Makromol. Chem. 183,2399-2413 (1982) 2399

The Cationic Polymerization of 2-Alkenylfurans, 2 a)

A Side Reaction Leading to Polyunsaturated Products

Rub& Aharez, Alessandro Gandini*b), Ricardo Martinez

Centro Nacional de Investigaciones Cientificas, Apartado 6990, La Habana, Cuba

(Date of receipt: January 5, 1982)

SUMMARY: A spectroscopic and conductimetric study of the cationic polymerization of 2-vinylfuran (1)

and 2-methyl-5-vinylfuran (2) showed the existence of an important side reaction originating from a hydride-ion shift from an unsaturated polymer molecule to an active species. The resulting allylic carbocation, in equilibrium with a doubly unsaturated polymer molecule, can react further and the repetition of the mechanism on progressively more conjugated species leads to the formation of a series of highly charge-delocalized carbenium ions absorbing throughout the visible region of the spectrum (and giving high electrical conductivity) and of neutral polyconjugated polymer molecules. Since the hydride-ion abstraction occurs from the tertiary carbon atom of the vinylic chain, vinylidene polymers of monomers such as 2-isopro- penylfuran (3) and 2-isopropenyl-5-methylfuran (4) are not susceptible to it. Indeed, their cationic polymerization proceeds without colour formation and conductivity increase.

Introduction

Our interest in the polymerization of furanic monomers l ) brought us to investigate the behaviour of 2-alkenylfurans with anionic2), Ziegler-Natta3), and cationic4-@ initiators. The latter systems gave the most interesting results and were therefore studied more comprehensively. In the first paper of this series’) we described the general features of the rather complex phenomenology which characterizes the inter- action of these monomers with typical cationic initiators. We now wish to report systematically on the mechanisms and kinetics related to the polymerization processes and to two major side reactions which alter substantially the structure of the products. The present communication deals with one of these secondary events, namely the formation of polyunsaturations on some polymer chains. Of the four monomers studied, viz. 2-vinylfuran (l), 2-methyl-Svinylfuran (2), 2-isopropenyl- furan (3), and 2-isopropenyl-Smethylfuran (4), only the first two exhibited this particular behaviour which arises from the lability of the methine hydrogen (C-H attached to C2 of the ring) in the polymer chains.

a) Part 1 : cf. 3. b, Present address: Laboratoire de Chimie des Polym&res, Ecole Franqaise de Papeterie, B. P.

3, 38400 St. Martin d’Htres, France.

0025-1 16X/82/10 2399-1 5/$03.00

2400 R . Alvarez, A. Gandini, R. Martinez

Experimental Part

All experiments were carried out taking particular care to insure that the stringent require- ments for reliability imposed by the good practice of cationic polymerization were satisfied. Therefore, throughout this investigation we made use of classical high vacuum techniques and we thoroughly purified and dried all reagents.

Materials: Methylene chloride (from Schuchardt) was purified, dried and handled in the vacuum line by a standard procedure7). Trifluoroacetic acid (TFA, from Carlo Erba) was fractionally distilled on the vacuum line, generous head and tail fractions being discarded. The middle portion was collected over activated silica gel and stored thereafter over its own vapour pressure at room temperature and in the dark. From this reservoir attached to the vacuum line a known amount of TFA was transferred into a tipping device*) together with a measured volume of CH2C12, whenever a batch of breakable phials containing the acid solution was needed. By successive dilutions through tipping devices we then prepared phials containing from to

mol of TFA in 0.5 to 2 ml of CH2C12. Each operation in this cascade process was accompanied by a titration check of the calculated acid concentration which was always satisfactory within +2%. These solutions were stable over the three years’ duration of the investigation as shown by frequent titrations from random phials and by the reproducibility of a given kinetic experiment repeated every few months. Even the most concentrated solutions were homogenous down to -20°C. Below this temperature some of the TFA cjystallized out of the 0,2-2 mol.1-’ solutions while the most diluted mixtures, viz. less than mol-1-’, were homogeneous down to -78 “C. All the work to be described in this series of papers was carried out under completely homogeneous conditions.

2-Vinylfuran (1) was synthesized, purified, dried and stored as previously described’). It was vacuum distilled into the reaction vessels through a microburette. Breakable phials of undiluted 1 were prepared in a similar manner and their contents checked by the mid-point method”). Phials containing solutions of 1 in DCM were filled with a tipping device following the technique described above for the acid. The stability, physical properties and spectra of pure 1 gave already been reported and discussed ’).

5-Methyl-Zvinylfuran (2) was synthesized from 5-methyl-2-furaldehyde according to the standard technique applied for 1 ’ 9 ” ) and submitted to the same purification and drying treat- ments’). nZ0 = 1,5075; dzo = 0,935, d-78 = 1,012, do = 0,955, dSo = 0,905.

MS: m/e 108 (100, M’), 107 (64, M + - l ) , 65 (20, M + - CH,CO). UV (DCM) A,,, = 273 nm ( E = 1,43 . lo4 1 . mol-’ . cm-’).

The Cationic Polymerization of 2-Alkenylfurans, 2 2401

IR (liq. film): 1 640 (v,=~, vinyl), 1450 and 1365 (6CH3), 780, 890, 1020, 1520 and 1570 cm-' (furan ring).

(m; 1 H, H4), 5,98 (d; 1 H, H3), and 6,30 (4; 1 H, HC). 2-Isopropenylfuran (3) was synthesized following Bachman's procedure 12) and processed in

the same way as the two preceeding monomers; b. p. 128°C at 1011 mbar (760 Torr);

MS: m/e 108 (100, M'), 79 (41, M + - HCO), 93 (30, M + - CH,). UV (DCM): A,,, = 264

IR (liq. film): 1640 (v,,,, isopropenyl), 1455 and 1385 (dCH,), 750, 810, 890, 1010, 1 170

'H NMR (CDCI,; TMS): 6 = 2,23 ( s ; 3 H, C_H,), 4,94 (4; 1 H, Hb), 5,52 (4; 1 H, Ha), 5,82

do = 1,5003; d-' , = 1,029, do = 0,962, dZo = 0,944, dS0 = 0,915.

nm ( E = 1 ,40. lo4 1 . mo1-l . cm- I ) .

and 1500 cm-' (furan ring).

(m; 2H, H3 and H4), and 7,30 (d; 1 H, H5). and

by a new route developped in our laboratory. The latter involved the preparation of 5-methyl-2- furyllithium from butyllithium and 2-methylfuran in THF at - 30 "C and its subsequent reac- tion with acetone at room temperature to yield 75% of 2-(5-methyl-2-furyl)-2-propanol in 1 h. The dehydration of this alcohol with acetic anhydride and pyridine under standard conditions yielded 4 in nearly quantitative yields. It should be pointed out that the above alcohol slowly dehydrates on standing to give 4. In our experience 80% of conversion is obtained in a few weeks at 10°C. The purification, drying and vacuum handling of this last monomer followed the same pattern as for the others. nZo = 1,5035; d-" = 1,018, do = 0,953, d20 = 0,936, da = 0,917. MS: m/e 122 (100, Mi), 107 (37, M + - CH,), 121 (32, M+ - 1). UV (DCM): A,,, = 273nm(& = 1,43.1O4rnol.1-'.cm-').

IR (liq. film): 1640 (vC=,, isopropenyl), 1450 and 1 360 (6CH3), 780, 880, 1 020, 1530 and 1 600 cm - ' (furan ring).

5,33 (s; 1 H, Hb), 5,82 (m; 1 H, H4), and 5,98 (d; 1 H; H3). The purity of the four monomers was checked by GLC and was in all cases better than

99,95%. None of them suffered any detectable thermal polymerization at room temperature when kept for several months in sealed reservoirs, neither did they show any sign of decomposi- tion.

Furan (F, from Fluka) and 2-methylfuran (MF, from Fluka) were fractionally distilled and the middle fraction transferred on the vacuum line where they were stored on calcium hydride in reservoirs provided with Hoke metal valves.

Techniques: Most experiments were carried out in devices like the one shown in Fig. 1 . Typi- cally, monomer and solvent were vacuum distilled into the reactor after this had been charged with a phial containing the catalyst solution, joined to the line and evacuated for several hours at lo-' mbar. The reactor was then sealed off the line and its contents allowed to reach room temperature (25 k 2 "C). The phial was crushed and the resulting mixture vigorously shaken for a few seconds. The quartz and the conductivity cells were filled with the reacting solution and measurements could be taken within about 15 s from the time of mixing. While the electrical conductivity was continuously recorded, two modes of monitoring the evolution of the elec- tronic absorption spectrum were used, depending on the relative rate of the reactions studied. With slow processes the full spectrum was taken repeatedly, while with faster ones continuous scanning at a fixed wavelength allowed a more precise kinetic study. The latter mode was completed with the recording of full spectra once the reaction had slowed down and most of the variation at the chosen wavelength had been recorded.

At the end of each run the device was opened to the atmosphere, the solution neutralised with aqueous alkali and the spectrum of the DCM phase scanned. Then this neutral solution was reacidified with TFA or other strong acids and a new spectrum taken. This cycle was often repeated to check the reversibility of the processes involved. Finally, the neutral DCM solution was vacuum dried to isolate the polymer.

to 5 . mol. I - ' , respectively.

'H NMR (CDCI,; TMS): 6 = 2,Ol (d; 3H, C_H,), 4,94 (9; 1 H, Ha), 5,50 ( s ; 1 H, Hb), 6,26

2-Isopropenyl-5-methylfuran (4) was synthesized as reported by Fetizon and Baranger

'H NMR (CDCI,; TMS): 6 = 1,94 (d; 3 H, C€I3), 2,27 ( s ; 3 H, 5 - C_H3), 4,77 (9; 1 H, Ha),

The monomers' and acid concentrations in this study ranged from 0,2 to 1,0 and from

2402

n RJ. Alvarez, A. Gandini, R. Martinez

Fig. 1. High-vacuum device for following the evolution of the electronic spectrum and of the electrical conductivity during and after the polymerization of alkenylfurans. (a): 1 cm quartz cell; (b): 1 mm quartz cell; (c): quartz-to-Pyrex graded seal; (d): Pyrex-to-soda glass graded seal; (e): conductivity cell (Pt electrodes, constants from 0,002 to 0,02 cm-' depending on acid concentration used); (f): sintered-glass filter; (g): catalyst phial; (h): connection to vacuum line and seal- off point

Conductioity and spectra of starting materials: The specific conductivity of monomer solu- tions were always lower than 10 * a- ' . cm ' . TFA solutions in CH,Cl, gave values ranging from to lo-' SZ-' . cm-' depending on the concentration, in good agreement with the figures reported by Bolza and Treloar 14) for ethylene chloride solutions.

The UV spectra of the four monomers were very similar (see above) and the acid solutions gave a weak maximum at 233 nm.

Results and Discussion

Before entering into a detailed description of the results relevant to this part of our work, it is useful to give a summary of the most salient features which characterize the system l/CH,Cl,/trifluoroacetic acid (TFA) at 0 to 40°C. Although some of these features are related to aspects which will be dealt with in following papers, it is im- portant to outline the general phenomenology of this particular system around room temperature since it will serve as reference point for all further discussions concerning the behaviour of each monomer, from the point of view of both the polymerization mechanisms and kinetics and the nature of the side reactions.

1 . 2.

3.

4.

5 .

All polymerizations went to completion. Titration of the acid at any stage of the reaction (including long periods after the completion of the polymerization) gave values identical to the initial concentration used. Treatment of the reaction mixture with solid potassium hydrogen carbonate produced the total removal of the acid used. Addition of a second portion of monomer at the end of a polymerization produced further polymerization, but with a much lower rate. Polymerization solutions turned from pale yellow at low conversions to an amber- like colour at later stages to dark brown towards the end of the polymerization.

The Cationic Polymerization of 2-Alkenylfurans, 2 2403

The colour continued to deepen even after complete conversion reaching dark blue-grey tonalities.

6 . The electrical conductivity of the polymerizing solutions grew steadily as the reaction proceeded and kept growing even after complete conversion to polymer had been reached.

7. Neutralization of these deeply-coloured conducting solutions induced a sudden partial bleaching and drop in conductivity which left a light orange-brown poorly conducting CH,Cl, phase. Re-acidification brought back the original colour and high conductivity and their magnitude was the higher the stronger the new acid solution. This cycle could be repeated ad. lib.

8. If an empty side-arm of the reaction vessel was cooled during the polymerization, an amount of each of the three initial components distilled into it as shown by the onset of a new (slower) polymerization accompanied by colour formation etc.

9. The yellow-brown polymers obtained were in fact oligomers with DP’s ranging between 5 and 20 and had important structural irregularities with respect to a normal vinylic chain structure the most important one being branching from the C5 ring position.

The particular processes relevant to this paper manifest themselves through colour formation and a concomitant large increase in conductivity. A systematic study of these two phenomena was, therefore, carried out on the system l/TFA/CH,Cl,.

Fig. 2 shows the time evolution of the UV-visible absorption spectrum for a typical slow reaction. The growing intensity of the overall spectrum and the progressive ap- pearance of a new band at longer wavelengths continued even after 100% conversion into polymer had been achieved. This indicates that some of the species accumulating from the reaction acquired a progressively higher degree of conjugation and that monomer molecules are not needed for this colour-forming process to proceed. The neutralization and reacidification cycle operating on the final mixture is shown in Fig. 3 for a typical long-lasting experiment. Scanning the increase in optical density with time at any fixed wavelength always produced S-shaped curves like that shown in Fig. 4. The dependence of the acceleration period and of the valve of the maximum rate of the reaction upon the initial monomer and acid concentrations was studied at room temperature. The relationships obtained clearly point to a mechanism involving the interaction of active species with polymer molecules which bear a terminal un- saturation. However, these correlations obviously include in a specific form the kinetic parameters pertaining to the actual polymerization process and will, therefore, be dealt with in a subsequent paper discussing the overall kinetics of this complex system.

The conductivity increased with time much in the same way as a chosen optical density, as shown in Fig. 5 . Again, all reactions displayed an S-shaped behaviour and the value of the specific conductivity increased steadily even after complete monomer consumption had been reached. The proportionately high values of K towards the end of all reactions indicated that a substantial fraction of the initial amount of added acid must have been converted into an ionic form, i. e. the trifluoroacetate anion or its acid homoconjugate. The time necessary to arrive at the maximum d d d t and the

2404 R. AIvarez, A. Candini, R. Martinez

L Inm

Fig. 2. tion of 1. [l], = 0,70 mol . I-’; [TFA], = 6,O * ( - - -): 1 cm cell; (-): 1 mm cell; (a): 150 min; (b): 25 h; (c): 17 days; (d): 42 days

Typical evolution of the UV-visible spectrum during and after a “slow” polymeriza- mol . I-’; T = 25 ‘C; solvent: CH,C12;

actual value of that rate were correlated to the initial monomer and acid concentra- tions, and the derived expressions confirmed those obtained spectroscopically (see above).

The system 2/TFA/CH,C12 at room temperature behaved essentially in the same way as that involving 1. All the features of colour formation with S-shaped curves and progressive appearance of new bands at longer wavelengths (Fig. 6) and of in- crease in conductivity were observed. Thus, the processes leading to polyconjugation and ion formation are common to these two monomers.

These features were also noticed when other cationic initiators (BF, - Et,O, SnCI,, TiCl,, I,, etc.) and other solvents (ClCH,CH,Cl, CCl,, etc.) were used for the poly- merization of 1 and 2 at 0 to 40°C. They must, therefore, be considered as a general characteristic of the cationic polymerization of these two monomers since, from our experience, their radical and coordination polymerizations gave products free of any polyconjugation (UV spectra characterized by a single maximum around 235 nm, no absorption in the visible).

In order to pinpoint the origin of this ionogenic, colour forming reaction we carried out some experiments with the systems furan (F)/TFA/CH,Cl, and 2-methylfuran (MF)/TFA/CH,CI, at room temperature using F, MF, and TFA concentrations slightly higher than the highest ones used in the experiments with 1 and 2. No

The Cationic Polymerization of 2-Alkenylfurans, 2 2405

a 3 Ol 0 -

2

1

0

-1

E. /nm Fig. 3. Combined spectrum of the solution of the experiment shown in Fig. 2, after opening the device under nitrogen. Successive dilutions allowed to scan down to 300 nm in a 1 cm cell. (a): Just after opening (45 days from mixing the reactants); (b): after neutralization (DCM phase); (c): after re-acidification of the CH,Cl, phase to give [TFA] = 3 . lo-' mol . I- '

t/min Fig. 4. Typical variation of the optical density during and after a polymerization of 1. [l], = 0,40 mol. I - ' ; [TFA], = 4,O. mol . l- ' ; T = 25OC; solvent: CH,CI,; 1 cm cell

R. Alvarez, A. Gandini, R. Martinez

160 tlmin

Fig. 5 . in Fig. 4

The evolution of the electrical conductivity during and after the polymerization shown

7

460 500 6dO 700 800 i. I n m

Fig. 6. Typical evolution of the UV-visible spectrum during and after a polymerization of 2. [210 = 0,53mol.I- '; [TFA], =2 ,3 .10 -3mol - l - ' u p t o 4 h , thensecondacidphialbroken to give a total [TFA] of 7.7' 10 mol . 1 - '; T = 25 "C; solvent: CH2C12. Spectra taken from a 1 cm cell for the first 2 hours' reaction; from a 1 mm cell thereafter. Times as indicated on tracings

appreciable change in colour or conductivity was noticed within several days and F and MF were recovered quantitatively indicating that under our experimental condi-

The Cationic Polymerization of 2-Alkenylfurans, 2 2407

tions the polymerization through the furan ring to give polyunsaturated oligomers does not take place. Indeed in order to observe such processes one needs acid concen- tration orders of magnitude higher15-”).

That the opening of the furan ring was not the cause of colour and conductivity was confirmed by a careful examination of the IR and NMR spectra of the resulting poly(1) and poly(2). All the characteristic features of the heterocycle were preserved in these products.

The search moved then to the possibility of hydride ion abstraction from the tertiary carbon atom of the vinylic chain. We followed the polymerizations of 3 and 4 by TFA in CH,Cl, by spectroscopy and conductivity. Complete conversion was achieved in both systems without the development of any colour or conductivity (above that of the acid). The polymerized solutions were also inert for several days after the end of the polymerizations. The products gave UV spectra exhibiting a band at around 230 nm (unconjugated furan ring) and a very weak feature at 270 - 275 nm typical of the conjugation of the ring with one double bond, i.e. terminal unsatura- tions. Thus, the substitution of the methine hydrogen with a methyl group completely eliminates the phenomenology under discussion.

On the basis of the results described it seems reasonable to formulate the following mechanism for the formation of progressively more conjugated polymer molecules both in the form of protonated and neutral species in the cationic polymerization of 1 and 2 between 0 and 40°C.

A. An active species A (in the form of free ions, ion pairs or polarized ester molecule) abstracts a hydride ion from the tertiary carbon atom adjacent to the terminal double bond of a polymer molecule B typically formed in a spontaneous or monomer transfer reaction.

A

c

The result of this primary interaction is the formation of a saturated polymer chain C and of a charged species D consisting of an allylic-type carbenium ion at the end of another polymer molecule (together with the anion previously accompanying the active center). The driving force of this reaction is the increase in stability involved in the generation of a more delocalized positive charge (more resonant structures) in one

2408 R . Alvarez, A. Candini, R . Martinez

of the products. Indeed, the allylic-type ion formed is expected to absorb at a longer wavelength than the original chain carrier and the unsaturated polymer molecule and its higher stability should result in a high degree of dissociation, i. e. electrical conduc- tivity of the free ions (mostly B o given its higher mobility). This process must, there- fore, be viewed as the first step in the generation of ionic chromophores.

B. The conjugated carbocation D formed in reaction (1) must be considered as existing in equilibrium with the corresponding neutral doubly unsaturated polymer molecule E.

U + Be K z . . .-CtI=C-CH=CH -0 + HB

The proton expulsion characterizing the forward direction of (2) is an equivalent of a spontaneous transfer reaction. The existence of equilibrium (2) allows the formula- tion of an interaction similar to ( l ) , but giving the next generation in the family of polyconjugated carbocations F and corresponding neutral polymer molecules G , viz.:

A + . . .-CHz-CH-CH=C-CH=CH

o \ o \ 33 E

G

The hydride ion abstraction-proton expulsion cycle can continue from the triply unsaturated polymer molecule formed in reaction (2a) to give still more delocalized carbenium ions in equilibrium with chains containing four conjugated terminal double bonds. Then, a further generation of such species would slowly arise, etc. I t must be pointed out that the hydride ion can be abstracted by any carbenium ion and not only be the propagating species A, provided that the polymer molecule from

The Cationic Polymerization of 2-Alkenylfurans, 2 2409

which it is taken gives rise to a positive species possessing a higher degree of charge stabilisation. The net result of this mechanism is that for every new unsaturation arising in a polymer molecule one is lost in another, the original growing centers be- coming fully saturated and thus unable to participate any further in the reactive sequence.

We can now turn to the list of features given at the beginning of this section and discuss how the proposed mechanism can account for them.

1. There is always some free acid available for initiation and therefore complete conversions are attained under all conditions.

2. and 3. The addition of a strong base or KHCO, displaces all equilibria (2) between stable carbenium ions and deprotonated polyunsaturated polymer molecules, thus regenerating all the initial acid used, which is neutralized by the base.

4. The lower polymerization rate observed upon a second monomer addition implies a decrease in the initiation potential after the first polymerization, i. e. a termina- tion reaction. The formation of stable carbocations and the corresponding counter ions B- must indeed be considered as a termination since the acid tied up in such species is not entirely available for initiation. In other words, the amount of free acid and of acid still included in a true propagating species decreases as the ionogenic cycles produce progressively more stable entities, and equilibria (2) are correspondingly shifted to the left. We observed, in fact, that the rate of the second polymerization was the lower the longer we waited after the completion of the first one. With TFA, kinetic data showed that up to 30% of the acid initially added could end as B - (i. e. H(CF,COO), ) coupled with the stable carbenium ions. Further support for the existence of equilibria (2) came from the observation that upon a second monomer addition the deeply coloured and highly conducting solution was partly bleached while the conductivity dropped, indicating that the acid used up in the initiation made equilibria (2) shift to the detriment of the various carbenium ions (chromophores). Later, however, as the second poly- merization proceeded the deep colour (and high conductivity) was progressively restored and eventually reached higher intensities and more pronounced batho- chromic shifts than those recorded just before the second monomer addition, and this occurred because the system now contained a higher polymer concentration.

5 . and 6 . The sequence of spectra shown in Fig. 2 can be explained in terms of a progressive increase in chromophore concentration accompanied by an increase in their degree in conjugation which produces the bathochromic shift and the slow appearance of new peaks at longer and longer wavelengths. The chomophores are both the carbenium ions and the polyunsaturated macromolecules. The increase in conductivity typified by the plot in Fig. 5 arises from the accumulation of free ions from the above process which induces the formation of progressively more stable (and therefore more dissociated) carbocations. Note also that the value of equilib- rium constants K, decreases with increasing charge delocalisation (the nucleo- philicity of the neutral polymer molecule increases with its degree of polyconjuga- tion) and therefore as the process advances the relative importance of ionic species is also favoured by this effect.

241 0 R. Alvarez, A. Gandini, R. Martinez

7.

The autoacceleration in both absorbance and conductivity (Figs. 4 and 5) is due to the need of unsaturated polymer molecules in the primary reaction. These accumulate in the reaction medium as the polymerization proceeds and since the increase in their concentration is much more important than the decrease in the concentration of active species during the first stage of the polymerization process, the rate of reaction (1) will increase from zero at time zero to a maximum value when the product of the concentrations of both reactants reaches a maxi- mum due to the build-up of polymer," Subsequently the overall rate decreases fol- lowing the end of the polymerization and the depletion of the reactants, and in particular the formation of saturated polymer molecules. The quantitative treat- ment of these kinetics will be given in a subsequent paper together with the kinetics of the polymerization and of the alkylation processes.

The mechanism proposed also explains why monomer is not needed for this ionogenic reaction to occur and why this continues as long as the system is kept from the atmosphere. From the results shown in Fig. 3 it can be argued that neutralization of the solu- tion containing both chromophores eliminates the carbocations with a corre- sponding increase in the concentrations of unsaturated polymer molecules (total shift to the right of equilibria (2)). The spectrum of the neutral solution (Figs. 3 and 7) is thus the sum of the spectra of polymer molecules of structure H with n varying from 0 to 6 .

H

For n = 0, A,, = 240 nm (unconjugated furan ring); for n = 1, A,, = 275 (see spectra of monomers and of poly(3) and poly(4)); for n = 2, 3, 4 and 5 assignements are more difficult because each species must have more than one absorption bandI8). However, on the basis of the work of Raijendam et al. 19) on the comparison of UV and visible spectra of compounds of the series I and H+CH=CH+H, it is reasonable to assume that each 2-(furan-2-yl)vinylene unit in our polymers is equivalent to three conjugated double bonds. Thus, the band at about 350 nm probably belongs to the chromophore with n = 2 and that around 440 nm to one with n = 3. The lowest energy band in neutralized solutions from very long reactions was at about 800 nm which is compatible with n = 6 (see Fig. 7).

The decreasing intensity of the peaks as one moves to higher wavelengths is a clear indication that the relative concentration of the various unsaturated polymer molecules falls steadily as n increases, as expected from the mechanism proposed.

The spectra obtained before neutralization and after re-acidification are too complex to be decomposed, but the lowest energy bands extending into the near

The Cationic Polymerization of 2-Alkenylfurans, 2 241 1

l l n m

l l n m

Fig. 7. UV-visible spectrum of a poly(1) isolated after a long-lasting ionogenic reaction. The different regions were scanned by successive dilutions with CH,CI,. T = transmittance

IR to about 1 OOO nm, again reflect the existence of carbocations with very high charge delocalization, corresponding to the protonated form of unsaturated polymer molecules with n = 6 or more. No previous data are available for such ions, but similar aromatic-polyenic carbenium saltsM) give strong bands in the visible with bathochromic shifts of the same magnitude as those observed in this study for an equivalent increase in the degree of conjugation.

The restauration of the original spectrum upon re-acidification reflects the partial reprotonation of the polyunsaturated sequences and the increase in its intensity as a function of the acid strength confirms the occurrence of equilibria (2).

Points 8 and 9 are more relevant to problems directly related to the initiation mechanism and to the other important side reactions, both to be discussed in further papers.

The similarity of behaviour of 1 and 2 is logical within the framework of the proposed mechanism which also accounts for the lack of ionogenic reactions with 3

241 2 R. Alvarez, A. Gandini, R. Martinez

and 4, i.e. monomers which give vinylidenic polymers lacking mobile methine hydrogen atoms.

The hydride ion shift-acid expulsion process invoked here is not new in cationic polymerization. Gandini and Plesch2') observed similar interactions at the end of polymerizations of styrene by perchloric acid, and Prosser and Young2*) reported a similar phenomenology during and after the polymerization of indene. In both instances, however, the relative rates of the ionogenic reaction were much lower than those measured in the present study: with styrene it took one to several days to observe the formation of allylic carbocations (polymerization being complete within 1 h at the most) and with indene the formation of those type of ions was also relative- ly slow compared with the rate of polymerization. This quantitative assessment is very important because if one can legitimately call post-polymerization reactions the processes leading to slow colour formation in the two systems quoted above, with 1 and 2 the build-up of conjugated sequences begins as soon as some polymer is formed and thus at high monomer conversions the polymerization product is already heavily contaminated with polyunsaturated sequences which impart to it an orange to brown colour.

One is, therefore, dealing with an important side reaction occurring during the polymerization as well as after it and which is intrinsic to the cationic polymerization of 1 and 2. Indeed, previous brief reports by various authors on the cationic poly- merization of 123-27) always mentioned the formation of coloured polymers, but no rationalization of this phenomenon had ever been put forward, not even in qualitative terms.

A. Gandini, Adv. Polym. Sci. 25, 47 (1977) 2, A. Gandini, C. D. Hernandez, Polym. Bull. 1, 221 (1978) ') A. Gandini, C. D. Hernandez, Makromol. Chem., Rapid Commun. 2, 351 (1981) 4, R. Alvarez, A. Gandini, R. Martinez, J. Polym. Sci., Polym. Lett. Ed. 13, 385 (1975) 5, A. Gandini, R. Martinez, J. Polym. Sci., Polym. Symp. 56, 79 (1976) 6, A. Gandini, R. Martinez, R. Sanchez, Rev. CENIC, Cienc. Fis. 10,13 (1979); Chem. Abstr.

') W. R. Longworth, P. H. Plesch, M. Rigbi, J . Chem. SOC. 1958, 451 ') A. Gandini, P. H. Plesch, J. Chem. SOC. 1965, 6019 ') R. Alvarez, A. Gandini, R. Martinez, P. J . Ortiz, C. S. Perez, Rev. CENIC, Cienc. Fis. 5,

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J. W. Reijendam, M. J. Janssen, Tetrahedron 26, 1303 (1970) '') J . W. Reijendam, G. J. Heeres, M. J. Janssen, Tetrahedron 26, 1291 (1970) 'O) K. Hafner, H. Pelster, Angew. Chek. 73, 342 (1961)

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The Cationic Polymerization of 2-Alkenylfurans, 2 241 3

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