([Eta]6-N-alkylcarbazole) ([eta]5-cyclopentadienyl) iron hexafluorophosphate salts in photoinitiated and thermal epoxy polymerization.

INTRODUCTION

UV curable systems are categorized as either radical or ionic polymerizations. Photoinitiated radical polymerizations have been investigated for a considerable period of time, but photoinitiated cationic and anionic polymerizations have received little attention. The slow development of this field has been largely due to the dearth of suitable photoinitiators to catalyze ionic polymerizations. Crivello and Lam have discovered that onium salts with complex metal halide counterions are efficient photoinitiators for the polymerization of a variety of cationically polymerizable monomers (1-4). The increasing commercial and technical demand for cationic photopolymerization has led to an interest in the design and synthesis of optimized photopolymerization systems (5-9).

Ferrocenium salts are also attractive as photoinitiators for cationic polymerization because they possess absorptions in the middle region of the UV spectrum (10), (11). The absorptions and photoactivities of ferrocenium salts can be modified through structural changes of the ligands (12-14). In a previously published article, we have reported that a carbazole-bound ferrocenium salt was highly efficient as a photoinitiator in the photopolymerization of epoxides (15), However, it was found to have poor solubility in most cationically polymerizable monomers, especially in those of commercial interest. Thus, its use in practical applications is limited.

In this work, to improve the solubility of ferrocenium salts in epoxides, two CFS photoinitiators bearing C4 and C8 alkyl chains on the nitrogen atom have been prepared, C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6]. With real-time infrared spectroscopy, the study of the photopolymerization of epoxides ERL-4221 and TDE-85 using C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] as photoinitiators has been conducted. With differential scanning calorimetry (DSC), the thermal initiating activities for the cationic ring-opening polymerization of epoxides also have been studied.

EXPERIMENTAL

Materials

All starting materials used in the preparation of ferrocenium salts were reagent grade and used without purification unless otherwise noted. The epoxy compounds used in this work were 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ERL-4221) and di(2,3-epoxypropyl)3,4-epoxy-1,2-cyclohexanedioate (TDE-85). ERL-4221 and TDE-85 were obtained from Tianjin Synthetic Matertial Research Institute (Tianjin, China) and used as received. Scheme 1 summarizes the abbreviations and structures of the epoxy compounds employed in this study.

[ILLUSTRATION OMITTED]

In this study, Cm-CFS [PPF.sub.6], is used to designate the cyclopentadiene-Fe-carbazole hexafluorophosphate (C0-CFS PF6), cyclopentadiene-Fe-N-butylcarbazole hexafluorophosphate (C4-CFS [PPF.sub.6]) and cyclopentadiene-Fe-N-octylcarbazole hexailuorophosphate (C8-CFS [PPF.sub.6]) where Cm represents alkyl chains. Their corresponding structures are shown in Scheme 2.

[ILLUSTRATION OMITTED]

Preparation of [cyclopentadienyl-Fe-N-butylcarbazole] [F.sub.6] (C4-CFS [PF.sub.6])

All ferrocenium salts were prepared via a ligand exchange reaction between one ring of ferrocene and arene, according to the following procedures.

Ferrocene (m = 20 g. n = 0.1 I mol), N-butyl carbazole (m = 6.69 g, n = 0.03 mol), anhydrous aluminum chloride (Al[Cl.sub.3]) (m = 30 g, n = 0.22 mol) and aluminum powder (m = 1 g, n = 0.037 mol) were added into a 500 ml three-necked round-bottom flask under nitrogen in cyclohexane (V = 80 ml). The reaction mixture was then heated under reflux, with rapid stirring, in a nitrogen atmosphere for 24 h. The reaction flask was then cooled to room temperature and placed in an ice bath, and the reaction mixture was slowly hydrolyzed with water (V = 50 ml). The mixture was filtered, and the aqueous layer was separated after washing several times with cyclohexane (V = 50 ml). The aqueous layer, which is air stable, was then filtered into a concentrated aqueous solution of potassium hexafluorophosphate ([KPF.sub.6]) (m = 4 g in 20 ml of water). The precipitated solid was filtered and purified through a column of aluminum oxide. After the column chromatography, a yellow solid was obtained. The solid was recrystallized from a warm acetone-ether solution.

[Cyclopentadienyl-Fe-N-butyicarbazole] [PF.sub.6] (C4-CFS [PF.sub.6]), mp: 178-180[degrees]C. IR (KBr, [cm.sup.-1]): 3110(C = C), 2959(=C--H), 2873(--C--H), 1622(C=C), 1550(C=C), 1475(C=C), 1330(--C--H), 834([PF.sub.6.sup.1]). (1) H NMR (C[D.sub.3]COC[D.sub.3], ppm): 7.82-6.37(m, 8H, benzene). 4.72(s. 5H, Cp), 4.53(t, 2H, N--C[H.sub.2]--), l.67-l.56(m. 4H, --C[H.sub.2]--C[H.sub.2]--), 1.04(t, 3H, --C[H.sub.3]).

Preparation of [cyclopentadienyl-Fe-N-octylcarbazole] [PF.sub.6] (C8-CFS [PF.sub.6])

[Cyclopentadienyl-Fe-N-octylcarbazole] [PF.sub.6] (C8-CFS [PF.sub.6]) was prepared by the reaction of ferrocene with N-octylcarbazole in a manner similar to C4-CFS [PF.sub.6].

[Cyclopentadienyl-Fe-N-octylcarbazole] [PF.sub.6] (C8-CFS [PF.sub.6]), mp; 127-129[degrees]C. IR (KBr, [cm.sup.-1]): 3115(C=C), 2927(=C-H), 2851(--C--H), 1612(C=C), 1556(C=C), 1465(C=C), 1324(--C--N), 834([PF.sub.6.sup.-1]). (1) H NMR (C[D.sub.3]COC[D.sub.3], ppm): 7.82-6.37 (m, 8H, benzene)), 4.70(s, 5H, Cp), 4.525(t, 2H, N--C[H.sub.2]--), 1.69-1.54 (m. 12H, --C[H.sub.2]--), 0.887 (t, 3H, --C[H.sub.3]).

Photopolymerization Studies Using Real-Time Infrared (RTIR) Spectroscopy

The basic principle of RTIR spectroscopy consists of exposing the sample simultaneously to UV light, which induces the polymerization, and to an infrared beam, which measures the monomer concentration at any given time. The resulting decrease in the IR absorption band characteristic of that monomer is monitored continuously on a transient memory recorder. Because the absorbance increment is always proportional to the amount of monomer that has polymerized after a given exposure, and thus to the degree of conversion, the recorded RTIR trace actually corresponds to the conversion versus time curve.

TABLE 1. Solubility of the photoinitiators in epoxy at 30[degrees]C.

Photoinitiato      ERL-4221 solubility (g/g)  TDE-85 solubility (g/g)

CO-CFS [PF.sub.6]         & 0.005 (i)               &0.005 (i)
C4-CFS [PF.sub.6]           0.061 (s)                0.076 (s)
C8-CFS [PF.sub.6]           0.112 (s)                0.174 (s)

s, soluble; i. insoluble.

In this article, conversion data were obtained by monitoring the decay of the epoxy groups centered at 781 [cm.sup.-1] (CHO) during irradiation. The mixture of monomer and photoinitiator was applied between two KBr crystals and irradiated with a UV spot light sources (Rolence-100 UV, Taiwan, China) at room temperature. The light intensity on the surface of samples was 70-75 mW/[cm.sub.2]. For each sample, the series of RTIR runs were repeated three times.

Thermally Initiated Cationic Polymerizations

Studies of thermally initiated cationic polymerizations were performed with a Perkin Elmer Pyris 1 differential scanning calorimeter. Samples containing 1.0 wt% of the CFS photoinitiator dissolved in the appropriate monomer were headed in the calorimeter from 50 to 250[degrees]C in [N.sub.2] at a rate of 10[degrees]C/min.

Instruments

The (1)H NMR spectra were recorded on a Bruker AV600 unity spectrometer operated at 600 MHz using [CD.sub.3][COCD.sub.3] as the deuterated solvent. FTIR spectra were recorded on a Nicolet 5700 instrument (Thermo Electron Corporation, Waltham, MA). UV-Vis absorption spectra were recorded in [CH.sub.2][Cl.sub.2] solution on a Hitachi U-3010 UV-Vis spectrophotometer (Hitachi High-Technologies Corporation, Tokyo, Japan). Light intensity was recorded by a UV light radiometer (Photoelectric Instrument Factory, Beijing Normal University, Beijing, China). DSC thermograms were measured using dynamic DSC analysis (Perkin-Elmer Pyris 1).

RESULTS AND DISCUSSION

Solubility of Photoinitiator in Epoxides

The solubility of the photoinitiator is very important in the application of photopolymerization. Table 1 lists the solubilities of C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] in the epoxies ERL-4221 and TDE-85. For comparison, the solubility of C0-CFS [PF.sub.6] is also given. From that table, it can be seen that the long-chain alkyl groups attached to the nitrogen atom of carbazole greatly enhance the solubility of ferrocenium salts, especially in polar solvents and epoxides. C8-CFS [PF.sub.6] has the highest solubility in epoxides. Because the amount of photoinitiator used is usually less than 5%, C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] can be consider to be soluble in epoxies ERL-422 and TDE-85.

UV-Vis Absorption Spectra of C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6]

To predict whether a photoinitiator can initiate an epoxy system under a high-pressure mercury lamp, it is very important to consider its UV spectrum, especially in the region above 300 nm. UV-Vis absorption spectra of C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] in C[H.sub.2][Cl.sub.2] are given in Fig. 1. For comparison, the absorption, the absorption spectra of cyclopentadienyl-Fe-cymene hexafluorophosphate (I-261) and C0-CFS [PF.sub.6] also are shown. These spectra were recorded at identical molar concentrations (1 X 10~4 M), enabling a direct comparison of asbsorbance.

[FIGURE 1 OMITTED]

The UV-Vis absorption spectra of C0-CFS [PF.sub.6], C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] are similar to each other, and their absorptions are much stronger than that of I-261. The strongest absorption lies in the region of 200-300 nm, which belongs to [pi]-[pi] transition. The compounds also show absorptions at longer wavelengths, ranging from 300 to 400 nm which belong to d-d transition of iron arene complexes. Introducing an N-alkyl group red shifts the absorptions of C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] by approximately 5 nm.

Figures 2 and 3 depict the UV absorption spectral changes of C4-CFS [PF.sub.6 and C8-CFS [PF.sub.6], respectively, in [CH.sub.2][Cl.sub.2] irradiated by a 365 nm high-pressure mercury lamp. The absorption curves of N-butyl carbazole and N-octyl carbazole in [CH.sub.2][Cl.sub.2] are also shown. Faster spectral changes were observed with irradiation. The arrows in the figures show the change tendencies of the peaks with increasing irradiation time. It is evident that photolysis of CFS ferrocenium salts occurs after UV irradiation.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In Fig. 2, an increase in absorption at 238 nm was observed, whereas absorption at 350-400 nm, caused by d-d transition, decreased. Furthermore, the absorption peaks at 294 and 334 nm appeared promptly after irradiation began. The same figure shows that the strongest absorptions, at 238, 294. and 334 nm, also belong to N-butyl carbazole.

Previously reported mechanisms of the photolysis of ferrocenium salts (11) indicate that ferrocinium salts undergo photolysis to generate an iron-based Lewis acid upon the loss of the arene ligand. Coordination of this latter species with an epoxy monomer is followed by ring-opening polymerization. The ring-opening and the polymerization reaction may start in the ligand sphere of the iron(II) cation due to its lack of electrons. Thus, it may be theorized that C4-CFS [PF.sub.6] undergoes photolysis to generate an iron-based Lewis acid upon the loss of the arene ligand N-butyl carbazole. The same observation can be obtained from Fig. 3. Scheme 3 proposes the mechanism for the photolysis of CFS [PF.sub.6] photoinitiator.

[ILLUSTRATION OMITTED]

Photoinitiating Activity Comparison

In this investigation, RTIR spectroscopy was employed to evaluate the efficiencies of C4-CFS [PF.sub.6] and the C8-CFS [PF.sub.6] photoinitiators in the cationic polymerizations of two typical monomers, ERL-4221 and TDE-85. TDE-85 and ERL-4221 are important epoxy monomers because they are commercially available and are employed in many applications for cationic polymerization.

The conversion vs. time plots for the polymerizations of ERL-4221 and TDE-85 induced by C4-CFS [PF.sub.6] C8-CFS [PF.sub.6] and I-261 are shown in Fig. 4. Although the plateau value gives the final epoxy group conversion, the slope of the curve gives an indication of the polymerization rate. C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] are more efficient than I-261. The conversion of epoxides and the polymerization rate initiated by C8-CFS [PF.sub.6] were slightly lower than that initiated by C4-CFS [PF.sub.6]. The reason for this may be the low diffusivity caused by the long alkyl group.

[FIGURE 4 OMITTED]

Ferrocinium salts undergo photolysis to generate an iron-based Lewis acid, which can coordinate with an epoxy monomer, causing epoxy ring-opening polymerization. Because this process involves an epoxy bond with an unsaturated iron center, the steric effect has a large influence on the photoactivities. Cycloaliphatic epoxides with a smaller steric effect are more reactive than aliphatic epoxides. ERL-4221 possesses two cycloaliphatic epoxy groups, and TDE-85 possesses only one cycloaliphatic epoxy group. Therefore, TDE-85 is less reactive than ERL-4221 in cationic photopolymerization.

[FIGURE 5 OMITTED]

ERL-4221 was chosen as the monomer for the further studies of the novel CFS ferrocenium salts because it had a higher final conversion than TDE-85. The plots of conversion vs. irradiation time of ERL-4221 incorporating different concentrations of C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] are shown in Figs. 5 and 6. The polymerization rate and final conversion increased with the increase of CFS ferrocenium salt concentration. This is due to higher concentrations generating more cations during irradiation, which leads to the increase in polymerization rate and final conversion. When the concentration of CFS photoinitiator is above 4%, the increase in polymerization rate slows down.

[FIGURE 6 OMITTED]

Photosensitization of the CFS Photoinitiators

One means of effectively increasing the activity of a ferrocenium salt photoinitiator is the use of photosensitization. Fortunately, the photoinitiating activity of CFS salts can be sensitized with the addition of benzoyl peroxide (BPO).

[FIGURE 7 OMITTED]

[ILLUSTRATION OMITTED]

[FIGURE 8 OMITTED]

The photoinitiated polymerization of ERL-4221 was performed in the presence and absence of BPO with C8-CFS [PF.sub.6] as the photoinitiator. The results are shown in Fig. 7. The figure shows a marked increase in the photopolymerization rate of ERL-4221 in the presence of 1% BPO, compared with the absence of BPO. Increasing the amount of BPO further produced an increase in the monomer conversion. When a mix of 1% C8-CFS [PF.sub.6] + 1% BPO was used as photoinitiator, the photopolymerization rate of ERL-4221 was almost equal to that of 3% C8-CFS [PF.sub.6].

Thermally Induced Cationic Polymerizations

Recently, the use of a number of sulfonium salts for thermally induced cationic polymerization has been reported (16), (17). Ferrocenium salt photoinitiators are shelf-stable in the presence of even highly reactive monomers and oligomers, and can be kept for extended periods of time in the dark. An important property of photoresists is the inherent thermal stability of the photoinitiator, as pre and postexposure bakes can influence the physical state of the material. This is particularly true for cationic photoinitiator/epoxy blends because cross-linking of unexposed areas must be avoided to maximize the photoresist contrast. To assess the likelihood of cross-linking and the capacity for thermally induced cationic polymerization, DSC was employed (18).

Thermally induced polymerizations of ERL-4221 and TDE-85 were examined with I wt% of C4-CFS [PF.sub.6], and C8-CFS [PF.sub.6]. The results are shown in Fig. 8 and Table 2. The thermally induced activities of C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] are nearly equivalent. The onset temperature of polymerization initialed by C8-CFS [PF.sub.6] is slightly higher than that of C4-CFS [PF.sub.6]. For TDE-85, the peak temperature of polymerization is approximately 169[degrees]C, and nearly full polymerization occurs, as indicated by a large exothermic peak obtained. In contrast, the thermally induced polymerization of the less reactive ERL-4221 begins at approximately 166[degrees] C, and only a small exothermic peak can be observed.

CONCLUSIONS

Two carbazole bonding ferrocenium salt photoinitiators, [cyclopentadiene-Fe-N-buylcarbazole] hexalluoro-phosphate {C4-CFS [PF.sub.6]) and [cyclopentadiene-Fe-/V-octylcarbazoie] hexafiuorophosphate (C8-CFS [PF.sub.6]) were prepared. These compounds can act as photoinitiators of cationic polymerizations of ERL-4221 and TDE-85 directly upon irradiation with a high-pressure mercury lamp. Studies with real-time infrared spectroscopy have shown that C4-CFS and C8-CFS photoinitiators exhibit high efficiency in the polymerization. BPO sensitizer is very effective in developing the photoinitiating activity of CFS in the polymerization of both ERL-4221 and TDE-85. UV absorption spectral changes of C4-CFS [PF.sub.6] and C8-CFS [PF.sub.6] in C[H.sub.2][C1.sub.2] after irradiation showed that CFS [PF.sub.6] undergoes photolysis to generate an iron-based Lewis acid upon the loss of the arene ligand N-alkyl carbazole. DSC studies have shown that C4-CFS and C8-CFS photo-initiators can also be employed as thermal initiators for the cationic ring-opening polymerization of epoxides at moderate temperatures.

TABLE 2. Properties of mixtures of arene iron sails and epoxides from
D.S.C. scans.

                    Onset          Peak
                 temperature   temperature
Photoinitiator/  ([degrees]C)  ([degrees]C)  [DELTA]H(J/g)
epoxide

C4-CFS              114.80        166.99          99.14
[PF.sub.6]/
ERL-42210

C4-CFS              114.81        169.99         295.05
[PF.sub.6]/
TDE-85

C8-CFS              121.04        166.89          85.89
[PF.sub.6]/
ERL-422I

C8-CFS              119.94        170.36         242.71
[PF.sub.6]/
TDB-85

ACKNOWLEDGMENTS

The authors acknowledge Dr. Nie Jun for supplying with RTIR instrument.

REFERENCES

(1.) V. Crivello and J.H.W. lam, J. Polym, Sci. Symp., 56, 383 (1976).

(2.) J.V. Crivello and J.H.W. Lam, J. Polym. Sci., Chem. Ed., 18, 1021 (1980).

(3.) J.V. Crivello, U.S. Patent 3,981,897 (1976).

(4.) J.V. Crivello and J.H.W. Lam, J. Polym. Sci., Chem. Ed., 17, 2877 (1979).

(5.) J.V. Crivello and J.L. Lee, J. Polym, Sci., Chem. Ed., 28, 479 (1990).

(6.) J.V. Crivello, J. Polym. Sci: Part A: Polym Chem., 37, 4241 (1999).

(7.) J.V. Crivello and M. Jang, J. Phpiochem. Photobio. A: Chem., 159, 173 (2003).

(8.) B. Falk, M.R. Zonca, and J.V. Crivello., J. Polym, Sri.: Parr A: Polym. Chem., 43. 2504 (2005).

(9.) C.H. Park. S. Takahara. and T. Yamaoka, Polym, Adv. Technol. 17, 156 (2006).

(10.) K. Meier, N. Buehler, H. Zweifel, G Berner. and F. Lohse, Eur. Patent 94,915 (1983).

(11.) K. Meier and H. Zweifel, J, Image. Sci., 30. 174 (1986).

(12.) F. Lohse and H. Zweifel. Adv. Polym. Sri., 78. 69 (1986).

(13.) T. Wang. Y.L. Huang, and Sh. J. Shi. Chem . J. Chinese Univ., 24, 735 (2003).

(14.) T. Wang. L.J. Ma, P.Y. Wan, J. Photockem. Plutobio. A: Chem.. 16. 377 (2004).

(15.) T. Wang, B. Sh. Li, and L.X. Zhang. Polym, Int.. 54. 1251 (2005).

(16.) O. Shimomura, I. Tomita, and T. Endo. J. Polym. Sci.: Part A: Polym. Chem., 38, 18 (2000).

(17.) S. Ark, g. Heo, and D. Suh, J. Polym. Sci.: Part A: Polym. Chem., 41, 2393 (2003).

(18.) S. Murai, Y. Nakano, and S. Hayase. J. Appl. Polym. Sci., 80, 181 (2001).

Tao Wang, (1) Zhiquan Li, (1) Ying Zhang, (1) Kafssi Hassan, (1) Xiaoning Wang(2)

(1) State Key Lab of Chemical Resource Engineering, College of Science, Beijing University of chemical Technology, Beijing 100029, People Republic of china

(2) College of Material Engineering, Beijng Institute of Fashion Technology, Beijing 100019, People's Republic of China

Correspondence to: Tao Wang; e-mail: [e-mail]

Contact grant sponsor: National Natural Science Foundation (China); contract grant number: 20676012.

DOI 10.1002/pen.21329

Published online in Wiley InterScience (www.interscience.wiley.com).

[C] 2009 Society of Plastics Engineers