the incorporation of aroyl peroxide fragments in polystyrene

JOURNAL OF POLYMER SCIENCE VOL. XLII, PAGES 129-137 (1960)

The Incorporation of Aroyl Peroxide Fragments in Polystyrene *t

RALPH L. DANNLEY and HENRY J. ESSIG, Morley Chemical Laboratory, Western Reserve University, Cleveland 6, Ohio

I. INTRODUCTION

When the polymerization of styrene is initiated with aroyl peroxides, both aryl and aroyloxy groups are found in the purified polymers. Rigor- ous saponification gives a quantitative determination of the ester groups introduced by aroyloxy radicals. Using this analytical procedure, Bartlett and Cohen' reported that use of pchlorobenzoyl and pbromobenzoyl peroxide as initiators in styrene polymerization led to polymers in which 88% and MOJO, respectively, of the incorporated halogen appeared as halo- aroyloxy groups.

Later work2 proved that p-chlorobenzoyl peroxide is capable of reacting with preformed polystyrene as well as with monomer. The reaction with polymer leads primarily (96a/,) to the introduction of p-chlorobenzoyloxy groups. It became obvious that in studying the incorporation of aroyl peroxide fragments care must be taken to differentiate between the reaction of initiator with monomer and that with polymer.

The work of Bartlett and Cohen presented an unexplained difference in incorporation of the aroyloxy group with p-chlorobenzoyl and p-bromobenzoyl peroxides. The present work was undertaken to determine whether (a) this difference arose in chain initiation or reaction of initiator with preformed polymer, ( b ) even greater differences would be found using other peroxides, and (c) a correlation of structure and reaction could be obtained.

11. EXPERIMENTAL

Reagents

Commercial styrene was distilled under reduced pressure just before use. Lucidol dibenzoyl and di-p-chlorobenzoyl peroxides, assayed at 99.6% and 96.7y0, respectively, were used without further purification.

* This paper is based on a portion of the thesis to be submitted by Henry J. Essig in partial fulfillment of the requirementa for the degree of Doctor of Philosophy in the Graduate School of Western &serve University.

tPresented at the 134th meeting of the American Chemical Society a t Chicago, Illinois, September, 1958.

129

130 R. L. DANNLEY AND H. J. ESSIG

The following substituted diaxoyl peroxides were prepared in each case by the dropwise addition of a solution of the corresponding acid chloride to a rapidly agitated, cold, aqueous, sodium peroxide solution. The crude product was recrystallized and dried in a vacuum oven.

Recrystallized Purity, M.p., Lit. m.p., Peroxide Solvent from % "C. "C.

Di-pbromobenzoyl' Benzene Methanol 100 143.5 144 Di-piodoben~oyl~ Benzene Toluene 98.2 151 157 Di-p-nitrobenzoyP Toluene Toluene 97.1 155 156

The p-iodobenzoyl chloride was obtained in 95.4% yield by heating phosphorous pentachloride (0.08 mole) and the corresponding acid (0.08 mole) for 75 min. in a steam bath. The product distilling at 147.8"C./20 mm. was collected.

Polystyrene prepaxed by benzoyl peroxide-initiated polymerization served as both an analytical control and as a starting material for the poly- meric reaction products obtained in these experiments. The purified polymer was dissolved in dioxane and hydrogenated over a palladium on char- coal catalyst. Two hydrogenated polystyrenes were prepared, purified, and subjected to the frozen benzene technique16 and the following data (chlorine and bromine not differentiated in the halogen titration) obtained.

Molecular Chlorine, Bromine, Iodine, Nitrogen, weight % % % % 14,450 - 0.36 - 0.20 19,940 0.42 0.95 0.08 0.24

Analytical Procedures

Preparation of Polystyrene Polymers for Analysis. These polymers were purified for analysis by four to six successive precipitations from methanol. Approximately 5-10% solutions of these polymers in benzene were first filtered and the resulting filtrates slowly added to a vigorously agitated, tenfold excess of methanol. The precipitates were then collected and vacuum dried in an oven at 60°C. for several hours. The samples were subjected to the frozen benzene technique prior to analysis or molecular weight determination.

Saponification Procedure. The homogeneous saponification technique of Bartlett and Cohen was utilized in these experiments. By mixing equal volumes of approximately 10% of the respective polystyrene in toluene with sodium ethoxide (1 g. sodium/lO ml. absolute ethanol) a clear, homogeneous solution is obtained. The length of reflux time varied as indicated in Tables 1-111.

AROYL PEROXIDE FRAGMENTS IN POLYSTYRENE 131

TABLE I Initiation of Monomeric Styrene Polymerization with Aroyl Perorides

Di-p- Di-p Di-p- bromobenzoyl iodobenzoyl nitrobenzoyl

Peroxide concn., mole/mole mono-

Styrene concn., mole/l. solvent Polymerization time, hr.8 Polymer yield, % Molecular weight Corrected analysis

mer unit

Purified polymer, %b Control polymer, y0b,o Control after saponification, y0b*d

Saponification time, hours Corrected analysis, hydrolyzed poly-

Content of aroyloxy groups, yo Content of aryl groups, yo

mer, %b

0.005 1.76 8.5 83.2 12,000

0.73 0.360 0.320 95.0

0.06 91.8 8.2

0.005 2.00 17.0 60.1 20,000

1.80 0.08' -

96.0

0.38 78.9 21.1

0.0062 2.00 20.0 13.0 14,800

0.42 0.204 -

96.0

0.03 93.0 7.0

a Periods of time varied to give su5cient quantities of polymer. b Content of Br, I, or N (in per cent) in respective compounds. 0 Control polymer is hydrogenated polystyrene. d Saponification for 72 hours. 0 Molecular weight 14,450.

Molecular weight 19,940.

TABLE I1 Reaction of Diaroyl Peroxides with Solutions of Polystyrene

Di-p- Di-p- Di-p bromobenzoyl iodobenzoyl nitrobenzoyl

Peroxide concn., mole/mole mono-

Monomer unit concn., mole/l. solvent Peroxide decomposition time, days Molecular weight

mer unit

Initial Final

Purified polymer, %. Control polymer, %*

Saponification time, hr. Corrected analysis hydrolyzed poly-

Content of aroyloxy groups, % Content of aryl groups, yo

Corrected analyses

mer, W

0.0669 1.35 12

14,450'~ 17,780

0.96 0.36b

120

0.50 48.0 52.0

0.0671 1.37 12

19,9400 24,000

1.48 0.080

118

0.47 68.2 31.8

0.0691 1.37 12

14, 450b 19,150

0.01 0.20b

im 0.10 0 0

a Content of Br, I, or N (in per cent) in respective compounds. b Hydrogenated polystyrene control polymer. 0 Hydrogenated polystyrene control polymer.


TABLE I11 Bulk Reaction of Aroyl Peroxides with Polystyrene

Di-p- Di-p- Di-p- chlorobenzoyl bromobenzoyl nitrobenzoyl

Peroxide concn., mole/mole monomer unit 0.0672 0.0670 0.0672

Peroxide decomposition time, days 12 12 12 Molecular weighta

Initial 19,940 19,940 19,940 Final 14,805 15,915 15,860

Purified polymer, %b 1.53 4.30 0.32 Control polymer, yoa,b 0.42 0.95 0.24

Corrected analyses

Saponification time, hr. 120 120 120 Corrected analysis, hydrolyzed poly-

Content of aroyloxy groups, % 70.0 55.3 3.0 Content of aryl groups, % 30.0 44.7 97.0

mer, %b 0.46 1.92 0.31

Hydrogenated polystyrene control polymer. Content of C1, Br, or N (in per cent) in respective compounds.

Chlorine and Bromine. Analysis for chlorine and bromine content of the polymers was obtained by preliminary combustion in a quartz tube, the acidic products being absorbed in sodium carbonate-hydrogen peroxide. The halides were then determined by amperometric titration7 with 0.005N silver nitrate. Reproducibility in the order of 0.05 was obtained.

Iodine. Iodine analysis following a similar combustion step was determined by the Liepert8 method, in which ultimately iodine is titrated with 0.005N sodium thiosulfate. Reproducibility to within 0.02 was obtained.

Nitrogen. The nitrogen content of these polymers was obtained by a micro-Dumas t e~hn ique .~ After a three-tube catalytic combustion step, nitrogen is determined by a displacement and gravimetric method. Repro- ducibility is within 0.05%.

Molecular Weights

Molecular weights of these polystyrenes were determined viscometrically by use of the Flory-Krigbaum'O relationship for unfractionated polystyrene: [ q ] = 1.03 X 10-4M0.74. Viscosity measurements were determined in benzene at 25°C. with the use of a Ubbelohde'l dilution viscometer and at concentrations of 0.5-3 g./dl. Intrinsic viscosity [qj was determined from a plot of reduced viscosity versus concentration.

Solution Polymerization of Styrene with Diaroyl Peroxides

Solution polymerizations of styrene initiated by di-p-bromobenzoyl, di- p-iodobenzoyl, and di-p-nitrobenzoyl peroxides were carried out in refluxing benzene. The resulting polymers were purified and each subsequently analyzed for its respective substituent and its molecular weight determined.


The polymers were then subjected to saponification for 96 hr. and thereafter purified and analyzed. The data are reported in Table I, and the analytical results have been corrected relative to the polystyrene control.

Reaction of Diaroyl Peroxides with Polystyrene Solutions

Benzene solutions of the control polystyrene (benzoyl peroxide-initiated) were refluxed for twelve days in the presence of di-p-bromobenzoyl, di-p- iodobenzoyl, and di-p-nitrobenzoyl peroxide. After this period, addition of potassium iodide to a 1-ml. aliquot of each reaction mixture indicated complete peroxide decomposition. The polymers were then purified, each analyzed for its respective substituent, and its molecular weight determined.

The polystyrenes treated in this manner were then subjected to a saponification treatment for approximately 120 hr. They were then purified and once again analyzed for the substituent. The data, again corrected for the inherent content of the control polymer, are reported in Table 11.

Bulk Reactions of Diaroyl Peroxides With Polystyrene

A mixture of polystyrene (benzoyl peroxide-initiated) and di-p-chlorobenzoyl, di-p-bromobenzoyl, or di-p-nitrobenzoyl peroxide was rapidly dissolved in a minimum quantity of benzene at room temperature and then immediately frozen in a Dry Ice-acetone bath. After a vacuum sublima- tion of the benzene from an ice-water bath over a period of several days, homogeneous, powdery peroxide-polymer mixtures were obtained. These dry mixtures were then subjected to a twelve-day decomposition period in a container jacketed with refluxing benzene. Potassium iodide again indicated complete peroxide decomposition after this time interval. The polymers were then purified, analyzed, and their molecular weights determined.

They were then subjected to a 120-hr. saponification, thereafter purified, and subsequently analyzed. These data, adjusted for the appropriate control corrections, are listed in Table 111.

Measurement of the Decomposition of Peroxides in the Presence of Poly- mer or Monomer

Dilute (0.012M) solutions of di-p-chlorobenzoyl, di-p-bromobenzoyl, and di-p-nitrobenzoyl peroxides in benzene were heated to reflux, and the dis- appearance of peroxide was measured by titration,12 while the carbon dioxide evolved was determined by absorption in Ascarite. In each experiment, complete decomposition of the peroxide occurred. The carbon dioxide liberated was 0.416 and 0.236 mole/mole of di-p-chlorobenzoyl peroxide, 0.248 and 0.285 mole/mole of di-p-bromobenzoyl peroxide, and 0.269 and 0.964 moles/mole of di-p-nitrobenzoyl peroxide in the presence of monomer and polymer, respectively.

134 R. L. DANNLEY AND H. J. MSIG

Efficiency of Di-p-nitrobenzoyl Peroxide as an Initiator

Two solutions of 26 g. (0.25 mole) of styrene in 125 ml. benzene were heated to reflux. To one was added 0.515 g. (0.0015 mole) of di-p-nitrobenzoyl peroxide. After 20 hr., the polymer was precipitated by addition of methanol. The peroxide-initiated reaction yielded 2.4 g. of polystyrene, compared to 1.1 g. for the control reaction.

III. RESULTS AND DISCUSSION

Polymerization of Styrene Monomer with Aroyl Peroxides

It has now been found that initiation of the polymerization of styrene by catalytic quantities of di-p-bromo- and di-p-nitrobenzoyl as well as di -p- anisoylla and di-p-chlorobenzoyl peroxide2 leads to incorporation of catalyst fragments which are primarily (88-93%) aroyloxy groups. In each case, the polymers produced are of similar molecular weight although the yield and time required for reaction varies. Price, Kell, and Krebsa have reported that di-p-nitrobenzoyl peroxide is an inhibitor and not an initiator for styrene polymerization. The present work proves conclusively that this peroxide does have catalytic activity. Di-p-iodobenzoyl peroxide differs, as an initiator, from the other aroyl peroxides in two respects: it produces a polymer which contains appreciably smaller percentages of aroyloxy fragments (79a/,) and is of higher molecular weight. However, it is kn0wn14 that o-iodobenzoyl peroxide undergoes an intramolecular rearrangement to a type of iodosobenzoate compound. It is entirely possible that intermolecular rearrangement of a similar type occurs with t,he para isomer. The differences observed with the use of di-p-iodobenzoyl peroxide may, therefore, be due to its action as an iodoso derivative rather than an aroyl peroxide. Since its classification is ambiguous, the iodobenzoyl peroxide was not used in additional experiments.

Reaction of Aroyl Peroxides with Polystyrene in Benzene Solution

The reaction of di-p-bromobenzoyl peroxide with polystyrene led to incorporation of 52% of the initiator fragments as p-bromophenyl groups. This explains the difference in figures reported in the present work and that of Bartlett and Cohen' for initiation of polymerization by di-p-bromobenzoyl peroxide. Bartlett and Cohen used large quantities of the peroxide (11.5 mole-yo) in their polymerization. The peroxide, therefore, not only served as a polymerization catalyst but was present in sufficient quantity to react with the polymer formed. They reported that of the 10.7% bromine incorporated, 64% was present in aroyloxy groups. This value is a logical intermediate between the %yo and 48% figures now reported for the initiation and polymer reaction steps.

The inability of di-p-nitrobenzoyl peroxide to introduce any initiator fragments into polystyrene was an unexpected development. It was not


due to a lack of decomposition of the peroxide, for titration and measurement of carbon dioxide evolution showed that all of the di-pnitrobenzoyl peroxide decomposed, yielding approximately 2 moles (1.93 exactly) of p- nitrophenyl radical per mole of peroxide. The pnitrophenyl radical is ob- viously capable of abstracting hydrogen from the polymer (resulting in a molecular weight change) but incapable of combining with polymer radicals. There are no p-nitrobenzoyloxy radicals available for a termination reaction, and therefore nitrogen is not incorporated.

It is surprising that the bromophenyl radical, which is produced in smaller concentration than the pnitrophenyl radical, is incorporated in polystyrene to a greater degree. This difference in degree of incorporation was thought to be a result of the scavenging effect of the benzene solvent. The pnitrophenyl radical is more electrophilicl6 than the bromophenyl radical, and it would therefore seem more logical that it react with the electron-rich benzene nucleus. The influence of the halogen substituent upon a phenyl radical has been reportedls to be inductive in nature; therefore, the chlorophenyl radical should be more electrophilic than the bromophenyl radical. Although the trend of reaction is as predicted, it is surprising that the bromo- and chlorobenzoyl peroxides exhibit such a marked difference in their reaction with polymer (48801, and 96y0 aroyloxy group incorporation, respectively).

The di-p-iodobenzoyl peroxide also reacts in an anomalous fashion which may be ascribed to its possible rearrangement to an iodoso compound.

An appreciable increase in molecular weight (see Table 11) occurred in every experiment when polystyrene was treated with an aroyl peroxide. Aryl or aroyloxy radicals from the peroxide undoubtedly abstract hydrogen atoms from polystyrene. The polymer radicals thus produced may, in some instances, partially combine with peroxide fragments or dispropor- tionate, but apparently there is always suflicient dimerization to yield an overall increase in molecular weight. The dimerization may be aided by stabilization of the polymer radicals through coordination with the benzene.

Bulk Reaction of Aroyl Peroxides with Polystyrene In these bulk reactions appreciable percentages of aryl radicals were in-

corporated in the polymer in every case studied. These data emphasize the scavenging reaction of the solvent benzene for aryl radicals when the reactions are performed in solution.

In the bulk reactions the molecular weight of the polymers always decreased. Polymer radicals were undoubtedly formed through hydrogen abstraction by peroxide fragments. Since there was no benzene present to coordinate and stabilize these radicals, and their rate of diffusion is undoubtedly slow, appreciable depolymerization must take place before a termination step occurs.

The authors would like to express their appreciation to Mr. James R. Kubik of the B. F. Goodrich Company Research Center, Brecksville, Ohio, for performing the nitrogen analyses reported in this paper.


References 1. Bartlett, P. D., and S. G. Cohen, J. Am. Chem. SOC., 65,543 (1943). 2. Dannley, R. L., and E. L. Kay, J. PoZymer Sn‘., 19,87 (1956). 3. Price, C. C., R. W. Kell, and E. Krebs, J. Am. Chem. Soc., 64, 1103 (1942). 4. Cooper, W., J. Chem. Soc., 1951,3106. 5. Price, C. C., and E. Krebs, Org. Syntheses, 23,65 (1943). 6. Lewis, F. M., and F. R. Mayo, Ind. Eng. Chem., Anal. Ed., 17,134 (1945). 7. Laitinen, H. A., W. A. Jennings, and T. D. Parks, Ind. Eng. Chem., Anal. Ed., 18,

8. Leipert, T., Mikrochemie ver. Mikrochim A&, 3, 147 (1938). 9. Semi-Automdic Micro. Dumas Nitrogen, B. F. Goodrich Research Center, Brecks-

335 (1946).

ville, Ohio. 10. Krigbaum, W. R., and P. J. Flory, J . Polymer Sci., 11,37 (1953). 11. Craig, L. H., T. M. Rogers, and D. A. Henderson, Can. J. Research, B25, 333

12. Swain, C. G., W. H. Stockmayer, and J. T. Clark, J. Am. Chem. SOC., 72, 5426

13. Bevington, J. C., J. Toole, and L. Trossarelli, Trans. Faraday SOC., 54, 863

14. Leffler, J. E., C. C. Petropoulos, and R. D. Faulkner, Chem. & I d . (London), 1956,

15. Dannley, R. L., and M. Sternfeld, J. Am. Chem. SOC., 76,4543 (1954). 16. Cadogan, J. I. G., D. H. Hey, and G. H. Williams, J. Chem. SOC., 1955, 1425.

(1947).

(1950).

(1958).

1238.

Synopsis Initiation of the solution polymerization of styrene by catalytic quantities of p-chloro-,

pbromo-, and p-nitrobenzoyl peroxides leads to incorporation of catalyst fragments which are primarily (88-95%) aroyloxy groups. Use of p-iodobenzoyl peroxide gives polymers of somewhat higher molecular weight and lower (79%) aroyloxy fragment content. This anomalous behavior of the iodo compound may be due to its rearrangement to an iodosobenzoate-type compound. Treatment of polystyrene in benzene solution with these same peroxides leads to incorporation of catalyst fragment6 (96% chlorobenzoyloxy and 48% bromobenzoyloxy groups. p-Nitrobenzoyl peroxide is not incorporated under these conditions. This is due to a scavenging action of the benzene solvent. pIodobenzoy1 peroxide again did not fit a general pattern and introduced 68% aroyloxy p u p s . In all of these reactions an increase in molecular weight of the polymer occurred. Treatment of bulk polystyrene with the peroxides led to incorporation of 70 yo chlorobenzoyloxy, 55y0 bromobenzoyloxy, and 3% pnitrobenzoyloxy groups. In these reactions the molecular weights of the polystyrene samples always decreased.

R 6 W 6 L’initiation de la polym6risation en solution du styrene par des quantit4s catalytiques

de peroxydes de pchloro-, p-bromo- et pnitrobenzoyle conduit B l’incorporation de fragments de catalyseur qui sont primitivement (-95%) des groupes aroyloxy. L’emploi de peroxyde de p-iodobenzoyle fournit des plymeres de poids molkculaire un peu plus 6lev6 et de teneur en fragments aroyloxy plus bas (78%). Ce comportement anonnal du compos6 iod6 peut &re dB il son rbarrangement en un compos6 du t,ype iodo-benzoate. Un traitement du polyst,yr&ne en solution dam le benzene avec ces m&mes peroxydes conduit il l’incorporation de fragments de catalyseur des groupes chlorobenzoyloxy il 96% et bromobenzoyloxy A 48%. Le peroxyde de p-nitrobenzoyle n’est pas introduit comme fragments dans ces conditions. Cela est dd il la capture de radicaux par le sol- vant benz6nique. Le peroxyde de p-iodobenzoyle n’a pss non plus donne un sch6ma


gen6ral et introduit sS% de groupes aroyloxy. Dana toutes ces reaction8 un accroisse- ment du poids mol&ulaire du polymbre a lieu. Le traitement du polystyrbne en bloc avec les peroxydes a conduit $. 70% de groupe chlorobenzoyloxy, 55% de groupe bromobenzoyloxy et 3 yo de groupe p-nitrobenzoyloxy respectivement. Dana ces rbctions, les poids moleculsires des Bchantillons de polystyrhe diminuent toujours.

Zusammenfassung Die Anregung der Styrolpolymerisation in Losung durch katalytische Mengen von p-

Chlor-, p-Brom und p-Nitrobenzoylperoxyd fiihrt zum Einbau von Katalyeatorbruch- stiicken und zwar in erster Linie (8&95%) von Aroyloxygruppen. Verwendung von p-Jodbenzoylperoxyd ergibt Polymere von etwas hoherem Molekulargewicht und niedri- gerem Gehalt (79%) an Aroyloxybruchstiicken. Dieses anomale Verhalten der Jodver- bindung kann durch ihre Umlagerung zu einer Verbindung vom Jodbenzoattyp bedingt sein. Behandlung von Polystyrol in Benzollosung mit den gleichen Peroxyden fiihrt zum Einbau von Katalysatorbruchstucken, die zu 96% aus Chlorbenzoyloxybzw. 48% aus Brombenzoyloxygruppen bestehen. p-Nitrobenzoylperoxyd wird unter diesen Bedingungen nicht in Form von Bruchstiicken eingebaut, was auf die Abfangwirkung des Losungsmittela Benzol zuriickzufiihren ist, p-Jodbenzoylperoxyd passt auch in diesem Fall nicht in das allgemeine Schema und fiihrte zum Einbau von 68% Aroyloxygruppen. Bei allen diesen Reaktionen trat eine Zunahme des Molekulargewichts des Polymeren auf. Behandlung von Polystyrol in Substanz mit diesen Peroxyden fiihrte zu 70% Chlorbenzoyloxy-, 55% Brombenzoyloxy- bzw. 3% pNitrobenzoyloxygruppen. In diesem Falle nahme das Molekulargewicht der Polystyrolproben stets ab.

Received August 24. 1959

the incorporation of aroyl peroxide fragments in polystyrene

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