JOURNAL OF POLYMER SCIENCE T'OL. XLV, PAGES 105-110 (1960)

An Investigation of Lead Tetraacetate, N-Bromosuccinimide, p-Toluenesulfonyl Azide,

Benzenediazonium Fluoborate, and Saccharine-Dimethylaniline as Chain Initiators

RALPH L. DANNLEY and MANUEL ESAYIAN,* Morley Chemical Laboratory, Western Reserve University, Cleveland, Ohio

I. INTRODUCTION

The reactions of lead tetraacetate, 1,2 N-bromo~uccinimide,~-~ and aryl- sulfonyl azides6 are generally considered to proceed by free-radical mecha- nisms. Aryldiazonium salts in general may react by either ionic or polar mechanisms, depending on the nature of the solvent, temperature, e t ~ . ~ - ~ Saccharine-tertiary amine mixtures have recently been proposed'O as free-radical initiators for vinyl monomers. The present work was under- taken primarily to test whether thermal decomposition of these com- pounds in equimolar mixtures of styrene and methyl methacrylate would give free radicals capable of acting as initiators. This particular monomer mixture was used because the composition of the initial polymer formed is characteristic of the mechanism of polymerization, l1 varying from -99% polystyrene for carbonium ion reagents to equimolar copolymer with free- radical initiators to -99% polymethyl methacrylate with carbanion catalysts.

11. EXPERIMENTAL

Reagents

Styrene and methyl methacrylate were distilled under reduced pressure just prior to use. Boron trifluoride etherate, dimethylaniline and N-bromo- succinimide were used without purification. Benzenediazonium fluo- borate,12 m.p. 100" (lit. m.p. lOO"), lead te t raa~etate , '~ m.p. 177-179" (lit. m.p. 175-BOO), and p-toluenesulfonyl azide,14 m.p. 22" (lit. m.p. 22") were prepared just before use. Saccharine was recrystallized from aqueous ethanol until its melting point was 228" (lit.15 m.p. 228-229").

* This paper is based on a portion of the thesis submitted by Manuel Esayian in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Western Reserve University. Presented before the Division of Polymer Chemistry a t the 134th meeting of the American Chemical Society a t Chicago, Illinois, September, 1958.

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INVESTIGATION OF CHAIN INITIATORS 107

Specialized Techniques

All antioxidant runs were performed on a high vacuum line attaining a pressure not greater than 2.5 X mm. over frozen material on three successive degassings. The reactions were quenched and analyzed by standard techniques. l 1

The copolymerization data listed in Table I were obtained in degassed systems with purified monomers. The boron trifluoride polymerization was performed to prove the proper use of the techniques through agreement with literature values."

Several homopolymerizations were performed as shown in Table 11. Antioxidant conditions and purified monomers were used. A crystalline precipitate was formed in the styrene experiment which from its melting point and infrared spectrum was proved to be succinimide.

TABLE I1 Homopolymerizations with Different Catalysts

Catalyst, concn., Temp., Time, Yield, wt.-% of

Catalyst Monomer mole-cj, "C. hr. monomer used

N-Bromosuc- Styrene 5 . 9 =t 0.4 I00 3 0 cinimidea

None Styrene - 100 3 6 . 6 N-Bromosuc- Methyl metha- 5 . 0 zk 0.3 95-100 1 16.5 f 1.5

cinimide crylateb None Methyl metha- - 95-100 1 4.5

crylate

3 Crystalline precipitate (succinimide) formed but no gas was evolved on quenching. b Unreacted .V-bromosuccinimide left :it end of reaction.

111. DISCUSSION

Lead Tetraacetate

This material is a very inefficient initiator. The reaction is free radical in mechanism but the quantity of polymer (6.9%) obtained in a given time is not much greater than that obtained in a control experiment (3.4%) with- out catalyst. The catalyst is not stable in the absence of acetic acid, and in initial runs the color of lead oxide was observed before the samples were heated to cause polymerization.

An additional experiment was performed in which an acetic acid solu- tion of lead tetraacetate was used. The reaction mixture was colorless even after polymerization had taken place. An equivalent amount of acetic acid was added to the control run to eliminate the acid as a factor in the reaction. Again the lead tetraacetate was found to have a slight but definite catalytic action to initiate free-radical polymerization. In both the absence or presence of acetic acid the initiating species could be either the

108 R. L. DANNLEY AND M. ESAYIAN

methyl or the acetoxy radical, both of which are known to form on thermal decomposition of lead tetraacetate.

p-Toluenesulfonyl Azide

This compound does initiate polymerization by a free-radical mecha- nism, but the activity is extremely low. It requires careful repetition of procedures to differentiate between the quantity (9%) of polymer obtained in the presence of catalyst and the quantity (5%) obtained in its absence. The polymerization was probably initiated by the p-CH3CsH,SO2N : diradical which is formed by thermal decomposition of the azide.

Benzenediazonium Fluoborate

This compound is a good initiator, but the polymerization occurs by a carbonium ion instead of a free-radical mechanism. The initiating species could theoretically be the diazonium ion itself , the corresponding phenyl ion obtained from the diazonium ion by loss of nitrogen, fluoboric acid, boron trifluoride, or hydrogen fluoride.

The diazonium ion can be eliminated from consideration as the catalytic fragment, for the purified polymers are colorless. The crude products did have a color indicative of azo impurities, however.

The phenyl carbonium ion, which has been proposed as an intermediate in other is a logical initiator. Ultimately, however, chain termination reactions must result in the formation of either fluoboric acid (which is unstable) or boron trifluoride, and possibly hydrogen fluoride. All of these products could also initiate carbonium ion polymerization. Boron trifluoride could be obtained as well by direct dissociation of the fluoborate ion. Therefore many Lewis acids could be produced and the initiating species is not identified.

Saccharine-Dimethylaniline

This combination of compounds has been described'O as an initiator system, but in the present work has been found to be of extremely low activity (1% conversion in 1 hr. a t SO0). It definitely reacts by a free- radical mechanism.

N-Bromosuccinimide

This compound reacts by a homolytic mechanism although generally used in conjunction with a free-radical generator. It has now been found to be an inhibitor instead of a catalyst for the copolymerization of styrene and methyl methacrylate. At elevat.ed temperatures ( looo) the N-bromo- succinimide undergoes reaction, a t least in part, to yield succinimide and hydrogen bromide, but the inhibitor action is still marked.

Homopolymerizations were attempted to determine whether both monomers were involved in the inhibitor action. As seen in Table 11, N- bromosuccinimide inhibits the polymerization of styrene but catalyzes the

INVESTIGATION 01: CHAIN INI'TIATORS 109

polymerization of methyl methacrylate. A logical interpretation of these data is that styryl radicals (C6HbCHCHA) formed by any of several possible mechanisms must necessarily undergo a chain termination step with N-bromosuccinimide or its fragments. The styryl radical should be a better electron donor than the radical from methyl methacrylate and would be expected to combine readily with electron-accepting bromo- succinimide fragments. This interpretation explains why in the mixed monomer systems even the homopolymerization of the ester was prevented. The scavenging effect of styrene for N-bromosuccinimide probably leads predominantly to bromination of the styrene. The exact mechanism of this bromination with stoichiometric quantities of N-bromosuccinimide has been investigated but not completely elucidated by several groups of workers.16

References 1. Fieser, L. F., and F. C. Chang, J . Am. Chem. SOC., 64,2043 (1942). 2. Fieser, L. F., R. C. Clapp, and W. H. Daubt, J . Am. Chem. SOC., 64,2052 (1942). 3. Ziegler, K., G. Schenck, E. W. Krockow, A. Siebert, A. Wenz, and H. Weber, Ann.,

551, 1 (1942); K. Ziegler, A. Spath, E. Schaaf, W. Schumann, and E. Winklemann, Ann., 551, 80 (1942).

4. Schmid, H., and P. Karrer, Helv. Chim. Acta, 31, 1067 (1948). 5. Ettlinger, M. G., and L. F. Fieser, J . Biol. Chem., 164, 451 (1946). 6. Dermer, 0. C., and M. T. Edmison, Chem. Revs., 57, 99 (1957); 0. C. Dermer

7. Nesmeyanov, A. N., and L. G. Makarova, Dokludy Akud. NaukS.S.S.R., 87,417

8. Nesmeyanov, A. N., and L. G. Makarova, Bull. Acad. Sci. U.R.S.S., Classe sci.

9. Makarova, L. G., and E. A. Gribchenko, Zzvest. Akad. Nauk S.S.S.R., Otdel.

and M. T. Edmison, J. Am. Chem. SOC., 77,70 (1955).

(1952).

chim., 1947,213.

Khim. Nauk, 1958, 693. 10. Lal, J., R. Green, and S. Ellis, J . Polymer Sci., 24, 75 (1957). 11. Walling, C., E. R. Briggs, W. Cummings and F. R. Mayo, J . Am. Chem. Soc.,

12. Bala, G., and G. Schiemann, Ber., 60B, 1188 (1927). 13. Inorganic Syntheses, Volume I, 1st Ed., McGraw-Hill, Kew York, 1939, p. 47. 14. Curtius, T., J. prakt. Chem., 125,303 (1930). 15. Kempf, R., J. prakt. Chem., 78,201 (1908). 16. Buckles, R. E., R. C. Johnson, and W. J. Probst, J . Org. Chem., 22,55 (1957).

72,48 (1950).

Synopsis

The thermal decomposition of several common materials in equimolar mixtures of styrene and methyl methacrylate has determined the initiator efficiency of the materials and permitted classification of the mechanisms as ionic or free radical. Both lead tetra- acetate and p-toluenesulfonyl aaide are such weak catalysts for homolytic polymerization that careful work is required to differentiate catalyzed and blank runs. The proposed catalyst system, saccharine-dimethylaniline, is also homolytic in nature but is even less efficient (1% conversion in 1 hr. at 60"). Benzenediazonium fluoborate is a good initiator but is carbonium ion in nature. The active species is not proved but may be the phenyl carbonium ion. N-Bromosuccinimide is an inhibitor not only for the monomer mixture but also for the homopolymerization of styrene. It is a catalyst however for the homo- polymerization of methyl methacrylate. Apparently styryl free radicals are efficiently

110 €1. L. DANNIEY AND M. ESAYlhN

scavenged by N-bromosuccinimide or its fragments to terminate the radical chain. The principal products are probably the brominated materials.

Rbum6 La d6composition thermique de plusieurs substances courantes dam des melanges

6quimol6culaires de styrene et de mkthacrylate de m6thyle a permis de determiner le pouvoir initiateur de ces substances et de classifier les m6canismes en mecanismes ioniques e t par radicaux libres. Le t6tranc6tat.e de plomb et l’azoture de p-tolubnesul- fonyle sont de si faibles catalyseurs pour line polym6risation homolytique eu’il faut travailler avec soin pour voir la difference entre une rdaction catalys6e et une reaction de contr6le. Le systeme catalyseur propos6: saccharine-dim6thylaniline est aussi de nature homolytique mais est encore moins efficient (1% de conversion en 1 h. A 60”). Le fluoroborate de benzenediazonium est un bon initiateur mais agit par formation d’ion carbonium. L’esphce active peut &re Pion carbonium, mais cela n’est pas prouv6. Le N-bromosuccinimide est un unhibiteur non seulement pour le melange de monombre mais aussi pour la homopolym6risation du styrbne. C’est cependant un catalyseur pour la homopolym6risation du m6thacrylate de m6thyle. I1 semble que les radicaux styryles libres soient capt6s d’une faCon efficace par le N-bromosuccinimide ou ses fragments avec terminaison de la chaine radicalaire. Les produits principaux sont probablement des mat6riaux brom6s.

Zusammen fassung Bei der thermischen Zersetzung einiger Stoffe in aquimolaren Mischungen von Styrol

und Methylmethacrylat wurde ihre Wirksamkeit als Starter bestimmt und damit eine Klasszzierung des Mechanismus als ionisch oder radikalisch ermoglicht. Sowohl Bleitetraacetat als auch p-Toluolsulfonylazid sind so schwache Katalysatoren fur homolytische Polymerisation, dass nur sorgfaltiges Arbeiten eine Unterscheidung der katalysierten von den Blindversuchen gestattet. Das als Katalysator vorgeschlagene System Saccharin-Dimethylanilin hat ebenfalls homolytischen Charakter, ist aber noch weniger wirksam (1% Umsatz in einer Stunde bei 60°C). Benzoldiazonium- fluorborat ist ein guter Starter, aber von der Natur eines Carboniumions. Als aktive Komponente scheint, obwohl es nicht bewiesen wurde, das Phenylcarboniumion wirksam zu sein. N-Bromsuccinimid wirkt nicht nur in der Monomermischung, sondern auch bei der Homopolymerisation des Styrols als Inhibitor. Fur die Homopolymerisation des Methylmethacrylats ist es jedoch ein Katalysator. Offenbar werden Styrylradikale durch N-Bromsuccinimid oder seine Bruckstiicke wirksam abgefangen und so die Radi- kalkette abgebrochen. Die Hauptprodukte sind wahrscheinlich die bromierten Stoffe.

Received September 30, 1959 Revised March 18, 1960