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Electro-initiated cationic polymerizations—III
European Polymer Journal, Vol [2, pp. 247 to 250 Pergamon Press 1976. Printed in Great Britain.
ELECTRO-INITIATED CATIONIC POLYMERIZATIONS III POLYMERIZATION OF ANETHOLE IN THE PRESENCE OF TETRA-ALKYLAMMONIUM PERCHLORATES* P. CERRAI, G. GUERRA and M. TRICOLI Nucleo di Ricerca del C.N.R. presso Flstituto di Chimica lndustriale ed Applicata delFUnixer~it~ di Pisa, Via Diotisalvi 2, 56100 Pisa, Italy
(Received 18 July 1975) Abstract The electro-polymerization of trans-anethole in 1,2-dichloroethane solution with tetraethyland tetrabutylammonium perchlorates as supporting electrolytes was studied: the dependences of pobmerization rates and molecular weight on some experimental parameters were determined. To investigate the electrolytic formation of initiating species, the anolyte from the electrolysis of the supporting salt in the absence of monomer was examined; the presence of electrolytically produced perchloric acid was ascertained unequivocally. However, owing to the polarographic behaviour of anethole, which shows an oxidation wave at lower anodic potential than the solvent salt couple, it was not possible to postulate any reaction mechanism based on a single initiation reaction. The fact that the polymerizations were carried out without control of the anodic potential and some of the kinetic results indicate that two parallel initiation reactions (direct monomer oxidation and 9rotonation by electrolytically formed perchloric acid) can occur. This procedure was always followed in the i.r. detection of perchloric acid. The anolyte containing ampoule was attached to the vacuum line, the break-seat crushed, the solution distilled at room temperature and the distillate divided into two portions. A portion was checked for strong acidity by triphenylmethanol as already described [9] and a second extracted by several portions of water in a Teflon-stopcock separating funnel. The water solution was neutralized with LiOH and evaporated to dryness. The lithium salt was redissolved in acetonitrile and examined by a Perkin Elmer model 521 Grating Infra-red Spectrophotometer. Polyanethole was recovered by low-temperature precipitation in methanol; molecular weights were determined by a Mechrolab model 301A vapour osmometer in benzene solution at 37 c'. Constant current intensities were supplied by the appropriate inst~rument described in Part 1 [8].
INTRODUCTION
This paper deals with electrochemically initiated polymerizanon of trans-/3-methyl-p-methoxystyrene (anethole) in the presence of t e t r a e t h y l a m m o n i u m perchlorate (TEAP) or t e t r a b u t y l a m m o n i u m one (TBAP) as supporting electrolytes in 1,2-dichloroethane solution. Perchlorate a n i o n containing electrolytes are often used in electro-initiated cationic polymerizations of cyclic a n d vinyl m o n o m e r s a n d initiation by anodically formed HC104 is proposed for most such systems [1--5]. E X P E R I M E N T %1.
MateriaLs Anethole (C. Erba RS) and 1,2-dichloroethane (C. Erba) were purified and stored as previously described [6]. TEAP (C. Erba RS) and TBAP (Eastman Kodak) were pumped under vacuum for several hours before use.
5
+'L
Apparatus and procedure All the experiments were carried out in 1,2-dichloroethane solution at 25: using a high vacuum technique. The kinetics were determined dilatometrically in reaction cells similar to those already described, both single [6] and divided by a sintcred glass disc I-7]. Those electrolyses which needed recovery of electrolysed solutions under vacuum were carried out in the cell shown in Fig. 1. The cell (1) was filled under vacuum by the techniques already described [8] and then sealed off at (2). After the electrolysis, the apparatus was inverted in order to transfer anodic and cathodic solutions in the two ampoutes (3) without mixing. The ampoules were then sealed off at (4) and attached to the vacuum line. * Paper presented at the IUPAC XXIII International Symposium on Macromolecules, Madrid, 1974. Part II, see Ref. [7].
( Fig. 1. Electrolysis cell. 247
CERRAI,G. GUERRAand M. TRICOLI
P.
248
0-7-0"6
I00
t
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e
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0-3 0.2
b
a
O.I 0
0 I0
20
30
40
50
60 '70 80 T i m e , min
90
I00 I10
5
120
Fig. 2. Conversion-time curves for electro-polymerizations of anethole (0'5 M) in a divided cell under continuous electrolysis. Electrolyte, 15 × 10 3 mole/1 TEAP; current intensities: (a) 0.01 mA; (b) 0'02 mA; (c) 0.10 mA; (d) 0.25 mA; (e) 0.80 mA.
RESULTSANDDISCUSSION When a constant intensity current is passed through 1,2-dichloroethane solutions of anethole and T E A P or TBAP, a polymerization starts immediately only in the anode compartment of a divided reaction cell. The same solutions w.ere stable for 12 hr in the absence of current. During the polymerization, a brown coating is observed on the anode and a yellowish-brown colour develops around the electrode. The dilatometric conversion-time curves present a sigmoidal shape; maxim u m rates evaluated at the inflection points of the curves, R ..... are tabulated. The shape of the curves indicates nonsteady-state polymerizations with continuous production of active centres, starting without detectable induction periods. Conversion-time curves at different current intensities are shown in Fig. 2. The logarithmic plot in Fig. 3, whose slope is 0.45, shows a non-linear dependence o f / ~ , on the current intensity. The acceleration times, t*, of the curves in Fig. 2 are inversely proportional to the current intensity, as shown in Fig. 4. The kinetic analysis of the polymerization curves to evaluate the dependence of the instantaneous reaction rate, R,, on the instantaneous monomer concentration, mr (i.e. the internal order with respect to monomer), is not simple. Assuming the relationship R t = k t m T , which takes into account the progressive increase of active species, it is possible to obtain a differential plot which shows an internal second order in monomer for most curves (Fig. 5).
tmin/F
15
Fig. 4. Dependence of the acceleration times, t*, on the current intensity, i, for electro-polymerizations of anethole (0.5 M) in a divided cell under continuous electrolysis. Electrolyte, 15 x 10 -3 mole/l TEAP. An important feature of the described polymerizations is the presence of a marked "after effect", the reaction maintaining the same rate after the current is switched off. When using single cells with systems otherwise identical to those described previously, much slower polymerizations are obtained and the conversiontime curves do not show acceleration. The conversion-time curves for polymerizations carried out in a divided and a single cell, switching off the current at partial conversion, are shown in Fig. 6. The lower reaction rate in a single cell can be ascribed to retardation by C1- anions produced in the cathodic decomposition of the solvent, previously ascertained in electrolyses of 1,2-dichloroethane solutions of tetraalkylammonium salts in the absence of monomer
1-8,103. The influence of the initial monomer concentration on the polymerization rate using a divided cell and maintaining the current constant is shown in Fig. 7. The dependence is linear only up to 0'7 mole/l of monomer and then markedly decreases. Supporting
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I
-5"5
-5
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Fig. 3. Dependence of the polymerization rate, R~x, on the current intensity, i, for electro-polymerizations of anethole (0.5 M) in a divided cell under continuous electrolysis. Electrolyte, 15 mole/1 TEAP.
× 10-3
-0"9
I
-0.6
f
-0.7
I
-0'6
log ( I - x )
Fig. 5. Internal order with respect to the monomer from a modified differential plot for an anethole (l.0 M) electropolymerization in a divided cell under continuous electrolysis. Electrolyte, 15 x 10.3 mole/l TEAP; current intensity, 0-3 mA. R, and X, polymerization rate and fractional conversion at time t.
Electro-initiated cationic polymerizations--III
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Fig. 6. Conversion-time curves for electro-polymerizations of anethole in a divided (curve 1) and a single cell (curve 2) under continuous electrolysis. Dashed lines, polymerization course after switching off the current. electrolyte concentrations from 5 × 10-3 to 25 x 10-3 mole/1 do not influence the reaction rate. Some experiments were carried out to study the influence of anolyte stirring on the polymerization. Figure 8 shows that the stirring markedly enhances the polymerization rate. Very probably the initiating species accumulate in the neighbourhood of the anode in the absence of stirring; they can interact with the monomer as a result of stirring, causing a faster and no longer diffusion limited polymerization. The internal order with respect to the monomer is not constant during the whole polymerization in the experiments with stirring at lower current intensities. There was also a study of the dependence of the molecular weights of the polymers obtained in the experiments involving continuous electrolysis on the current and the initial monomer concentration. The number average molecular weights, ranging from 1500 to 2500, seem to show no simple dependence on the current, whereas a first Mayo plot suggests monomer transfer and other chain breaking reactions. Some polymerization experiments under continuous electrolysis are summarized in Table 1. As a marked after-effect had been detected, polymerizations started by short pulses of current were
E/
Fig. 8. Conversion-time curve of a polymerization of anethole (0"25 M) in a divided cell under continuous electrolysis (i = 0.5 mA) before and after stirring the anolyte at time t~; dashed line, the same polymerization in the absence of stirring. Electrolyte, 15 x 10 3 mole/1 TEAP. carried out, so that the reaction mixture was supplied with controlled quantities of electricity, Q, between 6'04 x 10 -5 and 3'02 x 10 -4 F/1. S-shaped conversion-time curves were again obtained. Figure 9 shows. a linear dependence of Rmax on Q above a threshold value of about 6 x 10-5 F/1. The molecular weights summarized in Table 2 are higher than the preceding ones and show no definite dependence on Q. This non-dependence, together with high current efficiencies (some tens of polymer moles per Faraday), confirm that the polymerization is transfer-governed. The marked after-effect indicates that termination is negligible. A natural termination involving the combination of the C10 4 counter-ions with the active centres is quite unlikely owing to the poor nucleophilicity of C102, [11]. Termination by the cathodically produced C1- anions can be neglected since the migration of such anions in the anodic compartment is retarded by the sintered disc and no inverse dependence of molecular weight on current (and hence on C I - concentration) was found. Concerning the nature of transfer processes, it seems certain that there is transfer to monomer (as suggested by the molecular weights) and quite probTable I. Electro2polymerization of anethole under continuous electrolysis* Exp. no.
mo (mole/l)
1 2 3 4 5 6 7 8 9 l0 11 12 13
0-5 0'5 0"5 0'5 0'5 0'5 0"5 0'5 0'25 0"50 0.75 1'00 1"25
i × 10 6 Rmax× 10 3 (F/1 min) (mole/1 min) M,
x 10-3t
to
0
I
I
0"5
I'O mo,
mole/t
Fig. 7. Dependence of the polymerization rate, Rm,x, on the initial monomer concentration, m0, for electro-polymerizations of anethole in a divided cell under continuous electrolysis. Electrolyte, 15 x 10 3 mole/l TEAP; current intensity, 0.3 mA.
0-62 0"93 1-24 3"11 6-22 15"50 31-11 49-70 18"70 18"70 18.70 18.70 18"70
1"56 0"94 2"52 3-93 4-83 7'27 9"03 10.63 3.90 7'94 11'81 12.80 11'20
1.47 2"10 1'95 1"64 2'05 2"16 1'87 1.91 2"05 2"15 2-18 2"22 2'28
* Other conditions: electrolyte, 15 x 10 3 M TEAP. + Estimated from vapour pressure osmometry in benzene solution.
250
P. CERRAI,
G. GUERRAand M. TR1COLI Table 2. Electro-polymerization of anethole by initial pulses of current*
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2~
rl.
0
,
I
I 2 QxlO 4, F/1
I
I 3
Fig. 9. Dependence of the polymerization rate, Rmax, on the quantity of electricity, Q, for electro-polymerizations of anethole in a divided cell by pulses of current. Electrolyte, 15 × 10 - 3 mole/l TBAP; pulse length, 60 sec. ably transfer to C l O t anions, the high concentration of which is also in favour of propagation by ion pairs. Regarding the nature of the initiation process, electrolyses of TBAP solutions in the absence of monomer with the same current intensities, current densities and electrolysis times as in the electro-polymerizations indicate anodic formation of HC104. The anolyte was distilled under vacuum to separate the volatile materials from the residual TBAP. The acidity detected in the distilled solution was neutralized by LiOH. Infra-red analysis of the so obtained lithium salt revealed two absorption bands at 626 and 1100 cm -1 in acetonitrile, corresponding to the va and v4 bands of ClOg in the same solvent [12]. The last results seem to suggest that the polymerization may be initiated under our experimental conditions by the anodically formed HC104. Moreover, anethole polymerization chemically initiated by HCIO,~ in 1,2-dichloroethane have rates in good agreement with those of the pulsed current electropolymerizations. However, an anodic polarogram of the anetholeTBAP-1,2-dichloroethane system shows a monomer oxidation wave at a potential less positive than the electrolyte-solvent system [13]. Hitherto there is no direct evidence that the anethole oxidation produces radical cations able to initiate the polymerization; nevertheless, the polarographic result gives an indication that a direct electro-initiation is also possible. Since the described experiments were carried out in two-electrode cells with no control of the anodic potential, it is likely that both initiation mechanisms operate. This could explain both the fractional order of the maximum rate with respect to the current intensity and the non-constant internal kinetics observed in some polymerizations; the tentative kinetic pattern previously proposed [14], based on a single initiation process and an unlikely spontaneous termination, must be re-examined. CONCLUSION
From the results, it appears that different initiation processes can occur simultaneously in the absence of a rigorous control of the anodic potential, i.e. under
Exp. no.
i (mA)
14 l5 16 17 18
1.0 2.0 3.0 4.0 5.0
Q x 104 Rmax x 10 3 (F/I) (mole/1rain) M,, × 1 0 - 3 t 0.604 1.207 1.811 2-415 3-018
0.34 5-50 12.43 20.16 24-69
3.9 2.6 4.7 2.7 3-5
* Other conditions: m0 =0"5 M; I-TEAP] = 15 x 10 -3 M; pulse length = 60 sec. 5 Estimated from vapour pressure osmometry in benzene solution. the experimental conditions frequently used by most authors in electro-polymerizations. Hence it follows that there is need for rigorous control of the anodic potential, as reported by other authors [15], in order to obtain an anethole polymerization initiated by direct electron transfer only. Conversely, the choice of a less oxidizable monomer might enable a polymerization initiated by electrolytically formed HCIO4 to occur at controlled anodic potential. The study of the electro-polymerization of anethole at controlled anodic potential corresponding to the oxidation wave of the monomer will be object of a future paper. Ackmm'ledgement--The work was carried out with financial support of the Consiglio Nazionale delle Ricerche IC.N.R.) of Italy. REFERENCES
1. B. L. Funt and T. J. Blain, J. Polym. Sci. A1, 8, 3339 (1970). 2. S. Nakahama, S. Hino and N. Yamazaki, Polym. J. 2, 56 (1971). 3. J. W. Breitenbach, O. F. Olaj and F. Sommer, Adv. Polym. Sci. 9, 170 (1972). 4. G. Pistoia, Europ. Polym. J. 10, 279 (1974). 5. G. Mengoli and S. Valcher, Europ. Polym. J. 10, 959 (1974); G. Mengoli and G. Vidotto, J. appl. Polym. Sci. 18, 3095 (1974). 6. P. Cerrai, P. Giusti, G. Guerra and M. Tricoli, Europ. Polym. J. 10, 1141 (1974). 7. P. Cerrai, P. Giusti, G. Guerra and M. Tricoli, Europ. Polym. J. 11, 101 (1975). 8. P. Cerrai, P. Giusti, G. Guerra and M. Tricoli, Europ. Polym. J. 10, 1195 (1974). 9. G. Guerra, F. Andruzzi, P. Cerrai and M. Tricoli, Chimica Ind., Milano 57, 495 (1975). 10. M. v. Stackelberg and W. Stracke, Z. Elektochem. 53, 118 (1948); D. K. Durmanov and I. A. Bagotskaya, Elektrokhimiya 5, 1236 (1969); K. Nyberg, Acta chem. scand. 24, 1609 (1970). 11. D. C. Pepper and P. J. Reilly, J. Polym. Sci. 58, 639 (1962). 12. M. Kinugasa, K. Kishi and S. Ikeda, J. phys. Chem. 77, 1914 (1973). 13. L. Nucci, P. Cerrai and G. Guerra, unpublished results. 14. G. Guerra, P. Cerrai, G. Barbini and M. Tricoli, Chimica Ind., Milano 55, 541 (1973). 15. L. Turcot, A. Glasel and B. L. Funt, Polym. Lett. 12, 687 (1974); U. Akbulut, J. E. Fernandez and R. L. Birke, J. Polym. Sci., Polym. Chem. Ed. 13, 133 (1975).
ELECTRO-INITIATED CATIONIC POLYMERIZATIONS III POLYMERIZATION OF ANETHOLE IN THE PRESENCE OF TETRA-ALKYLAMMONIUM PERCHLORATES* P. CERRAI, G. GUERRA and M. TRICOLI Nucleo di Ricerca del C.N.R. presso Flstituto di Chimica lndustriale ed Applicata delFUnixer~it~ di Pisa, Via Diotisalvi 2, 56100 Pisa, Italy
(Received 18 July 1975) Abstract The electro-polymerization of trans-anethole in 1,2-dichloroethane solution with tetraethyland tetrabutylammonium perchlorates as supporting electrolytes was studied: the dependences of pobmerization rates and molecular weight on some experimental parameters were determined. To investigate the electrolytic formation of initiating species, the anolyte from the electrolysis of the supporting salt in the absence of monomer was examined; the presence of electrolytically produced perchloric acid was ascertained unequivocally. However, owing to the polarographic behaviour of anethole, which shows an oxidation wave at lower anodic potential than the solvent salt couple, it was not possible to postulate any reaction mechanism based on a single initiation reaction. The fact that the polymerizations were carried out without control of the anodic potential and some of the kinetic results indicate that two parallel initiation reactions (direct monomer oxidation and 9rotonation by electrolytically formed perchloric acid) can occur. This procedure was always followed in the i.r. detection of perchloric acid. The anolyte containing ampoule was attached to the vacuum line, the break-seat crushed, the solution distilled at room temperature and the distillate divided into two portions. A portion was checked for strong acidity by triphenylmethanol as already described [9] and a second extracted by several portions of water in a Teflon-stopcock separating funnel. The water solution was neutralized with LiOH and evaporated to dryness. The lithium salt was redissolved in acetonitrile and examined by a Perkin Elmer model 521 Grating Infra-red Spectrophotometer. Polyanethole was recovered by low-temperature precipitation in methanol; molecular weights were determined by a Mechrolab model 301A vapour osmometer in benzene solution at 37 c'. Constant current intensities were supplied by the appropriate inst~rument described in Part 1 [8].
INTRODUCTION
This paper deals with electrochemically initiated polymerizanon of trans-/3-methyl-p-methoxystyrene (anethole) in the presence of t e t r a e t h y l a m m o n i u m perchlorate (TEAP) or t e t r a b u t y l a m m o n i u m one (TBAP) as supporting electrolytes in 1,2-dichloroethane solution. Perchlorate a n i o n containing electrolytes are often used in electro-initiated cationic polymerizations of cyclic a n d vinyl m o n o m e r s a n d initiation by anodically formed HC104 is proposed for most such systems [1--5]. E X P E R I M E N T %1.
MateriaLs Anethole (C. Erba RS) and 1,2-dichloroethane (C. Erba) were purified and stored as previously described [6]. TEAP (C. Erba RS) and TBAP (Eastman Kodak) were pumped under vacuum for several hours before use.
5
+'L
Apparatus and procedure All the experiments were carried out in 1,2-dichloroethane solution at 25: using a high vacuum technique. The kinetics were determined dilatometrically in reaction cells similar to those already described, both single [6] and divided by a sintcred glass disc I-7]. Those electrolyses which needed recovery of electrolysed solutions under vacuum were carried out in the cell shown in Fig. 1. The cell (1) was filled under vacuum by the techniques already described [8] and then sealed off at (2). After the electrolysis, the apparatus was inverted in order to transfer anodic and cathodic solutions in the two ampoutes (3) without mixing. The ampoules were then sealed off at (4) and attached to the vacuum line. * Paper presented at the IUPAC XXIII International Symposium on Macromolecules, Madrid, 1974. Part II, see Ref. [7].
( Fig. 1. Electrolysis cell. 247
CERRAI,G. GUERRAand M. TRICOLI
P.
248
0-7-0"6
I00
t
A 4°41 E
e
J
50
0-3 0.2
b
a
O.I 0
0 I0
20
30
40
50
60 '70 80 T i m e , min
90
I00 I10
5
120
Fig. 2. Conversion-time curves for electro-polymerizations of anethole (0'5 M) in a divided cell under continuous electrolysis. Electrolyte, 15 × 10 3 mole/1 TEAP; current intensities: (a) 0.01 mA; (b) 0'02 mA; (c) 0.10 mA; (d) 0.25 mA; (e) 0.80 mA.
RESULTSANDDISCUSSION When a constant intensity current is passed through 1,2-dichloroethane solutions of anethole and T E A P or TBAP, a polymerization starts immediately only in the anode compartment of a divided reaction cell. The same solutions w.ere stable for 12 hr in the absence of current. During the polymerization, a brown coating is observed on the anode and a yellowish-brown colour develops around the electrode. The dilatometric conversion-time curves present a sigmoidal shape; maxim u m rates evaluated at the inflection points of the curves, R ..... are tabulated. The shape of the curves indicates nonsteady-state polymerizations with continuous production of active centres, starting without detectable induction periods. Conversion-time curves at different current intensities are shown in Fig. 2. The logarithmic plot in Fig. 3, whose slope is 0.45, shows a non-linear dependence o f / ~ , on the current intensity. The acceleration times, t*, of the curves in Fig. 2 are inversely proportional to the current intensity, as shown in Fig. 4. The kinetic analysis of the polymerization curves to evaluate the dependence of the instantaneous reaction rate, R,, on the instantaneous monomer concentration, mr (i.e. the internal order with respect to monomer), is not simple. Assuming the relationship R t = k t m T , which takes into account the progressive increase of active species, it is possible to obtain a differential plot which shows an internal second order in monomer for most curves (Fig. 5).
tmin/F
15
Fig. 4. Dependence of the acceleration times, t*, on the current intensity, i, for electro-polymerizations of anethole (0.5 M) in a divided cell under continuous electrolysis. Electrolyte, 15 x 10 -3 mole/l TEAP. An important feature of the described polymerizations is the presence of a marked "after effect", the reaction maintaining the same rate after the current is switched off. When using single cells with systems otherwise identical to those described previously, much slower polymerizations are obtained and the conversiontime curves do not show acceleration. The conversion-time curves for polymerizations carried out in a divided and a single cell, switching off the current at partial conversion, are shown in Fig. 6. The lower reaction rate in a single cell can be ascribed to retardation by C1- anions produced in the cathodic decomposition of the solvent, previously ascertained in electrolyses of 1,2-dichloroethane solutions of tetraalkylammonium salts in the absence of monomer
1-8,103. The influence of the initial monomer concentration on the polymerization rate using a divided cell and maintaining the current constant is shown in Fig. 7. The dependence is linear only up to 0'7 mole/l of monomer and then markedly decreases. Supporting
-4'4--
/
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-'4.E --
n,.' m
-~OI
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,--.I/iXln-5 ,--,
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4. -3"°1-- I'~" -6"5
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[
I
-5"5
-5
I -4.5
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Fig. 3. Dependence of the polymerization rate, R~x, on the current intensity, i, for electro-polymerizations of anethole (0.5 M) in a divided cell under continuous electrolysis. Electrolyte, 15 mole/1 TEAP.
× 10-3
-0"9
I
-0.6
f
-0.7
I
-0'6
log ( I - x )
Fig. 5. Internal order with respect to the monomer from a modified differential plot for an anethole (l.0 M) electropolymerization in a divided cell under continuous electrolysis. Electrolyte, 15 x 10.3 mole/l TEAP; current intensity, 0-3 mA. R, and X, polymerization rate and fractional conversion at time t.
Electro-initiated cationic polymerizations--III
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/
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Time, min 0
I
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I
I0
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I
I
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40
50
60
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Fig. 6. Conversion-time curves for electro-polymerizations of anethole in a divided (curve 1) and a single cell (curve 2) under continuous electrolysis. Dashed lines, polymerization course after switching off the current. electrolyte concentrations from 5 × 10-3 to 25 x 10-3 mole/1 do not influence the reaction rate. Some experiments were carried out to study the influence of anolyte stirring on the polymerization. Figure 8 shows that the stirring markedly enhances the polymerization rate. Very probably the initiating species accumulate in the neighbourhood of the anode in the absence of stirring; they can interact with the monomer as a result of stirring, causing a faster and no longer diffusion limited polymerization. The internal order with respect to the monomer is not constant during the whole polymerization in the experiments with stirring at lower current intensities. There was also a study of the dependence of the molecular weights of the polymers obtained in the experiments involving continuous electrolysis on the current and the initial monomer concentration. The number average molecular weights, ranging from 1500 to 2500, seem to show no simple dependence on the current, whereas a first Mayo plot suggests monomer transfer and other chain breaking reactions. Some polymerization experiments under continuous electrolysis are summarized in Table 1. As a marked after-effect had been detected, polymerizations started by short pulses of current were
E/
Fig. 8. Conversion-time curve of a polymerization of anethole (0"25 M) in a divided cell under continuous electrolysis (i = 0.5 mA) before and after stirring the anolyte at time t~; dashed line, the same polymerization in the absence of stirring. Electrolyte, 15 x 10 3 mole/1 TEAP. carried out, so that the reaction mixture was supplied with controlled quantities of electricity, Q, between 6'04 x 10 -5 and 3'02 x 10 -4 F/1. S-shaped conversion-time curves were again obtained. Figure 9 shows. a linear dependence of Rmax on Q above a threshold value of about 6 x 10-5 F/1. The molecular weights summarized in Table 2 are higher than the preceding ones and show no definite dependence on Q. This non-dependence, together with high current efficiencies (some tens of polymer moles per Faraday), confirm that the polymerization is transfer-governed. The marked after-effect indicates that termination is negligible. A natural termination involving the combination of the C10 4 counter-ions with the active centres is quite unlikely owing to the poor nucleophilicity of C102, [11]. Termination by the cathodically produced C1- anions can be neglected since the migration of such anions in the anodic compartment is retarded by the sintered disc and no inverse dependence of molecular weight on current (and hence on C I - concentration) was found. Concerning the nature of transfer processes, it seems certain that there is transfer to monomer (as suggested by the molecular weights) and quite probTable I. Electro2polymerization of anethole under continuous electrolysis* Exp. no.
mo (mole/l)
1 2 3 4 5 6 7 8 9 l0 11 12 13
0-5 0'5 0"5 0'5 0'5 0'5 0"5 0'5 0'25 0"50 0.75 1'00 1"25
i × 10 6 Rmax× 10 3 (F/1 min) (mole/1 min) M,
x 10-3t
to
0
I
I
0"5
I'O mo,
mole/t
Fig. 7. Dependence of the polymerization rate, Rm,x, on the initial monomer concentration, m0, for electro-polymerizations of anethole in a divided cell under continuous electrolysis. Electrolyte, 15 x 10 3 mole/l TEAP; current intensity, 0.3 mA.
0-62 0"93 1-24 3"11 6-22 15"50 31-11 49-70 18"70 18"70 18.70 18.70 18"70
1"56 0"94 2"52 3-93 4-83 7'27 9"03 10.63 3.90 7'94 11'81 12.80 11'20
1.47 2"10 1'95 1"64 2'05 2"16 1'87 1.91 2"05 2"15 2-18 2"22 2'28
* Other conditions: electrolyte, 15 x 10 3 M TEAP. + Estimated from vapour pressure osmometry in benzene solution.
250
P. CERRAI,
G. GUERRAand M. TR1COLI Table 2. Electro-polymerization of anethole by initial pulses of current*
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2~
rl.
0
,
I
I 2 QxlO 4, F/1
I
I 3
Fig. 9. Dependence of the polymerization rate, Rmax, on the quantity of electricity, Q, for electro-polymerizations of anethole in a divided cell by pulses of current. Electrolyte, 15 × 10 - 3 mole/l TBAP; pulse length, 60 sec. ably transfer to C l O t anions, the high concentration of which is also in favour of propagation by ion pairs. Regarding the nature of the initiation process, electrolyses of TBAP solutions in the absence of monomer with the same current intensities, current densities and electrolysis times as in the electro-polymerizations indicate anodic formation of HC104. The anolyte was distilled under vacuum to separate the volatile materials from the residual TBAP. The acidity detected in the distilled solution was neutralized by LiOH. Infra-red analysis of the so obtained lithium salt revealed two absorption bands at 626 and 1100 cm -1 in acetonitrile, corresponding to the va and v4 bands of ClOg in the same solvent [12]. The last results seem to suggest that the polymerization may be initiated under our experimental conditions by the anodically formed HC104. Moreover, anethole polymerization chemically initiated by HCIO,~ in 1,2-dichloroethane have rates in good agreement with those of the pulsed current electropolymerizations. However, an anodic polarogram of the anetholeTBAP-1,2-dichloroethane system shows a monomer oxidation wave at a potential less positive than the electrolyte-solvent system [13]. Hitherto there is no direct evidence that the anethole oxidation produces radical cations able to initiate the polymerization; nevertheless, the polarographic result gives an indication that a direct electro-initiation is also possible. Since the described experiments were carried out in two-electrode cells with no control of the anodic potential, it is likely that both initiation mechanisms operate. This could explain both the fractional order of the maximum rate with respect to the current intensity and the non-constant internal kinetics observed in some polymerizations; the tentative kinetic pattern previously proposed [14], based on a single initiation process and an unlikely spontaneous termination, must be re-examined. CONCLUSION
From the results, it appears that different initiation processes can occur simultaneously in the absence of a rigorous control of the anodic potential, i.e. under
Exp. no.
i (mA)
14 l5 16 17 18
1.0 2.0 3.0 4.0 5.0
Q x 104 Rmax x 10 3 (F/I) (mole/1rain) M,, × 1 0 - 3 t 0.604 1.207 1.811 2-415 3-018
0.34 5-50 12.43 20.16 24-69
3.9 2.6 4.7 2.7 3-5
* Other conditions: m0 =0"5 M; I-TEAP] = 15 x 10 -3 M; pulse length = 60 sec. 5 Estimated from vapour pressure osmometry in benzene solution. the experimental conditions frequently used by most authors in electro-polymerizations. Hence it follows that there is need for rigorous control of the anodic potential, as reported by other authors [15], in order to obtain an anethole polymerization initiated by direct electron transfer only. Conversely, the choice of a less oxidizable monomer might enable a polymerization initiated by electrolytically formed HCIO4 to occur at controlled anodic potential. The study of the electro-polymerization of anethole at controlled anodic potential corresponding to the oxidation wave of the monomer will be object of a future paper. Ackmm'ledgement--The work was carried out with financial support of the Consiglio Nazionale delle Ricerche IC.N.R.) of Italy. REFERENCES
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