Electrochemically initiated polymerization of vinyl-substituted five-membered nitrogen-containing aromatic heterocycles

Polymer Science U.S.S.R. Vol.32, No. 1, pp. 150-154,1990

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ELECTROCHEMICALLY INITIATED POLYMERIZATION OF VINYL-SUBSTITUTED FIVE-MEMBERED NITROGENCONTAINING AROMATIC HETEROCYCLES* V. A . LOPYREV, T. N. KASHIK, T. G . ERMAKOVA, E . I. BRODSKAYA, L. G . KOLZUNOVA a n d N. YA. KOVARSKII Irkutsk Institute of Organic Chemistry, Sibir Branch, U.S.S.R. Academyof Sciences;and Institute of Chemistry, Far-Eastern ScientificCentre, U.S.S.R. Academyof Sciences (Received 2 A ugust 1988)

Electrochemically initiated polymerization of 1-vinylpyrrole, 1-vinylimidazole,and 1-vinyl-l,2,4-triazole was studied. Normal polymerization of the vinylic double bond was found to proceed at low current densities. A critical current density exists for each monomer at which the heterocyclic ring begins to be cleaved; the critical current density increases in the series pyrrole < imidazole< 1,2,4-triazole.Copolymers are formed at current densities exceeding the critical value; they also contain--in addition to normal monomeric units----polyconjugatedsegments resulting from the cleavage of the heterocyclicring. ELECTROCHEMICALLY initiated polymerization (ECIP) of vinyl-substituted nitrogen-containing aromatic heterocyclic compounds has not yet been studied. A single paper exists which deals with ECIP of 1-vinylimidazole on a germanium cathode, proceeding at a current density of 520 mA/ cm 2 [1]. The authors have found that, in addition to a product formed by polymerization of the vinylic double bonds, a polymer with polyconjugated structure is also formed by cleavage of the heterocycle,

L--CH--CH',--

,~

CH----CH2

I

I1

The product of vinyl group polymerization was claimed to be separated [1] by precipitation with water from a concentrated acetone solution. This observation aroused our suspicion since compound I is known to be readily soluble in water [2, 3]. We therefore decided to study in more detail ECIP of 1-vinylimidazole and of some other vinyl-substituted five-membered nitrogencontaining aromatic heterocycles. 1-Vinylimidazole (VI), 1-vinylpyrrole (VP), and 1-vinyl-l,2,4-triazole (VT) were prepared by methods described in Refs [4--6]. Poly(1-vinylimidazole), poly(1-vinylpyrrole), and poly(1-vinyl1,2,4-triazole) standards were prepared by radical polymerization according to Refs [2, 7, 8]. Electrolysis of the monomer, in which lithium perchlorate was dissolved, was carried out at ambient temperature, using the potentiostat P-5827M and a single-cell glass electrolytic cell. A germanium plate (10 cm 2) was the cathode, the anode was made of platinum. The polymers were isolated by precipitation: PVI and PVT with acetone, PVP with hexane. The precipitate was filtered off and

*Vysokomol. soyed. A32: No. 1,149--153, 1990. 150

Electrochemically initiated polymerization

151

vacuum-dried at room temperature to constant weight. IR spectra were registered on a Specord IR-75 spectrophotometer (KBr pellets, 2-10 mg per 800 mg of KBr). UV spectra of prepared polymers dissolved in dimethylformamide ( c - - 3 % ) were registered on a Specord UV-VIS spectrophotometer in the region 260-450 nm. The polymers were fractionated at 20°C by fractional dissolution [9] in dimethylformamide (solvent)-acetone (non-solvent). When the experimental conditions described in Ref. [1] were exactly reproduced, a brownish product was isolated by precipitation with acetone; this polymer was previously considered to have structure II. However, all attempts to isolate any other polymer by precipitation of concentrated acetone solutions with water, as described in Ref. [1], proved to be unsuccessful. IR spectra of the precipitate insoluble in acetone exhibited bands characteristic of the imidazole ring (920 cm-1), vinyl groups (970 and 1640 cm-1), and also a band typical for a polyconjugated structure (1600-1700 cm-1). The simultaneous presence of free vinyl groups and of intact imidazole rings indicates that the product is either a mixture of two homopolymers or a copolymer containing both types of units. To resolve this problem we fractionated the precipitate into four fractions after carefully washing it with water, the assumption being that polymer I from a mixture with polymer II will dissolve in water. IR spectra proved the existence of both units (I and II) in all four fractions; thus, the product is a copolymer having the structure

[ Ill

UV spectra of solutions of these copolymers showed a long-wavelength tail (extending up to 450 nm), similar to that found in the spectra of polyenes [10]. The content of type I units in the copolymer was calculated from the intensity of the band of the imidazole ring (920 cm-1), using as standards samples known to contain only structure I units. The content of type I units in the copolymer is

DcPs x 100%, DsPc

A = --

where Dc and Ds are the respective optical densities of characteristic IR bands of the copolymer and the standard, Pc and Ps are the corresponding weights (2-10 mg). The relative error was +5%. The data collected in Table 1 show that fractionation has essentially no effect on sample T A B L E 1.

C O N T E N T OF TYPE

I STRUCTURES

IN I-VINYLIMIDAZOLE COPOLYMERS PREPARED BY

ELI~CTROCHEMICAL INITIATION

(i = 750 mA/cm2, r = 15 h) Copolymer

Content of type I structures, % (band at 920 cm- l)

PVI, non-fractionated

62

PVI, fractions:

60 60 56 58

1

2 3 4

152

V . A . LOPYREVet al.

TABLE 2.

CONTENTOF TYPEIV STRUCTURESIN 1-VINYLPYRROLEAND I-VINYL-1,2,4-TRIAZOLECOPOLYMERS (i = 1000 mAJcm2, ¢ = 15 h) Content of type IV structures, %

Copolymer

from the band at 870 cm -1

from the band at 1000 cm -1

PVT, non-fractionated

80

80

PVT, fractions:

75 76 76 80 35*

75 74 74 80 35t

1 2 3 4 PVP, non-fractionated * From the band at 708 cm- L. tFrom the band at 1300 cm -1.

composition, thus confirming that a copolymer is indeed formed. Electrochemically initiated polymerization of monomers VT and VP led to analogous results (Table 2). It is thus apparent that electrochemically initiated polymerization of VI, VP, and VT at current density 500 mA/cm 2 results in the formation of copolymers containing structural units of two types:

tl I I Y\ NI CH=CH2

d- e[--Y=CH--X=:CH--NI k --CH--CH2--_J,t IV

(VI: X - - N , Y = C H ;

]

CH=CH2 m V

VP: X ~ - Y - - - C H ; VT: X ~ - Y - - - N ) .

The yield P of copolymers is plotted in Fig. 1 against the current density i and the time of electrolysis t. The yield rises with increasing i and t, being always higher for VP than for VI or VT. The relative content of monomeric units in the copolymers is seen to be independent of time; on the other hand, the current density is a factor decisive for the copolymer composition. Figure 2 shows that the content of type IV units decreases with increasing current density. At the same current density the pyrrole ring is cleaved more readily than the imidazole or the triazole ring, in accord with the changes in aromaticity of the rings, which increases in the series pyrrole < imidazole < 1,2,4-triazole [11]. Note that a critical value of current density corresponding to the onset of ring cleavage exists for each monomer. Thus, electrochemical initiation of VI polymerization with current densities below 350 mA/cm 2 leads to homopolymer I. Copolymer III is formed at current densities between 350 and 1750 mA/cm 2, whilst at still higher values of current density black, insoluble products are formed with IR spectra resembling those of compounds with graphite-like structure [12]. The formation of these products is due to secondary reactions of the pendent vinyl groups. Analogous results have been obtained with VP: homopolymer IV is formed below the current density of 250 mA/cm 2, whilst initiation with current densities between 250 and 1500 mA/cm 2 results in the formation of copolymers; thereafter the product is a black, insoluble mass. Homopolymer IV is formed from VT up to i = 450 mA/cm 2 (Fig. 3a); initiation with i = 450-2000 mAJcm 2 leads to

E l e c t r o c h e m i c a l l y initiated p o l y m e r i z a t i o n

153

~ g

t :.0-

I ,

500

1500 2500

2

Io

20

FIG. I. Yield of copolymers prepared from 1-vinylpyrrole (1), 1-vinylimidazole (2), and 1-vinyl-l,2,4triazole, plotted against the current density (a) and against the duration of electrolysis (b).

Iv, %

/00 ~~x~ 00

2o 500

1500

vx10-2,

2300

cm

7

i, m A / c m 2

Fxo. 2.

FIG. 3.

FiG. 2. The effect of current density on the content of type IV monomeric units in copolymers prepared from 1-vinylpyrrole (1), l-vinylimidazole (2), and l-vinyl-l,2,4-triazole (3). FIG. 3. IR spectra of polymers prepared from 1-vinyl-l,2,4-triazole: (a) type IV homopolymer, i = 450 mA/cm2; (b) copolymer containing units IV and V, i = 1000 mA/cm2, (c) graphite-like structure, i = 2000 mA/cm2.

c o p o l y m e r s (Fig. 3b), whilst still higher current densities cause the f o r m a t i o n of graphite-like p r o d u c t s (Fig. 3c). Thus, by varying the current density o n e can control the p o l y m e r i z a t i o n process and change the ratio of individual m o n o m e r i c units in the c o p o l y m e r ; i n c o r p o r a t i o n of either of t h e m can be s u p p r e s s e d entirely. Translated by M. KUBff~

REFERENCES 1. A. L. TRIFONOV, J. SCHOPOV, D. J. KOLEV and J. TSONOV, Compt. Rend. Acad. Bulg. Sci. 28: 767, 1975. 2. G. G. SKVORTSOVA, E. S. DOMNINA, N. P. GLAZKOVA, Yu. N. IVLEV and N. N. CHIPANINA, Vysokomol. soyed. AI4: 587, 1972 (Translated in Polymer Science U.S.S.R. 14: 3,660, 1972). 3. A. I. KOKORIN, A. S. POLINSKII, V. S. PSHEZHETSKII, N. P. KUZNETSOVA, T. G. ERMAKOVA, V. A. LOPYREV and V. A. KABANOV, Vysokomol. soyed. A27, 1834, 1985 (Translated in Polymer Science U.S.S.R. 27: 9, 2060, 1985).

154

E . M . ANTIPOV et al.

4. M. F. SHOSTAKOVSKII, G. G. SKVORTSOVA, N. P. GLAZKOVA and E. S. DOMNINA, Khimia geterotsykl, soyed. No. 6, p. 1070, 1969. 5. B. A. TROFIMOV, A. I. MIKItALEVA, S. E. KOROSTOVA, A. N. VASIL'EV and A. N. BALABANOVA, Khimia geterotsykl, soyed. No. 2, p. 213, 1977. 6. L. P. MAKHNO, T. G. ERMAKOVA, E. S. DOMNINA, L. A. TATAROVA, G. G. SKVORTSOVA and V. A. LOPYREV, U.S.S.R. Pat. 464,584. 7. B. A. TROFIMOV, T. T. MINAKOVA, G. A. TANDURA, A. I. MIKHALEVA and S. E. KOROSTOVA, Vysokomol. soyed. B22: 103, 1980 (Not translated in Polymer Science U.S.S.R.). 8. L. A. TATAROVA, T. G. ERMAKOVA, V. A. LOPYREV, N. F. KEDRINA, E. F. RAZVODOVSKII, A. A. BERLIN and N. S. ENIKOLOPOV, U.S.S.R. Pat. 647,310. 9. Fraktsionirovanie polimerov (Polymer Fractionation), M. M. Cantow (Ed.), p. 61. Moscow, 1971. 10. I. A. DRABKIN, V. I. TSARYUK, M. I. CHERKASHIN, P. P. KISILITSA, M. G. CHAUSER, A. I. CHIGIR' and A. A. BERLIN, Vysokomol. soyed. AI0: 1727, 1968 (Translated in Polymer Science U.S.S.R. 10: 8, 1998, 1968). 11. A. F. POZHARSKII, Khimia geterotsykl, soyed. No. 7, p. 867, 1985. 12. Prikladnaya infrakrasnaya spektroskopia (Applied Infrared Spectroscopy). D. M. Kendal, (Ed.), p. 376. Moscow, 1970.

PolymerScience U.S.S.R. Vol.32, No. 1, pp. 154-161,1990

Printed in GreatBritain.

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STRUCTURE OF POLYETHYLENE IN ORIENTED BINARY BLENDS ANNEALED ABOVE ITS MELTING TEMPERATURE* E . M. ANTIPOV, S. A . KUPTSOV, S. A . BELOUSOV a n d E . V. KOTOVA A. V. Topchiev Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences (Received 8 August 1988)

Structure of polyethylene dispersed by various methods (during synthesis, via a solution or a melt) in a matrix of another polymer [polypropylene, poly(tetrafluoroethylene), polystyrene] was studied by smalland wide-angle X-ray diffraction, by electron microscopy and DSC. The ability of the matrix to impart orientation to dispersed polyethylene (PE) during its crystallization depends on the nature of the second polymer, on the interaction between the two components, and on the dispersity of PE in the blend. All these factors influence the type of texture appearing in the material after annealing and cooling as well as the mechanism of PE crystallization. IT HAS been already shown [1-3] that oriented binary polyethylene ( P E ) - p o l y p r o p y l e n e (PP) blends p r e p a r e d by various means (during the synthesis, via a solution or a melt), annealed above the PE melting t e m p e r a t u r e , preserve the initial orientation of PE chains, provided the interaction between the dispersed material and the PP matrix is sufficiently large. Similar effects, although less well pronounced, were observed also in blends PE-poly(tetrafluoroethylene) (PTFE) and P E polystyrene (PS) [4, 5]. The necessary intimate contact between the components was mediated in some cases [1-3] by a di-block copolymer which participated by its respective blocks in crystallites of the two components, in other instances [4-7] it was achieved byradiation-induced crosslinking of the *Vysokomol. soyed. A32: No. 1,154-161, 1990.