The fluorescence of polybenzyl

European

Polymer Journal, 1969, Vol. 5, pp. 307-314. Pcrgamon Press. Printed in England.

THE FLUORESCENCE

OF POLYBENZYL

BRYAN ELLIS,* P. G. WHITE* and R. N. YOUNG~ Departments of Glass Technology* and Chemistry,t The University, Sheffield (Received 17 August 1968) Abstract-Polybenzyl, prepared by treating benzyl chloride with stamk chloride in the presence of air at room temperature, is fluorescent. The fluorescence and absorption spectra of this polybenzyl have been determined. From these spectral studies together with the reactions of polybenzyl with butyl lithium and sodium/potassium alloy, it has been established that the most probable fluorescent unit is a benzyl substituted 9 phenyl or 9:lO diphenyl anthracene. A possible mechanism is given for the formation of this fluorescent group via the formation of a carbonium ion of diphenyl methane type.

INTRODUCTION THE FORMATIONof

polybenzyls, from benzyl compounds in the presence of FriedelCrafts catalysts, has been known for a long time. (l) There is renewed interest in these polymers because they can be cross-linked to form thermo-setting resins(2) (the Phillips “Friedel-Crafts Polymers”) which have high thermal stability and are readily fabricated into laminates suitable for structural applications. The benzyl compounds which have been used for the preparation of polybenzyls are benzyl halides, alcohols and ethers(j) but by far the most thorough investigation has concerned the polymerization of benzyl chloride catalysed by stannic chloride.(4* 5, Recentlyc6) it has been reported that a linear polybenzyl has been prepared using aluminium chloride at low temperatures (- 100’) whereas at ambient temperatures or above cross-linked products are usually, but not always, obtained with this catalyst.(5) The detailed structure of the polybenzyl prepared under “normal” conditions, e.g. benzyl chloride with stannic chloride at room temperature, is unknown but it is established that predominantly it contains phenyl groups linked by methylene bridges.(s) The structure (I) was suggested by Haas and co-workers”) since the i.r. spectrum Ph Ph

Ph Ph

2

L

CH~CC

PhC”&&#

Wz CHz 6h bh (1)

307

308

BRYAN ELLIS, P. G. WHITE and R. N. YOUNG

of polybenzyl indicates that a large proportion of the phenyl groups are only monosubstituted. They pointed out that the reactivity of an aromatic ring to electrophilic substitution increases with the degree of substitution, thus the polymer consists of a few (2,3 or 4) fully substituted aromatic rings with pendant benzyl groups. Structure (I) has a number of shortcomings. It does not take account of the steric overcrowding due to adjacent benzyl groups, nor does it completely account for the i .r . spectrum,“’ for the high intensity of the U.V.absorption band at 270 mp, for the limited degree of polymerization (sa) (E = 25) and in particular the pronounced fluorescence of these polybenzyls. The fluorescence has been observed previously by Valentine and Wintert4) and Parker (5) but the fluorescence spectrum was not examined and hence its origin could not be assigned. Because of the use of these polymers at high temperatures where thermal degradation and oxidation@* g, occur, it is important to characterise particular chemical groups which might influence the degradation. The fluorescent centres may well play a part in the initiation of the degradation of polybenzyl or the Phillips resins prepared from them. In this paper we report the fluorescence spectrum of a polybenzyl prepared from benzyl chloride with stannic chloride as the Friedel-Crafts catalyst; we consider the possible structures responsible for the fluorescence and propose a mechanism for the formation of the fluorescent groups. EXPERIMENTAL Benzyl chloride and stannic chloride were standard laboratory reagents and were used without purification. Spectroscopic grades of benzene and chloroform were used for determination of the fluorescence and absorption spectra respectively. Tetrahydrofuran was purified by distillation and was rigorously dried by storing over benzophenone ketyl under high vacuum. Butyl lithium was prepared under high vacuum by reaction of lithium metal with a benzene solution of butyl chloride. A typical preparation of tluorescent polybenzyl was the following ?* ‘) Benzyl chloride 50 g (O-395 moles) was treated with O-65 g (2.5 x IO-” moles) stannic chloride at room temperature, the HCl produced being allowed to escape. After 5 hr the red solid mass was dissolved in dioxan; inorganic residue was removed by filtration and the polybenzyl was recovered by precipitation with ice cold water. The polybenzyl was dried at room temperature under vacuum, and the weight of the recovered purified polymer was 15 g. When prepared in the absence of oxygen, using suitable apparatus and techniques, polybenzyl was not fluorescent. Metallation reactions were carried out under high vacuum in a Pyrex vessel which was attached to a quartz cell enabling spectra of the reaction mixture to be recorded directly. The number average molecular weight of 2240 + 110 was determined with a Mechrolab (1301A) vapour phase osmometer. Previous determinations, reported in the literature, range from several hundred up to 4OOO.(3-s) The absorption spectra were determined using an Optica and an Ultracord recording spectrophotometers. The fluorescence spectra were determined using a phase sensitive method described by Watson and Parke”O) with exciting tungsten light chopped at 200 c/s.

RESULTS AND DISCUSSION (a) Fluorescence spectra The fluorescence spectrum of polybenzyl in benzene solution is given in Fig. 1. It is broad with a main maximum at 449 rnp and a second peak at 432 mp. The position and general characteristics of the fluorescence spectrum of polybenzyl indicate that a highly conjugated centre is present, probably an anthracene unit with either the 9 or both 9 and 10 positions substituted. (ri) Thus, the fluorescence spectrum of 9:lO

The Fluorescence of Polybenzyl

309

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Diphenyl anthracene

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430 Wavelength,

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I

470

FiG. 1. Fluorescent emission spectra of polybenzyl and 9:10 diphenyl anthracene in benzene solution. (Note: emission scales are different for these two compounds) diphenyl anthracene in benzene solution has been determined and is given in Fig. 1 for comparison with that of polybenzyl (the intensity scales are different). The correspondenc~ is not exact, nor would it be expected to be, since the fluorescent centre in polybenzyl will contain substituent benzyl groups. The effect of such bvnzyl groups 40--

6O

c

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8O

100250

270

Wavelength,

290 m~,

I

310

Fro. 2. Spectrum, 250-300 m~, of polybenzyl in chloroform (0"06352 g/l).

310

BRYAN ELLIS, P. G. WHITE and R. N. YOUNG

would be to increase the wavelength and broaden the fluorescence spectrum, which is in accord with the observed differences.

(b) Absorption spectra The u.v. spectrum of polybenzyl is given in Fig. 2; there is a strong band at 270 mp of high intensity (E = 1.23 × 104 for polybenzyl with a molecular weight of 2240 or • - 500 per benzyl unit). This is in substantial agreement with that reported by Valentine and Winter who commented on this abnormally high extinction coefficient. However, it should be noted that relatively small changes in structure may cause considerable alterations to extinction coefficients, compare for example mesitylene (e -----220) and 1, 2, 4 trimethylbenzene (E = 700). In the spectra of some of their polybenzyls, Valentine and Winter (4) found two weak bands in the region of 300-400 mp which they suggested might be associated with either anthracene or 9:10 dihydro anthracene terminal groups. However, the latter suggestion cannot be correct since 9:10 dihydro anthracene does not have absorption bands in this region. The spectra of our polybenzyl and 9:10 diphenyl anthracene (both in chloroform) are given in Fig. 3. While not identical, these two spectra are sufficiently similar for

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1~o. 3. Spectra, 320--420rap, polybenzyl (8.75 g/l) and 9:10 diphenyl anthracene (0.1443 g/l) in chloroform.

The Fluorescenceof Polybenzyl

311

the shifts to be accounted for by an argument similar to that used for the shifts in the fluorescent spectra. Using the measured extinction coefficient at 377 m/z of 9:10 diphenyl and the observed intensity of the polybenzyl band at 387 m/z, we have calculated that the polymer would contain 1.5 per cent by weight of 9:10 diphenyl anthracene units.

(c) Metallation reactions While the above results suggest that 9:10 diphenyl anthracene is a likely structure, there remains the possibility that other derivatives of anthracene could have similar spectra. Furthermore, while the absorption spectrum of polybenzyl does not indicate the presence of 9:10 dihydro-anthracene, it is conceivable that this grouping is present. These possibilities are conveniently investigated by measurement of the visible absorption spectra of the ions formed on metallation, even in the presence of substituent benzyl groups. A suspension of the liquid alloy of sodium and potassium in tetrahydrofuran slowly metallates 9, 10 dihydroanthracene in the 9-position to yield a red solution (A=a. = 450 m/z). If this procedure is conducted in the presence of a large excess of butyl lithium, the red anion is formed more rapidly and undergoes further reaction to give a blue solution with absorption bands at 715 and 615 m/z which correspond respectively to those recorded for the mono- and di-negative ions of anthracene, t~2) The following sequence of reactions accounts for these observations:

No - K

Antit~'/me-

-(- Anl~hr(r.,ene : -

~', Treatment of the polymer with butyl lithium and Na-K alloy initially produced a yellow colour due to a broad absorption band with a centre at about 380 n~. A strong band at 450 m/z appeared on further reaction, but at no stage was there any evidence for absorption at 715 and 615 m/z as would have occurred if there had been a detectable concentration of either anthracene or 9:10 dihydroanthracene units in the original polymer. Reaction of a mixture of 9:10 dihydroanthracene and polybenzyl (1:10 w/w) with butyl lithium and Na-K alloy in tetrahydrofuran give the same peaks at 615 and 715 m/z shown for 9, l0 dihydroanthracene together with a strong band at 450 m/z corresponding to the simultaneous metallation of the polybenzyl to yield a substituted diphenyl methyl ion. Thus, the original polybenzyl contains less than an estimated total of 0.2 per cent anthracene and 9:10 dihydroanthracene units. A survey of the mechanistically possible derivatives of anthracene showed that only substitution in the 9 or the 9:10 positions by a phenyl group is in fact reduced by sodium metal to a yellow anion with an absorption spectrum similar to that observed in the initial stages of the polymer reduction. Metallation of a 9 or 9:10 benzyl substituted anthracene would take place at the methylene group. This anion, by analogy with the naphthyl methyl anion, (~a) would be expected to be highly coloured with an

312

BRYAN ELLIS,P. G. WHITE and R. N. YOUNG

intense long wavelength absorption and would not appear yellow as observed in the metallation of polybenzyl. Thus, these metaUation reactions show that any anthracene residue in our polybcnzyl must be predominantly substituted in either the 9 or the 9:10 positions by phenyl groups.

(d) Reaction mechanisms From the previous discussion, it is clear that a reaction mechanism to account for the formation of the highly fluorescent species must lead to the formation of substituted anthracene units. Mechanistically, it is extremely difficult to account for the presence of substituents other than bvnzyl or phenyl. In our opinion, the evidence supports the presence of phenyl substituents but it does not conclusively exclude the presence of bcnzyl substituents. The mechanism leading to benzyl substitutions is a straight-forward Friedel--Crafts reaction: however, the formation of phenyl substituents is not so obvious and the following scheme indicates how it may occur:

PhCI-~L

.~

A

H

E

F

Ox m

G

A methylene group in the polymerizing polybenzyl A can form ions B of the diphenyl methyl cation type by reaction with stannic chloride. The characteristic red colour of the reaction mixture (Fig. 4) is supporting evidence for the presence of ions B. The geometry of the ion B is favourable for cyclization to the dihydroanthracene derivative C. In the presence of molecular oxygen, C may directly aromatize

The Fluorescenceof Polybenzyl

-_

~

w

i

t

h

313

'Intensity increases extent of

c" .9 50

I-

IOO

SO0

I

4OO Wavelength,

I

500 m~.

FIG. 4. Spectrum, 300-500 mt~, benzyl chloride with stannic chloride during preparation of polybenzyl.

to the 9-phenyl anthracene residue D. Alternatively, further Friedel-Crafts electrophilic substitution with E to produce F may precede oxidation resulting in an anthracene residue G which is phenylated in positions 9 and 10. When polybenzyl is prepared in the absence of oxygen, the reaction mixture does not show the characteristic visible fluorescence supporting the above mechanism since oxidation is required for aromatization to a species fluorescent at these wavelengths. In this scheme no attempt has been made to specify the exact catalyst species involved; in the preparation of the polybenzyl no steps were taken to use absolutely dry reagents and so the stannic chloride must be hydrated or hydrolysed to some extent.(5°) It has been suggested that water is necessary as a co-catalyst for these reactions, (4) although this was not established beyond doubt. However, in our ease it was desirable to ensure polymerization by allowing the presence of small adventitious amounts of water. If very wet reaction conditions were used, e.g. stale stannic chloride, polymerization did not occur and the catalyst was precipitated from solution. During our preparation of polybenzyl, a characteristic red colour appears and the absorption spectrum of the polymerizing mixture (Fig. 4) has a broad band at ~ 347 mt~ due to a benzyl carbonium ion and at 480 mt~ probably due to the diphenyl methyl carbonium ion. Valentine and Winter(4) showed that the characteristic red colour, which develops during polymerization, is also formed on treating purified polybenzyl with stannic chloride; we have confirmed this observation. They also showed that simple aromatic compounds (benzene, toluene, etc.) did not give the red colour on treatment with stannic chloride; however, we have found that diphenylmethane gives a similar red colour immediately on addition of stannic chloride, supporting our suggestion that a diphenylmethyl carbonium is present during the formation of polybenzyl. Finally, it should be noted that the temperature of polymerization will affect the

314

BRYAN ELLIS, P. G. WHITE and R. N. Y O U N G

detailed structure of the polymer formed, thus of a crystalline polymer was prepared at --100 ° by Kennedy and IsaacsonJ 6~ Acknowledgements--This work forms part of an investigation of the properties of thermally stable polymers under Ministry of Technology contract no. PD/31/032. We are grateful to Mr. E. Cole for assistance with fluorescence measurements. "Crown Copyright, reproduced with the permission of the Controller, Her Majesty's Stationery Office."

REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

(11) (12) (13)

R. L. Shriner and L. Berger, 3". Org. Chem. 6, 305 (1941). L. N. Phillips, Trans. Piast. Inst. Lomt. 32, 298 (1964). H. C. Haas, D. I. Livingston and M. Saunders, 3". Polym. Sci. 15, 503 (1955). L. Valentine and R. W. Winter, J. chem. Soc. 4768 (1956) and also Simposio Int. di Chimica Maeromol., supplement to Ricerca Sci. 25, 95 (1955). D. B. V. Parker, Ph.D. thesis, London 1966, (a) J. chem. Soc. (B) 471 (1967). ]. P. Kermedy and R. B. I ~ o n , J. Macromolek. Chem. 1, 541 (1966). B. Ellis and P. White, Unpublished data, but see also Refs. 4 and 5. R. T. Conley, 3". appl. Polym. Sci. 9, 1107 (1965). J. H. Lady, I. K ~ and R. E. Adams, J. appl. Polym. S¢i. 3, 71 (1960). A. I. Watson anti S. Parke, Br.J. appl. Phys. 17, 963 (1966). I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules. Academic Press (1965). P. Balk, G. J. Hoijtink and J. W. H. Schreurs, Recl. Tray. chim.Pays.BasBelg. 76, 813 (1957). M. Szwarc, Proc. R. Soc. 279, 260 (1964).

R ( n ~ L - L e polybenzyle obtenu en traitant le chlorure de benzyle par le chlorure de staunique/L l'air et ~ la temperature ordinaire est fluorescent. On a d6termin6 les spectres d'absorption et de fluor--,~cence de ce polym~re. A p r ~ avoir ~tudi~ ces spectres ainsi que les r~actions du polybenzyle avec le butyle fithium et un alliage sodium-potassium on conclut que l'unit6 fluorescente est probablemerit un ph~nyl-9 ou on un diph~nyl-9, 10-anthracene benzyl6. On propose nn mb:anisme pour la formation de ce compos6 qui met en jeu un ion carbonium de type diph6nylm6thane. Somm~rio---II polibenzile, preparato trattando cloruro di benzile con cloruro di stagno in presenza di aria a temperatura ambiente, ~ fluorescente. Sono stati determinati la fluorescenza e gli spettri di assorbimento di questo polibenzile. Da queste indagini spettrografiche insieme con le reazioni del polibenzile con litio butile e una lega sodio/potassio, ~ stato stabilito c h e l a pit': probabile unitA fluorescente ~ un bcnzile sostituito 9 fertile o 9:10 difenile antractme. E' dato un possibile meccanismo per la formazione di questo gruppo fluorescente per mezzo della formazione di uno ione di carbonio del tipo difenihnetano. Zummmeafasmmg--Polybenzyl, hergesteUt durch Behandlung von Benzylchlorid mit Zinn-IVcblorid in Gegenwart yon Luft bei Raumtemperatur, ist fluoreszierend. Die Fluoreszenzund Absorptionsspektren dieses Polybenzyls wurden bestimmt. Aus diessen spektralen Untersuchungen in Verbindung mit Reaktionen des Polybenzyls mit BuWllithium und Natrium/Kaliumlegierung lieB sich feststellen, dab die fluoreszierende Einheit sehr wahrscheinlich ein Benzyl-substituiertes 9-Phenyloder 9:10 Diphenylanthrazen darstellt. Ffir die Bildung dieser fluoreszierenden Gruppe wird ein mSglicher Mechanismus angegebcn, der fiber die Bildung eines Carbonium-Ions des Diphenylmethan-Typs verliuft