Vinyl Monomers Bearing Chromophore Moieties and Their Polymers. I I . Fluorescence and Initiation Behavior of N-(4- N’,N’-dimethylaminophenyl)maleimide and Its Polymer

HUI CAI, XIAO-HUI HE, DONG-YING ZHENG, JIAN QlU, ZI-CHEN LI, and FU-MIAN LI”

Department of Chemistry, Peking University, Beijing 100871, China

SYNOPSIS

A maleimide bearing electron-donating chromophore, N-(4-N,N-dimethylaminophenyI)- maleimide (DMAPMI) was synthesized from N,N-dimethylaminoaniline and maleic an- hydride in the presence of acetic anhydride and sodium acetate. DMAPMI can be easily copolymerized with vinyl acetate (VAc). In addition, it can be easily homopolymerized by UV light irradiation or by using AIBN or BPO as an initiator. The fluorescence spectra of DMAPMI and its polymer or copolymer were recorded and compared at the same chro- mophore concentrations. I t was observed that the fluorescence emission intensity of DMAPMI was much lower than those of its polymers. This may be due to the occurrence of intermolecular charge transfer interaction between the electron-donating dimethylami- nophenyl moiety and acrylic electron-accepting carbon-carbon double bond in the monomer. The model compound, N-(4-N,N-dimethylaminophenyl)succinimide (DMAPSI), which has no carbon-carbon double bond, displayed the same fluorescence behavior as DMAPMI polymers. The fluorescence of DMAPMI polymers and DMAPSI can be quenched by elec- tron-deficient compounds such as AN, TCNE, MMA, etc. All these results supported the above conclusion. This is another example of the “fluorescence structural self-quenching effect” termed by us previously and demonstrates again that this phenomenon is not an accidental but a general one for acrylic monomers bearing electron-donating chromophores. Study of the initiation behavior of DMAPMI and its polymer showed that they could initiate the photopolymerization of AN, by combination with BPO, they could also initiate the thermopolymerization of vinyl monomers such as MMA. 0 1996 John Wiley & Sons, Inc. Keywords: N-(4-N,N-dimethylaminophenyl)maleimide fluoroescence structural self- quenching effect * initiation - polymerization

INTRODUCTION acrylic monomers bearing electron-donating chro-

Much attention has been given to exciplexes as re- action intermediates in photoinduced electron transfer and photochemical reactions. In the field of polymer chemistry, studies were most concen- trated on homopolymers or copolymers capable of energy migration and exciplex formation.’-3 Our re- search efforts, however, have been directed toward the investigation of exciplex formation in monomers containing both electron donor and acceptor groups, and the influence on their polymerization and pho- tochemical behavior. In our previous work, several

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A Polymer Chemistry, Vol. 34.1245-1250 (1996) 0 1996 John Wiley & Sons, Inc. CCC OSS7-624X/96/071245-06

mophore moieties, most of which are derivatives of monocarboxylic acid, such as N,N-dimethylami- nobenzyl methacrylate (DMABMA); N-(N,N-di- methylaminopheny1)acrylamide (DMAPAA)? 8- acryloyloxyquinolines (A&)> N-acryloyl-N-phenyl piperazines (APP),7 and N-methacryloylethyl-N- methylaniline (MEMA): were synthesized and studied photochemically. It was observed that the fluorescence emission intensities of these monomers were always dramatically lower than those of their corresponding polymers or copolymers at the same chromophore concentration. This phenomenon, termed “fluorescence structural self-quenching ef- fect,” is the result of intra- or intermolecular exci- plex formation under UV irradiation. For this rea-

1245

1246 CAI ET AL.

son, the monomers can also act as photosensitizers to initiate their own photopolymerization and also the photopolymerization of some electron-deficient monomers such as MMA and AN.

In this article, our idea is to introduce an electron- donating moiety into unsaturated dicarboxylic acid to know if it shows the same photochemical prop- erties and initiation behavior as those of the acrylic monomers. This will also broaden the understanding of fluorescence structural self-quenching phenom- enon. Thus, a novel monomer N-(4-NP,N-dimethy1- aminopheny1)maleimide (DMAPMI) was synthesized and polymerized, the fluorescence and initiation be- havior of DMAPMI and its polymers were studied.

EXPERIMENTAL

Materials

Maleic anhydride (Beijing Chemical Co.) and N,N- dimethyl-1,4-phenylenediamine (Aldrich) were used as received. Itaconic anhydride (IAn) and citraconic anhydride (CAn) were prepared by the dehydration of the corresponding acids with phosphorus pentox- ide. IAn was a white crystal, mp 71-72°C. 'H-NMR (CDCI,, TMS, ppm): 3.58-3.63 (t, 2H), 5.89-5.95 (t, lH), 6.52-6.59 (t, 1H). CAn was a colorless liquid, bp 53-54"C/l.O mmHg, ng=1.4714. 'H-NMR (CDC13, TMS, ppm): 2.20 (d, 3H), 6.63-6.70 (q,lH). All the reagents and solvents were purified by con- ventional methods. The solvents used for spectral measurement were verified to have no interference.

Synthesis of DMAPMI and DMAPSI

DMAPMI and its model compound DMAPSI were synthesized according to the method of Cava et al.:'

to 0

v 0

0

DMAPSI

DMAPMI

A reddish-brown needle crystal with mp 151-153°C. 'H-NMR(CDC13, TMS, ppm): 2.97(s, 6H), 6.80(s, 2H), 6.71-7.21(m, 4H). IR(KBr, cm-'): 1690, 1510, 675. (The spectrum was collected in Sadtler, D 12201 K.) MS(E1 source): 216 (rnle).

ANAL. Calcd: C, 66.67%; H, 5.56%; N, 12.96%. Found C, 66.17%; H, 5.85%; N, 12.71%.

DMAPSI

A white crystal with mp 223-225°C. 'H-NMR (CDCI3, TMS, ppm): 2.83(s, 4H), 2.96(s, 6H), 6.68- 7.16(m, 4H). IR(KBr, cm-'): 1690, 1515, 1290.

Homopolymerization of DMAPMI

A glass tube was charged with known amounts of DMAPMI, DMF, and AIBN or BPO. The ampoule was then sealed under vacuum after nitrogen was purged. After several hours of heating (for AIBN, at 60°C; for BPO, at 25OC), a white polymer powder was obtained by precipitation from methanol. The photopolymerization of DMAPMI was carried out without any sensitizer under a high-pressure mer- cury arc lamp.

Copolymerization of DMAPMI with Vinyl Acetate(VAc)

A mixture of DMAPMI and VAc in DMF (DMAPMI: VAc = 1 : 2 in molar ratio) together with 0.5% of AIBN was heated in a sealed glass tube at 60°C for 4 h. The polymer was precipitated and further purified from methanol several times.

Kinetic Study of the Polymerization of AN and MMA

The kinetics of the polymerization of AN and MMA was followed by dilatometric method. For the pho- topolymerization of AN, the UV light source was a high-pressure mercury arc lamp filtered by potas- sium chromate solution.

Absorption and Fluorescence Spectra

The UV spectrum was recorded on a Shimadzu UV- 250 Spectrophotometer. The fluorescence emission spectrum was recorded on a Hitachi M-850 Fluo- rescence Spectrophotometer at room temperature. The slit width of both monochrometers were 5 nm. All the solvents and reagents were void of interfering impurities.

VINYL MONOMERS BEARING CHROMOPHORE MOIETIES 1247

370 410 450 490

A lmi

Figure 1. Fluorescence emission spectra of (1) DMAPSI, (2) P(DMAPM1-VAc), (3) P(DMAPMI), and (4) DMAPMI. A,, = 312 nm, solvent, DMF, [chromo- phore], 1.0 X mol/L.

RESULTS AND DISCUSSION

Polymerization of DMAPMI

Like maleic anhydride, DMAPMI easily underwent free radical copolymerization with vinyl acetate (VAc), elementary analysis showed that the content of DMAPMI unit in the copolymer was 47.8%. The molecular weight of the copolymer was measured to be 10,000 by GPC method. As an aromatic tertiary amine, DMAPMI could form redox system when combined with BPO to initiate its own thermopo- lymerization even at room temperature. In addition, DMAPMI could also be homopolymerized at 60°C by using AIBN as an initiator. This was proved by following the diappearance of the double bonds of DMAPMI monomers, the molecular weight of the polymer was 4500 as measured by GPC method. This phenomenon was due to the existence of electron- donating group in the molecule.

Under UV irradiation, DMAPMI could easily undergo photopolymerization without any sensitizer. The polymer obtained proved to have similar struc- ture and molecular weight to that obtained via ther- mopolymerization. Further study showed that, in the cases of some other 4-substituted N-arylmaleimides which have no electron-donating groups such as N - (4-nitrophenyl)maleimide, N- (4-acetoxyphenyl) male- imide, N-(4-hydroxyphenyl)maleimide, and N-(4- methylphenyl)maleimide, the photopolymerization could not take place without the participation of a sensitizer. Thus, it could be concluded that dimethylaminophenyl group which acted as an elec-

tron-donating moiety in DMAPMI did play an im- portant role in the sensitizing process of DMAPMI p hotopolymerization.

The mechanism for the photopolymerization of DMAPMI might therefore be suggested as follows:

0 I

0 1

Fluorescence Behavior of DMAPMI and I ts Polymers

The fluorescence emission spectra of DMAPMI, P(DMAPMI), P(DMAPM1-VAc), and DMAPSI in DMF ([chromophore J = 1 X mol/L) are shown in Figure 1. It was obvious that the fluorescence in- tensity of the monomer was much lower than those of its homopolymer, copolymer, and its model com- pound DMAPSI. To confirm that such a low fluo- rescence intensity of the monomer is not caused by concentrational factors, we recorded the fluorescence emission of DMAPMI and P(DMAPM1) at different concentrations. The results are shown in Figure 2. It could be seen clearly that the fluorescence inten- sities of P(DMAPM1) were always higher than those of DMAPMI. This phenomenon is quite different from the concentrational effect; it should be attrib- uted to the coexistence of electron-deficient double bond of vinyl group and electron-donating chro- mophore in the monomer. In the presence of such

-7 -5 - 3 -1 1-

Figure 2. Relative fluorescence intensities of (1) DMAPMI and (2) P(DMAPM1) at different concentra- tions. X., = 312 nm; A,, = 420 nm.

1248 CAI ET AL.

7 1

I

I I , , I I I I I n 2.0 4.0 6 .0 8.0

c x lo2 ( l m l / L l

Figure 3. Stern-Volmer plots of P(DMAPM1-VAc) quenched by: a (1) MAn, (2), CAn (3), MA (4) MMA; b (1) TCNE, (2) FN, ( 3 ) FM, (4) AN, (5) MAN.

an acrylic carbon-carbon double bond which could accept an electron from the excited electron-donat- ing chromophore, the fluorescence emission of the chromophore was quenched due to exciplex forma- tion. As reported in our previous articles, this phe- nomenon is not an accidental one but is commonly observed in the acrylic monomers bearing electron- donating chromophore. This quenching effect, brought about by structural factors, was termed as “structural self-quenching effect” to differ from the “concentrational self-quenching effect.””

To further make it clear that electron-deficient double bonds do quench the fluorescence emission of the chromophore, and to find the relationship be- tween the quenching efficiency of the quenchers and the degree of electron-deficiency of the double bond, several quenchers with electron-deficient double bonds were chosen. The orders of their electron de- ficiency or the electron acceptivity are as follows:

1.

2.

3.

4.

Methyl methacrylate (MMA) < Methyl ac- rylate (MA). Citraconic anhydride (CAn) < Maleic an- hydride (MAn). Methylacrylonitrile (MAN) < Acrylonitrile (AN) < Fumaronitrile (FN) < Tetracy- anoethene (TCNE). Dimethyl fumarate (FM) < Fumaronitrile (FN).

When any of these quenchers was added to the solutions of P(DMAPM1-VAc), the fluorescence intensities of the polymers were found to decrease regularly. The Stern-Volmer plots are given in Fig-

ure 3. The slopes of the straight lines, i.e., the Stern- Volmer constants, which indicate the quenching ef- ficiency of various quenchers are summarized in Ta- ble I. It was obvious that the quenching ability was greatly influenced by the electron-deficiency of the quenchers. Generally, the more electron-deficiency the double bond is, the easier it is to form exciplex and thus to quench the fluorescence more efficiently. But an inconsequent phenomenon was observed when the quenching efficiencies of MMA, MAN, MA, and AN were compared respectively. Despite their less deficient nature of double bond, MMA and MA showed higher quenching efficiencies than MAN and AN. Such a phenomenon might be ascribed to the “structural compatibility” of MA and MMA with repeating polymer units. That is, MA and MMA are both acrylic compounds, just like DMAPMI which is an acrylic polymer. As a result, they might have stronger interactions with P(DMAPM1) and greater quenching constants than MAN and AN do. The same was true in the case of FM and FN. A similar phenomenon was also observed in our previous work^.^.^

Sensitizing and initiation Behavior for the Polymerization of AN and MMA

As mentioned above, AN could quench the fluores- cence of P(DMAPM1-VAc), which implied the ex- ciplex formation between the electron-deficient double bond of AN and the electron-donating aro- matic tertiary amino chromophore. Therefore, the use of DMAPMI, P(DMAPMI), P(DMAPM1-VAc), and DMAPSI as sensitizers for the photopolymer- ization of AN was examined.

As an example, Figure 4 shows the conversion-time plots for the polymerization of AN at 30°C by varying the concentration of DMAPMI [for P(DMAPM1- VAc) and DMAPSI, similar figures were obtained]. After calculation the overall rate of the polymerization (R,) from the slopes of these straight lines, the de- pendence of Rp on the concentration of different sen- sitizers were obtained as shown in Figure 5. By the

Table I. Different Quenchers for P(DMAPM1-VAc)

Fluorescence Quenching Constants of

e Kq r e Kq r Quencher (eV) (M-’) Quencher (eV) (M-’)

MAN 0.68 0.88 MMA 0.40 4.18 AN 1.23 5.1 MA 0.64 20.1 FM - 269.8 IAn - 270.5 FN 2.73 165.0 CAn - 285.3 TCNE - 20400 MAn 3.69 291.5

VINYL MONOMERS BEARING CHROMOPHORE MOIETIES 1249

same method, the dependence of Rp on the concen- tration of AN could also be obtained. The exponential of concentration of sensitizers and AN for the rate equation of polymerization are summarized in Table 11. The overalI activation energies for the polymerization of AN sensitized by DMAPMI, P(DMAPMI), P(DMAPM1-VAc), and DMAPSI were calculated by varying polymerization temperature from 2545°C re- spectively. Relevant results are also given in Table 11.

To clarify the mechanism for the photopolymer- ization, several other aromatic tertiary amines such as N,N-dimethyltoluidine (DMT) and N,N-di- methylaniline (DMA) were used as sensitizers for comparison. Results showed that Rp (%/min) for AN was in the order of DMT (0.048) = DMA (0.048) > DMAPSI (0.043) > P(DMAPM1) (0.015)

The order was in accordance with that of the ca- pability of charge transfer complexes formation of these sensitizers with TCNE: DMT (695 nm) > DMA (650 nm) > DMAPSI (620 nm) > DMAPMI (590 nm). Therefore the mechanism of photopoly- merization for AN was proposed as follows:

> P(DMAPM1-VAC) (0.011) > DMAPMI (0.004).

AN [.. T?--(@N CH3]* - ton radical - Polymer CH3

After the photopolymerization of AN, the DMF solution of polymer was poured into a large excess of methanol to give the polymer P(AN). Since the sensitizers, including DMAPMI and P(DMAPMI), were soluble in chloroform, while P(AN) was insol- uble, the polymer was extracted by chloroform sev-

0 1 0 2 0 3 0 4 0 5 0

Time (mm )

Figure 4. Conversion-time plots for the polymerization of AN sensitized by DMAPMI. [DMAPMI] (X10-3 mol/ L): (1) 4.57, (2) 6.28, (3) 7.98, (4) 9.68, (5) 11.4. T = 3OoC, [AN] = 4.47 mol/L.

l o i

o i l L I 05 0 8 0 7 08 09

Ig[ssnsit~rer] + 3

Figure 5. lgR,-[Sensitizer] plots for the polymeriza- tion of AN: (1) DMAPSI, (2) P(DMAPM1-VAc), (3) P(DMAPMI),(4) DMAPMI.

era1 times to eliminate the residual DMAPMI and P(DMAPM1). Then the product was dissolved in DMF again and its UV absorption was recorded. The absorptions at 265 nm shown in the UV spectra of P(AN) were the same as those of the sensitizers. The intensities remained unchanged after several times of dissolution-precipitation and treatment by chloroform. This phenomenon indicated that the sensitizers not only sensitized the photopolymeri- zation, but also entered the polymer chains, i.e., participated in the polymerization of AN. It is ex- pected that these polymerizable sensitizers can be used in situ-curing of biomedical materials such as bone cement to reduce or avoid the release of ini- tiator residues.

As a polymerizable tertiary amine, DMAPMI, combined with BPO, could not only initiate its own polymerization at room temperature, but also ini- tiates the thermopolymerization of MMA. The rate equation could be obtained as follows:

Rp = kapp[DMAPMI]o~5[BPO]o~5[MMA]1-o, E, = 50.48 kJ/mol

As we know, under general circumstances, the polymerization initiated by redox system can take place at low temperature, and the kinetics equation could be expressed as:

Rp = kapp[Reductor]o~5[Oxidizer]o~5[Monomer]'~o,

E, = 40-60 kJ/mol

Thus, it is very clear that DMAPMI and BPO indeed form a redox system to initiate the polymerization of MMA and DMAPMI. Likewise, the rate equation for the polymerization of MMA initiated by P(DMAPM1)-BPO was obtained as follows:

Rp = ka,[P(DMAPMI)]0-5[BPO]0~5[MMA]1~0, E, = 63.12 kJ/mol

1250 CAI ET AL.

Table 11. Kinetics of Photosensitized Polymerization of AN, Rp = Kp X [Sensitizer]' X [ANIY

Sensitizer DMAPMI P(DMAPM1) P( DMAPMI-VAC) DMAPSI

X 0.10 0.22 0.27 0.44

Y 1.41 1.42 1.46 0.48 E, (kJ/mol) 21.3 19.8 20.7 18.3

From the data of activation energy, we can find that as an initiation system for thermopolymeriza- tion of MMA, P(DMAPM1)-BPO is less effective than DMAPMI-BPO. This result can be explained that, in P(DMAPMI), some N,N-dimethylamino phenyl groups may be entrapped in the coil structure of the polymer; thus the formation of redox system with BPO was not so easy.

As in the case of photopolymerization of AN, measurement of UV spectra of P(MMA) indicated that DMAPMI and P(DMAPM1) not only initiated the polymerization of MMA, but also entered the polymer chain.

CONCLUSION

Having both electron-donating chromophore and electron-deficient double bond, DMAPMI can form intermolecular exciplex under UV irradiation; thus the fluorescence intensity of the monomer is much lower than that of its polymer. This phenomenon demonstrates again that fluorescence structural self- quenching effect is not an accidental but a general phenomenon for acrylic monomers bearing electron- donating chromophores.

The fluorescenceof P(DMAPM1) and P(DMAPM1- VAc) can be quenched by electron-deficient mono- mers; the quenching efficiency of the quenchers de- pends not only on their electron-drawing capability but also on their structural compatibilities with the polymers.

DMAPMI can easily undergo not only copoly- merization with VAc, but also homopolymerization initiated by AIBN at 6OoC, or by BPO under a redox process at room temperature. It can also undergo

photopolymerization without any sensitizer. The mechanism of these polymerizations are deduced as exciplex formation.

DMAPMI as well as its homopolymer and co- polymer can initiate the photopolymerization of AN. When they are combined with BPO, a redox initi- ation system can be formed to initiate the thermo- polymerization of vinyl monomers such as MMA, AN, etc. The entrance of these initiators into the polymer chains indicates their prospective appli- cation on situ-curing of biomedical materials.

REFERENCES AND NOTES

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2. S. Tazuke and H. L. Yuan, J. Phys. Chem., 86,1250 (1982).

3. K. Iwai, K. Yamamoto, and F. Takemura, Macro- molecules, 18, 1021 (1985).

4. S. K. Wu, Y . C. Jing, F. M. Li, andX. D. Feng, Polym. Bull., 8, 275 ( 1982).

5. S. K. Wu, Q. Q. Zhu, F. M. Li, and X. D. Feng, Pho- tographic Sci. Photochem., 4, 28 (1984).

6. L. Wang, F. M. Li, and X. D. Feng, Photographic Sci. Photochem., 3, 42 ( 1987).

7. F. M. Li, S. J. Chen, 2. C. Li, and J. Qiu, J . Polym. Sci. Part A: Polym. Chem., to appear.

8. S. J. Chen, D. J. Guo, K. Y. Qiu, and F. M. Li, Acta Polym. Sinica, 4, 501 ( 1990).

9. M. P. Cava et al., Bull. Chem. SOC. Jpn., 14, 173 ( 1939).

10. S. K. Wu and F. M. Li, in New Trends in Photochem- istry of Polymers, Elsevier, New York, 1985, p. 85.

Received January 31, 1995 Accepted November 3, 1995