ELECTROINITIATED POLYMERIZATION OF STYRENE I. ELECTROLYSIS OF ALKALI METAL NITRATES IN DIMETHYLFORMAMIDE

CONTAINING STYRENE MONOMER'

B. LIONEL FUNT AND SHARON WALKER LAURENT Parker Clzenzistry Laboratory, Universily of Manitoba, Winnipeg, Maniloba

Received June 18, 1964

ABSTRACT

Electrolysis of alkali metal salts in dimethylformamide containing styrene monomer produced polymerization. The rate of reaction increased with increase in current and monomer concentration and varied dramatically with choice of initiating salt. The alkali metal nitrates from sodium to cesium produced polymer but lithium was ineffective. KNO3 was the most efficient of the alkali metal salts tested.

The effects of inhibitors on the rates of polymerization together with analysis of copolymer composition established that polymerization proceeded by an anionic mechanism.

A detailed study with ICNO3 solution showed that relatively high electrical efficiencies were attained and that the number-average molecular weights ranged between 2 000 and 5 000.

Although there are now a number of reports of polymerization induced by electrolysis of solutions of monomer, most investigations have been confined to the polymerization of methyl methacrylate. Only fragmentary references to the polymerization of styrene have been made.

In 1952 Goldschmidt and Stock1 (I) obtained polystyrene by electrolysis of solutions of monomer in anhydrous fatty acids containing the corresponding fatty acid salts. The major part of the product of the reaction consisted of dimer and trimer, and only 0.09 g of a "semi-colloid" of molecular weight 3 200 was obtained from the electrolysis of a solution containing 20 g of styrene. Das and Palit (2) reported a trace of polymer formed under similar conditions in propylene glycol.

A brief note by Yang, McEwen, and Kleinberg (3) reported on the initiation of styrene polyinerization a t a cathode. These workers electrolyzed a pyridine solution of NaI and monomer. An unstated quantity of polymer was obtained with molecular weight of 1 800. In the light of present knowledge it is probable that this was the first electrically initiated anionic polymerization. Kolthoff and Ferstandig were not successful in obtaining poly- styrene with their redox systems, although acrylonitrile was polymerized with great efficiency. Failure to obtain polystyrene was also reported by Friedlander, Swann, and Marvel (4), and the field has recently been reviewed in excellent detail (5, 6, 7).

Exploratory experiments were conducted in our laboratories with the salt and solvent systems which we used for the anionic polymerization of acrylonitrile (8) and the free radical polymerization of methyl methacrylate (9). I t became apparent that polystyrene can be formed by the electrolysis of anhydrous solutions in dimethylformamide if a suitable salt is employed. The present report represents an attempt to discover the effects of various parameters on the yield and configuration of the polymer formed by this method.

EXPERIMENTAL

The procedures and apparatus were based on those previously described by us.

Materials Dimethylformamide (DMF), A.R. grade, was purified by mixing with 10% benzene previously dried

'Presented at the Twelfth Canadian High Polymer Forum. Ste. Margz~erite, Quebec. M a y 1964.

Canadian Journal of Chemistry. Volume 42 (1964)

2728

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FUNT AND LAURENT: POLYMERIZATION OF STYRENE. I 2729

over CaH?. .4fter standing 24 h the mixture was fractionally distilled, and the fraction boiling a t 150-152 "C was collected over BaO, allowed to stand 24 h, and redistilled a t reduced pressure (10).

Styrene nlonomer was passed through chromatographic alumina and then distilled a t reduced pressure. Salts were A.R. grade and were dried for 48 h before use.

Poly~nerisations Electrically regulated constant current power supplies wereemployed. For voltages up to 40 V and currents

to 100 mA the Power Designs type 4005 units were used. For high voltages and low currents to a maximum of 15 mA specially designed units from Mr. W. G. Hoyle of N.R.C. were employed. For some experiments manual adjustment of large dropping resistors powered by a 250 V d-c. regulated line proved satisfactory. In such instances the output was continually monitored by a pen recorder.

The polynlerization cells were of a basic test tube type. Two platinum electrodes each of dimensions 1 in. X 1 in. and spaced 1 cm apart were sealed into a 34/45 inner joint and fitted into an outer joint test tube with a sidearm. The cells were purged with N? and sealed. A magnetic stirring bar agitated the solutions during electrolysis. A bank of four such cells was operated simultaneously in a constant temperature bath. One geared motor rotated a series of magnets which provided uniform, constant stirring for all cells. Samples were withdrawn with a hypodermic syringe through a serum cap fitted over the sidearm of each cell.

For kinetic analysis samples were precipitated in cold methanol, filtered, vacuum dried for 24 h, and weighed.

IVith all experinlental runs blank experiments were performed, in which sample cells were allowed to stand for the full reaction time without passage of current. Precipitation of the contents showed that there was no detectable polymer formation in the absence of current.

Coloring of the reaction mixture during electrolysis of nitrate salts was noted previously and also occurred in this work. The characteristic color of the "living" styryl anion was not observed, and color formation is not attributed to this source.

RESULTS

With all parameters held constant except the nature of the salt, the maximum amount of polynler was formed in solutions saturated with KN03. The data are given in Table I

1 I together with the solubilities of salts in the system.

j TABLE I

Relative yields of polystyrene with various salts a t a current of 100 mA for 47 h

Solubility of salt, g/100 ml % conversio~l of

Salt In D M F In solution monomer

The favored position of KN03 is puzzling. I t is not the most soluble salt on either a molar basis or a weight basis, but its superiority to the other nitrates has been confirmed in a large number of experiments. On the other hand, all potassium salts will not give uniform polymer production a t the cathode. This aspect of the polymerization mechanism deserves further investigation. Our attempts with polarographic measurements have not produced unequivocal results.

To determine the propagation mechanism, polymerizations were conducted in the presence of the free radical inhibitors t-butylcatechol and p-benzoquinone. Very high concentrations of inhibitor were used to insure tha t electrolytic destruction of the inhibitor would not invalidate the results. Even with 2.4% by weight of t-butylcatechol the degree of conversion was reduced only modestly in con~parison with the uninhibited solution. The data are presented in Table I1 and indicate tha t the reaction occurs primarily by a non-radical mechanism.

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2730 CANADIAN JOURNAL OF CHEMISTRY. VOL. 42. 1964

TABLE I1

T h e effect of inhibitors on the yield of polystyrene

Concentration Electrolysis Current, Initial monomer % Inhibitor of inhibitor, q;b time, h m A concentration, % conversion

2 .4 20 50 40 32.3 Benzoquinone 0 21 ' 100 40 19

4 . 0 21 100 40 35 0 28 100 40 28 4.0 28 100 40 36.1

In an equimolar mixture of styrene and methyl methacrylate i t is well known that the initial copolymer formed is approximately 50y0 polymethylmethacrylate in a radical copolymerization, but only 1 yo styrene in an anionic reaction (11).

A study of this point was made using 14C-tagged monomer for ease of analysis. The data presented in Table I11 show tha t the free radical contribution is apparently confined to the first few yo of polymerization, whereas a t later stages the reaction is anionic. This is consistent with the inhibitor studies and may reflect some hydrogen atom initiation of the free radical reaction in the early stages.

TABLE 111 The copolymerization of a mole-to-mole mixture of styrene and methyl methacrylate

in a ICN03-DlMF system

Reaction time, % Activity of styrene Activity of copolymer, yo free radical h conversion monomer, counts/g min counts/g min reaction

A systematic study was performed of the production of polymer a t currents of 25, 50, and 100 mA for initial concentrations of 20,30, and 40 volume yo of monomer in dimethyl- formamide solutions saturated with KNOB. The results are shown in Figs. 1 and 2 for the 30 and 40% systems.

Although the data show conclusively a dependence of polymerization rate on current and initial monomer concentration, i t was not possible to assign a simple order dependence of the rate on these parameters. When the data are expressed in terms of the degree of conversion of the monomer initially present in the reaction mixture, they show an asymp- totic approach to a maximum a t about 50% conversion. This apparently reflects a wastage of some of the monomer in the formation of low molecular weight products which are not separated by our precipitation techniques. Hence the rate of monomer disappearance and the rate of polymer production are not equivalent in the present situation, and this fact results in a further complication in the kinetic analysis.

The molecular weights were measured on an absolute number-average basis with a Mechrolab vapor pressure instrument. The data in Table IV show no pronounced influence of the conditions of polymerization on the length of the polymer chain. The molecular weight is apparently increased by increase in monomer concentration and in temperature.

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F U N T A N D LAURENT: POLYMERIZATION OF STYRENE. I

100 ma I

T ime (hours) Time (hours)

FIG. 1. Formation of polymer a t inscribed currents for a 40 volun~e yo solution of styrene in dimethyl- formainide containing ICNOJ as electrolyte.

FIG. 2. Fornlation of polymer for a 30 volume solution of styrene in dimethylformamide containing I<N03 as electrolyte.

However, changes in the initiating salt or the presence of free radical inhibitors exerted only a minor influence on the molecular weight.

TABLE IV Molecular weight analysis

Styrene mo?omer Number-average concentration, Current, Temperature, molecular weight,

volume yo Salt m A "C g/mole

900 2 soo* 2 300t

-- - -

*Wit11 benzoquinone a s inhibitor. t \ V ~ t h 1-butylcatechol as inhibitor.

The absolute electrical efficiency can be determined from Faraday's Laws, in terms of moles of polymer per faraday. The data are given in Table V and indicate a high electrical efficiency of approxin~ately 20y0 of the theoretical inaximuln during the early stages of the reaction, although this percentage falls with time as the monomer becomes depleted.

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CANADIAN JOURNAL OF CHEMISTRY. VOL. 42, 1964

TABLE V The efficiency of electrolytic initiation in a 30 volume % solution

Current, Electrolysis Molecular weight m A time, h of polymer, g/mole 7, efficiency

Infrared spectra were taken of samples of the electrically produced polynler. The data confirmed that the polymer thus produced was identical to tha t formed by conventional free radical polymerization.

I t is evident that the KN03-dimethylformamide system represents the best medium thus far reported for the polynlerization of styrene. Although the electrical efficiency is high, the system is not amenable to the type of kinetic analysis applied to our previous work on anionic polymerization of acrylonitrile (8). Nevertheless, we believe that the formation of a radical ion and its subsequent polymerization most probably represents the reaction path.

ACKNOWLEDGMENTS

The authors thank the National Research Council of Canada for a grant and the Camille and Henry Dreyfuss Foundation for financial assistance.

REFERENCES

1. S. GOLDSCHMIDT and E. STOCKL. Chem. Ber. 85, 630 (1952). 2. M. N. DAS and A. R. PALIT. Sci. Cult. Calcutta, 16, 34 (1950). 3. J. Y. YANG, W. E. MCEWEN, and J. KLEINBERG. J. Am. Chem. Soc. 79, 5833 (1957). 4. H. 2. FRIEDLANDER. S. SWANN. and C. S. MARVEL. 1. Electrochem. Soc. 100. 408 (1953). . ,

W. BREITENBACU and C. H. 'SRNA. Pure Appl. Chkm. 4,245 (1962). . Y. FIOSHIN and A. P. TOMILOV. Plasticheskie Massy, 10, 2 (1960). 7. A. P. TOMILOV and M. Y. FIOSHIN. R u s ~ . Chem. Rev. 32,30 (1963). 8. B. L. FUNT and F. D. WILLIAMS. J. Polymer Sci. A, 2, 865 (1964). 9. B. L. FUNT and K. C. Yu. J. Polymer Sci. 62, 359 (1962).

10. E. G. ROCHOW and A. B. THOMAS. J. Am. Chem. Soc. 79, 1843 (1957). 11. C. WALLING, E. R. BRIGGS, W. CUMYINGS, and F. R. MAYO. J. Am. Chem. Soc. 72, 48 (1950).

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