radical polymerization of ring-substituted trans-cinnamonitriles and formation of ladder spin...

Macromol. Chem. Phys. 195, 601-608 (1994) 60 1

Radical polymerization of ring-substituted trans-cinnamonitriles and formation of ladder spin polymer

Hitoshi Tanaka *, Yoshimasa Kikukawa, Maki Sataki, Tsuneyuki Sato, Tadatoshi Ota

Department of Chemical Science and Technology, Faculty of Engineering, Tokushima University, 2-1 Minamijosanjima, Tokushima 770, Japan

(Received: March 29, 1993; revised manuscript of May 27, 1993)

SUMMARY: The polymerization of ring-substituted trans-cinnamonitriles has been studied with radical

initiators such as peroxides and azo compounds at 60- 130 "C. 3,4,5-Trimethoxycinnamonitriles (3,4,5-MOCN) yield a homopolymer with a molecular weight of about lo4 in contrast to the unsubstituted cinnamonitrile. The activation energy of the 3,4,5-MOCN polymerization is 106 kJ/mol and the polymerization rate is proportional to the 0,45th power of the initiator concentration. Radical copolymerization of 3,4,5-MOCN yields copolymers with vinyl monomers such as styrene and acrylonitrile. ESR studies suggested that the primary radical from an azo initiator attacks at the olefinic carbon of the cinnamonitrile bearing the cyano group. Heating of the 3,4,5-MOCN homopolymer produces a ladder spin polymer with hydroaniline units in the main chain.

Introduction

It has been reported that fi-substituted acrylates such as dialkyl fumarates can homopolymerize by a radical initiator I ) . However, it is known that fi-substituted styrenes such as fi-alkylstyrenes 2, and stilbene 3, give no homopolymer by radical initiators because of steric hindrance. For example, only a slight amount of stilbene is incorporated in the end of the polymer chain when an initiator giving an oxygen- centered primary radical is used in the copolymerization3). It is also well known that cinnamic compounds such as unsubstituted cinnamonitrile, cinnamic acid, and its ester easily undergo anionic polymerization, but they can not give a homopolymer by a radical initiator 4,5).

In the course of our work on the polymerization of push-pull olefins, namely, capto- dative-substituted (geminal substitution by both electron-withdrawing and donating groups on the carbon) olefins6s7), we found that vicinally push-pull substituted olefins such as 3,4,5-trimethoxycinnamonitrile (3,4,5-MOCN) can give a homopolymer by both azo- and peroxy-initiators. The present paper describes the radical polymerization of ring-substituted cinnamonitriles.

In addition, organic spin polymers have recently attracted much interest, not only as catalysts and organic conductors but also as organic ferromagnets*). In the present paper we also deal with the formation of a ladder spin polymer by thermal cyclization of the 3,4,5-MOCN polymer.

0 1994, Hiithig & Wepf Verlag, Basel CCC 1022-1352/94/$08.00

602 H. Tanaka, Y. Kikukawa, M. Sataki, T. Sato, T. Ota

Experimental part

Alkyl- and alkoxxy-substituted cinnamonitriles were prepared by condensation of appro- priately substituted benzaldehyde and acetonitrile or deuterated acetonitrile according to the method used for the synthesis of unsubstituted cinnamonitrile ’). Commercially available 4-di- methylamino- and 4-methylthiocinnamonitriles were purified by recrystallization and distillation, respectively. 4-Chloro- and 4-cyano-substituted cinnamonitriles were obtained by reaction of cyanoacetic acid and the correspondingly substituted benzaldehydes following the thermal decarboxylation route of Kobuke et The physical properties of the cinnamonitriles used in this study are summarized in Tab. 1. Initiators such as dimethyl 2,2’-azobisisobutyrate (MAIB), 2,2’-azobis(2,4,4-trimethylpentane) (ATMP), 2,2’-azobis(2-methylpropane), 1 , l ’-azobis( 1 q c l o - hexanecarbonitrile) (ACCN), and di-tert-butyl peroxide (mBP) were commercially available and purified by ordinary methods.

Tab. 1. Physical properties of cinnamonitrilesa)

Monomer R2 R3 R4 R5 R6 m.p. Elemental analysis b, in “C

C H

3,4,5-MOCN H OMe OMe OMe H 95-96 65,56(65,74) 5,97(5,98) 2,3,4-MOCN OMe OMe OMe H H 53-54 65,62 (65,74) 6,Ol (5.98) 2,4,6-MOCN OMe H OMe H OMe 129- 132 65,60 (65,74) 5,93 (5,98) 3,5-MOCN H OMe H OMe H 105-107 70,02 (69,83) 5,96 (5,86)

2-MOCN OMe H H H H 110‘) 75,34 (75,45) 5,79 (5,70) 4-MOCN H H OMe H H 62-63 75,48 (75,45) 5,80 (5,70)

4-MSCN H H SMe H H 76-77 68,62 (68,53) 5,16 (5,18) 4-MACN H H NMe, H H 168-169 76,58 (76,71) 6,99 (7,02) 4-MeCN H H Me H H 69-72 83,76 (83,88) 6,30 (6,34) 4-CNCN H H CN H H 220 78,06 (77,91) 4,16 (3,92)

3,4-MDCN H -0CH20- H H 89-91 69,48 (69,36) 4,11 (4,07)

4-BOCN H H OBU H H 40-41 77,55 (77,58) 7,32 (7,51)

4-ClCN H H C1 H H 84-85 66,12 (66,07) 3,90 (3,70) UCN H H H H H 20-21 83,74 (83,69) 5,47 (5,46)

R6. .R’

NC-CH = C H G - R 4

R2 R3

b, Calculated values in parentheses. ‘) Boiling point at 3 mmHg.

Polymerization was carried out in a sealed ampoule with shaking at a given temperature. The ampoule with the required amounts of reagents was degassed several times by the freeze-thaw method and then sealed under vacuum and placed in a constant temperature bath. The resulting polyincr was isolated by pouring the contents of the ampoule into a large amount of methanol. Electron spin resonance (ESR) spectrum was recorded on a JEOL FE-2XG spectrometer equipped with an X-band microwave unit and 100 kHz field modulation. The g-value was determined by comparison with that of Fremy’s salt in an aqueous solution of K,C03. The ‘H NMR spectrum

Radical polymerization of ring-substituted trans-cinnamonitriles . . . 603

was measured on a JNM-EX400 (400 MHz) spectrometer. Number- and weight-average molecular weights (a, and a,,,) of the polymers were determined by size exclusion chromatography (SEC) using a Toyosoda HLC-802A (calibration with standard polystyrenes) in tetrahydrofuran at 38 "C.

Results and discussion

Polymerizations of cinnamonitriles by radical initiators are summarized in Tab. 2. It is obvious from this table that some of the methoxy-substituted cinnamonitriles,

Tab. 2. Radical polymerization of ring-substituted cinnamonitriles at various temperatures for 10 h

Monomer Initiator a) Solvent b, Temp. Yield in "C in 070

3,4,5-MOCN

2,3,4-MOCN

2,4,6-MOCN

3,5-MOCN

3,4-MDCN 4-IMOCN

2-MOCN

4-BOCN

4-MSCN

4-MeCN

4-MACN 4-CNCN 4-ClCN

UCN

MAIB ATMP ATMP ACCN DTBP None MAIB DTBP MAIB DTBP MAIB ATMP DTBP MAIB MAIB ATMP DTBP MAIB DTBP MAIB DTBP MAIB MAIB MAIB DTBP MAIB MAIB MAIB DTBP MAIB DTBP

Dioxane None None None None None Dioxane None Dioxane None Dioxane None None Dioxane Dioxane None None Dioxane None Dioxane None Dioxane Dioxane Dioxane None Dioxane Dimethylformamide Dioxane None Dioxane None

60 120 130 105 130 130 80

130 80

130 80

120 130 80 80

120 130 80

130 80

130 60 80 60

130 80 80 80

130 80

130

a) MAIB: Dimethyl 2,2'-azobisisobutyrate, ATMP: 2,2'-azobis(2,4,4-trimethylpentane), ACCN: 1 ,1 '-azobis( 1 -cyclohexanecarbonitrile), DTBP: di-fert-butyl peroxide. [Initiator] = 1,3 * 10 -2

mol/L. [Monomer] = 1 3 mol/L for solution polymerization. 2,


namely, 3,4,5-MOCN, 3,5-MOCN, 4-MOCN, 4-BOCN, and 4-MSCN, undergo a radical homopolymerization. Especially, 3,4,5-MOCN can polymerize to give a homopolymer in higher yield than the other methoxy and methylthio analogues by both azo and peroxy initiators, and the yield increases with increasing temperature. However, SEC measurements on the polymerization mixture indicated that the other cinnamonitriles gave no long-chain polymer and produced only small molecules, and methyl 3,4,5-trimethoxycinnamate also does not give a homopolymer under experimental conditions similar to those of 3,4,5-MOCN. It has been reported that stilbene is very reactive toward oxygen-centered radicals but very unreactive toward carbon-centered radicals such as the 2-cyano-2-propyl radical3). This is contrary to the behavior of 3,4,5-MOCN, i.e., 3,4,5-MOCN can react even with the carbon radicals produced by the decomposition of the azo initiators to give a homopolymer. Smaller steric hindrance of 3,4,5-MOCN than in case of methyl 3,4,5-trimethoxycinnamate and stilbene seems to be the reason of such reactivity.

'H NMR spectrum of poly(3,4,5-MOCN) showed broad absorptions around 3,7 and 6,2 ppm due to the methoxy and o-protons on the benzene ring, respectively, but no peaks at 5,71 and 7,24 ppm due to the olefinic protons of the monomer, suggesting the polymerization proceeded through the opening of the C=C double bond of the cinnamonitrile. The molecular weights of several cinnamonitrile polymers obtained are listed in Tab. 3. m,, of poly(3,4,5-MOCN), i.e., 9500, corresponds to a degree of polymerization of about 43, and it did not vary with the reaction time. The latter fact supports the steady state polymerization of 3,4,5-MOCN. Mn and the yield of poly(3,4,5-MOCN) obtained at 130 "Care higher than those obtained at 105 "C, as can be seen in Tabs. 2 and 3, suggesting the reaction temperature is an important factor for the homopolymerizability.

To confirm the polymerization mode of 3,4,5-MOCN, the dependence of the polymerization rate on the initiator concentration and the reaction temperature was examined using ACCN as an initiator. Fig. 1 shows time-conversion curves for the polymerization of 3,4,5-MOCN in dioxane at 80- 105 "C. The conversion increases with increasing reaction time and temperature. Fig. 2 represents the dependence of the polymerization rate on temperature. From this figure the overall activation energy of the polymerization was estimated to be 106 kJ/mol, which is similar to the value of a

Tab. 3 . Molecular weight of poly(cinnamonitri1e)s obtaineda)

Monomer Polymerization I o - 3 . a,, ~o-~.M, Mw/Gn temp. in "C

3,4,5-MOCN 130 9 s 3,4,5-MOCN 105 393 3,5-MOCN 130 495 4 .MOCN 130 3.1 4-MSCN 80 3,9

a) The polymers used in this experiment are those obtained by the polymerization at the corresponding temperature shown in Tab. 2.


Fig. 1. Fig. 2.

Fig. 1. Time-conversion curves for the polymerization of 3,4,5-MOCN with ACCN at various temperatures in dioxane. (0) 105, (0) 98, ((3) 90, and (A) 80°C. [ACCN] = 1,3 * lO-’mol/L and [3,4,5-MOCN] = 1,5 mol/L

Fig. 2. temperature. [ACCN] = 1,3 .

Dependence of the polymerization rate (R,) of 3,4,5-MOCN on the polymerization mol/L and [3,4,5-MOCN] = 1 3 mol/ L in dioxane

Fig. 3. Dependence of the polymerization rate (R,) of 3,4,5-MOCN on the ACCN concentration in dioxane at 105 “C. [3,4,5-MOCN] = 1.5 mol/L

-2.0 -1.5 -1.0 loglo{[ACCNl/~mol~ L-’)]

conventional radical polymerization9). Fig. 3 shows the dependence of the polymerization rate on ACCN concentration. From the slope the polymerization rate is proportional to the 0,45th power of the initiator concentration. These facts suggest the polymerization occurs via radical mechanism with bimolecular termination. Moreover, the molecular weight of poly(3,4,5-MOCN) was much affected by the initiator concentration: a,, diminished from 3 300 to 1 900 when ACCN concentration was increased from 1,3 - to 7,O. mol/L.

Radical copolymerizations of 3,4,5-MOCN with various olefins are summarized in Tab. 4. It is obvious that 3,4,5-MOCN can copolymerize with most of the olefins except


Tab. 4. Copolymerization of 3,4,5-MOCN and olefins at 60"Ca)

Olefin

Styrene Vinyl acetate Acrylonitrile &Methylstyrene Methyl methacrylate Maleic anhydride

- Yield [3,4,5-MOCN] in copolymer M" in Vo in mol-%

18,8 31,3 0,3 27,O 2,4 19,8 0

0

- 13,6 3,4

-

66 000

2 300

30000

-

-

- a) Copolymerization was carried out in dioxane for 10 h using dimethyl azobisisobutylate

(1 . lo-* mol/L) at a mole ratio [3,4,5-MOCN]/[olefin] = 1.

a-methyl styrene and maleic anhydride, to give a copolymer containing both mono- meric units, and electron-donating comonomers such as styrene are more incorporated in the copolymer than electron-withdrawing comonomers such as acrylonitrile. The poor reactivity of 3,4,5-MOCN toward poly(methy1 methacrylate) radical is similar to that of cinnamic acid'", and it seems to be due to steric hindrance as well as to an electronic effect. That is, a large steric repulsion is expected between a,a-disubstituted radicals (such as poly(methy1 methacrylate) radical) and a,pdisubstituted monomers (such as 3,4,5-MOCN) in the propagation step. Large repulsion is further expected in the copolymerization systems of 3,4,5-MOCN with a-methylstyrene and maleic anhydride, which is in agreement with the experimental results, as shown in Tab. 4. However, 4-MOCN can copolymerize with maleic anhydride to give a copolymer with a,, = 2200 in 5,4% yield under the same conditions as in Tab. 4, in contrast to 4-BOCN and 3,4,5-MOCN. Thus, an insignificant difference in the monomer structure may affect the copolymerizability.

Fig. 4 shows the ESR spectra of the radical generated from the reaction of 3,4,5-MOCN and 2,2'-azobis(2-methylpropane) in dioxane under UV irradiation at ambient temperature. The spectrum (A) observed for 3,4,5-MOCN exhibits a doublet of doublet of triplet, which agrees with the computer-simulated spectrum (B) which is calculated by assuming a styryl type radical (Eq. (1)) with ua-H (=CH-Ar) = 15,35,

(NC-CH=) = 5,76, and u , . ~ (aromatic o-position) = 4,89 G. The spectrum (C) observed for P-deuterated 3,4,5-MOCN (NC-CD=CH-Ar) indicates a doublet of triplet, as expected. Recently, Yamada et al. ' I ) and Viehe et al. ") determined u ~ - ~ =


Fig. 4. ESR spectra of the radicals generated by the reaction of 2,2'-azobis(2-methylpropane) and (A) 3,4,5-MOCN and (C) /3- deuterated 3,4,5-MOCN in dioxane under UV irradiation at ambient temperature. Spectrum (B) is simulated for (A) by using the lineshape functions: 0,8 G linewidth and 90% Gaussian (10% Lorentzian)

17,70, = 15,50, and ao.H = 4,80 G for poly@-methoxystyrene) radical and aa.H = 15,60, = 17,25, and ao.H = 4,95 G for p-methoxyphenethyl radical, respectively. The values of au-H and ao.H obtained for 3,4,5-MOCN are similar to those of the small molecule, i. e., p-methoxyphenethyl radical. However, the value of ap.H for 3,4,5-MOCN is much deviating from that for poly@-methoxystyrene) and p- methoxyphenethyl radicals. This may be due to the reduced dihedral angle between the /3-hydrogen and the carbon pz orbital caused by the bulky 8-substituents, the tert- butyl and the cyano group.

Thermogravimetry of poly(3,4,5-MOCN) indicated that the polymer began to decompose near 277 "C, and maximum degradation rate of the polymer was observed at 364 "C. It was also found that the weight loss of the polymer was 53,4% at 364 "C, and a residue of 25,8% remained even after heating at 500°C. The IR spectrum of the residue obtained by heating poly(3,4,5-MOCN) at 350 "C for 10 min showed a peak at 1625 cm-' due to -C=N- moiety, as well as a small sharp peak at 2240 cm-' due to CN group, suggesting formation of a polymer with partial hydroaniline ladder structure, as shown in Eq. (2). The elemental analyses before and after heating the

NC H Ar Ar Ar Ar Ar I I I I I

I I A

I t -c-c- d

N Ar

(Ar: 3,4,5-trimethoxyphenyl)


Fig. 5. ESR spectrum of the radical formed in the ladder polymer

polymer at 350°C were as follows; C: 62,52, H: 5,95, N: 6,40, and C: 67,58, H: 4,18, N: 5,60%, respectively. Increase of C content and decrease of H and N contents in elemental analyses and the IR spectral change observed by heating are similar to the results obtained for poly(acrylonitri1e) 1 3 ) , which forms a hydroaniline ladder structure followed by graphitization (carbonization) by further heating. It is of interest that the ladder polymer produced shows an ESR absorption near 3290 G (g = 2,0034, line width = ca. 6 G) at ambient temperature, as indicated in Fig. 5 , and only a very small portion (< 1 Yo) is sensitive to permanent magnetism, implying formation of a domain with high spin state in the ladder structure of the polymer.

One of the authors (H. T.) acknowledges gratefully the partial support of ZCI Japan Science Foundation.

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radical polymerization of ring-substituted trans-cinnamonitriles and formation of ladder spin...

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