JOURNAL OF POLYMER SCIENCE: PART A-l VOL. 5, 945-964 (1967) Polym erization of Arom atic Nuclei. XII. Oligom erization of Halobenzenes by Alum inum Chloride-Cupric Chloride PETER KOVACIC, JOHN T. UCHIC, and LI-CHEN HSU, Department of Chemistry, Case Institute of Technology, Cleveland, Ohio 44-106 Synopsis Polymerization of chloro- and fluorobenzene by aluminum chloride-cupric chloride produced highly colored oligomers. Chlorobenzene reacted under the standard condi­ tions, i.e., 6/1/0.5 (molar ratio) of aromatic/catalyst/oxidant at 60°C. for 1 hr., to give a red solid in 14% yield. Evidence concerning the structure was obtained from elemental analyses, infrared and ultraviolet spectra, dechlorination, oxidation, solubility, molecular weight, and color. The data indicate that the backbone chain consists of an o-polyphenyl structure with chlorine atoms situated at the 4-positions. Polynuclear regions presumably comprise part of the structure. Molecular weight data pointed to an average of 10-12 units per chain. The coupled product from fluorobenzene was very similar to the chlorobenzene oligomer in most respects. In contrast to the chlorobenzene case, there was evidence of propagation occurring to some extent by attack ortho to the fluorine. Bromobenzene produced brominated p-polyphenyl apparently by dispro­ portionation to benzene which then functioned as the monomer. An oxidative cationic mechanism (<r polymerization) is proposed for the nuclear coupling. INTRODUCTION Benzene has been polymerized to p-polyphenyl by various reagents, viz., aluminum chloride-cupric chloride,1 ferric chloride,2 and molybdenum pentachloride.3 Excellent yields were obtained in the aluminum chloride- cupric chloride system.4 In a similar fashion, other aromatic monomers have been studied in this laboratory: biphenyl,5 p-terphenyl,5 naphthalene,6 mesitylene,7 and m- and p-xylene.8 Bilow and (Miller9 prepared fusible polyphenyls by the oxidative cationic polymerization (in the molten state) of m-terphenyl, o-terphenyl, 1,3,5-triphenylbenzene, or mixtures of the terphenyls with biphenyl and benzene. Naphthalene10 has also been converted to a polychloronaphthyl, and triphenylene to chlorinated poly- triphenylene11 in a ferric chloride-sodium chloride-potassium chloride eutectic mixture. Benzene, anthracene, dibenzofuran,10 and p-terphenyl- triphenylene were also employed in this system, but the nature of the prod­ ucts was not disclosed. The objective of our study was to extend the polymerization scheme to the halobenzenes and to study the derived products in order to establish their nature and elucidate the reaction pathway. 945 A«®)* I t n.fl. 2510 946 P. KOVACIC, J. T. UCIIIC, AND L.-C. IfSU The previous literature on halogenated polyphenyls is comparatively scant. Lawlor and Miville12 prepared a chlorinated polyphenyl of unde­ fined structure by treating hexachlorocyclohexane with an aromatic compound in the presence of aluminum chloride. With trichlorobenzene, they obtained a black, friable, solid product composed of 45% chlorine. In an attempt to synthesize pure p-polyphenyl from p-dichlorobenzene by the Wurtz-Fittig method, Goldfinger isolated a polymer having an average degree of polymerization of 34 and analyzing for approximately 6 chlorine atoms per molecule.13 Using a different approach, iUcCall and Hughes14 produced chlorinated and brominated terphenyls by reacting a benzene- sulfonyl halide with a halobenzene in the presence of a copper catalyst. Fluorinated polyphenyls have also been synthesized. Hellmann and co- workers16 condensed brominated or iodinated tetrafluorobenzene via the Ullmann reaction to an oligomer, the benzene-insoluble fraction of which had an average of 8-10 units. Wright and Fielding16 treated dichlorotetra- fluorobenzene with activated copper and obtained a solid which they be­ lieved to be C1(C«F4) i«C1. Halogenation of p-polyphenyl17 by various reagents, e.g., antimony pentachloride, chlorine-aluminum chloride, or bromine-aluminum bromide has been effected. In addition, bromication of the lower members in the polyphenyl series has been studied.18 RESULTS AND DISCUSSION Polymerization of the halobenzene monomers was performed with mono- mer/aluminum chloride/cupric chloride in 6/1/0.5 molar ratio at 60°C. for 1 hr. In all cases, highly colored material resulting from nuclear coupling was obtained. Various approaches were utilized in characteriza­ tion of the products, including elemental analyses, molecular weight de­ termination, infrared and ultraviolet spectroscopy, oxidative degradation, dehalogenation, solubility, and color. By analogy to the oxidative cationic mechanism1 (a polymerization)19 for formation of p-polyphenyl, the reaction pathway shown in eqs. (1) H20 + Al Cl3 H20 — AlCl 3 H+AlCl;i(OHr (1) X X X POLYMERIZATION OF AROMATIC NUCLEI. XII 017 and (2) is postulated for halobenzene polymerization (X = F or Cl). In this scheme, the para position is employed for fixation of the electrophile in initiation and propagation. Since halobenzencs are susceptible to ortho-para attack, the alternative possibility of ortho participation should also be considered [eq. (3)] It should be made clear that, for polymerizations in this general category, little is known about the nature of the oxidation stop and the stage at which it occurs. Chlorobenzene Reaction Variables. The effects of variation in time, temperature, and concentration on product yield and color were determined. Chlorobenzene polymerized in fairly low yield (Table I) to an orange-red solid on exposure to aluminum chloride-cupric chloride. A cationic mechanism is in keeping with the less favorable yield relative to benzene.4 In the initial phases of the study, various conditions were utilized in an attempt to find a system which would provide the optimum yield and, at the same time, an oligomer with little or no color. We theorized that color reflects the occurrence of undesirable, accompanying transformations. The following observations apply: (I) at a given temperature, a 2:1 ratio of catalyst: oxidant provided a better yield than did a 1:1 ratio (analogous to benzene polymerization)4; (2) higher yields were favored by an increase in temperature; (8) in all cases, the color varied from orange to a dark red. Previously with very mild conditions (30°C., 0.5 hr.) and monomer/catalyst/oxidant in 12/1/0.5 molar ratio, a yellow solid was obtained (3% yield).20 On the basis of the data, the indicated standard conditions for polymerization were chosen: monomer/aluminum chloride/ 948 P. KOVACIC, J. T. UCII1C, AND L.-C. IISU cupric chloride = 6/1/0.5 (molar ratio) at 60°C. for 1 hr. Although opti­ mum yields were realized at the higher temperatures, the more drastic conditions were avoided in order to minimize side reactions and thus pro­ vide a more homogeneous product. TABLE I Oligomerization of Chlorobenzene Oligomer CdhCl/AlCI:,/ CuCU, Temp., Time, Yield, molar ratio °C. hr. % Color 12/1/1.0 80 1 11.5 Orange-red 12/1/0.5 80 1 23.4 Dark orange 10/1/0.5 80 1 15.5 Orange-red 8/1/0.5 80 1 17.3 Orange-red 6/1/1.0 80 1 7.2 1lark red 6/1/0.5 80 0.5 23.2 Dark red 6/1/0.5 SO 1 IS Orange-brown 6/1/0.5“ 80 1 7 Orange-red 4/1/0.5 80 1 15.1 Orange-red 2/1/0.5 80 1 16 1 lark brick red 20/1/0.5 60 1 14.5 1 lark orange 12/1/0.5 60 1 11.3 1 lark orange 6/1/0.5 60 2 14.2 Orange-red 6/1/0.5 60 1 13.9 Orange-red 6/1/0.5 50 2 11.8 Red 6/1/0.5 40 6 8.8 Orange-red 6/1/0.5 40 1 7.1 Orange-brown 12/1/0.5 30 12 6.7 Red-brown 6/1/0.5 30 12 5.6 Red-brown a Pulverized cupric chloride. Elemental Analyses and Color. The analytical re;suits for the chloro- benzene oligomer are shown in Table II. The sum of carbon, hydrogen, and halogen fell in the range 94.8-98.2%. In all cases per cent chlorine did not differ appreciably from the theoretical value, suggesting that chlorina- tion via the metal halide did not occur. This result is in contrast to the polymerization of other monomers in the same system, wherein small amounts of halogenation were shown to take place1'4'6 (see also the results for fluorobenzene). Note that the C/Cl atomic ratio is near the theo­ retical value of 6 calculated for (C6H3C1)„. Carbon analyses were somewhat at variance with the calculated figures, but the most significant discrepancy existed in the amount of hydrogen present, the values being 23-33% below theory. This is strikingly shown by the C/(H -f- Cl) atomic ratios, 1.8-2.0, which diverge considerably from theory (1.5). Hydrogen loss can most easily be explained on the basis of polynuclear formation. POLYMERIZATION OF AROMATIC NUCLEI. XII 949 Cl IV It is obvious that such a cyclization reaction can theoretically occur at any stage following trimerization, and to varying degrees. Ring closure is also possible when propagation takes place through a combination of ortho and para attack. This type of structure might also arise during termination. Another point in favor of the existence of polynuclear re­ gions is provided by the color of the product. It is pertinent that the oligomer prepared under very mild conditions, i.e., 30°C. for 0.5 hr., possessed a yellow color in contrast with the darker hue characteristic of the 60°C. product. It may well be that cyclization diminishes with decrease in reaction temperature. It should be borne in mind that the polynuclear moiety is a much more potent chromophore than the analogous polyphenyl entity. Alternatively, p-quinoid units may be partially respon­ sible for the observed color.21 Since an increase in chlorine content is known to darken the color of p-polyphenyl,21 perhaps halogen contributes to oligomer color in the present case.