Chinese Chemical Letters  2015, Vol.26 Issue (01):21-25   PDF    
Benzoylformamides as new photocaged bases for photo-latent anion polymerization
Ming-Hui Hea, Mei-Lin Sub, Zhao-Hui Yua, Guang-Xue Chena, Zhao-Hua Zengb, Jian-Wen Yangb     
a State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou 510640, China;
b Institute of Polymer Science, DSAPM Lab, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
Abstract: Benzoylformamide (BFA) derivatives are proposed as new photocaged bases with good solubility in epoxy resin. Initially their structures were confirmed by 1HNMR, 13C NMR, and elemental analysis. Next, we detail their thermal stability, solubility behavior, and photolysis products. Furthermore, the model photo-latent anion polymerization (AP) of epoxide system in the presence of BFA-dBA (N,N-dibenzyl-2-oxo-2-phenylacetamide) as a photocaged base has been investigated, and excellent photopolymerization profile is obtained.
Key words: Anion polymerization     Photocaged superbase     Photopolymerization    
1. Introduction

In recent years,photopolymerizations have received revitalized interest as they promise a wide range of economic and ecological advantages,while photoinitiators (PIs) and photoinitiating systems (PISs) have been the subject of intense studies [1, 2, 3, 4, 5, 6, 7]. However,the design and preparation of highly versatile PIs, which are able to initiate versatile PISs such as free radical polymerization (FRP),cationic polymerization (CP),and free radical promoted cationic photopolymerization (FRPCP),is an on-going challenge in chemical and material sciences with many applications in a wide range of industrial fields ranging from radiation curing,imaging,and optics technologies to medicine and microelectronics areas [8]. In this context,many new structures of PIs were reported recently [9, 10, 11, 12, 13, 14, 15, 16, 17] and exhibited a good radical initiating ability. Nevertheless,there is still room for the continuous discovery of new versatile PIs.

Indeed,the already reported photosensitive quaternary ammonium salt (QAS) [18, 19, 20, 21],as an archetypical photocaged base,not only can liberate the amine molecules that allow for anionic polymerization of epoxides,but also immediately initiate FRP of acrylates [22, 23, 24]. However,this type of photocaged base is not soluble in monomers and common organic solvents,limiting its practical use in UV curing applications [25]. Improving the solubility attained or meeting new or promising possibilities for applications require a continuous search of new structures. Following our works in the area of photocaged bases [26, 27, 28, 29, 30, 31], we explore here the possibility of using benzoylformamide (BFA) derivatives as photocaged bases of anionic polymerization.

As known,photochemistry of BFA involved in photocyclization or asymmetric synthesis has already been investigated in organic chemistry [32, 33, 34, 35, 36]. Previous researchers had elucidated that the photocleavage of BFA usually processed the identical pathway, yielding oxazolidin-4-ones,β-lactams,and mandelic acid derivatives [37, 38]. However,to the best of our knowledge,BFAs have not been reported as PIs in photopolymerization. Furthermore,few reports have been concerned with Norrish type II photolysis products of BFAs possessing alicyclic-amine [39, 40],and we speculate novel heterocyclic photolytic compounds may be produced. The involved mechanisms will be investigated by UV- vis spectra,NMR,CG-MS,ESI-MS,and RTIR experiments. 2. Experimental

Methyl benzoylformate (MBF,>99.0%) was provided by Changzhou Tronly (China). Piperidine (PD,≥99.5%),4-benzylpiperidine (BPD,98%),morpholine (ML,99%),pyrrolidine (PRL,99%), N-isopropylaniline (iPA,99%),dibenzylamine (dBA,98%),and oxalyl chloride (98%) were purchased from Aladdin-reagent (China). Bisphenol A epoxy resin (E51,Blue Star New Chemical Material) was used as received. All other chemicals used were analytical grade and used without further purification.

The NMR spectra were obtained on a Varian 300 MHz spectrometer with CDCl3 and TMS as the solvent and internal standard,respectively. UV-vis absorption spectra were obtained on an Agilent 8453 spectrophotometer. Thermogravimetric (TG) tests were performed in the 40-500 ºC range,using a TG-209 Netzsch thermogravimetric analyzer at a heating speed of 20 ºC/min under N2 atmosphere. Elemental analysis was obtained on an Elementar Vario EL analyzer. Gas chromatography-mass spectrometry (GC-MS) spectra were obtained from a Finnigan Voyager GC-MS. Electrospray ionization mass spectra (ESI-MS) were acquired on a Thermo Finnigan LCQ DECA XP ion trap mass spectrometer,equipped with an ESI source. Epoxide conversions were monitored by Nicolet 5700 Fourier transform infrared spectroscopy.

Representative procedure for synthesis of benzoylformamides except BFA-iPA: Methyl benzoylformate (10 mmol) in methanol (10 mL) was added dropwise to a stirred mixture of amine (30 mmol) and methanol (20 mL) at 55 ºC and then stirred for 3 h, distilled to concentrate the solution,and crystallized in the refrigerator. The crystal was filtered,washed with cooled methanol,and dried in vacuum to give white crystals.

1-(Phenylglyoxylyl)piperidine (BFA-PD): Yield: 30.3%. 1H NMR (300 MHz,CDCl3): δ 1.54-1.58 (m,2H),1.69-1.73 (m,4H),3.28- 3.32 (t,2H,J = 6.0 Hz),3.70-3.72 (m,2H),7.48-7.53 (t,2H, J = 6.0 Hz),7.61-7.66 (t,1H,J = 6.0 Hz),7.93-7.96 (d,2H, J = 9.0 Hz). 13C NMR (300 MHz,CDCl3): δ 24.7,25.8,26.6,42.5, 47.4,129.2,129.7,133.4,134.9,165.5,192.0. Anal. calcd. for C13H15NO2: C,71.87; H,6.96; N,6.45%. Found: C,71.59; H,7.31; N, 6.36.

1-(Phenylglyoxylyl)-4-benzylpiperidine (BFA-BPD): Yield: 39.2%. 1H NMR (300 MHz,CDCl3): δ 1.14-1.36 (m,2H),1.59- 1.64 (m,1H),1.78-1.82 (m,2H),2.54-2.56 (d,2H,J = 6.0 Hz),2.69- 2.76 (t,1H,J = 12.0 Hz),2.94-3.03 (t,1H,J = 15.0 Hz),3.50-3.54 (d, 1H,J = 12.0 Hz),4.61-4.66 (d,1H,J = 15.0 Hz),7.08-7.11 (d,2H, J = 9.0 Hz),7.14-7.19 (t,1H,J = 9.0 Hz),7.23-7.27 (t,2H,J = 6.0 Hz), 7.45-7.50 (t,2H,J = 6.0 Hz),7.58-7.63 (t,1H,J = 6.0 Hz),7.90-7.92 (d,2H,J = 6.0 Hz). 13C NMR (300 MHz,CDCl3): δ 32.0,32.7,38.5, 41.8,43.2,46.6,126.3,128.5,129.2,129.7,133.4,134.8,139.8, 165.5,192.0. Anal. calcd. for C20H21NO2: C,78.15; H,6.89; N,4.56%. Found: C,77.75; H,7.04; N,4.15.

1-(Phenylglyoxylyl)morpholide (BFA-MP): Yield: 27.8%. 1H NMR (300 MHz,CDCl3): δ 3.35-3.38 (t,2H,J = 3.0 Hz),3.62-3.65 (t,2H,J = 3.0 Hz),374-3.80 (m,4H),7.46-7.51 (t,2H,J = 9.0 Hz), 7.60-7.65 (t,1H,J = 6.0 Hz),7.92-7.95 (d,2H,J = 9.0 Hz). 13C NMR (300 MHz,CDCl3): δ 42.0,46.6,66.9,67.0,129.2,129.8,133.4, 135.0,165.5,191.1. Anal. calcd. for C12H13NO3: C,65.74; H,5.98; N, 6.39%. Found: C,65.56; H,6.11; N,6.25.

1-(Phenylglyoxylyl)pyrrolidine (BFA-PrD): Yield: 34.5%. 1H NMR (300 MHz,CDCl3): δ 1.89-2.02 (m,4H),3.40-3.45 (t,2H, J = 6.0 Hz),3.63-3.68 (t,2H,J = 6.0 Hz),7.46-7.52 (t,2H,J = 9.0 Hz), 7.59-7.65 (t,1H,J = 9.0 Hz),7.96-8.00 (d,2H,J = 9.0 Hz). 13C NMR (300 MHz,CDCl3): δ 24.3,26.2,45.5,46.9,129.1,129.9,133.0, 134.8,165.0,191.6. Anal. calcd. for C12H13NO2: C,70.92; H,6.45; N, 6.89%. Found: C,70.71; H,6.48; N,6.35.

N,N-Dibenzyl-2-oxo-2-phenylacetamide (BFA-dBA): Yield: 42.8%. 1H NMR (300 MHz,CDCl3): d 3.86 (s,4H),7.18-7.37 (m, 10H),7.43-8.01 (m,5H). 13C NMR (300 MHz,CDCl3): d 50.1,128.6, 129.0,130.2,131.5,133.6,134.3,170.6,194.9. Anal. calcd. for C22H19NO2: C,80.22; H,5.81; N,4.25%. Found: C,80.11; H,5.94; N, 4.16.

N-N-Isopropylbenzoylformanilide (BFA-iPA): To a CH2Cl2 solution (20 mL) of benzoylformic acid (3.00 g,30.0 mmol) prepared from methyl benzoylformate by basic hydrolysis with aqueous NaOH solution (quantitative) was added catalytic amount of DMF (1 drop) and oxalyl chloride (1.9 mL,22.0 mmol). The reaction mixture was stirred for 4 h at room temperature until generation of gases stopped and then cooled to 0 ºC. To this was added a solution of N-isopropylanilin (3.2 mL,29.9 mmol) and Et3N (7.0 mL, 50.2 mmol). The reaction mixture was stirred at room temperature for 1 h and then quenched by adding water. Organic materials were extracted 3 times with ethyl acetate,and then the combined extracts were washed successively with a 5% HCl aqueous solution and saturated NaCl solution,dried over anhydrous MgSO4, evaporated to give the corresponding product,and recrystallized from ethyl acetate to give colorless crystals. Yield: 35.4%. 1H NMR (300 MHz,CDCl3): d 1.22-1.24 (d,6H,J = 6.0 Hz),5.01-5.15 (m,1H, J = 6.0 Hz),7.05-7.78 (m,10H). 13C NMR (300 MHz,CDCl3): d 21.2, 47.0,128.8,129.1,129.4,131.2,133.8,134.2,135.7,166.8, 190.0. Anal. calcd. for C17H17NO2: C,76.38; H,6.41; N,5.24%. Found: C,75.89; H,6.50; N,5.16.

Photodecomposition of BFA-dBA: First,BFA-dBA in acetonitrile (1 ×10-4 g/mL) was put into a quartz cell. Next,the photodecomposition was carried out by optical cable-directed UV lamp (RWUVA- Φ200U,Runwing Co. China) over time (0 min,1 min,2 min, 3 min),then UV-vis spectra were measured.

Photoinitiated thermal anion polymerization: Mixtures of BFAdBA (5 ×10-4 mol) and a commercial epoxide E51 (1 g) were spread on KBr plates. The mixtures were irradiated on the UV curing machine for 10 min. In all photochemical experiments,the optical cable-directed UV lamp operating in the 200-400 nm range (RW-UVA-Φ200U,Runwing Co.,China) was used as the irradiation source,and the light intensity at the surface level of the cured samples was 20 mW/cm2 measured by a UV-radiometer (type UVA, Photoelectric Instrument Factory,Beijing Normal University). Then some of the mixtures were baked at high temperature. The results were observed visually and measured quantitatively by FTIR spectra. 3. Results and discussion

In this paper,we endeavor to synthesize BFAs possessing alicyclic-amine in order to test the photochemical characteristics. Furthermore,their Norrish type II photolysis products may involve basic heterocyclic compounds,which is of particular current interest as they can be exploited in the design of new photocaged bases. Hence,BFAs 1-5 (Scheme 1) with different amide substituents were successfully synthesized from methyl benzoylformate (MBF) by transesterification reaction with secondary amine (Scheme 2a) [41]. Due to the weak basicity of Nisopropylanilin, transesterification reaction with MBF was difficult, but BFA-iPA can be prepared according to the literature (Scheme 2b) [42]. The structures of BFAs 1-6 were confirmed by 1H NMR, 13C NMR,and elemental analysis.

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Scheme 1. Structures of the benzoylformamides (BFAs) 1-6.

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Scheme 2. Synthesis of BFAs 1-6.

Thermal stability is one of the important parameters of photocaged bases. When the stability of the initiator is very low, the pot life may be short,making the system less useful. Indeed, the observed order of thermal stability for BFAs is BFA-BPD (246.6 ºC) > BFA-iPA (188.6 ºC) > BFA-MP (172.2 ºC) > BFA-PD (168.6 ºC) > BFA-PrD (164.6 ºC) > BFA-dBA (139.2 ºC) (Fig. 1, Table 1). Noting that the initial decomposition temperatures of all BFAs are above 135 ºC,we conclude the BFAs have good thermal stability at room temperature. Among the BFAs studied, BFA-BPD is the most stable one,while BFA-dBA has the worst thermal stability. In the paper,we focus on the photochemistry properties of BFAs. Actually,since the thermal stability of BFAs is quite different,this may be used for thermal latent anion polymerization of epoxy resin,and detailed research is worthy of further exploration.

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Fig. 1. Thermogravimetric profiles of BFAs with heating rate at 20 ºC/min under N2

Table 1
Thermal stability of BFAs.

Introducing the amide group to benzoyl skeleton drastically changes the solubility behavior of the BFAs. Of these photocaged bases,liquid BFA-PrD is readily soluble in the bisphenol A epoxy resin E51,BFA-PD,BFA-BPD,BFA-MP,BFA-iPA and BFA-dBA showed moderate solubility after adequate mixing at 50 ºC. Thus, BFA derivatives as photocaged base exhibit sufficient solubility in E51,and may provide a promising future for its practical use in UV curing applications.

The absorption spectra of the investigated BFAs in chloroform are given in Fig. 2. All BFAs exhibit similar absorption characteristics with maxima at 286 nm and 355 nm,and a tail over 400 nm,exhibiting a red shift compared with that of the parent compound,methyl benzoylformate (MBF,λmax = 255 nm), due to the presence of the amide substituents on the benzoyl skeleton.

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Fig. 2. UV-vis absorption spectra of BFAs in chloroform (1 ×10-4 g/mL).

In the paper,BFA-dBA was selected here for the model mechanistic investigation,i.e. this photocaged base was characterized both by the UV absorption properties and photopolymerization (see below). As shown in Fig. 3,photolysis of BFA-dBA in chloroform was investigated by UV-vis spectroscopy. BFA-dBA exhibits strong UV absorption with molar extinction coefficients of 1.52 × 103 L mol-1 cm-1 at 288 nm,which is attributed to the π-π* transition; the maximum at 368 nm with molar extinction coefficients of 7.38 × 101 L mol-1 cm-1 can be explained by the n-π* transition. Furthermore,distinct decreases in absorption bands at 288 nm and 368 nm were observed with prolonged irradiation time.

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Fig. 3. UV-vis spectral changes of BFA-dBA in chloroform (1 ×10-4 g/mL) being irradiated over time.

The photolysis products formed from irradiated BFA-dBA in chloroform were identified by 1H NMR,followed by GC-MS and ESI-MS assisted product analysis. From Fig. 4,most proton signals in BFA-dBA weakened after 30 min irradiation,meanwhile new protons Ha (10.0 ppm),Hb (8.3 ppm),and Hc (4.8 ppm) appeared. According to the literature [43],we can speculate that the abovementioned new protons should correspond to -CHO in benzaldehyde and -CH=N- and -CH2-Ph in N-benzylidenebenzylamine, respectively. Normalized by an internal standard peak at TMS,the ratios of the photolysis products were calculated by the internal standard method. As shown in Scheme 3,after 30 min irradiation, the yields of benzaldehyde and N-benzylidenebenzylamine were 48.5% and 21%,respectively. Due to a superposition with the parent molecule BFA-dBA,the signals of N-dibenzylamine are difficult to detect by NMR. Moreover,GC-MS study of the photolysis products confirmed the formation of benzaldehyde (M' = 106) and Nbenzylidenebenzylamine (M' = 195). Astonishingly,basic photolysis compounds from irradiated BFA-dBA were detected by pHindicator paper. Since GC-MS failed to detect an amine compound, ESI-MS was employed to probe photogenerated base in Fig. 5. It should be noticed that the detection of protonated dibenzylamine dBA+ (m/z = 198.2) after irradiation was direct evidence for the formation of dBA from BFA-dBA,and thus BFA-dBA also can act as a novel photocaged base. We speculate that the photolysis of BFAdBA may undergo Norrish type II elimination reaction,and the photolysis products of BFA-dBA are identified as benzaldehyde,Nbenzylidenebenzylamine, N-dibenzylamine and some unknown substances. Taking the foregoing points into consideration,the photolysis products of BFA-dBA can be listed in Scheme 3.

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Fig. 4. 1H NMR spectral changes of BFA-dBA (a) before and (b) after 30 min irradiation.

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Fig. 5. The protonated N-dibenzylamine from irradiated BFA-dBA in chloroform (0.01 mmol/L) detected by ESI-MS.

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Scheme 3. Photolysis products of BFA-dBA in chloroform on excitation at 365 nm.

This BFA-dBA also can provide a promising future for its application to photolatent amine-catalytic ring-opening polymerization (ROP) of epoxide systems. The chemical transformation in an E51 film containing BFA-dBA was followed by monitoring the characteristic band of the epoxide ring at 915 cm-1 in FT-IR spectra to obtain some information concerning the photo-latent amine catalytic reaction (Fig. 6). After UV-exposure of 10 min,no alteration of the spectral shape (Fig. 6,curve 2) was observed after 60 min at room temperature compared to the pristine E51 (Fig. 6,curve 1),suggesting that the crosslinking of the epoxide is not essentially induced before post-exposure baking. When the film was heated at 120 ºC after UV-exposure,the peak intensity of the epoxide at 915 cm-1 gradually decreased over time (curve 3-4). This situation is visualized by plotting the decreased peak area as a function of heating time,as given in Fig. 7. No marked spectral alteration was observed within 9 min,but the absorption band of epoxide abruptly decreased thereafter,and 80% conversion was obtained after 100 min baking. These results indicate that the crosslinking reaction of E51 can be induced by the photogenerated dibenzylamine from BFA-dBA.

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Fig. 6. IR spectra of E-51 thermally cured by irradiated BFA-dBA (pristine E51 - curve 1,the spectral shape for 60 min at room temperature after UV-exposure of 10 min - curve 2,bakes at 120 ºC for 20 min - curve 3,250 min - curve 4).

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Fig. 7. Conversion vs. time curves for photo-latent amine-catalytic reaction of E51 heating at 120 ºC after UV-exposure of 10min with the light intensity of 20mW/cm2. Experimental conditions: BFA-dBA = 5 ×10-4 mol,E51 = 1 g.
4. Conclusions

Benzoylformamides (BFAs) derivatives as novel photocaged baseswere designed and synthesized. Initially their structureswere confirmed by 1H NMR,13C NMR,and elemental analysis. Next,we detail their thermal stability,solubility behavior and photolysis products. The results show that BFAs have good thermal stability at room temperature and exhibit sufficient solubility in epoxy resin E51. Meanwhile,photolysis products of BFA-dBA are identified as benzaldehyde,N-benzylidenebenzylamine,N-dibenzylamine and some unknown substances. Furthermore,the model photo-latent anion polymerization (AP) of epoxide system in the presence of BFA-dBA (N,N-dibenzyl-2-oxo-2-phenylacetamide) as a photocaged base has been investigated,and 80% conversionwas obtained at 100 min baking after UV-exposure of 10 min. This photodecomposition reaction is of particular interest as it is can facilitate the design of new photocaged bases with very promising properties.

Acknowledgments

This research was financially supported by National Natural Science Foundation of China (No. 21374135), China Postdoctoral Science Foundation (Nos. 2013M542178 and 2014M562183), the Open Foundation of the State Key Laboratory of Pulp and Paper EngineeringinSouthChinaUniversityofTechnology(No.C713043z), and the Fundamental Research Funds for the Central Universities (No. 2013ZB0025).

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