Polymerization ofN-vinylpyrroles: Recent achievements

ISSN 1560�0904, Polymer Science, Ser. B, 2014, Vol. 56, No. 4, pp. 443–463. © Pleiades Publishing, Ltd., 2014.Original Russian Text © A.I. Mikhaleva, M.V. Markova, I.V. Tatarinova, L.V. Morozova, B.A. Trofimov, 2014, published in Russian in Vysokomolekulyarnye Soedineniya, Ser. B, 2014,Vol. 56, No. 4, pp. 401–422.

443

INTRODUCTION

Interest in the chemistry of pyrroles, specificallyN�vinylpyrroles, is continuously increasing. Two fun�damental monographs [1, 2] highlighting variousaspects of the chemistry and physical chemistry of pyr�roles were published more than three and a halfdecades ago. Since then, the stream of general andanalytical publications devoted to the synthesis, reac�tivity, and properties of compounds of the pyrroleseries has grown [3–39]. This fact reflects the ever�increasing comprehension of the important role thatpyrrole structures play in biochemistry, the search fordrugs, and the creation of modern high�tech materi�als. Accordingly, the steam of journal publications, forexample, [40–44], related to the diverse lines of pre�parative, theoretical, and applied chemistry of pyrroleis growing.

The still almost unknown N�vinylpyrroles areinteresting but poorly studied monomers for the prep�aration of the corresponding polymers and oligomerswith a set of properties useful for practice.

Although N�vinylpyrroles show promise for prac�tice, the preparation of these monomers and theirpolymerization have been hampered for a long time bytheir difficult synthesis. The situation changedabruptly after the discovery and beginning of the sys�tematic elaboration of the reaction of ketoximes (thesimplest ketone derivatives) with acetylene in superba�sic catalytic systems (of the KOH/DMSO type) toyield the formation of pyrroles and N�vinylpyrroles viaone step of preparation (one�pot synthesis) [4–7, 9–16, 18, 22, 23, 29, 35, 45]. This synthesis is included inmonographs [17, 46], encyclopedias [47], and manu�als [48, 49] as the Trofimov reaction [47, 50–53]).Expensive and exotic N�vinylpyrroles became inex�pensive and accessible monomers of the pyrrole series.

At present, these compounds have become the subjectof intense research of many scientific teams as prom�ising monomers, intermediate products for fineorganic synthesis, and drug precursors [54–67].

Accessible substituted pyrroles and N�vinylpyrroleshave become a rewarding field of testing and applica�tion of modern concepts of theoretical chemistry andreactivity [7].

The polymers of N�vinylpyrroles show promise as abasis for the design of new special semiconductor andphotosensitive materials and diverse charge�transfercomplexes às well as synthetic dyes and pigmentswhose properties are close to those of natural dyes andpigments. For example, N�vinylpyrrole is used to pre�pare conducting ladderlike polymers [68]. They aresynthesized through the polymerization ofN�vinylpyrrole via double bonds followed by oxidationof pyrrole fragments [69, 70]. Recently, the oxidativepolymerization of N�vinylpyrrole in the presence ofFeCl3 was performed and the resulting soluble poly�mers were found to show high optical absorption in thevisible region and strong luminescence both in solu�tion and in the solid state [71]. The polymers ofN�vinylpyrrole and of several methyl�substitutedvinylindoles were synthesized in order to replace poly�vinylcarbazole, which is usually used in organic pho�torefractive materials [72, 73]. Poly(N�vinylpyrroles)and polyindoles—such as poly(N�vinylindole),poly(1,4�bis[2�(N�vinyl)pyrrolyl]benzene), poly(N�vinyl�4,5�dihydrobenz[g]indole), poly(N�vinyl�2,5�diphenylpyrrole), poly(9�methyl�3�(N�vinylpyrrole�2�yl)carbazole), poly(N�vinyl�2�(1�anthracenyl)pyr�role), and poly(N�vinyl�2�(2�anthracenyl)pyrrole)—have been patented as base layers for organic lumines�cent displays and devices emitting in the blue part ofthe visible�light spectrum [74].

Polymerization of N�Vinylpyrroles: Recent AchievementsA. I. Mikhaleva, M. V. Markova, I. V. Tatarinova, L. V. Morozova, and B. A. Trofimov*

Favorsky Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, ul. Favorskogo 1, Irkutsk, 664033 Russia*e�mail: [email protected]

Received October 24, 2013;Revised Manuscript Received January 27, 2014

Abstract—The data on the polymerization of N�vinylpyrroles published for the most part during the pastdecade are systematized and summarized. Radical, cationic, and anionic polymerization of the mentionedmonomers, which became easily accessible owing to the discovery and systematic development of their directone�step synthesis from ketoximes and acetylene in the superbasic catalytic system KOH/DMSO (the Trofi�mov reaction), are discussed. Special attention is given to the physicochemical properties of the polymers(conductivity, paramagnetism, photosensitivity, and optical characteristics).

DOI: 10.1134/S1560090414040071

REVIEW

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POLYMER SCIENCE Series B Vol. 56 No. 4 2014

MIKHALEVA et al.

This review is devoted to the analysis and summa�rization of the data on the polymerization (radical,cationic, and anionic) of N�vinylpyrroles that wereobtained for the most part during the past decade andwhich were not included in previous monograph [7]and review [8]. In some cases, earlier studies are citedfor the sake of comparison. Special attention is givento the discussion of the physicochemical properties ofthe polymers and oligomers in terms of their use inpractice.

RADICAL POLYMERIZATIONOF N�VINYLPYRROLES

The radical polymerization of N�vinylpyrroles hasbecome the subject of systematic studies since the dis�covery of their one�step synthesis from ketoximes andacetylene [7, 45]. The synthesis of new pyrrole�con�taining polymers is stimulated by their potential valuefor practice (e.g., as components of semiconductorand photosensitive materials for electrophotography,holography, etc. [68–70, 74]).

The homopolymerization of N�vinylpyrroles wasstudied in [75–79]. The reaction was initiated by

AIBN (2–5 wt %) or UV radiation (for N�vinyl�2�phenylpyrrole [76]); the reaction temperature was 60–80°C. As a result, linear oligomers containing polyeth�ylene units in the main chain and alkyl(aryl)pyrrolegroups in side chains were synthesized.

(1)

Here, R1 = Ph, 1�naphthyl, or 2�naphthyl; R2 = H,Ph, n�С5Н11, n�С7Н15, or n�С9Н19; and R3 = H or Ph.

The oligomers show solubility in organic solvents(benzene, acetone, chloroform), their yields lie in therange 11–92% (depending on the substituents of thepyrrole ring and the conditions of polymerization),and their molecular masses do not exceed 3000.

The low degrees of polymerization of the oligomersare probably due to the attack of growing radical A atthe neighboring phenyl group, which is accompaniedby the formation of less radical species B, in whichspin is distributed also over the benzene ring [77].

(2)

Similar reactions of spin transfer to the neighboringheteroatomic group were observed in the radical poly�merization of N�vinylindole and its substituted deriv�atives [80]. The formation of additional cyclic struc�tures in the polymerization of N�vinylpyrroles is con�sistent with the data on their cyclopolymerization[81], which confirm that the change in entropy inintramolecular cyclization reactions is smaller thanthat in intermolecular chain�growth reactions. Stablerand longer lived macroradical B may be involved inmacrochain transfer to the monomer along with theprimary growing radical; as a result, end groups oftypes C and D form.

(3)

The presence of the aromatic substituent in posi�tion 2 of the pyrrole ring (N�vinyl�2�phenylpyrrole)leads to the steric shielding of N�vinyl groups and, as aconsequence, to the low activity of the monomer inradical polymerization. (The yield of oligo(N�vinyl�2�phenylpyrrole) does not exceed 40%; M = 2100–3000[76].)

The low activity of N�vinyl�2,5�diphenylpyrrole (ayield of no more than 11%; M = 1400 [75]) is probablydue to the limited accessibility of the monomer to theinitiating growing radical shielded by phenyl groups.

Because of the spatial interaction between substit�uents, the introduction of substituents into position 3

NR1

R2

R3 NR1

R2

R3

n

,

N

R

C

H

H2C

R N

R

C

H

H2C

R N

R

C

H

H2C

R N

R

C

H

H2C

R

A B

••

••

N

R

RA

C

–[H•]

N

R

C

H

H2C

R N

R

C

H

H2C

R•

–[H•]

D


POLYMERIZATION OF N�VINYLPYRROLES: RECENT ACHIEVEMENTS 445

of the pyrrole ring increases the angle of rotation of thephenyl ring in position 2 relative to plane of the pyrrolering and, hence, decreases conjugation in the mono�mer. As a consequence, the stability of the growingradical decreases and the rate of oligomerization andthe yield of oligomers increase. The yields of the oligo�mers of N�vinyl�3�alkyl�2�phenyl�, N�vinyl�2,3�diphenyl�, and N�vinyl�2,3,5�triphenylpyrroles are87, 92, and 27%, respectively [75, 77, 79].

It is possible that, in the case of N�vinyl�3�alkyl�2�phenylpyrroles, polymerization is promoted also bythe hydrophobic interaction of alkyl radicals orientingmonomers in space, so that more polar fragments ofmolecules carrying N�vinyl groups become closer.

Moreover, the incorporation of long�chain alkylradicals into position 3 of the pyrrole ring of N�vinyl�2�phenylpyrroles improves the solubility of the result�ing oligomers. They show solubility not only in ben�zene, dioxane, and chloroform, as does oligo(N�vinyl�2�phenylpyrrole), but also in n�hexane and ether.

It appears that the low activity of N�vinyl�2�naph�thylpyrroles may additionally be attributed to spatialfactors: The bulky naphthyl substituent stericallyblocks the radical center in the growing macroradical.The yields of the oligomers of N�vinyl�2�(1�naph�thyl)� and N�vinyl�2�(2�naphthyl)pyrroles are as lowas 17–30%; the molecular masses are 2100–2500 [82].

Despite their low molecular masses, the oligomersof N�vinyl�2�arylpyrroles feature the properties ofhigh�resistance organic semiconductors: Their con�ductivities are 10–13–10–14 S/cm. These valuesincrease by 4–7 orders of magnitude after doping withiodine vapor.

As was shown for oligo(N�vinyl�2�phenylpyrrole),the optimum time of doping with iodine vapor was72 h. The content of iodine in the oligomer was as highas 64.7%, and its conductivity increased to 1.3 ×10⎯6 S/cm. A further increase in the time of contact ofiodine vapor with the oligomers insignificantlyaffected their conductivity values [77].

Oligo(N�vinyl�2�phenylpyrrole) possesses para�magnetism, which is characterized by an asymmetricline in the EPR spectrum [76]. Its paramagnetism, likethat of the conjugated oligomer, is probably due to thepreservation of low�activity long�lived radicals inthem. Their increased stability may be rationalized bythe effective distribution of unpaired electronsbetween pyrrole and benzene rings (reaction (2),structure B).

The formation of such stable and, hence, less reac�tive radicals ensures the low activity of N�vinyl�2�phe�nylpyrrole in radical polymerization. The concentra�tion of paramagnetic centers in the oligomer is low,namely, on the order of 1016 spin/g. The value of the gfactor (2.0062) differs appreciably from the purely spinvalue (2.0023), a circumstance that indicates the con�tribution of spin–orbital interactions of unpaired elec�trons with nitrogen atoms in the initial oligomer.

Asymmetry of the singlet corresponding to the oli�gomer (A : B = 1.6) at a low conductivity (10–14 S/cm)is associated with the superposition of signals due tovarious paramagnetic centers. The study of the char�acteristics of the EPR signal of the oligomers as a func�tion of the power of microwave radiation in the range0.200–63.250 mW showed that, as the power of themicrowave radiation is increased by a factor of 100(from 0.200 to 20.0 mW), the asymmetry parameterchanges, the signal becomes symmetric, and its widthchanges. These results confirm that there are at leasttwo types of paramagnetic centers in the oligomer. Thefirst type, with a smaller g factor, is saturated at ahigher rate. The second signal is practically unsatur�ated in this range, but, after a further increase in thepower of microwave radiation to 63.25 mW, this signalwidens.

For oligo(N�vinyl�2�phenylpyrrole) doped withiodine vapor for 24 h at room temperature, a spin echowas observed. This fact made it possible to determinethe times of spin–lattice relaxation (Т1) and spin–spinrelaxation (Т2).

The presence of two relaxation components indi�cates a complex character of interaction between theunpaired electrons and the environment and agreeswith the suggestion that two types of paramagneticcenters occur in the considered oligomers. The time of

the spin–lattice relaxation, Т1 ( = 4791.5 and =18 037.6 ns), is much higher than the time of the spin–

lattice relaxation, Т2 ( = 117.0 and = 736.4 ns).This situation is typical for magnetically dilute para�magnetics [83].

Introduction of a long�chain alkyl or phenyl sub�stituent into position 3 of the pyrrole ring disturbs thecoplanarity of pyrrole and benzene rings and, hence,leads to decreased conjugation in the monomer. As aconsequence, stabilization of the radical decreases andthe rate of oligomerization and the yield of oligomersincrease. The oligomers of N�vinyl�3�alkyl� andN�vinyl�2,3�diphenylpyrroles are practically diamag�netic. (There are no signals in the EPR spectra.) Afterdoping with iodine vapor for 72 h, the oligomersacquire paramagnetic behavior: The EPR spectraexhibit practically identical weak signals with g factorsof ~2.006.

The paramagnetic behavior of the oligomers dopedwith iodine vapor may be explained by the transfer ofelectrons from π�donor (2�phenylpyrrole) fragmentsto the acceptor, the dopant (I2). As a result, there isgeneration of ion�radical pairs (polarons [84, 85]),which serve as charge carriers providing conductivityand paramagnetism.

T 1' T 1''

T 2' T 2''

446


MIKHALEVA et al.

, (4)

Here, R1 = Ph, 1�naphthyl, or 2�naphthyl; R2 = H,Ph, n�C5H11, n�C7H15, or n�C9H19; and R3 = H or Ph.

Relatively low concentrations of paramagnetic cen�ters in the doped oligomers (not above 0.99 × 1017 spin/g)at high doping levels (the iodine contents are as high as53–65%) and increases in conductivity to 10–7–10⎯6 S/cm suggest the polaron–bipolaron dynamics.Upon attaining a sufficiently high concentration,polarons (cation radicals) recombine to form diamag�netic bipolarons [86, 87].

(5)

The oligomers of N�vinyl�2�phenyl�, N�vinyl�3�alkyl�2�phenyl�, and N�vinyl�2,3�diphenylpyrrolefluoresce in the near�UV spectral region [76, 77, 79].Spectroscopic characteristics were investigated withthe use of 2�phenylpyrrole, 3�heptyl�2�phenylpyrrole,and 2,3�diphenylpyrrole, whose π systems modelchromophores of the studied oligomers.

Positions of the band maxima in the fluorescencespectra of the oligomers and pyrroles taken as stan�dards well fit each other owing to the absence ofthrough conjugation along polymer chains.

The UV spectra of N�vinyl�2�phenylpyrrole oligo�mers taken in an acetonitrile solution show a consid�erable shortwave shift in the longwave band (277 nm),relative to the corresponding band of 2�phenylpyrrole(286 nm) [88, 89]. This fact may be explained by anincrease in the dihedral angle in the 2�phenylpyrrolefragment as a result of the steric interaction of the phe�nyl substituent with the polymer backbone.

Disturbance of the coplanarity of pyrrole and ben�zene rings during the incorporation of bulky alkyl sub�stituents into position 3 of the pyrrole ring caused ahypsochromic shift of the longwave band (a ππ* tran�sition) by ≈ 500 cm–1 (n�hexane).

As evidenced by the DTA data, the oligomers ofN�vinyl�2�arylpyrroles are stable: A 10% loss in theweight of the N�vinyl�2�phenylpyrrole oligomer isobserved at 300°С; a 10% loss in the weight of the 2,3�diphenylpyrrole oligomer is observed at 400°C.

Replacement of the phenyl substituent in position2 of the pyrrole ring with the naphthyl substituent like�wise increases the thermal stability of oligomers of

N�vinyl�2�(1�naphthyl)� and N�vinyl�2�(2�naph�thyl)pyrroles to 320 and 380°C, respectively [81].

The oligomers of N�vinyl�2,5�diphenylpyrrole arepatented as new efficient luminescent materials;therefore, they show promise for the creation of con�ducting electrochromic layers in electro�opticaldevices (thin�film displays, fluorescent sensors) [74].Poly(N�vinyl�2,3�diphenylpyrrole) is claimed to be aphotosensitive material with good adhesion and a lowsoftening temperature [90].

Dipyrroles with internal labile π systems are sub�stantially new monomers for the synthesis of polypyr�roles with increased levels of conductivity and electro�chemical stability [91, 92]. These synthetic metalsform the basis for high�tech materials (data carriers,nonlinear optical devices, thin�film flexible color dis�plays, field transistors) [93, 94].

Of special interest is a dipyrrole that carries twoN�vinyl groups in each pyrrole ring: 1,4�bis[2�(N�vinyl)pyrrolyl]benzene. However, only its electri�cal polymerization has been studied [93].

Later [95], polymers of 1,4�bis[2�(N�vinyl)pyrro�lyl]benzene with side N�vinyl groups were preparedvia radical polymerization in the presence of AIBN(70°C, 4–72 h) with yields of up to 34%. Their molec�ular masses depend on polymerization conditions andamount to Мw = 9500–11 500 and Мn = 2400–2800;their polydispersity values are 3.96–4.11. As evi�denced by IR, UV, and 1H NMR spectroscopy, theycontain blocks of two types, in which structures withfree N�vinyl groups predominate by a factor of 2 to 6.

(6)

N

R2

R1 R3+•

n

N

R2

R1 R3

nI2

I2.–

N

R2

R1N

R2

R1N

R2

R1

N

R2

R1

⎝ ⎠⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎛ ⎞

⎝ ⎠⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎛ ⎞

⎝ ⎠⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎛ ⎞

I– I– + I2I2•–

•+•+I2•– + ++

N

N

N

NN

N

p

nm



Free N�vinylpyrrole groups in the polymers ensuretheir subsequent functionalization and crosslinking toyield three�dimensional crosslinks. This feature maybe used for the formation of photosensitive and con�ducting layers.

The above�described polymers are paramagnetic:The EPR spectra show asymmetric singlets. (The con�centration of unpaired electrons is ~1018 spin/g, ΔН =7.6 H, and g = 2.0037.) The paramagnetic behavior ofthe polymers is probably associated with the fact thatkinetically low�activity macroradicals, which are notinvolved in recombination, are preserved in stronglybranched oligomeric structures owing to steric shield�ing. They possess luminescent properties, feature afluorescence band at 386 nm (an excitation wave�length of 290 nm, acetonitrile), and show promise for

the design of organic semiconductors and nanolayersof optoelectronic materials.

Azopyrroles may be used as precursors for the syn�thesis of semiconductor polymers with narrow bandgaps [96] and electro�optical materials [97]. In [98–100], polyconjugated conducting materials were pre�pared via the electrical polymerization of arylazopyr�roles.

A new group of monomers—azo dyes of the pyr�role series, 2�arylazo�1�vinylpyrroles—polymerizeduring heating (80°C, 5 h, benzene) both in theabsence of the initiator and in the presence of AIBN(2 wt %, 80°C, 50 h, benzene) to produce deeply col�ored oligomers with yields of up to 92% [101, 102].

(7)

Here, R1 = R2 = R3 = H, R1 = Me and R2 = R3 = H,R1 = Ph and R2 = R3 = H, R1 – R2 = (CH2)4 andR3 = H or R1 – R2 = (CH2)4 and R3 = OEt.

During heating, arylazo groups of the monomersundergo partial decomposition and generate radicals

that can initiate polymerization accompanied by theformation of homopolymers (self�initiated polymer�ization) [102]. The recombination of these radicalsyields N�vinyl�2�phenylpyrroles. The presence of theoligomers was confirmed via 1H NMR spectroscopy.

(8)

The generation of radical centers in macromole�cules via the above scheme and their recombinationlead to the crosslinking of chains and, thus, explain theformation of insoluble fractions in the oligomers.

The thus�prepared oligomers are paramagnetic(N = 1.6 × 1019–2.6 × 1019 spin/g, ΔH = 20.030–20.062 G), and their conductivity values range from1.0 × 10–13 to 2.0 × 10–14 S/cm. After doping withiodine, these values increase to 2.5 × 10–6–5.6 ×10⎯6 S/cm [102].

A number of new pyrrole oligomers containingaldehyde groups capable of easy transformation intoother functions were prepared via the polymerizationof N�vinylpyrrole�2�carbaldehydes (2 wt % AIBN,60–80°C, 24–50 h) [103].

(9)

Here, R1 = Ph and R2 = H, R1 = 2�thienyl andR2 = H, or R1 ⎯ R2 = (CH2)4.

R1N

R3R2N

N

R3R2

R1 N

N

N

n

R3NR2

R1 NN

R2

R1 NR3 R3

R2

R1 N

· + · –N2

80°C

R1

R2

N

m

OR1

R2

N OR1

R2

N

n

448


MIKHALEVA et al.

The oligomers with pyrrole, tetrahydroindole, anddihydrobenz[g]�indole rings and aldehyde groups inside chains are soluble red�brown powders (yields ofup to 27%, M = 1800–3200) [103].

The low activity of N�vinylpyrrole�2�carbalde�hydes in radical polymerization is probably related tospatial hindrances encountered at the stage of chaingrowth. In addition, the active radical center is par�tially blocked sterically. Moreover, it appears thatintra� and intermolecular interactions of the growingradical with neighboring carbonyl substituents exertthe inhibitory effect on polymerization [103].

The oligomers are paramagnetic, and their EPRspectra show slightly asymmetric narrow singlets(ΔН = 5.8–7.8 G); the corresponding concentrationsof unpaired electrons are 1017–1018 spin/g. The valuesof the g factor (2.0024–2.0031) are close to the dataknown for free radicals.

The oligomers of N�vinylpyrrole�2�carbaldehydesinteract with aniline and form Schiff bases (1 wt %CF3COOH, 20–80°C, 5 h, 1.5 moles of aniline permole of oligomer (per unit)) [103].

(10)

Here, R1 = Ph or 2�thienyl and R2 = H, or R1 – R2 = (CH2)4.The degrees of conversion are as high as 78% (1H NMR).Condensation with ethanethiol conducted in the presence of CF3COOH yields poly(N�vinylpyrrole�2�carbalde�

hydethioacetals) (1 wt % CF3COOH, 20–60°C, 3–8 h, benzene) with a conversion of up 33% [103].

(11)

Here, R1 = Ph or 2�thienyl and R2 = H, or R1⎯R2 = (CH2)4.

N O ON

n

OPhN

OPh

Ph

N

ON

•

•

R1

R2

ON

ON

NPhR1

R2

N R1

R2

ON

NNPh ON

n

n

m

m n – m

n – m

+ H2NPh CF3COOH

–H2O

+ H2NPh CF3COOH

–H2O

R1

R2

ON

ONN

SEt

SEtO

N

SEt

SEtR1

R2

NR1

R2

ON

n – mmn

n+ EtSH

+ EtSH

CF3COOH

–H2O

CF3COOH

–H2O



Radical (0.2–2 wt % AIBN, 60–80°C, 4–25 h)and thermal (60–80°C, 12–25 h) polymerization ofN�allenylpyrrole, N�allenyl�2�phenylpyrrole, andN�allenyl�4,5,6,7�tetrahydroindole occurs via both

the 1,2� and 2,3�positions of the allenyl group andresults in the formation of soluble reactive oligomerscontaining various ethylene, including polyvinylene,fragments [104, 105].

(12)

Here, R1 = R2 = H, R1 – R2 = (CH2)4, or R1 = H andR2 = Ph.

The presence of polyvinylene fragments III and IVin macromolecules of poly(N�allenylpyrroles) isrelated to the prototropic isomerization of olefingroups in structures I and II.

The quantitative ratio of blocks of various struc�tures (estimated from the integral intensities of thecorresponding signals in the 1H NMR spectra)depends on the initiation procedure [104].

Compared to N�vinylpyrroles, N�allenylpyrrolesare more active in radical polymerization and formhigher molecular mass products. For example, undersimilar conditions (1 wt % AIBN, 80°C, 5 h), the yieldand molecular mass of poly(N�allenyl�4,5,6,7�tet�rahydroindole) (97% and 4400, respectively) [104] aremuch higher than those for poly(N�vinyl�4,5,6,7�tet�rahydroindole) (51% and 2800) [7, 8]. An even largerdifference in activity was observed for N�allenyl� andN�vinyl�2�phenylpyrroles. The yield of poly(N�alle�nyl�2�phenylpyrrole) was as high as 71% [104],whereas the yield of poly(N�vinyl�2�phenylpyrrole)did not exceed 18% [7, 8].

The EPR spectra of all synthesized oligomersexhibit signals typical for polyconjugated systems,namely, symmetric singlets; the concentration ofunpaired electrons is 3.1 × 1016–8.8 × 1016 spin/g; andΔH lies in the range 0.97–0.98 G [104, 105].

Polymer layers based on N�allenyl�4,5,6,7�tet�rahydroindole are charged to a potential of 80 V in thepositive corona discharge and possess an electropho�

tographic sensitivity of (S 0.5) 0.024 m2 J–1 with aspectral maximum at 380 nm. Sensitization with vari�ous electron acceptors leads to an increase in thecharge on the layer to 280 V and causes a marked gainin electrophotographic sensitivity and a bathochromicshift in the absorption maximum to 440 nm [104].

The resulting polyfunctional reactive oligomerscarrying pyrrole and ethylene rings, including polyvi�nylene fragments, offer additional possibilities for tar�geted change in conductivity and other electrophysi�cal properties. These oligomers may be used as mac�romonomers for further polymerization andcopolymerization and as active binders for the synthe�sis of conducting materials.

RADICAL COPOLYMERIZATIONOF N�VINYLPYRROLES

The copolymerization of N�vinyl�4,5,6,7�tetrahy�droindole with styrene [106], vinyl� andvinylidenechloride [107–109], vinyl acetate [110],derivatives of acrylic acids (acrylonitrile, acrylamide,and methyl methacrylate) [111, 112], diethyl maleate[113], ethanolamine vinyl ether [114], cholesterolvinyl ether [115], and butyl vinyl ether and poly(butylvinyl ether) [116, 117] were investigated.

The radical copolymerization of N�vinyl�4,5,6,7�tetrahydroindole М1 with styrene М2 (1.5 wt % AIBN,60°C, 48 h, DMF) gave rise to white powderlikecopolymers with a yield of up to 23% (an intrinsic vis�cosity of 0.289–0.440 dL/g; Tm = 99–151°C) [106].

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R2

R1 N

R2

R1N

R2

R1N

R2

R1N

Me

R2

R1N

.nmlk

I II III IV

NN +

m n

450


MIKHALEVA et al.

Reactivity ratios r1 and r2 are 0.067 ± 0.006 and4.3 ± 0.7, respectively. These data indicate that theactivity of N�vinyl�4,5,6,7�tetrahydroindole is lowerthan that of styrene. As evidenced by the calculation ofthe microstructural parameters, the copolymers as arule comprise styrene blocks (up to 38 units) separatedby single units of the indole component. A shortsequence of N�vinyl�4,5,6,7�tetrahydroindole units(up to three units) appears if its content in the initialmixture is high.

The copolymerization of N�vinyl�4,5,6,7�tetrahy�droindole with vinyl chloride and vinylidenechloride(1.5 wt % AIBN, 60–80°С, 6–10 h) occurs at all ini�tial monomer ratios [107]. The analysis of the IR spec�tra and the compositions of the copolymers showedthat, in this case, the dehydrochlorination of macro�molecules occurs (the degrees of dehydrochlorinationare up to 90% and 30% for vinyl chloride andvinylidenechloride, respectively) [107, 108].

(14)

Copolymerization occurs via two steps. At the firststage, the copolymer forms via the radical mechanism;then, HCl that evolved as a result of the eliminationreaction enters into a reaction with N�vinyl�4,5,6,7�tetrahydroindole and forms the immonium cation [7,118]. The subsequent electrophilic attack of this cat�ion at the pyrrole ring of the monomer leads to the for�mation of the dimer and/or the oligomer of N�vinyl�4,5,6,7�tetrahydroindole that is then involved in radi�cal copolymerization. A similar attack of the immo�nium cation at the pyrrole ring of the copolymer mac�romolecule yields graft copolymers [109]. These

copolymers (a yield of up to 63%, a content of the4,5,6,7�tetrahydroindole component of 51–98 mol %)are distinguished by enhanced thermal stability and alow rate of thermal decomposition relative to those ofPVC. Moreover, they can form concentrated solutionsin organic solvents [108, 109].

The copolymers of N�vinyl�4,5,6,7�tetrahydroin�dole with vinyl acetate (VA) (1–2 wt % AIBN, 50–70°C, 5–50 h) are synthesized at any initial compo�nent ratio and are always enriched with N�vinyl�4,5,6,7�tetrahydroindole units [110].

(15)

The maximum reaction rate, ultimate conversion,and molecular mass were observed for systems con�taining 90% 4,5,6,7�tetrahydroindole monomer.There is no induction period on the kinetic curves ofthe process; however, there is a well�defined ultimateconversion, which is probably related to consumptionof the active N�vinyl�4,5,6,7�tetrahydroindole [110].The yield of the copolymer is up to 76%, the molecularmass is 1200–4600, and the maximum fraction ofvinyl acetate in the copolymers is 18% [110].

The rate of copolymerization is described by theequation v = 7.39 [M]1.18 [I]0.51; the activation energyis 98.7 kJ/mol. The overestimated reaction order withrespect to the monomer may be attributed to the

dependence of the initiation rate on the monomerconcentration or to the formation of low�activity rad�icals that is due to chain transfer to benzene [110].

The reactivity ratios r1 = 5.09 and r2 = 0.18 and thecalculation of the copolymer microstructure suggestthat the macromolecules consist of N�vinyl�4,5,6,7�tetrahydroindole blocks whose lengths change from 2to 46 units separated by one to two vinyl acetate units[110].

The copolymers of N�vinyl�4,5,6,7�tetrahydroin�dole with derivatives of acrylic acids (acrylonitrile,acrylamide, and methyl methacrylate) were synthe�sized in the presence of AIBN (0.3 wt %, 60°C, 5–24 h, DMF) [111].

N Cl

N

Me

NN Cl

+ –HClnmlk

O

CH3ONN

O

CH3O+

nm



(16)

Here, R1 = CH3 and R2 = , or R1 = H and R2 = ⎯≡N or .

It was found [111, 112] (calculation based on 13CNMR data) that the maximum fractions of N�vinyl�4,5,6,7�tetrahydroindole in the copolymers withmethyl methacrylate, acrylamide, and acrylonitrileare 0.48, 0.39, and 0.21 molar fractions, respectively.The reactivity ratios of N�vinyl�4,5,6,7�tetrahydroin�dole in all the mentioned monomer pairs are belowunity [111, 112].

The alkaline hydrolysis of copolymers of N�vinyl�4,5,6,7�tetrahydroindole with methyl methacrylate(a twofold excess of NaOH, 200°C, 6 h) and withacrylonitrile and acrylamide (50 wt % NaOH, 100°C,2 h) produced carboxyl�containing water�solublepolymers of N�vinyl�4,5,6,7�tetrahydroindole withyields up to 71% [111].

(17)

Here, R1 = CH3 and R2 = , or R1 = H and R2 = ⎯≡N or .

Potentiometric titration and viscometric measure�ments revealed that these copolymers possess theproperties of weak polymer electrolytes in aqueoussolutions. The conformations of macromolecules arespecified by the hydrophobic interactions involving4,5,6,7�tetrahydroindole units [111].

The radical copolymerization of N�vinyl�4,5,6,7�tetrahydroindole with diethyl maleate (0.3 wt %AIBN, 60°C, 24 h) yields copolymers in which theamount of N�vinyl�4,5,6,7�tetrahydroindole unitsvaries within 0.14–0.76 molar fractions; the constantsof relative activity of the monomers are 0.24 ± 0.01and 0.48 ± 0.02, respectively [113].

(18)

The yield of the copolymers shows an extremumdependence on the composition of the initial mixture.This value is maximum (64%) near the equimolar ratio of

the monomers. This is usually the case when the charge�transfer complex forms between the monomers and thepolymerization yields the alternating copolymer [119].

N R2

R1

R2R1N+

m n

OCH3

O

NH2

O

R2R1N

p

R1

NaO

ON

mnm n

OCH3

O

NH2

O

N

COOC2H5

COOC2H5

N COOC2H5C2H5OOC+

nm

452


MIKHALEVA et al.

The copolymers form complexes with copper, cad�mium, cobalt, and magnesium ions in aqueous solu�tions [113, 120].

As was shown via potentiometry and EPR spectros�copy, the coordination of metal with carboxyl groupsof the copolymers is favorable for the compact confor�mation of the copolymer stabilized by hydrophobicinteractions of 4,5,6,7�tetrahydroindole units. Whencarboxyl groups are neutralized via the addition of analkaline, the copolymer undergoes a conformational

transition that is accompanied by partial dissociationof the complex.

The alkaline hydrolysis of ether units (boiling inethanol with a twofold excess of NaOH, 30 min, puri�fication via dialysis followed by lyophilic drying)yielded water�soluble copolymers of N�vinyl�4,5,6,7�tetrahydroindole with sodium maleate [113, 120].

The copolymerization of N�vinyl�4,5,6,7�tetrahy�droindole with ethanolamine vinyl ether was studied(0.5–3 wt % AIBN, 60–80°C, to 30 h) [114].

(19)

The yield of the copolymers was as high as 93%,their molecular masses varied from 900 to 3700, andthe maximum share of vinyl ether in the copolymerswas 25%. An increase in the content of vinyl ether inthe reaction mass leads to decreases in the rate of theprocess, the monomer conversion, and the molecularmasses of the copolymers.

The kinetic study of the copolymerization ofN�vinyl�4,5,6,7�tetrahydroindole with ethanola�mine vinyl ether demonstrated that the reaction ratemay be described by an equation including thebimolecular termination of growing chains: v =K [M]1.5 [I]0.5. The activation energy is 83.8 kJ/mol[114]. The constants of the relative activities of theabove monomer pair are 6.5 and 0.05 for N�vinyl�4,5,6,7�tetrahydroindole and vinyl ether, respec�tively. Calculation of the structure of macromole�

cules with the use of these data shows that thecopolymers are composed of large blocks (up to12 units) of N�vinyl�4,5,6,7�tetrahydroindole sepa�rated by single units of ethanolamine vinyl ether[114].

Layers obtained on the basis of the copolymers arecharacterized by an integral electrophotographic sen�sitivity on the order of 10–4–10–5 (lx s)–1. Doping ofthe copolymers with 2,4,7�trinitrofluorenone resultsin a gain in sensitivity by two orders of magnitude[114].

Optically active copolymers of N�vinyl�4,5,6,7�tetrahydroindole with cholesterol vinyl ether (2 wt %AIBN, 70°C, 50 h, benzene) [115] synthesized withyields of 19–66 % are potential materials for register�ing LC systems and drugs.

(20)

NH2

ONNNH2

O+

nm

N

Me Me

Me

O

Me

Me

N

Me Me

Me

O

Me

Me

+nm

Me



The contents of cholesterol in the copolymers are 0.05–0.09 molar fractions, and the molecular masses are 1000–

1750. The value of specific rotation [α] of the copolymers varies from –4 to ⎯9 [115].

The copolymerization of N�vinyl�4,5,6,7�tetrahydroindole with butyl vinyl ether (2 wt % AIBN, 80°C, up to 26 h)yields oligomers with molecular masses of up to 1200 in the form of white powders that do not melt until 350°Cand show solubility in benzene, dioxane, and chloroform [117].

(21)

The oligomers are always enriched in N�vinyl�4,5,6,7�tetrahydroindole units, while the content of butyl vinylether units changes from 0.21 to 0.25 molar fractions [117]. The reactivity ratios of N�vinyl�4,5,6,7�tetrahy�droindole and butyl vinyl ether are 1.30 and 0.13, respectively.

The examination of 1H and 13C NMR spectra showed that the oligomerization is complicated by intramolec�ular cyclization involving the carbon atom at position 2 of the pyrrole ring.

(22)

The low activity of vinyl ether in radical copoly�merization excludes the synthesis of copolymerspredominantly containing units of this monomer[121]. When the commercial poly(vinyl butyl ether)used in medicine under the name Vinilin(the Favorsky–Shostakovsky balsam) was employedas a macromonomer [122], we managed to preparecopolymers of N�vinyl�4,5,6,7�tetrahydroindoleand its dimer (N�vinyl�2�[1�(1�4,5,6,7�tetrahy�

droindolyl)ethyl�4,5,6,7�tetrahydroindole), inwhich the content of vinyl butyl ether units was ashigh as 55–86% [116]. The reaction is based on theuse of “internal unsaturation” of poly(alkyl vinylethers), that is, the presence of double bondsformed by specific processes of chain terminationand transfer during the cationic polymerization intheir macromolecules [123].

25D

N OC4H9N

OC4H9

+nm

N

CH2N

CH CH2H2C

N

CH

HC CH2H2C

N

N

CHN

CH CH2H2C–[H•]

•

•

CH

454


MIKHALEVA et al.

(23)

Here, or .

The above copolymers make it possible to preserve the advantages of Vinilin and, at the same time, to modifyit and thus widen its application areas.

“Inner�colored” polymer materials were prepared via the copolymerization of N�vinyl�4,5,6,7�tetrahydro�2�(4�ethoxy)phenylazoindole and N�vinyl�2�phenylazopyrrole with N�vinylpyrrolidone (2 wt % AIBN, 70–80°C, 30–45 h, a yield of 3–89%) [101, 102].

(24)

Here, R1 = R2 = R3 = H, or R1–R2 = (CH2)4 and R3 = OEt.

The solutions of the copolymers containing3 mol % azo monomer are bright yellow (acetone,ethanol, chloroform) or red (dioxane, DMSO, aceto�nitrile) [101].

As was shown for N�vinyl�2�phenylazopyrrole,owing to radicals generated during the partial decom�position of the azo group (via the mechanism developedfor their homopolymerization, reaction (8)), N�vinyl�2�phenylazopyrroles are involved in copolymerizationwith N�vinylpyrrolidone (80°C, 45 h, benzene) andfunction as both comonomers and initiators [102].

The reaction accelerates when additional amountsof AIBN (2 wt %) are added. The yield of the copoly�

mers achieves 89% and decreases abruptly as the frac�tion of pyrrole in the reaction mixture is decreased.The compositions of the copolymers are close to thecomposition of the monomer mixture, but in the caseof thermal initiation, the copolymers with a higher (by13–18 mol %) content of N�vinyl�2�phenylazopyr�roles form [102].

N�Vinylpyrrole�2�carbaldehydes are readilyinvolved in copolymerization with styrene, N�vinylpyr�rolidone, and ethylene glycol vinyl glycidyl ether(2 wt % AIBN, 80°C, 50 h) to yield functionalized sol�uble copolymers in the form of powders or resins [124].

OBuOBu OBuOBu

OBuOBuOBu OBuOBu

RR

BuO

OBu OBu OBu

R

R

+ B–

E

–[HOBu]

F

E or FCH2=CHR

or

–[HB]

n n

mlkmlk

k p k ml – 1

p

N

R =

Me

NN

O N

R3R2

R1

N

NN

+

R3R2

R1

N

NNO N

nm



(25)

Here,

or and R2 = H, or R1–R2 = (CH2)4; , , or .

The yield of the copolymers is as high as 98%, thecontent of pyrrole units is up to 88%, and the molecu�lar mass is up to 4800 [124].

The copolymers form at any initial comonomerratio. In all cases except that of ethylene glycol vinylglycidyl ether, the yield of the copolymers declines asthe fraction of carbaldehyde comonomer in the reac�tion mixture is increased.

The copolymers are high�resistance organic semi�conductors. Their conductivity values are 10–13–10⎯14 S/cm. Doping of the copolymers with iodine

leads to an increase in conductivity values to 10–7–10⎯11 S/cm. The content of iodine in the samplesattains 64%; that is, electrophilic iodination of thecopolymers occurs along with doping [124].

N�Allenylpyrrole and N�allenyl�4,5,6,7�tetrahy�droindole are involved in copolymerization withN�vinylpyrrolidone and ethylene glycol vinyl glycidylether (0.5–2 wt % AIBN, 60–80°C, 7–24 h) to giverise to polyfunctional copolymers with a yield of up to73% (molecular masses of up to 6800) [104, 105].

(26)

R1 = R2 = H or R1–R2 = (CH2)4; R3 = or .

The as�prepared soluble reactive copolymers areenriched in a more active comonomer, N�allenyl�4,5,6,7�tetrahydroindole [104, 105].

For N�allenylpyrrole and N�vinylpyrrolidone, thereactivity ratios are 2.26 and 0.80, respectively [105].Calculation of the microstructure of copolymer mole�cules performed on the basis of these constants showedthat the copolymers are composed of blocks of 2–20N�allenylpyrrole units separated by single (1–3) N�vinylpyrrolidone units.

In terms of the Q – e scheme, the values of Q =0.016 and e = –2.03 found for N�allenylpyrrole indi�cate a higher (relative to N�vinylpyrrolidone) conjuga�tion of the double bond with the heteroatom andstronger electron�donor properties of the heterocyclewith respect to the allene group [105].

CATIONIC POLYMERIZATIONOF N�VINYLPYRROLES

As opposed to N�vinylindole and N�vinylcarba�zole, which, in the presence of cationic catalysts,polymerize via vinyl groups, N�vinyl�4,5,6,7�tetrahy�droindole transforms in the presence of hydrochloricacid via a substantially different route to produce thedimer of N�vinyl�2�[1�(4,5,6,7�tetrahydroin�dolyl)ethyl]�4,5,6,7�tetrahydroindole with a yield of20% [7, 125]. As acid is replaced with trimethylchlo�rosilane, the yield of the dimer increases to 70%. In[118, 126–132], the range of catalysts for the dimer�ization of N�vinyl�4,5,6,7�tetrahydroindole was wid�ened appreciably and included diverse Brønsted andLewis acids: alkyl halosilanes, chlorides of metals (tin,iron, titanium), boron trifluoride etherate, and variousmineral and organic acids (HCl, H2SO4, HNO3,AcOH). In this case, the yield of the dimer increased

OR1

R2

N R3 R3R1

R2

O N

+ nm

R1 = S

ON

R3 = O

OO

R2

R1 N

.

R3 R3

R2

R1N

R2

R1 N

R2

R1

N

R2

R1N

Me

+ k l m n p

OO

OON

456


MIKHALEVA et al.

from 70% (2 wt % alkyl halosilanes as catalysts, roomtemperature, 50 h) [118, 128, 132] to quantitativewhen a fivefold excess of acetic acid was used (roomtemperature, 2 h) [129, 132].

Dimers of N�vinyl�2,3�alkylpyrroles (2 wt %HCl, Me3SiCl, 20°C, 4 h) were synthesized as well[127, 128]. Their yield in the studied pyrrole seriesincreased with lengthening of the alkyl radical inposition 3 of the pyrrole ring (H < Me < Pr) andeventually reached 53% [133].

Along with dimers, polymers of unexpected struc�tures containing alternating pyrrole and ethylideneunits in the backbone were produced via cationic poly�merization.

The process occurs via the attack of the immo�nium cation at the pyrrole ring of the next monomermolecule and forms unusual oligomers (throughdimer formation) containing alternating pyrroleand ethylidene groups.

(27)

Here, R1 = Me, Pr, or Ph and R2 = H, Me, Pr, Et, or Ph.

For example, the cationic polymerization ofN�vinyl�2�phenyl� and N�vinyl�2,3�diphenylpyrroles(Me3SiCl, CF3COOH, BF3OEt2, HCl, WCl6, FeCl3,1–2 wt % LiBF4–dimethoxyethane (DME) complex,20–80°C, 24–48 h) forms oligomers with a yield of upto 63% (for poly(N�vinyl�2�phenylpyrroles), themolecular masses are 1400–1700; for poly(N�vinyl�2,3�diphenylpyrroles), the molecular masses are 800–3100) [76, 79].

In accordance with the gel�permeation�chroma�tography data (figure), the molecular�mass distribu�tion of N�vinyl�2,3�diphenylpyrrole (Me3SiCl) is uni�form: For 69% oligomer molecules, М = 800–1200(n = 3–5); for 23%, М = 2100–3100 (n = 9–15). Inaddition, the molecular�mass�distribution curveshows peaks of low�molecular�mass (up to 800) andhigh�molecular�mass (up to 47 000) fractions (in total

~8%). As is seen from the figure, UV and refractomet�ric detectors yield close molecular�mass distributionsof the oligomer [79].

The above mechanism of polymerization confirmsthe isolation (with a yield of up to 34%) and identifi�cation of corresponding dimers [76, 79].

As evidenced via DTA, the above oligomers arethermally stable and their macromolecules decomposein the range 350–650°C, whereas for the oligomersprepared with the use of AIBN, this range is 310–530°C [76, 79]. They possess the paramagnetic behav�ior corresponding to high concentrations of paramag�netic centers from 4.9 × 1017 to 8.7 × 1017 spin/g andare high�resistance organic semiconductors featuringconductivity (10–13 S/cm). Doping of the oligomerswith iodine promotes a gain in their conductivity to10–6 S/cm. The oligomers fluoresce in the near�UV

R2

R1N

R2

Me

R1N

R2

Me

R1N

R2

R1N

R2R2

R1

HR1

N

Me

NH+

++

+

R2R2

R1

R1

Me

N

Me

N

R2R2

R1

R1

N

Me

NH+

+

–[H+]

R2R2R2

R1R1

HR1

N

Me

N

Me

N

R2

R1

Me

N

R2R2

R1

R1

N

Me

N–[H+]

+

n

R2

R1N



spectral region (λ = 355–388 nm, acetonitrile, 1,4�dioxane) [76, 79].

The cationic polymerization of 1,4�bis[2�(N�vinyl)pyrrolyl]benzene (Me3SiCl, BF3

.OEt2, theLiBF4–DME catalytic system), as in the case ofother N�vinylpyrroles, occurs via the alternateinvolvement of double bonds and pyrrole rings in

the process [95]. In this case, three�dimensionalinsoluble polymers containing alternating pyrroleneand ethylidene units in the main chain predomi�nantly form. The maximum content of the solublefraction containing mostly blocks with sideN�vinylpyrrolylphenyl groups is 28%; the molecularmass of the oligomer is 1500–1700.

(28)

The maximum yield of the polymers of 80% (only the insoluble fraction) was achieved when Me3SiCl wasused as a catalyst. The use of the catalytic system LiBF4⎯DME made it possible to obtain functionalized oli�gomers in which the content of the soluble fraction was a factor of 4 greater (although the total yield of the oli�gomer was 35%) [95].

2�Arylazo�1�vinylpyrroles react with protic (CF3COОH, HCl) and aprotic (BF3) acids and form oligo�meric deeply colored products composed of soluble (in benzene, chloroform) and insoluble fractions withyields of up to 73% [134].

(29)

NN

Me

N

N

Me

Me

N

N

n

m

m

ArNN

R2

R1 N

HA

ArNN

R2

R1 N

n

HA

(a)

102 103 104 105

Molecular mass

(b)

102 103 104 105

Molecular mass

Molecular�mass�distribution curves of the N�vinyl�2,3�diphenylpyrrole oligomer (Me3SiCl): records of (a) a UV detector and(b) a refractometric detector.

458


MIKHALEVA et al.

Here, Ar = Ph, C6H4OEt�p, or C6H4Br�p; R1 = H, Me, or Ph and R2 = H, or R1–R2 = (CH2)4.

As evidenced by 1H NMR and IR spectroscopy,oligomerization mostly occurs via the N�vinylgroup with partial involvement of the pyrrole ring.The process is accompanied by the capture of cata�lyzing acids by the azo group, a phenomenon that

leads to the deep polarization of the elementary unitwith transfer of the positive charge to the pyrrolering, which, as a result, becomes capable of furthertransformations accompanied by the crosslinking ofoligomer chains.

(30)

Here, Ar = Ph, C6H4OEt�p, or C6H4Br�p; A = Cl– or CF3COO–; R1 = H, Me, or Ph and R2 = H, or R1 – R2 = (CH2)4.

The above�described oligomers feature conductiv�ity values of 10–13–10–9 S/cm, which increase to 4.8 ×10–6 S/cm after iodination of the products with iodinevapor.

Organic–inorganic copolymers based on the prod�ucts of hydrolysis of methyltrichlorosilane with anumber of nitrogenous bases, such as N�vinylazole,N�vinylpyridine, and N�vinyl�4,5,6,7�tetrahydroin�dole, were synthesized with the use of the sol–gelmethod [135].

In the case of N�vinyl�4,5,6,7�tetrahydroindole(VTHI), the yield of hybrid copolymers was 79%. Inaccordance with [135], the composite contains thedimer of N�vinyl�4,5,6,7�tetrahydroindole, whereasin hydrochloric acid, the dimerization of N�vinyl�4,5,6,7�tetrahydroindole related to the hydrolysis ofmethyltrichlorosilane occurs. The composition of thecopolymer calculated from these data may beexpressed via the ratio CH3SiO1.5 : (VTHI)2 = 1.8 : 1.

The copolymers possess high thermal stability(a decomposition temperature of 325°C) and havehighly dispersed structures (globules with sizes of 86–97 nm predominate) and developed surfaces (a spe�cific surface of 31.1 m2/g) [135].

ANIONIC POLYMERIZATIONOF N�VINYLPYRROLES

It is generally agreed that N�vinylpyrroles readilypolymerize under the action of radicals, cations, orelectron acceptors [7, 39, 75, 118, 133, 136–138] butare stable in the presence of alkaline reagents [136–138] because the vinylation of pyrroles occurs insuperbasic media at elevated temperatures [7, 45].

Attempts to initiate the polymerization of N�vinyl�carbazole with the use of anionic catalysts failed [139];only recently. Natori managed to perform this processby using large amounts of BuLi as a catalyst [140].

ArNN

R2

R1 N

Ar

NN

R2R1

NF3B�

Ar NN

R2

R1N+

n

BF3 · OEt or LiBF4 · DME

–

,

NN

NAr

H

R1

R2

NN

NAr

H

R2

R1 NN

NAr

HR2

R1

NN

NAr

HR2

R1 NN

N Ar

HR2

R1 NN

NAr

HR2

R1

+

+ +

+++

A–

A– A–

A–A–

A–

n

n



On the basis of quantum�chemical calculations, itwas assumed that N�vinylpyrroles can polymerize viathe anionic mechanism as well [141, 142]. As wasshown for the first time [143, 144], N�vinyl�4,5,6,7�tetrahydroindole, N�vinyl�2,3�dimethylpyrrole, andN�vinyl�2�phenylpyrrole are involved in polymeriza�

tion under the action of strong bases during heating(5–15 wt % Na, KOH, 190°C, 12 h). In all cases, theproducts contained fractions soluble (a molecularmass of 800–1200) and insoluble in organic solvents.The maximum yield of the polymers was 47% [143,144].

, (31)

Here, R1 – R2 = (CH2)4; R1 = Ph and R2 = H, or R1 = R2 = Me.

In accordance with NMR, IR, and UV spectro�scopic studies, polymerization occurs via both N�vinylgroups of the monomers and endocyclic double bondsof the pyrrole ring and leads to formation of the oligo�mers containing the above�mentioned fragments andshort polyene blocks resulting from the splitting of NHpyrroles. The presence of free NH pyrroles (up to1.5% with respect to the weight of unreacted mono�mers) was evidenced by gas–liquid chromatographyand thin�layer chromatography. The oligomeric prod�ucts are practically free of the alkaline. Sodium metalis quantitatively recovered from the reaction [144].

The participation of oxygen in the reaction shouldbe excluded, because the yield of oligomers remainsalmost the same when the process is performed underan inert atmosphere (argon). Inhibition of the reactionwith hydroquinone (a twofold drop in yield from 36 to14%) indicates the participation of free�radical spe�cies, apparently anion radicals, in the formation ofmacromolecules.

The quantitative IR studies reveal that the fractionof structures formed via opening of endocyclic doublebonds of the pyrrole ring remains practically invariable(12–16%). Hence, this process is purely thermal; it isaffected by neither the type nor the amount of the usedalkaline reagents.

The above oligomers exhibit an EPR signal: Foroligo(N�vinyl�4,5,6,7�tetrahydroindole), the concen�tration of paramagnetic centers is 4.8 × 1016 spin/g(ΔН = 9.7 G); for oligo(N�vinyl�2�phenylpyrrole),this concentration is 1.3 × 1017 spin/g (ΔН = 8.9 G)[144].

CONCLUSIONS

Studies of the polymerization of N�vinylpyrrolespersisted in the recent decade, being largely performedin the laboratory of the authors of this review. This ten�dency was stimulated by the increasing accessibility ofinitial monomers obtained from ketones (ketoximes)and acetylene via one step preparation in the superba�

sic catalytic systems MtOH–DMSO (Mt = Li, Na, K,or Cs). In this case, an important circumstance is thepossibility to widely vary substituents in the pyrrolering of N�vinylpyrroles and, hence, the possibility tovary the reactivity and properties of the resulting poly�mers.

The specific feature of the polymerization ofN�vinylpyrroles (both radical and cationic) is thatlow�molecular�mass oligomers containing on average10–20 monomer units are formed. This effect may beexplained by a decline in the activity of growing radi�cals that is related to partial transfer of spin density onthe pyrrole ring (in the case of radical polymerization)or capture of primary cations by the pyrrole part of amolecule. In this case, either chain transfer to themonomer occurs or the chain is terminated owing tothe radical or cationic attack of neighboring pyrrolerings accompanied by the formation of new cyclicstructures.

In the case of cationic polymerization, the result�ing immonium cation attacks the next monomer mol�ecule in the α position of the pyrrole ring rather thanat the N�vinyl group. Eventually, dimers predomi�nantly form and the corresponding oligomers belongto the previously unknown group of polypyrroles, inwhich pyrrole rings alternate with ethylidene frag�ments.

Thus, the polymerization of N�vinylpyrroles (radi�cal, cationic, and anionic) first opened a simple way topreviously inaccessible oligopyrroles, which in fact arehighly reactive macromonomers capable of furtheroxidative polymerization via pyrrole rings accompa�nied by the formation of conducting polypyrroles withmacronetwork or carcass structures. The unexpectedproperty of oligo(N�vinylpyrroles) is that, despite lowmolecular masses and the absence of classical poly�conjugation, they possess paramagnetism and con�ductivity, which can be increased by several orders ofmagnitude via doping with electron acceptors, forexample, elementary iodine, which in some casesform covalent bonds. All these circumstances have

R2

R1

N

R2

R1N

R2

R1N

nl m

460


MIKHALEVA et al.

opened new possibilities for the synthesis of materialswith previously unknown electro� and photophysicalproperties.

The ability to form crosslinked structures and tocure under the action of acids and/or oxidizers makesthese oligomers promising binders for conductingcompositions, and the known ability of pyrrole rings toform complexes with various transition elements willprobably make it possible to use these oligomers as softmultimodal ligands in the design of catalysts with con�trollable activity and as selective sorbents of heavymetals.

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Translated by T. Soboleva

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