long-lived polymer radicals, 2. an esr study on the reactions of the propagating polymer radicals of...

Makromol. Chern. 181,2421 -2431 (1980) 242 1

Long-Lived Polymer Radicals, 2a)

An ESR Study on the Reactions of the Propagating Polymer Radicals of N-Methylacrylamide and N-Methylmethacryl- amide with Vinyl Monomers at Room Temperature

Hitoshi Tanaka, Tsuneyuki Sato, Takayuki Otsu *

Department of Applied Chemistry, Faculty of Engineering, Osaka City University, Sugimoto-cho, Sumiyoshi-ku, Osaka 558, Japan

(Date of receipt: November 13, 1979)

SUMMARY: Acrylamide (AAm), methacrylamide (MAm), N-methylacrylamide (NMAAm) and N-methyl-

methacrylamide (NMMAm) were found to yield long-lived propagating polymer radicals in the photo-sensitized polymerizations in 1 ,4-dioxane or benzene. The concentration of poly- (NMAAm) radicals reached 1 . 10- mol/l. Some post-effect was observed at room temperature in the photo-sensitized polymerization of AAm with di-tert-butyl peroxide in I ,Cdioxane, while no post-polymerization proceeded at room temperature in the polymerization of NMMAm in benzene. The reactions of poly(NMAAm) and poly(NMMAm) radicals with various vinyl monomers were found to produce long-lived propagating polymer radicals of the second monomers at room temperature. Polymer radicals of non-homopolymerizable monomers such as a-methylstyrene and 1 , I -diphenylethylene were easily formed in such a block-copolymerization matrix. The formation of the propagating polymer radicals of the vinyl monomers was investigated by means of ESR spectroscopy.

Introduction

Recently, we have preliminarily reported that long-lived propagating radicals of N- methylacrylamide (NMAAm) and N-methylmethacrylamide (NMMAm) are formed, when the amide monomers are polymerized at room temperature by di-tert-butyl per- oxyoxalate in adequate solvents such as benzene, carbon tetrachloride and 1,4-dioxane I). These propagating polymer radicals have been also reported to react easily with other vinly monomers to yield block-copolymers I).

Further, the reactions of the polymer radicals of the amide monomers with the second monomers have been found to give stable propagating polymer radicals of the second monomers at room temperature. Therefore, it is of interest to investigate the formation of the propagating radicals of vinyl monomers in such a block copolymerization matrix by means of ESR spectroscopy.

This paper is concerned with the result of the post-polymerizations observed in the photo-sensitized polymerizations of NMMAm and acrylamide (AAm) by di-tert-butyl

2422 H. Tanaka, T. Sato, T. Otsu

peroxide (DTBP), and with an ESR study on the reactions of the polymer radicals of NMAAm and NMMAm with various monomers at room temperature.

Experimental Part

N-Methylacrylamide (NMAAm) and N-methylmethacrylamide (NMMAm) were prepared by the reactions of methylamine with acryloyl chloride and methacryloyl chloride and purified by distillation. Other monomers, solvents and di-tert-butyl peroxide (DTBP) were used after usual purifications.

The reactions of the propagating polymer radicals of NMAAm and NMMAm with vinyl monomers were carried out as follows; a benzene solution (0,25 ml) of NMAAm (2,3 mol/l) or NMMAm (2,O mol/l) and DTBP (0,43 mol/l) was irradiated by a 100 W high pressure mercury lamp for about 2 h at room temperature in an ESR tube, which was degassed and sealed i. vac. To this reaction mixture, a second monomer was added by using the break-seal technique at room temperature.

After a given time, the ESR spectrum of the reaction mixture was recorded at room temperature by using a JES-ME-3X spectrometer with 100 kHz field modulation.

A 1,4-dioxane solution of acrylamide and DTBP in a degassed and sealed glass tube was irradiated by a I00 W high pressure mercury lamp and after a given time, the post-effect of this polymerization was investigated at room temperature in the dark. The post-polymerization of NMMAm was similarly carried out in benzene.

Results and Discussion

Radicals formed in the photo-sensitized polymerizations of NMAA m, NMMAm, AAm, methactylamide (MAm) and maleimide ( M h )

As reported previously I), the photo-sensitized polymerizations of NMAAm and NMMAm with DTBP are found to yield the long-lived propagating polymer radicals (1 and 2) of the monomers.

H I 7 H 3 . . . -CH,-F*

I co co I NH-CH,

1 2

. _. -CH,-C*

I NH-CH,

Fig. 1 shows the ESR spectrum of the reaction mixture obtained when the photo- sensitized polymerization of AAm was carried out in 1 ,Cdioxane at room temperature. The three line spectrum in the figure is assigned to the propagating polymer radical 3 of AAm. Further, two shoulder peaks are observed at both sides of this three line spectrum, while the propagating radical 1 of NMAAm has none of such shoulder peaks (see Fig. 6) . Appearance of such shoulders has been also reported for 3 formed in a solid state polymerization of AAm2). These shoulders may be due to a different conformation of 3.

Long-Lived Polymer Radicals, 2 2423

Fig. I Fig. 2

Fig. I . ESR spectrum of the reaction system acrylamide (AAm)/di-tert-butyl peroxide (DBPO)/dioxane after being irradiated for 3,7 h at room temperature; [AAm] = 2,8 mol/l, [DBPO] = 0,43 mol/l (SI unit: 1G = 10 T)

Fig. 2. ESR spectrum of the reaction system methacrylamide (MAm)/DBPO/dioxane after being irradiated for 4,3 h at room temperature; [MAm] = 2,4 mol/l, [DBPO] = 0,43 mol/l

Fig. 2 shows the ESR spectrum of the polymerization mixture of MAm. This spectrum is closely similar to that of 2 and assigned to the propagating radical 4 of MAm.

H I ?HI

co . . . -CH2-C . . . -CH,-F*

co I

I

NHz N H i

3 4

The polymerization of Mlm, an a,p-disubstituted monomer, was also investigated ESR spectroscopically in 1,4-dioxane. The result obtained by using DTBP as photo- sensitizer is shown in Fig. 3 . The same ESR spectrum was also observed when azodiisobutyronitrile was used as a photo-sensitizer. These findings indicate that the radical of the spectrum shown in Fig. 3 originates from the MIm monomer and may be assignable to the polymer radical (5) of MIm. As shown in the figure, this radical 5 is less stable under the present conditions as compared with the propagating polymer radicals of NMAAm, NMMAm, AAm and MAm.

0 I H

5


I 10 min af ter -

-

78 min

Fig. 3

" 0 2 4 6 8 20 Reaction time in h

Fig. 4

Fig. 3 . ESR spectrum of the reaction system maleimide (MIm)/DBPO/dioxane after being irradiated for 9,3 h at room temperature and then irradiation being stopped; [MIm] = 3,4 mol/l, [DBPO] = 0,36 mol/l

Fig. 4. Post-polymerization of AAm and N-methylmethacrylarnide (NMMAm) at room temperature; a) irradiation was stopped. Vacant mark: under irradiation, filled one: after stopping irradiation; 0: [AAm] = 3,52 mol/l, [DBPO] = 0,217 mol/l, in dioxane; 4: [NMMAm] = 1,96 mol/l, [DBPO] = 0,217 mol/l, in benzene; f3: [NMMAm] = 4,90 mol/l, [DBPO] = 0,217 mol/l, in benzene

Post-polymerizations in the photo-sensitized polymerizations of NMMA m and AAm at room temperature

As mentioned above, the amide monomers are found to be easily converted into stable propagating radicals when they undergo photo-sensitized polymerizations by DTBP in benzene or 1 ,Cdioxane. Therefore, it is of interest to investigate the post- polymerizations in the photo-sensitized polymerizations of the amide monomers.

Fig. 4 shows the results obtained in the post-polymerizations of AAm in 1,4-dioxane and of NMMAm in benzene at room temperature. As can be seen from the figure, a small post-effect was observed in the polymerization of AAm. However, no post-polymerization was found to proceed under the present conditions in the polymerization of NMMAm, in which the conversion of the pre-polymerization was changed. Since the systems become heterogeneous in these polymerizations, the post-


polymerization of NMMAm is expected to proceed at higher temperature3). The effect of the reaction conditions on the post-polymerizations is under study at the present time.

Reactions of poly(NMAAm) radical with some vinyl monomers at room temperature

The concentration of the long-lived propagating radical 1 was found to reach 1 * 10 mol/l when a benzene solution of NMAAm (2,3 mol/l) and DTBP (0,43 mol/l) was irradiated by a UV-lamp.

The reactions of the propagating radical 1 with vinyl monomers were carried out at room temperature and investigated by means of ESR spectroscopy.

Fig. 5 shows the ESR spectrum of the reaction mixture of 1 and methyl methacrylate (NMA). The three line spectrum of 1 changes with time, finally to that of the

Fig. 5 Fig. 6

Fig. 5 . ESR spectrum change observed in the reaction of poly(N-methylacrylamide), poly- (NMAAm), radical 1 with methyl methacrylate (MMA); a benzene solution (0,25 ml) of NMAAm (2,3 mol/l) and DBPO (0,43 mol/l) was irradiated for 2 h at room temperature and then a mixture of MMA (0,25 ml) and benzene (0,25 ml) was added

Fig. 6. ESR spectrum of the reaction system of 1 and isopropenyl methyl ketone (IPMK) after being reacted for 2 days at room temperature; a mixture of IPMK (0.2 ml) and benzene (0,4 ml) was added


propagating polymer radical 6 of MMA, which is closely similar to the nine line spectrum of poly(MMA) radical reported by other workers4. 5 ) . This finding indicates that 1 reacts slowly with MMA to be converted to 6 (Eq. (9).

. . l ~ ~ ~ - ; d f " ' ? ' + ~ M M A - co

NHCH, NHCH, n-I

1

, . . f 2 - { ~ - # H 2 f f 2 - ~ ~ 3 co I

NHCH, OCH, OCH, n m- 1

6

Fig. 6 shows the ESR spectrum observed in the reaction of 1 with isopropenyl methyl ketone (IPMK). This spectrum is similar to that of poly(MMA) radical and assigned to the propagating polymer radical (7) of IPMK.

The nine line spectrum of poly(MMA) radical has been explained as an overlap of a nine line spectrum and a five line one, which are due to a symmetrical conformation and a deformed one of the /3-methylene hydrogens in the poly(MMA) radical, re- spectively5). Comparison of the spectra of 6 and 7 indicates that the relative popula- tion of the deformed conformation to the symmetrical one is similar in 6 and in 7.

On the other hand, as shown in Figs. 2 and 10, the propagating polymer radicals of MAm and NMMAm show a five line spectrum, suggesting that these polymer radicals take mainly a deformed conformation.

It is very difficult to obtain the ESR spectra of the propagating polymer radicals of 1, l -diphenylethylene (DPE) and a-methylstyrene (a-MeSt), because these monomers have little homopolymerizability in usual radical polymerization. However, our block copolymerization matrix method can yield easily the propagating radicals of such monomers at room temperature.

Fig. 7 shows the ESR spectrum of the reaction mixture of 1 and DPE recorded at 50°C. The doublet spectrum observed is assigned to the propagating radical 8 of DPE formed by addition of one monomer to 1 (Eq. (ii)).

Long-Lived Polymer Radicals, 2

8

2427

(ii)

Fig. 7 Fig. 8

Fig. 7 . ESR spectrum of the reaction system of 1 and 1, I-diphenylethylene (DPE) after being reacted for 5 days; recorded at 50°C; a mixture of DPE (0,2 ml) and benzene (0,4 ml) was added

Fig. 8. ESR spectrum of the reaction system of 1 and a-methylstyrene (a-MeSt) after being reacted for a day at room temperature; 0,5 ml of a-MeSt was added

The polymer radical 8 has mainly a deformed conformation, since the doublet of this spectrum is considered to be due to interaction of the unpaired electron with only one of two P-hydrogens in the polymer radical.

Fig. 8 shows the ESR spectrum observed in the reaction of 1 and a-MeSt. This fine- splitted spectrum is quite different from those of the propagating polymer radicals of other a-methyl-substituted monomers such as MMA, IPMK, MAm and NMMAm. This difference suggests that polymer radical 9 takes a conformation other than those of 2 , 4 , 6 and 7. Poly(MMA) radical has been reported to show a spectrum of 13 lines in methyltetrahydrofuran at very low temperature, which is interpreted as a con- sequence of a slight distortion of the symmetrical conformation of the P-hydrogen~~).

(iii)

9


It is also possible that the radical 1 attacks the benzene ring of a-MeSt to afford the hexadienyl radical 10 (Eq. (iv))@.

Fig. 9 shows the ESR spectrum of the reaction system of 1 and phenylacetylene. The broad singlet spectrum observed may be assigned to polymer radical 11.

Fig. 9 Fig. 10

Fig. 9. ESR spectrum of the reaction system of 1 and phenylacetylene (PA) after being reacted for 8 days at room temperature; 0,6 ml of PA was added

Fig. 10. ESR spectrum change observed in the reaction of poly(NMMAm) radical 2 with acrylonitrile (AN); a benzene solution (0,25 ml) of NMMAm (2,O mol/l) and DBPO (0,43 mol/l) was irradiated for 2 h at room temperature and then a mixture of AN (0,17 ml) and benzene (0,33 ml) was added


Reactions of poly(NMMAm) radical with some vinyI monomers

The long-lived propagating polymer radical 2 is also formed by the photo-sensitized polymerization of NMMAm in benzene at room temperature (Fig. 10).

Fig. 10 shows the ESR spectrum of the system of 2 and acrylonitrile (AN). As shown in the figure, the five line spectrum of 2 was gradually converted into the three line spectrum of the propagating polymer radical (12) of AN, and then the latter triplet spectrum changed to a broad singlet one. A similar three line spectrum has been reported to be observed on y-irradiation of AN at low temperature by which AN

1 2

oligomer is considered to be formed'), while the occluded polymer radical formed in the usual polymerization of AN has been known to show a broad singlet spectrum8). The change in the spectrum of the reaction system of 2 and AN seems to originate from the change in the environment around the radical center of 12 depending on the number of AN monomeric units incorporated in the polymer radical.

Figs. I I and 12 show the ESR observed in the reactions of 2 with styrene (St) and N-vinyl-2-pyrrolidone (NVP). The propagating polymer radicals of St (13) and NVP

Fig. I I Fig. 12

Fig. I 1 . at room temperature; 0,5 ml of St was added

Fig. 12. ESR spectrum of the reaction system of 2 and N-vinyl-2-pyrrolidone (NVP) after being reacted for 46 h at room temperature; a mixture of NVP (0,17 ml) and benzene (0,33 ml) was added

ESR spectrum of the reaction system of 2 and styrene (St) after being reacted for 22 h


(14) are observed as three line spectra similar to that (15) of methyl acrylate, an a-mono-substituted monomer I).

13

14

NHCH, OCH, OCH, m-I

15

Fig. 13 shows the ESR spectrum observed in the reaction of 2 with N,N-dimethylacrylamide (NDMAAm). Besides the usual three line spectrum, another absorption is observed which is not present in the spectra of the polymer radicals generated from other a-monosubstituted monomers. This suggests that the polymer radical of

Fig. 13. ESR spectrum of the reaction system of 2 and N,N-dimethylacrylamide (NDMAAm) after being reacted for 14 days at room temperature; a mixture of NDAAm (0, I ml) and benzene (0,3 rnl) was added

NDMAAm (16) also takes another conformation. The polymer radical of NDMAAm has been found to have as high reactivity toward vinyl monomers as polymer radicals derived from non-conjugative monomers, although NDMAAm, a conjugative monomer, itself has a high reactivity for radical addition9). This high reactivity of

Long-Lived Polymer Radicals, 2 243 I

16

NDMAAm polymer radical may be ascribable to a reactive conformation of the radical, which seems to be responsible for the other absorption observed in Fig. 13.

' ) H. Tanaka, T. Sato, T. Otsu, Makromol. Chern. 180, 267 (1979) 2, C. Chachaty, A. Forchioni, J. Polym. Sci., Part A-I, 10, 1905 (1972) 3, S. Kiiciikyavuz, B. M. Baysal, J. Polyrn. Sci., Polyrn. Chern. Ed., 16, 2901 (1978) 4, M. Kamachi, M. Kohno, D. J. Liaw, S. Katsuki, Polyrn. J. 10, 69 (1978) 5 , M. Sakaguchi, J. Sohma, J. Appl. Polym. Sci. 22, 2915 (1978) 6, A. Hatano, Y. Iwase, K. Ashida, J. Macrornol. Sci.-Chern. 12, 647 (1978) ') R. Bensasson,'R. Marx, J. Polyrn. Sci. 48, 53 (1960)

C. H. Barnford, A. D. Jenkins, M. C. R. Syrnons, M. G . Townsend, J. Polym. Sci. 34, 18 I ( I 959)

9, B. Yamada, M. Yoshioka, T. Otsu, Kobunshi Ronbunshu 35, 795 (1978)

long-lived polymer radicals, 2. an esr study on the reactions of the propagating polymer radicals of...

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