Makromol. Chem., Rapid Commun. 8,223 -227 (1987)

CH, OCH, COZCH3 CH, I I I I

I I I I H3C-C-CHZ-C- C-CHZ-C-CH, R

223

la,4a b C d

CHzC(CH,), CH, COzCH, CN

Structure of model radicals of propagating polymer radicals complexed with SnC14

Hitoshi Tanaka*, Hirofumi Kato, Itsuo Sakai, Tsuneyuki Sato, Tadatoshi Ota

Department of Applied Chemistry, Faculty of Engineering, Tokushima University, Minami Josanjima 2-1, Tokushima 770, Japan

(Date of receipt: January 26, 1987)

Introduction

The effect of Lewis acids, such as ZnCh, BCI, , and SnCh , on the reaction rate and selectivity in a number of radical polymerization systems was examined, and also the effects on the acceleration and formation of alternating sequences were observed'-3). It is well-known that the coordination of a charged functional group to Lewis acids is the first step of promotion of such reactions, and the resulting complexed radical has been anticipated to play an important role in the peculiarity of the reaction. So far, a mode of coordination or a concrete role of the Lewis acids, however, has not been defined in detail yet, because of the unstability of a complexed radical.

In the preceding papers4-@, we have been able to detect the complexed radical intermediate directly by using a persistent captodative substituted radical, model radical of a propagating radical, by means of ESR spectroscopy, and clarified the variation of spin population and stereochemical conformation of the radical with the complexation with SnCI, . This communication deals with the ESR studies on some

R' 'CH-CN

R2'

R' 'CH-CO~CH,

CH, 3a: R' = R2 = CH,

R'/ 2

CH, R' 7H3 7cz.H~ j N 7H3 I

R' C'O,CH,

I H,C-C-CHz-C-C-CH C CH3 '- I- 1 = 2 H C C-CHZ-C*

- 1 I I I I CN CN SCzHs CN

4 5

0173-2803/87/$01 .OO

224 H. Tanaka, H. Kato, I. Sakai, T. Sato, T. Ota

complexed radicals with SnCh and binary systems of SnCh /metal salt. Furthermore, the stoichiometry and stability of the complexed raidicals are also examined by using the compounds shown above by means of NMR and IR spectroscopies. The result may be helpful in understanding the role of Lewis acids in radical polymerizations.

Experimental part

Materials: Our previous method6) and similar methods were adopted in the syntheses of 1 a and 1 b, l c , Id, respectively. la: colorless viscous liq. I H NMR (CDCI, ): 0,92 (s; 6H), 0,99 (s; 18H), 1,05 (s; 6H), 1,28 (s; 4H), 2,Ol (d; 2H), 2.39 (d; 2H), 3,68 (s; 6H), 3,73 ( s ; 6H).

l c : colorlessviscousliq. ' H NMR (CDCI,): 1,07(s; 6H), 1,15 (s; 6H), 2,10(d; 2H), 2,71 (d; 2H), 3,56 (s; 6H), 3,63 (s; 6H), 3,72 (s; 6H).

Id: whiteneedles, m.p. 113,5-115,5"C.'HNMR(CDC1,): 1,32(s;6H), 1,42(s;6H),2,20 (d; 2H), 2,40 (d; 2H), 3,64 (s; 6H), 3,76 (s; 6H).

2a, 2b, and 3 a (from Wako chemicals) were purified by distillation just before use. 2c was prepared according to a procedure described by Fuson and Wojcik'). Colorless liq.; b. p. 50°C/133 mbar. ' H NMR (CDCI,): 1,38 (d; 3H), 3,34 (s; 3H), 3,70 (s; 3H), 3,84 (9; 1 H).

3 b was prepared by the reaction of 4,5-diethylthio-2,7-dimethyl-2,4,5,7-octanetetracarbo- nitrile (5),) (400 mg) and I-pentanethiol(2 ml) at 100- 1 10°C for 20 h in a degassed ampoule. Colorless liq.; b.p. 150°C/13 mbar. ' H NMR (CDCI,): 1,34 (t; 3H), 1,45 (s; 3H), 1,50 (s; 3H), 1,90-2,30 (m; 2H), 2,82 (9; 2H), 3,72 (d-d; 1 H).

Decomposition reactions: Decompositions of 1 a - 1 d were carried out in an ESR cavity in a similar manner as reported6). ESR spectra were recorded on a JEOL JES-FE2XG spectrometer equipped with an X-band microwave unit and 100 kHz field modulation. Hyperfine splitting constants (a) were determined by comparison with those of Fremy's salt in K2C03 aqueous solution.

The determination of the equilibrium constant ( K ) by means of IR spectroscopy was carried out according to the procedure by Brown and Kubotas), according to which the K value was calculated by estimation of the change of the absorbance of the nitrile peak (2 230 - 2250 cm- I ,

or the carbonyl peak (1 720 - 1 750 cm - ) after the addition of SnCI, . Samples were prepared by adding SnCI, to the chloroform solution containing given amounts of the nitrile or the methyl ester in a drybox. IR spectra were recorded on a JASCO A-102 IR spectrometer applying a NaCl solution cell (length: 0,025 mm) at ambient temperature.

Samples for NMR measurements were also prepared in CDCI, solution in a drybox. ' H and I3C NMR spectra were obtained using JEOL GX-400 (400 MHz), Hitachi R-24B (60 MHz) or Hitachi R-42F"T (22,6 MHz) NMR spectrometers, respectively, in CDCI, at ambient tempera- ture.

Tetramethylsilane was used as internal reference.

Results and discussion

Dimers l a - Id are stable under UV light irradiation, but dissociate to the corre- sponding radicals 4a-d upon heating. Their ESR spectra can be easily observed within a temperature range of 1 10 - 200 "C.

It is of interest that SnCl, induces the dissociation of l a and similarly also of l b , i.e., even at ambient temperature. The ESR spectrum of radical 4a complexed with SnCl, is shown in Fig. 1. The following ESR parameters for the complexed radical are observed: %-H = 10,2 and 7,94, C Z . ~ ( O M ~ ) = 2,45, and a6.H (COOMe) = 2,12 G.

Structure of model radicals of propagating polymer. . . 225

Comparison with the free (uncomplexed) radical [(A) in Fig. 11, which has an equivalent $.H value of 10,3 G, indicates the restricted rotation of the Q-q bond in the complexed radical.

-10 G-

Fig. I . ESR spectra of 4 a in the absence (A) and presence (B) of SnC1, in 1,2,4-trichlorobenzene at 180 "C and in chlorobenzene at 2 4 T , respectively. [ l a ] = [SnCl,] = 1 3 mol . 1 - ' . Micro- wave power: 0,4 mW; modulation amplitude: 0,05 G

EtAlCI, also accelerates the dissociation of l a , and the ESR spectrum of the complexed radical can be detected at ambient temperature, but a conformational change is not observed, i.e., equivalent P-hydrogen, %-H = 8,81 G at 24°C. Other Lewis acids such as ZnC1, and AlEt, show only a negligible or small effect, and side reaction occurs in the presence of BCl, .

Fig. 2. shows the ESR spectra of radical 4b complexed with SnC1, and with the binary system SnCI,/TiCl, (mole ratio 1 :0,3). It is apparent that the addition of TiCl, reduces the concentration of the radical and relatively weakens the intensity of the central two lines of the spectrum.

Fig. 2. ESR spectra of 4 b in the presence of SnCl, (A) and SnC1, /TiCl, (B) in chlorobenzene at 24°C. [ lb ] = [SnCl,] = 1 mol-1- ' and [TiCl,] = 0,3 mol . 1 - ' . Microwave power: 0,4 mW; modulation amplitude: 1 G

226 H. Tanaka, H. Kato, I. Sakai, T. Sato, T. Ota

Such change of the line shape of the central two lines seems to suggest that the %.H values approach similar ones observed in the spectrum of radical 4 b complexed with SnCl, at higher temperature6). When other metal salts such as VOCl, or larger amounts of TiCl, are applied as co-metal salts, no complexed radical is observed in the ESR spectrum.

In contrast to l a and l b , l c and Id do not give any ESR spectrum, even in high concentrations of SnCl, at ambient temperature. In order to confirm such difference, the structure of the complexes was examined by means of NMR and IR spectroscopy.

Fig. 3 shows the variation of the ' H NMR spectra of l c with the addition of SnCl, . It is clear from this Figure that only two ester methyl protons at the outer positions are shifted significantly (K,I = 8,0 l/mol in CHCl,), whereas the shift of other protons is negligible.

Fig. 3. Variation of the I H NMR spectra of Ic with the addition of SnC1, in CDCl, at 24 "C. Mole fraction of SKI, : 0,05 (A), 0,47 (B), and 0,81 (C). [Ic] = 0,3 mol. l - '

6 in ppm 6 in ppm 6 in ppm

Steric hinderence may affect the complex formation. A similar tendency was also observed in the I3C NMR spectra of the system of 1 d and SnCl, , although a detailed study was not possible because of the poor solubility of the complex in an organic solvent. Consequently, lack of the complex formation of l c and I d seems to result in a negligible dissociation of the dimers.

Tab. 1 summarizes the equilibrium constants K, , and K12 of the complexes, where K,, = [AB]/([A] + [B]) and KI2 = [ABJ/([AB] + [B]), A: SnCl,, B: nitrile of methyl ester (2 and 3) in the following equations:

K I I K I I A i B -====T AB, A B t B - A%

In all cases, the values of Kl I are larger than those of K 2 , and especially 2b and 2c have large KII and very small K12 values. This implies that 2b and 2c form a 1 : 1 tight

Structure of model radicals of propagating polymer. . . 227

Tab. 1 . compounds and SnCI, in CHCI, at ambient temperature

Equilibrium constants (KII and K 1 2 ) of the complexes between hydrogenated model

Model compound K , I /(l/mol) K12 /(l/mol)

2a 2b 2c 3a 3b

5,2 (4,0)a) 28,2 36,5 4,6 (7,0)a) 1 3

a) Data from ref.9).

complex with SnCl, preferentially, which conforms to the ESR study previously reported6). The small Kl I value for 3 b may be explained by dipolar and steric repul- sions between the two complex sites R-CN-SnC1, . Such repulsion seems to become very important considering a penultimate effect in the propagation reaction of a complexed polymer radical, since even uncomplexed cyano groups show a fairly strong penultimate effect in addition reactionsio). The behavior of l c , Id, 3b, and tetranitrile 5,) in the complex formation also indicates the importance of the structure of a penultimate unit in a termination reaction, i. e., dipolar-dipolar repulsion and steric compression between neighboring cyano or carbonyl complex moieties seem to prevent the termination, especially combination, of propagating complexed radicals.

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*) M. Hirooka, Pure Appl. Chem. 53, 681 (1981) 3, H. Hirai, K . Takeuchi, M. Komiyama, J . Polym. Sci., Polym. Chem. Ed. 23, 901 (1985) 4, H. Tanaka, T . Ota, J. Polym. Sci., Polym. Lett. Ed. 23, 93 (1985)

H. Tanaka, Y. Yasuda, T. Ota, J . Chem. SOC., Chem. Commun. 1986, 109 6, H. Tanaka, I. Sakai, T. Ota, J. Am. Chem. SOC. 108, 2208 (1986) 7, R. C . Fuson, B. H . Wojcik, Org. Synth., COIL Vol. 2, 1950, p. 260

T. L. Brown, M. Kubota, J . Am. Chem. SOC. 83, 331 (1961) 9, B. Yamada, Y. Kusuki, T. Ota, Kogyo Kagaku Zasshi, 72, 364 (1969)

lo) S. A. Jones, G . S. Prementine, D. A. Tirrell, J. Am. Chem. SOC. 107, 5275 (1985)