->VI is reversible. COMPARISON WITH RESULTS OF OTHER WORKERS Bywater and Worsfold(zS~ studied essentially the same system; where our results overlap, they agree. These authors established that during polymerization there is no absorption down to about 305 m/z; this we have confirmed.(2a~ They also showed that during, but not after, the polymerization, the acid cannot be distilled out of the reaction mixture. This supports our view of ester formation, and release of acid during the ionogenic reactions following the polymerizations. They also showed very clearly the rise and fall of the concentration of unsaturated endgroups during and after polymerization. However, we cannot agree with their suggested reaction scheme. In particular, their penultimate equation in which a free proton appears to be released from an oligostyryl

72

A. GANDINI and P. H. PLESCH

cation is implausible. It might be combined with their last equation, in which the free proton is shown reacting with perchlorate anion, but then the resulting reaction orders would be incompatible with observation. The central question at issue is still whether pseudocationic polyrrrerisation, propagated by a large concentration of activated ester, is a real phenomenon sui generis or whether all these reactions can be explained in terms of very fast propagation by a very small concentration of conventional carbonium ions. The results of Enikolopyan and co-workers(27' 2s~ on the polymerization of styrene by perchloric acid at high pressures shed some new light on the problem. Essentially their kinetic results agree with those of Pepper and Reilly and of ourselves. The important feature of their findings is that the extent of acceleration by pressure is merely that which can be attributed to increase of dielectric constant of the solvent. There was no effect which could be attributed to increasing abundance of free ions by increased dissociation of ion-pairs. This means that, if the propagating species are ions, then they are all free ions even at normal pressure (which is reasonable), or the propagating species is non-ionic. Bywater and Worsfold's (2s) and our own (26) results show that for the S.D. ions the optical density and conductivity are linearly related (methylene dichloride solution, conc. 10-5-10-3 M) showing that under these conditions ion-pairing is unimportant. It follows, afortiori, that if the pseudocationic polymerizations were indeed propagated by ions at concentration less than 10-6 M, these would be entirely free ions. Therefore any alleged explanation of these reactions which involves ion-pairs is now ruled out. We emphasize that the strongest single argument against the view that the pseudocationic reactions are propagated by ions is the nil-effect of water on the rate. The strongly inhibiting effect of water on radiation-induced true cationic polymerizations is too well known to require further elaboration here. Other arguments in favour of the reality of pseudocationic polymerization, based on activation energies, co-polymerization ratios, electric field effects, etc. have been given (6) and will be elaborated elsewhere. Higashimura, Kanoh, and Okamura c29) drew attention to the fact that we, like all previous authors, had misinterpreted the spectrum given by styrene in presence of acids; this matter, like some of the queries raised in their paper, has now been resolved. However, they are mistaken in their criticism of our pseudocationic theory on the grounds that we had misinterpreted the spectra. The two questions are in no way logically related. Similarly, the paper of Gantmakher and Medvedev(3°~ confuses the two issues and adds nothing which clarifies the position. CONCLUSION Our studies have confirmed that the oligomerization of styrene by perchloric acid is both chemically and kinetically simple. However, the reactions which follow completion of the polymerization, and during which carbonium ions are formed and destroyed, are complicated. They can be rationalized in terms of equilibria involving ions, acid, double bonds, and esters; cyclization of olefinic oligomers and formation of polyenes by way of allylic ions add further complications. We have thus shown in some detail just why such systems must be treated with the greatest circumspection if they are to yield valid information.

Cationic and Pseudocationic Polymerization of Aromatic Olefins--II

73

Acknowledgements--We wish to thank the U.S. Rubber Company for a student.ship (to A. G.) and for other financial help, and the S.R.C. for a grant for equipment.

REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30)

A. Gandini and P. H. Plesch, 3". chem. Soc. 4826 (1965). (Part I). A. Gandini and P. H. Plesch, Proc. chem. Soc. 240 (1964). A. Gandini and P. H. Plesch, J. Polym. Sci. Part B, Polym. Lett. 3, 127 (1965). A. Gandini and P. H. Plesch, S.C.L Monograph, No. 20, Chemistry of Polymerisation Processes, S.C.I., London (1966). P. H. Plesch, Pure AppL Chem. 12, 117 (1966). P. H. Plesch, Polymer Preprints, A.C.S. Meeting, September (1966). V. Bertoli and P. H. Plesch, Chem. Commun. 625 (1966). R. O. Colclough and F. S. Dainton, Trans. Faraday Soc. 54, 886 (1958). A. Gandini and P. H. Plesch, 1. chem. Soc. 6019 (1965). W. R. Longworth, P. H. Plesch and M. Rigbi, J. chem. Soc. 451 (1958). R. H. Biddulph, P. H. Plesch and P. P. Rutherford, J. chem. Soc. 275 (1965). A. Gandini, P. Giusti, P. H. Pleseh and P. H. Westermann, Chemy Ind., 1225 (1965). R. H. Biddulph and P. H. Plesch, Chemy Ind. 1482 (1959). BiPI valves, obtainable from Polymer Consultants Ltd., High Church St., New Basford, Nottingham, England. J. R. Lind, J. J. Zwolenik and R. M. Fuoss, J. Am. chem. Soc. 81, 1557 (1959). A. Gandini and P. H. Plesch, J. chem. Sac. 4765 (1965). K. Hofner and H. Pelster, Angew. Chem. 73, 342 (1961). C. M. Fontana and G. A. Kidder, J. Am. chem. Soc. 70, 3745 (1948). P. V. French, L. Roubinek and A. Wassermann, J. chem. Soc. 1953 (1961). C. Spach, M. Levi and M. Szwarc, I. chem. Soc. 355 (1962). T. Inoue and S. Mima, Chemy high Polym. 14, 402 (1957). D. C. Pepper and P. J. Reilly, I. Polym. Sci. 58, 638 (1962). J. M. Barton and D. C. Pepper, J. chem. Soe. 1573 (1964). S. Bywater, private communication. S. Bywater and D. J. Worsfold, Can. J. Chem. 44, 1671 (1966). V. Bertoli, Thesis, Keele (1967). A. A. Zharov, A. A. Berlin and N. S. Enikolopyan, Int. Symp. Macromols., Prague, P 272 (1965). A. A. Zharov, V. V. Tatarintsev and N. S. Enikolopyan, VSsokomolek. Soyedin., 7, 1863 (1965); Translated in Polym. Sci. U.S.S.R. 7, 2044 (1965). T. Higashimura, N. Kanoh and S. Okamura, 3". Macromol. Chem. 1,109 (1966). A. R. Gantmakher and S. S. Medvedev, VSsokomolek. Soyedin. 8, 955 (1966).

R~,um~----On d~"rit des 6tudes spectroscopiques et conductim6triques de la r6action du styrene en presence d'acide perchlorique (principalement dans le chlorure de m~thyl~ne mais quelquesunes dans le chlorure d'6thyl6ne), pendant et apr~s la polym~risation. Pendant la polym~risation on ne peut d~celer aucun ion, mais h la fin de la r~action on observe d'abord des ions aralkyles et puis des ions aUyle. Une &ude quantitative a montr6 que la formation des ions d~bute lorsque la concentration du styrene est tomb~e b. quatre fois la concentration de racide. On en conclut qu'il faut quatre molecules de styrene pour stabiliser l'ester perchlorate d'(oligo)-styryle en solution. La cin~tique de la r~action ionog~ne qui suit la polym~risation passe du second au premier ordre. Elle est totalement inbib~e s'il y a darts la solution plus d'eau que d'acide, mais cette circonstance n'a pas d'influence sur la polym~risation elle m~me. A la fin de la polym~risation, la r~action ionog~ne ~tablit un ~quilibre qui fait intervenir de nombreuses espc~ces et qui se trouve lentement d~plac~ par formation de compos~s stables. La position de cet 6quilibre est renvers~e et les esp~ces ioniques disparaissent, lorsqu'on ajoute encore du monom~re au syst~me. On discute en d~tail la relation entre ce travail et les travaux pr~,.ed~nts d'autres auteurs. Sommario---Viene riportato uno studio spettroscopico e conduttometrico sulle reazioni deUo stirene in presenza di acido perclorico (per lo piia in cloruro di metilene, ma anche in dichloroetano), sia durante the dopo la polimedr~axione. Durante le polimerizzazioni non si osservano ioni, ma alia fine si formano ioni aralehilici (e in seguito allilici). I risultati quantitativi mostrano che la formazione degli ioni ini~ia quando la concentrazione dello stirene si ~ abbassata al quadrupio delia concentrazione dell'aeido; il risultato viene spiegato come una prova che l'estere (oligo) stirilperclorato ha bisogno di quattro molecole di stirene per essere stabilizzato in soluzione.

74

A. GANDINI and P. H. PLESCH

Le reazioni ionogeniche che avvengono dopo la polimeriw~zione cambiano la cinetica dal secondo al primo ordine. Esse vengono completamente inibite se la soluzione contiene pi/~ acqua the acido, bench~ ci6 non influenzi la polimerizzazione per se stessa. Alla fine della p o l i m e d ~ o n e , le reazioni ionogeniche raggiungono un equilibrio coinvolgente molti componenti e che ~ lentamente spostato dalla formazione di specie stabili; l'equilibrio pub essere retrogradato verso una maggiore concentrazione di composti non ionici per aggiunta di monomero fresco. Sono discusse in dettaglio le relazioni tra questo lavoro e quelli di altri autori. Zusammenfassung--Wir berichten tiber spektroskopische und konduktometrische Untersuchungen der Reaktionen yon Styrol in Anwesenheit von Perchlors/iure (die meisten in Methylendichlorid und einige in ~,thylendichlorid), sowohl w~ihrend als auch nach der Polymerisation. W~hrend der Polymerisation sind keine Ionen nachweisbar, gegen Ende aber werden Aralkyl- (und sp~.ter Allyl-) Ionen gebildet. Quantitative Ergebnisse zeigen, dab die Ionenbildung einsetzt, werm die Styrolkonzentration auf das Vierfache der S~iurekonzentration abgesunken ist. Dieser Befund bedeutet, dab der Ester (Oligo-) Styryl-Perchlorat zu seiner Stabilisierung in L6sung vier Molektile Styrol ben6tigt. W~hrend der auf die Polymerisation folgenden ionogenen Reaktion/indert sich deren Kinetik yon der zweiten zur ersten Ordnung. Diese Reaktion wird vollst/indig gehemmt, wenn die L6sung mehr Wasser als S~iureenthilt, obwohl dies die Polymerisation selbst nicht beeinfluBt. Die ionogene Reaktion fiihrt zu einem Gleichgewicht, an dem viele Komponenten beteiligt sind und das sich langsam durch Bildung stabiler Produkte verschiebt. Dutch emeute Zugabe yon Monomerem wird die Lage des Gleichgewichts wieder riickl~iufig in Richtung iiberweigend nicht-ionischer Produkte verschoben. Die Beziehungen zwischen unserer Arbeit und denen anderer Autoren wird eingehend diskutiert.