Phototriggered Base Proliferation: A Highly Efficient Domino Reaction

Article pubs.acs.org/Macromolecules

Phototriggered Base Proliferation: A Highly Efficient Domino Reaction for Creating Functionally Photo-Screened Materials Minghui He,†,‡ Xun Huang,† Zhaohua Zeng,† and Jianwen Yang*,† †

Institute of Polymer Science, DSAPM Lab, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China ‡ State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Phototriggered base proliferation as a highly efficient domino reaction is presented for creating functionally photo-screened materials, providing a strategy for the photopolymerization of shadow areas via chemically diffuse amines toward the nonirradiated areas during polymerization. By integrating proliferated amines with a peroxide initiator (dibenzoyl peroxide, BPO), phototriggered self-propagating polymerization of acrylate monomers in three-dimensional space was achieved. The advantages of this approach lie in its enhanced photosensitivity, increased propagating velocity, and elevated double-bond conversion (90%) while reducing the local high temperature and the minimum BPO concentration that sustain a traveling front. Astonishingly, the propagating velocity and local maximum temperature can be well-modulated by varied BPO concentration and the appropriate amount of BA-BPD (1-(9-fluorenylmethoxycarbonyl)-4-benzylpiperidine) concentration, respectively. Finally, functionally photo-screened material containing carbon nanotubes was successfully prepared by phototriggered base proliferation reactions.

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INTRODUCTION Phototriggered base proliferation reaction, an autocatalytic decomposition of base amplifiers (APs) triggered by photocaged base, was pioneered by Ichimura and Arimitsu in 2000.1 Reaction classified as “phototriggered” and “proliferated” displays several hallmarks that underlie their utility in photochemistry: high sensibility, fast proliferation rate, and short reaction times. 9-Fluorenylmethyl carbamate has emerged as the archetypical AP and has found promising applications in photosensitive materials.1−3 Practically, this system was used to prepare a negative working photoresist consisting of poly(glycidyl methacrylate), exhibiting marked improvement in the photosensitivity.1,2 A handful of other chemical compounds have also been recognized as APs, including but not limited to phenylsulfonyl carbamate4 and 3-nitropentan-2-yl carbamate.5 Unfortunately, the present base proliferation reactions have some limitations on the resolution power enhancement due to high amine volatility.3 Accordingly, Ichimura and Arimitsu endeavored to design Aps to liberate the corresponding macromolecular amines3,6 or inhibited the spreading behavior by dipping in a 2 wt % lactic acid aqueous solution. On the contrary, supposing that this unique proliferation characteristic can be well-utilized, phototriggered base proliferation reactions would be of great applied foreground in the material synthesis, especially in the functionally photo-screened materials, which could expand far beyond photoinitiated polymerization. © XXXX American Chemical Society

To date, photopolymerization has been primarily limited to the synthesis of thin-film materials, such as high-tech domains, optoelectronics, laser imaging, stereolithography, and nanotechnology as well as conventional areas of high performance coatings, inks, and adhesives.7 The major challenge in photoscreened materials such as dental filling material for a deep cavity, fabrication of 3D objects, shadow areas or deep layers of thick pigmented samples, and functional polymer composites arises from the light not being able to penetrate into deeper layers due to the light-intensity gradient.8,9 In addition, the requirement of a direct line by which a light beam can reach a surface is another major drawback for photopolymerization of shadow areas.10 These limitations prevent its use in a variety of applications. In this context, we envisaged that the unique properties of base proliferation reactions would allow for the highly responsive synthesis of photo-screened materials. Under photoirradiation at a localized region, the regenerated superbase from photocaged base can diffuse into unirradiated regions and effectively actuate base proliferation reactions in threedimensional space. Abruptly proliferated amines can be utilized to catalyze versatile types of organic reactions due to the nature Received: May 12, 2013 Revised: July 29, 2013

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dx.doi.org/10.1021/ma400983t | Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Chemical structures of QA-DBU, BA-BPD, BPO, TMPTA, and AA used in this study. (b) Schematic of phototriggered selfpropagating polymerization.

Figure 2. Mechanism of phototriggered self-propagating polymerization. triacrylate (TMPTA, Sartomer Company) were used as received. Multiwalled nanotubes (MWNT) was purchased from Chengdu Organic Chemical Company (China Academy of Sciences); hydroxylfunctionalized carbon nanotubes (CNT-OH) were synthesized according to the literature procedure.12 Conductive liquid silver paint was received from SAE Magnetics Ltd. Dichloromethane (CH2Cl2) was dried over calcium chloride, refluxed with calcium hydride (CaH2), and then distilled. All other chemicals used were analytical grade and used without further purification. The chemical structures of QA-DBU, BA-BPD, BPO, TMPTA, and AA are shown in Figure 1a. Methods. The NMR spectra were obtained on a Varian 300 MHz spectrometer with DMSO-d6 and TMS as the solvent and internal standard, respectively. A Nikon video camera was set up on one side of the test tube to record the initiation of the self-propagating polymerization and its velocity in cm/min. The temperature profile was measured by using an infrared (IR) thermal imaging camera (Fluke Ti32). Electrospray ionization mass spectra (ESI-MS) were acquired on a Thermo Finnigan LCQ DECA XP ion trap mass spectrometer, equipped with an ESI source; the obtained polymer at different regions was first immersed into DMSO (1 g mL−1) for 15 min, and then the filtered solution was measured. FTIR spectra were obtained on a Thermo Nicolet/Nexus 670 spectrophotometer and recorded from 32 scans with a resolution of 4 cm−1. Electrical conductivity of the composite material was calculated from the reciprocal of the resistivity measured by a Hewlett-Packard multimeter 34401 A (HP, Santa Clara, CA); the samples were coated with conductive silver paint to improve the contact between the probe and the polymer.

of amine molecules itself such as basicity, nucleophilicity, compatibility, and the more important reducibility. In this paper, by integrating proliferated amines with peroxide initiators, amine-mediated redox initiation system as the most effective method of generating free radicals under mild conditions would be immediately activated by itself, affording improved spatial and temporal control of the redox polymerization. More recently, we reported11 photoinitiated remarkable postconversion due to the persistent interaction of photoproduced longeval amine with peroxides in thin materials. Remarkably, the final conversion at 5 h was more than twice as high as the threshold value of conversion when the light source was extinguished. Thus, we speculate that the proliferated amine species derived from base proliferation reactions can exert more control over the self-propagating polymerization and then enhance propagation velocity in three-dimensional space, striving to provide a promising future for their application to photo-screened materials.

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EXPERIMENTAL SECTION

Materials. QA-DBU was synthesized according to the literature procedure.11 Dibenzoyl peroxide (BPO) and acrylic acid (AA, >99.5%) were purchased from Aladdin-reagent (China). BPO was purified by dissolving in CHCl3 at room temperature and precipitating by adding an equal volume of MeOH. 9-Fluorenylmethyl chloroformate (Fmoc-Cl, 98%, Shanghai Darui Finechem), 4benzylpiperidine (98%, Alfa Aesar Company), and trimethylolpropane B

dx.doi.org/10.1021/ma400983t | Macromolecules XXXX, XXX, XXX−XXX

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Preparation of 1-(9-Fluorenylmethoxycarbonyl)-4-benzylpiperidine (BA-BPD). To an ice cooled solution of Fmoc-Cl (2.00 g, 7.72 mmol) in CH2Cl2 (50 mL) was slowly added 4-benzylpiperidine (2.70 g, 15.4 mmol) in CH2Cl2 (20 mL), and then the mixture was stirred under cooling for 20 min and at room temperature for 1.5 h. The solution was washed thrice successively with water, a 5% HCl aqueous solution, and a saturated NaCl solution, dried over anhydrous MgSO4, evaporated to give a colorless viscous liquid, and then further purified by silica gel column chromatography using hexane:EtOAc (10:1) to give a pure product as light yellow oil. Yield: 85%. 1H NMR (300 MHz, DMSO, δ, ppm): 7.83 (d, 2H), 7.58 (d, 2H), 7.36 (m, 4H), 7.25 (d, 2H), 7.15 (m, 3H), 4.40 (d, 2H), 4.24 (t, 1H), 3.80 (d, 2H), 2.65 (m, 2H), 2.48 (d, 2H), 1.68 (m, 1H), 1.50 (d, 2H), 0.95 (m, 2H). 13C NMR (300 MHz, DMSO, δ, ppm): 154.89, 144.53, 141.44, 140.57, 129.63, 128.79, 128.24, 127.71, 126.46, 125.58, 120.75, 66.96, 47.69, 44.36, 42.90, 37.90, 32.07. IR (KBr, cm−1): 2920, 1699, 1445, 1276, 1237, 1177, 1117, 1080, 739. Anal. Found: C, 80.76; H, 6.82; N, 3.42. Calcd for C27H27NO2: C, 81.58; H, 6.85; N, 3.52%. Base Proliferation in Solution. To a DMSO-d6 solution of BABPD (100 mmol L−1) in a NMR tube was added amine (5 mmol L−1). The tube was sealed and subjected to real-time NMR measurement at different temperatures. The proton of the methylene group was used to monitor the consumption of the BA-BPD. Phototriggered Self-Propagating Polymerization Procedure. A typical procedure used as follows: QA-DBU (0.0715 g, 1.1 × 10−4 mol), BA-BPD, and BPO were dissolved in DMSO (2.5 mL) under ultrasonication, and then TMPTA (4 g) and AA (1 g) were added to this solution. Next, the resulting mixtures were placed in a 60 mm diameter Petri dish (12 mm height), and then the reactions were ignited by a UV source (RW-UVA-Φ200U, Shenzhen Runwing Company, China) with a dominant wavelength of 365 nm. The light intensity was 20 mW cm−2 measured by a UV radiometer (type UV-A, Photoelectric Instrument Factory, Beijing Normal University). The optical fiber cable was placed at about 1 cm from the center of the circular mixture, the obtained diameter of irradiation area was about 1 cm, and then held in position until a propagating front started, as shown in Figure 1b.

monomer acrylic acid (AA) was added to the polymerization system, an instantaneous self-propagating polymerization occurred. Second, photocaged superbase (QA-DBU) exhibits good absorption characteristics (Figure s1) and can liberate the superbase (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU) confirmed by our recent report.11 Moreover, this photoexcited quaternary ammonium salt (QA-DBU) can immediately initiate free radical polymerization of acrylates, which partly facilitates the diffusion of amine species more or less because of crowding-out effect of polymers. Third, the liberated amine from base amplifier should be strong enough to catalyze subsequent chemical reactions, while possessing lower volatility. Consequently, we designed 1-(9-fluorenylmethoxycarbonyl)-4benzylpiperidine (BA-BPD). Fourthly, base proliferation reaction is solvent-dependent.2 Namely, the higher the polarity of solvent, the faster the proliferation efficiency, and hence we utilized the strong polar dimethyl sulfoxide as the polymerizations medium. 2. Physically Diffuse Amine-Controlled Self-Propagating Polymerization. We first investigated the physically selfpropagating polymerization based on the photocaged superbase and readily available peroxides (Figures s2 and s3). Considering that crowding-out effect of polymer facilitated the diffusion of photogenerated amines from irradiated areas toward the nonirradiated areas during polymerization, it could be envisioned that the concentration of the diffuse superbases at the polymerization front will gradually reduce because of the corresponding increase of frontal area (Figure s4). Thus, for the given dimensions (60 × 3 mm), the irradiation time to initiate self-propagating polymerization (the induction period, Tstart) was 0.35 min, and the nonirradiated region was polymerized within 0.9 min. Remarkably, propagating velocity V first undergoes a gradual increase and then begins to sharply decrease (Figure 3). Similarly, as shown in spatial temperature

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RESULTS AND DISCUSSION 1. Outline Design. Our efforts were focused on how to realize self-propagating polymerization via phototriggered base proliferation and then successfully synthesize photo-screened materials. Specifically, as shown in Figure 2, the regenerated amine that liberates from photocaged base, as a phototrigger, should promptly actuate high effective base proliferation reactions, and copious amine species are thus generated like the domino reaction. By combining with peroxide initiators, the most effective redox polymerization would be immediately activated. In the same way, the physically diffused amine can open a new proliferation reaction cycle, consequently polymerizing acrylate monomers at shadow areas. In this paper, all polymerization reactions should been performed in open reactors in order to test the applicability of this technique to process conditions often found in practice, so the highly active implement has taken the following points into consideration. First, several preliminary runs were performed in order to verify whether secondary amine−peroxide redox polymerization could occur. Namely, we have found that, by the dropwise addition of 4-benzylpiperidine in the mixture of TMPTA and BPO at room temperature, the self-propagating polymerization of TMPTA could not happen. This can be explained by the fact that secondary amine can act as both nucleophiles and bases in secondary amine−acrylate Michael addition reaction; thus, amine−peroxide redox initiation may be inactivated. Hence, in order to block this unfavorable phenomenon, acid compound was additionally added as an inhibitor of Michael addition reaction. Practically, once the

Figure 3. Propagating velocity vs time. Experimental conditions: [QADBU] = 1.1 × 10−4 mol, [BPO] = 5.5 × 10−4 mol, DMSO = 2.5 mL, TMPTA = 4 g, AA = 1 g.

profiles (Figure s5), the maximum temperature of the front (Tmax) gradually increased from 137 to 165 °C along the radial direction. It is worth noting that instable front temperature is not usable to synthesize advanced materials. We further evaluated the effect of BPO/photocaged superbase molar ratios on the self-propagating polymerization (Figure s6). For lower BPO concentration (3:1, Figure s6e) or higher concentration (15:1, Figure s6a), no propagation occurred. For BPO concentration from 5:1 to 10:1, selfC

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exothermic polymerization system in the paper (≈150 °C), proliferation reaction would be more efficient. 3b. Base Proliferation Reactions Induced Propagating Polymerization. A representative time series of self-propagating polymerization based on base proliferation reactions in a Petri dish and the corresponding propagating velocity as a function of time are shown in Figures 5a and 5b, respectively. In this system, a clear circular front was formed. Furthermore, for the given dimensions (60 × 3 mm), the Tstart was just 0.2 min, a shorter induction period than the above physical diffusion, displaying higher photosensitivity. The nonirradiated region was polymerized within 0.45 min, showing great advantages in shortening the preparation time. It should be emphasized once more that our experiments are performed in open system in order to investigate the accessibility of self-propagating polymerization to the preparation of thin film materials. By this system, a quasi-sigmoidal propagating velocity dependence on time displays an autocatalytic propagation phenomenon, which can be explained below. In irradiation areas, a small amount of photogenerated DBU is used to catalyze the decomposition of BA-BPD (Figure s8), and then BA-BPD undergoes the autocatalytic E1cB elimination reaction, leading to the liberation of a large amount of 4-benzylpiperidine which can be directly evidenced by the detection of protonated 4-benzylpiperidine+ (m/z =176.1) (Figure s9), and begins to burst polymerization via amineinduced decomposition of peroxide (Figure s10). Because of the self-enrichment of amines toward the nonirradiated areas during polymerization, the persistent interaction of produced amines with peroxides results in the highly effective propagation polymerization (Scheme s2). Undoubtedly, frontal polymerization (FP), a mode of converting monomer into polymer via a localized reaction zone that propagates, has played a crucial role in this polymerization process. By integrating phototriggered base proliferation reaction with peroxide initiators, a novel approach of phototriggered redox frontal polymerization is thus achieved. Because of the lower activation energy of the highly effective redox initiation, the proliferated amines can provide the accessibility of facile FP and thus lead to the remarkable self-propagation polymerization. Figure 6a shows the IR thermal montages recorded in the present work, and the temperature of the propagating front as a function of radius region is shown in Figure 6b. It is known that fast fronts allow less time for heat loss, and consequently,

propagating polymerization can effectively actuate, but accompanied by local high temperature phenomenon due to the inhomogeneous diffusion of the superbase. The local maximum temperature is as high as 240.2 °C for BPO concentration of 5:1 (Table s1), which is harmful for preparing thermal sensitive materials. It thus could be predicted that the obtained polymer material would exhibit poor homogeneity and mechanical performance. 3. Chemically Diffuse Amine-Controlled Self-Propagating Polymerization. To overcome the above difficulties such as local high temperature areas and decreased propagating velocity, we introduce base proliferation reactions. 3a. Base Proliferation in Solution. Prior to evaluating propagating polymerization, base proliferation reaction in solution was recorded by 1H NMR (Scheme s1 and Figure s7). By monitoring the disappearing proton signals of methylene, the conversion curve was obtained in Figure 4.

Figure 4. Conversion of BA-BPD (100 mmol L−1) as a function of time in the presence of 4-benzylpiperidine (5 mmol L−1) in DMSO-d6 at 25 °C.

The approximately sigmoidal curve indicates that the decomposition of BA-BPD autocatalytically proceeds to lead to the proliferation of 4-benzylpiperidine at 25 °C; almost 90% conversion was obtained at 45 min. Astonishingly, when the reaction temperature is increased to 80 °C, the reaction profile was too fast to be measured by real NMR, indicating a complete reaction in 1000) of CNTs.13−15 However, the photo-screened feature prevents the use of photopolymerization in a variety of material preparation (Figure s13). In this work, we prepared CNT/polymer composites via phototriggered redox-induced self-propagating polymerization based on base proliferation reactions. Polymerization of acrylate could be instantly initiated with the effective interaction between the photogenerated amine and peroxide (Figure 9).

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (+86)-20-84111138 (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by National Natural Science Foundation of China (Grant No. 20974127). REFERENCES

(1) Arimitsu, K.; Miyamoto, M.; Ichimura, K. Angew. Chem. 2000, 112, 3567−3570. (2) Arimitsu, K.; Ichimura, K. J. Mater. Chem. 2004, 14, 336−343. (3) Igarashi, A.; Arimitsu, K.; Seki, T.; Ichimura, K. J. Mater. Chem. 2008, 18, 560−566. (4) Miyamoto, M.; Arimitsu, K.; Ichimura, K. J. Photopolym. Sci. Technol. 1999, 12, 315−316. (5) Arimitsu, K.; Ito, Y.; Gunji, T.; Abe, Y.; Ichimura, K. J. Photopolym. Sci. Technol. 2005, 18, 227−228. (6) Ichimura, K.; Igarashi, A.; Arimitsu, K.; Seki, T. J. Photopolym. Sci. Technol. 2004, 17, 433−434. (7) Mishra, M. K.; Yagci, Y. Handbook of Vinyl Polymers: Radical Polymerization, Process, and Technology; CRC Press: Boca Raton, FL, 2009. (8) Désilles, N.; Lecamp, L.; Lebaudy, P.; Bunel, C. Polymer 2003, 44, 6159−6167. (9) Ye, S.; Cramer, N. B.; Stevens, B. E.; Sani, R. L.; Bowman, C. N. Macromolecules 2011, 44, 4988−4996. (10) Ermoshkin, A. A.; Neckers, D. C.; Fedorov, A. V. Macromolecules 2006, 39, 5669−5674. (11) He, M. H.; Huang, X.; Huang, Y. G.; Zeng, Z. H.; Yang, J. W. Polymer 2012, 53, 3172−3177. (12) Gao, C.; Vo, C. D.; Jin, Y. Z.; Li, W.; Armes, S. P. Macromolecules 2005, 38, 8634−8648. (13) Guldi, D. M.; Rahman, G.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38, 871−878. (14) Moniruzzaman, M.; Winey, K. I. Macromolecules 2006, 39, 5194−5205. (15) Spitalsky, Z.; Tasis, D.; Papagelis, K.; Galiotis, C. Prog. Polym. Sci. 2010, 35, 357−401.

Figure 9. Sequence of image-processed video frames illustrating the preparation of functionally photo-screened materials (carbon nanotube/polymer composites). Experimental conditions: [QA-DBU] = 1.1 × 10−4 mol, [BA-BPD] = 1.65 × 10−4 mol, [BPO] = 5.5 × 10−4 mol, DMSO = 2.5 mL, TMPTA = 4 g, AA = 1 g, CNT = 0.01 g.

Because of the persistent interaction of produced amines by chemical diffusion with peroxide, remarkable self-propagating polymerization after irradiation, which is significant for radiation cross-linking of photo-screened materials, was thus initially obtained in phototriggered free radical polymerization. Unexpectedly, the composite containing 0.2 wt % CNTs reaches 92% conversion. However, when the amount of CNTs is increased to 1 wt %, the IR signal cannot measure the totally absorptive sample. The electrical conductivity increases by 6 orders of magnitude with 1 wt % CNTs (0.35 S/m) as compared to the pure PTMPTA (poly(trimethylolpropane triacrylate), 2.2 × 10−7 S/m).

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CONCLUSIONS We described a highly efficient strategy for creating functionally photo-screened materials by integrating phototriggered base proliferation reactions with a peroxide initiator. Under photoirradiation at a localized region, the regenerated superF

dx.doi.org/10.1021/ma400983t | Macromolecules XXXX, XXX, XXX−XXX