Friedel-Crafts Polymers—I. Formation and stability of polymers containing heterocycles

European Polymer Journal, 1968, Vol. 4, pp. 571-580.

Pergamon Press. Printed in England.

FRIEDEL-CRAFTS POLYMERS--I. FORMATION

AND STABILITY OF POLYMERS

CONTAINING

HETEROCYCLES

N. GRASSIE a n d I. G. MELDRUM Chemistry Department, The University of Glasgow, Glasgow W.2, Scotland

(Received 16 May 1968) Abstract--Polymers have been prepared by condensation of di(chloromethyl) benzene with benzene, thiophene, pyridine, indole, quinoline and pyrrole using SnCI4 as catalyst. Attempts to prepare a furan polymer were unsuccessful. Measurements have been made of the functionality of the aromatic groups, which is a measure of the degree of branching, in the benzene, indole and thiophene polymers. The i.r. spectra of all six polymers have also been measured and their Thermal Volatilization Analysis (T.V.A.) thermograms obtained. In the light of the results of these three kinds of measurements, relationships between structure and stability are discussed.

POLYMERS p r e p a r e d b y c o n d e n s a t i o n o f d i ( c h l o r o m e t h y l ) b e n z e n e with b e n z e n o i d a r o m a t i c substances, in the presence o f F r i e d e l - C r a f t s catalysts, have b e e n s h o w n ~1~ to be c o n s i d e r a b l y m o r e t h e r m a l l y stable t h a n p h e n o l i c resins a n d hence to have c o n s i d e r a b l e p o t e n t i a l c o m m e r c i a l a p p l i c a t i o n . T h e a i m o f the w o r k d e s c r i b e d in the p r e s e n t p a p e r was t o discover w h e t h e r p o l y m e r s m a y be similarly p r e p a r e d f r o m d i ( c h l o r o m e t h y l ) benzene a n d simple heterocyclic a r o m a t i c c o m p o u n d s , a n d if so, t o assess t h e i r relative stabilities. EXPERIMENTAL

Preparation of polymer~ Stannic chloride, pyridine (Analar), quinoline and indole were supplied by B.D.H. Ltd.; benzene (Analar), pyrrole, thiophene and o-dichlorobenzene were supplied by Hopkin & Williams; di(chloromethyl) benzene (DCMB) was obtained from Bush, Boake & Allen. They were all used without further purification. Polymers were prepared using the following standard procedure. Equimolar proportions of the heterocyclic substance and DCMB (2"0 g) were refluxed in the solvent, o-dichlorobenzene (10 g). If no reaction occurred in 1 hr, a few drops of a 10 per cent solution of SnC1, in o-diehlorobenzene were added. A benzene--DCMB polymer was prepared similarly for the purpose of comparison. Relevant data are presented in Table 1. It was established that under these reaction conditions there is no significant self-condensation of DCMB. Of the heterocyclics studied, only furan did not condense with DCMB. The pyridine, benzene and pyrrole polymers precipitated during reaction. The indole polymer remained soluble but was precipitated by toluene. The quinoline polymer was partially soluble; the yield quoted in Table 1 refers to the insoluble portion only. The reaction involving thiophene resulted in gelation of the mixture. The course of the reaction was followed by estimating the resulting HCI which was swept continuously from the reaction vessel by a stream of nitrogen. It was absorbed in 0.5 N Na2CO3 solution 571

N. GRASSIE and I. G. M E L D R U M

572

TABLE 1. PREPARATmNOF POLYMERS

Aromatic constituent

Yield

Pyridine Benzene Thiophene Indole Quinoline Pyrrole Furan

Polymer (g)

HCI ( ~ of theoretical)

1" 75 1- 6 1" 3 1" 1 0.7 0" 02 0

0 38 45 26 34 0 0

Catalyst none SnCL, SnCL, none none SnCh SnCI,

from which aliquots were withdrawn at intervals and titrated against standard acid. Typical reaction curves axe illustrated in Fig. 1. The ultimate yields of HCI and of polymer are expressed appropriately in Table 1.

t20 I N ~

"

100

29o

e 400

39o

I

-40 THIOPHENE -30 ~1"10"20

O0

300 'l

400 I

-- t30 B ~ "3 V

-40

" 150

100

50

I

200 !

QUINOLINE

-30

-20 .r ~

i

I

l Time,

I

I

rain

Fzo. 1. Evolution of HC1 during polymerization.

Friedel-Crafts Polymers--I

573

Infra-red spectra The KC1 disc technique was applied and measurements were made using a Perkin-Elmer "Infracord" instrument. A standard reference text was used for the interpretation of i.r. spectra.(s)

Thermal stability measurements Thermal stabilities were assessed by means of Thermal Volatilization Analysis (T.V.A.)ca) which involved temperature programmed heating (10°/rain) of a 25 mg sample in a continuously pumped vacuum and using the pressure of volatile products as a measure of the rate of degradation. RESULTS

Estimation of average functionality It is to be expected that the degree of branching or cross-linking in a polymer will affect its stability. In comparing the stabilities of these polymers, therefore, it would be useful to have some measure of this parameter. Unfortunately, the complete insolubility of these materials makes it impossible to apply conventional analytical methods. However, an approximate value of the average functionality, or degree of substitution, of the monomer units in the polymer, which is a direct measure of the degree of branching, may be obtained in the following way from the data in Table 1. In this derivation, a distinction is made between aromatic nuclei derived from DCMB molecules (Xunits) and those derived from the aromatic molecule (,4 units). The average functionality, f , of the A units in the polymer is defined by the expression, f _ _ total number of A - X links in the polymer total number o f A units in the polymer Also, Moles of HC1 produced during polymerization -----f x moles of A units incorporated in the polymer, thus,

WH/MB = fW~/MA or

WA = WB'M, d f ' M u

(1)

in which WH is the weight of HC1 produced, Wa is the weight of A units which have become incorporated into the polymer and MH and MA are the corresponding molecular weights. It should be noted at this point that substitution in the X units has been neglected. However, since these units are a structural feature common to all the polymers, the functionality values obtained, although approximate, will be in the correct relative order. Another important assumption is that all chloromethyl groups in the system have the same probability of reacting. Thus if n, is the number of chloromethyl groups initially present and nc is the number which have reacted, then the probabilities that any given chloromethyl group has or has not reacted are nc/n~ and 1 - (nc/n~) respectively. Similarly, the probabilities,/'2, PI and Po, that both, one or neither of the chlorine atoms in any given di(chloromethyl) benzene molecule have reacted are (n,/nt) 2, 211 -- (nJnt)] nc/nt and [1 -- (nJni)] z respectively. The number of moles of HC1 produced, WH/M~, is also given by,

WalMa = ncx + 2nx

(2)

574

N. GRASSIE and I. G. M E L D R U M

in which nx and ncx are the numbers of moles of xylenyl and monochloroxylenyl units in the polymer. But, ncx/nx = P1/P2 = 2[(n,/nc) -- 1].

(3)

Solution of Eqns. (2) and (3) leads to, ncx = Wn(1 -- ndnl)/Mn

and nx = n~ W , / 2 M , nl.

Since DCMB can only be incorporated into the polymer as xylenyl or monochloroxylenyl units, the total weight (W~) in the polymer of units derived from DCMB is given by, Wo = n x M x ÷ ncxMcx

in which M x and M c x are the molecular weight of the residues in the chains corresponding to nx and ncx. Substituting for nx and ncx WH n~Mx

(I

(4)

The total weight of polymer produced, Wp, is given by

w,=

w , + w,,.

(5)

Combination of Eqns. (1), (4) and (5) and subsequent rearrangement leads to,

[°i" (1 in which G, the observed fraction of the maximum possible yield of HC1 is given by, G -- WH/M~t _ WHMc __ ndnt. 2 Wc/Mc 2 WcM~ Wc is the initial weight of DCMB and M c its molecular weight. Values of G and Wp for each polymer may be obtained from Table I and Wc from the polymer preparation data, thus the functionality values in Table 2 have been calculated. TABLE 2. FUNCTIONALITYVALUF~ Polymer Benzene Indole Thiophene

Functionality 1" 0 l' 7 3" 4

Friedel-Crafts Polyraers--I

575

The absence of evolved HCI during the formation of the pyridine polymer precludes the calculation of a functionality value for this system, and is not surprising in view of the basic properties of the pyridine molecule. The data for the quinoline polymer lead to a negative value of functionality which is meaningless. This too may be traced to the basic properties of the quinoline molecule which reacts with a proportion of the HC1 thus preventing the materialisation of the full yield. The functionality value of approximately unity for the benzene polymer may seem surprising, implying as it does that a large proportion of the benzene molecules which have been incorporated into the polymer are only monosubstituted. This may be accounted for by the following reaction sequence which assumed that reaction at disubstituted aromatic units occurs preferentially, ® -k C1CH2I®--CH2CI ®ICH2--Q---CH2CI ®--CH2mQ--CH2--® ®ICHz~Q--CH2--®

\ CH2

\

®

\ CH2C1 ®--CH2--Q---CH2m®

\

CH2

\

®

\ CH2

\ ®

.4 X A ®~CH2--®---CH2--®

N CH2

N®x ] \an 2 CH2 \ ®X

®A

I CH2

[ CI

>

etc.

576

N. GRASSIE and I. G. MELDRUM

too

I Illllk

~,,,,.,-.,,~.

1

t',["lllll

J IillI

100

./-~"q

~i

,,,~>cl,~ ../, --" t/" "

°-

!

~PI'

i

I.I_LYI I q/Vl-i I I

11

J ,oi /.-

\

i .1,,,t.4 i-tt.

,

100 YRIr

INF

'o

IU'~/ 3000

I 2000 1500 Frequency, cm-1

Ill!l 1000

FiG. 2. Infra-red spectra of polymers.

the final polymer consisting of linear chains of alternating methylene groups and benzyl substituted benzene rings. X and A type nuclei are distinguished in the final formula. Long chain branching will occur as a result of reaction at a terminal aromatic ring and the extent of branching and hence the functionality is clearly governed by the relative rates of condensation of DCMB at disubstituted Xnuelei and monosubstituted A nuclei. The data in Table 2 suggests that the benzene polymer is substantially linear, that the thiophene polymer is highly branched and that the indole polymer has an intermediate structure.

Infra-red spectra Infra-red spectra of the various polymers are illustrated in Fig. 2. Because the polymers are insoluble, the KCI disc technique had to be used and since the polymers were difficult to grind to a fine powder the spectra are generally not of high quality. Nevertheless, they lead to certain comments which are relevant to the discussion of functionality in the previous section.

Friedel-Crafts Polymers~I

577

Only the pyridine polymer does not exhibit an absorption maximum at 810 cm -t although it is not clearly resolved in the pyrrole polymer. This maximum is assigned to the aromatic C-H out of plane deformation vibration for two adjacent hydrogen atoms as in 1,4 substituted benzene rings. Bands in the region of 2850 and 2900 cm -~, which are well defined in the benzene, thiophene and indole polymers and less well defined in the quinoline and pyrrole polymers, can be assigned to the methylene groups. These two pieces of evidence verify the incorporation in all these polymers ofp-xylenyl residues from the DCMB. Absorptions at 700 and 730 cm -~ in the benzene polymer are attributable to the aromatic C-H out of plane deformation involving five adjacent hydrogen atoms and the good definition and relatively high intensity may be taken as confirmation of the large concentration of pendant benzyl groups which is implied by the essentially linear structure of the benzene polymer indicated by functionality measurements. Thiophenes substituted in the 2-position absorb near 925, 853 and 800 cm -1. While there is a band in the spectrum of the thiophen polymer at 800 cm -1, there is no well defined absorption at either of the other two frequencies. Similarly, 3-substituted thiophenes absorb near 1530, 1410 and 1370 cm-L Since there is no absorption between 1510 and 1550cm -t and since bonds near 1425 and 1510cm -~ arc matched by corresponding bonds in the benzene polymer, there is no clear evidence of monosubstituted thiophene structures. This suggests that most thiophene nuclei are at least disubstituted which is in line with the greater functionality in the thiophene as compared with the benzene polymer. The indole and pyrrole polymers absorb at 3410 and 3340 cm -~ rcspectively, which are in the region expected for N-H bonds, thus showing that these centres are not particularly vulnerable to attack by DCMB and that substitution occurs clsewhere in the molecule. The very low yields of the pyrrole compared with the indole polymer appear to confirm that reaction occurs predominantly in the carbocyclic ring in the latter. In quinoline, absorption due to the three adjacent hydrogen atoms of the heterocyclic nucleus occurs at 804 cm -~ while the four hydrogen atoms in the carbocyclic nucleus result in absorption at 737 cm -t. In the quinoline polymer there is no maximum at 737 cm -x indicating that substitution has occurred in the carbocyclic nucleus. Since the xylenyl residue also absorbs near 800 cm -1, it is not possible to decide whether substitution has occurred in the heterocyclic nucleus but absorption near 770 cm -1 is attributed to three adjacent hydrogen atoms and suggests that carbocyclic nuclei are largely monosubstituted in a position adjacent to the heterocyclic ring. Compared with the other spectra, that of the pyridine polymer seems anomalous. The intense absorption near 3400 cm -~ is attributable to an N-H bond. This could arise if reaction occurs at the nitrogen atom to give the quaternary ammonium salt as the initial product, + C1--CH2 -- E)---CH2--N E) CI-

POlYmER 4/.~-.-a

578

N. GRASSIE and I. G. MELDRUM

Such compounds are known to rearrange on heating to give ring substituted pyridines, (') -F H -- N®--CH2--®--CH2--C1. C1The maxima at 685, 750 and 790 cm -~ may be accounted for by monosubstitution at the 3 (790 cm -I) and 4 (685 and 750 cm -1) positions. There is, however, no positive indication of methylene or xylenyl absorption at 2900 and 810 cm -1 respectively suggesting that xylenyl units may not be present. This would lead to the conclusion that pyridine hydrochloride units are linked directly or through benzene nuclei.

I

!

._g "8

PYRD IN IE i ~

~

U

"8 *)

°-

I

g

3~ ' mperature "C , FI~. 3. T.V.A. thermogtams of polymers.

Thermal degradation T.V.A. thermograms for the various polymers are illustrated in Fig. 3. All the polymers, except those derived from pyridine and quinoline, behave similarly in

Friedel-Crafts Polymers--I

579

giving rise to volatile products in a number of steps at lower temperatures; the main volatilization occurs from approximately 300 ° upwards. The amount of volatilization at the lower temperature is strongly dependent upon the length of pre-evacuation and preheating to which the polymer has been subjected and may be attributed to the progressive release of solvent and unchanged reagents which have been trapped in the infusible polymer. The temperature at which true degradation becomes discernible may be taken as a measure of thermal stability and for the various polymers these are, in increasing order, as follows:mindole, 290°; pyrrole, 300°; benzene, 330°; thiophene, 380 °. The characteristics of the T.V.A. thermograms for the pyridine and quinoline polymers are rather different suggesting very much less stable materials. This may be a reflection of the fundamental difference in structure revealed by i.r. spectral evidence. On heating to 500 ° the pyridine and quinoline polymers are almost completely volatilized. All the others give appreciable residues and the benzene and thiophene polymers are least altered in appearance.

DISCUSSION The great difference in stability of the pyridine polymer compared with the others is not surprising in view of the i.r. evidence which strongly suggests that fundamental structural differences also exist. The basic structural features of the other materials are clearly similar on the basis of the i.r. evidence. The question therefore arises as to what extent the minor differences in stability are a function of the aromatic molecule on the one hand and the extent of branching or cross-linking on the other. Comparing the i.r. and functionality evidence for the benzene and thiophene polymers it would appear that the superior stability of the latter can be satisfactorily accounted for in terms of a more compact branched structure. On the other hand, a comparison of the functionalities and thermal stabilities of the benzene and indole polymers suggests that there m a y be some inherent instability associated with indole. Clearly a satisfactory and comprehensive answer to these questions will depend upon detailed investigations of mechanism of polymerization and structure of the individual polymers, These are in progress for a number of the systems referred to above and will be reported upon in later publications. Acknowledgements--This work has been carried out under a research agreement with the Ministry

of Technology for which we are most grateful. We are especially indebted to Mr. L. N. Phillips of the Royal Aircraft Establishment, Farnborough, for his most valuable advice and assistance in getting the work under way. Crown Copyright, reproduced with the permission of the Controller, Her Majesty's Stationery Office.

REFERENCES (1) L. N. Phillips, Trans. Plastics Inst. 32, 298 (1964). (2) N. B. Colthup, L. H. Daly and S. E. Wibcrley, Introduction to Infrared and Raman Spectroscopy. Academic Press, New York (1964). (3) I. C. McNeil, Europ. Polym. J. 3, 409 (1967). (4) G. M. Badger, The Chemistry of Heterocyclic Compounds, p. 259. Academic Press, New York (1961).

580

N. GRASSIE and I. G. M E L D R U M

R ~ u n ~ - O n a pr~par~ des polym~res par condensation du di(chlorom6thyl) benz~ne avec le benzb~e, le thioph/me, la pyridine, rindole, la quinoline et le pyrrole en presence de SnCL, comme catalyseur. On n'a pas r6ussi a pr6parer de polym~re avec le furanne. On a d6termin6 la fonctionalit~, des groupes aromatiques qui est une mesuxe du degr6 de branchement dam les polym~'es du benz~ne, de l'indole et du thioph~ne. On a d6tennin6 les spectres infra-rouges des six polym~res et les thermogrammes de leur Analyse Thermique par Volatilisation (A.T.V.). On discute/t la lumi~'e des r~ultats obtenus les relations entre la structure et la stabilit6. Sommario--I polimeri sono stati preparati per condensazione di di(clorometil)benzene con benzene, tiofene, piridina, indolo, quinolina e pirrolo usando SnCl, come catalizzatore. Tentativi di preparare un polimero del furano sono staff vani. Sono state effettuate misure della funzionalit/t dei gruppi aromatici, che ~ misura del grado di ramificazione nei polimeri di benzene, indolo e tiofene. Sono staff anche misurati gli spettri infrarossi di tutti e sei i polimeri e sono stati ottenuti i loro termogrammi di anafisi di volatili77~one termica (TVA). Alla luce dei risultati di questi tre tipi di misure, sono discusse le relazioni tra struttura e stabilita. Zusammenfassung--Es wurden Polymere hergestellt durch Kondensation yon Dichlormethylbenzo mit Benzol, Thiophen, Pyridin, Indol, Chinolin und Pyrrol unter Verwendung yon SnCL, als Katalysator. Versuche, ein Polymeres wit Furan herzustellen, waren erfolglos. An den Benzol-, Indol- und Thiophen-Polymeren, wurden Messungen der Funktionalitat der aromatischen Gruppen, die ein MaB ffir den Verzweigungsgrad ist, vorgenommen. Von allen sechs Polyrneren wurden auch die Infrarotspektren gemessen und aus der thermiscben Verdampfungsanalyse (TVA) ihre Thermogramme erhalten. Angesichts der Ergebnisse dieser drei MeBmethoden werden die Beziehungen zwischen Struktur mad Stabilitiit diskutiert.