ISSN 15600904, Polymer Science, Ser. B, 2014, Vol. 56, No. 5, pp. 632–638. © Pleiades Publishing, Ltd., 2014.

FUNCTIONAL POLYMERS

A Novel High Efficiency Benzophenone Based Polymeric Photoinitiator from Ringopening Polymerization of Benzoxazine1 Jingsong Shia, Penghui Chena, Kemin Wanga,*, Jian Lub, and Jun Niec,** a

b

School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu, 213164, China Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou, Jiangsu, 213164, China c School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China *email: [email protected]; **email: [email protected] Received November 14, 2013; Revised Manuscript Received February 9, 2014

Abstract—A novel polymeric photoinitiator based on benzoxazine was synthesized by introducing 4hydroxy benzophenone, diglycolamine and paraformaldehyde into the macromolecular chain. Its molecular structure was characterized by 1H NMR, FTIR, and UV–Vis spectroscopy. Photopolymerization of tripropylene gly col diacrylate and trimethylolpropane triacrylate initiated by polybenzoxazine, benzoxazine, and benzophe none was studied by real time FTIR spectroscopy. The results obtained are discussed. DOI: 10.1134/S1560090414050121 1

INTRODUCTION

UV photopolymerization is a wellestablished method, which proceeds rather quickly even at room temperature or lower [1, 2] and which finds extensive application in the production of paper, board, metal coating, dryfilm resist, etc. [3–10]. Photoinitiators as one of the most important components have attracted a lot of attention. Among them, macroinitiators are of great interest, mainly due to their outstanding proper ties, such as reduced migration, masked unpleasant odors, and improved solubility and compatibility within the formulations [11]. Benzophenone (BP) and its derivatives are low cost and efficient Type II radical photoinitiators in vinyl polymerization, which have been used widely in the presence of amine coinitiator. It may be proposed that the incorporation of both BP and amine into one macromolecule allow to produce more reactive photo initiator.Previous studies have been focused on photo chemical properties of macrophotoinitiators, which contained BP [12]. In addition, recent work has found that chromophore in sidechain is more efficient than that in the main chain [13]. Therefore, in this paper, a macrophotoinitiator with sidechain BP derivative and coinitiator amine in the main chain was synthe sized by ringopening polymerization of benzoxazine. Two representative types of monomers with different functionality, difunctional monomer tripropylene gly col diacrylate (TPGDA) and trifunctional monomer trimethylolpropane triacrylate (TMPTA), were cho sen to be initiated by polymeric photoinitiators and 1 The article is published in the original.

the low molecular weight analogue, in order to obtain information on the effects of structure of amine on their photochemical properties. EXPERIMENTAL 4Hydroxy benzophenone (HBP) and paraformal dehyde were obtained from Aladdin Chemistry Co., Ltd. Diglycolamine (DGA) was from Aladdin Indus trial Corporation. Ethyl 4dimethylaminobenzoate (EDAB) was supplied by Chinese Medicine Group Chemical Reagent Co., Ltd. Trimethylolpropane tria crylate (TMPTA) and tripropylene glycol diacrylate (TPGDA) were purchased from Sartomer Company. Other chemicals are of analytical grade excepted as noted. Synthesis of Benzoxazine and Polybenzoxazine The photoinitiators were synthesized according to Scheme 1. Paraformaldehyde (0.10 mol) and DGA (0.05 mol) were dissolved in 200 mL of 1,4dioxane. The mixture was stirring for 0.5 h, then poured into 100 mL of 1,4 dioxane containing HBP (0.05 mol) and refluxed for 12 h. The reaction mixture was filtered and 1,4diox ane was evaporated under reduced pressure. Resulted oily product was dissolved in chloroform, washed for five times with 400 mL of 0.1 N NaOH aqueous solu tion and distilled water, respectively. Then, the chloro form solution was dried with anhydrous sodium sul fate. Benzoxazine as an orange oil (yield: 75.2%) was obtained by evaporation of the solvent.

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O OH +

O

H 2N

633

OH + (CH2O)n

OH *

O

N

n*

O N

O

OH

O

O

Bz

HO PBz Scheme 1. Synthesis of Bz and PBz.

filters with UV light intensity equal to 40 mW/cm2 (examined by Honle UV meter, Germany). The mix ture of acrylate and photoinitiator was smeared on a solid KBr slice and photopolymerization was carried out at room temperature. All concentrations were cal culated with respect to the monomer, unless noted elsewhere. Double bond conversion was monitored by recording the change of absorption area from 788 to 830 cm–1 during UV irradiation. The conversion (C) was calculated from the following equation: C ( % ) = ( 1 – B 2 /B 1 ) × 100, (1) where B1 is the =CH peak area before UV irradiation and B2 is the =CH peak area after UV irradiation.

Polybenzoxazine as orange solid was synthesized by thermally activated curing of benzoxazine at ca. 200°C for 1 h. According to GPC, Mn = 6 × 103, Mw/Mn = 1.37. Analysis NMR spectra were recorded on the Mercury Plus spectrometer 400 Hz with deuterated chloroform as solvent. FTIR spectra were recorded on a Nicolet iS5 spectrometer; the samples were prepared as cast films on KBr discs or as KBr pellets. UV–Vis spectra were recorded in acetonitrile by UV–Vis spectropho tometer Perkin–Elmer Lambda 20. Molecular weights were determined by GPC on a Perkin Elmer Series 200 apparatus using DMF as an eluent and PS stan dards for MW calibration. The photopolymerization kinetics was studied by realtime FTIR spectroscopy (Nicolet iS5, Thermo Company, USA) using EFOS Lite spot light source (5 mm crystal optical fiber, Canada) with 200–400 nm 1H

RESULTS AND DISCUSSION Synthesis of Photoinitiators 1 The H NMR spectra of Bz and PBz are shown in Fig. 1. In both spectra, the characteristic signals at 7.80–7.20 ppm were assigned to the hydrogen atoms

(a)

(b) OH

O

*[

O

a b

d

N

b

N c

e O

c

a

OH

]

n*

d

O

f

O e f

a HO

b R

Aromatics d+e

f

PBZOH

c

d

Aromatics

f e a, b

8

7

6

5

4

3

2

1

0 ppm

8

7

6

5

Fig. 1. 1H NMR spectra of (a) Bz and (b) PBz. POLYMER SCIENCE

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c

3

2

1

0 ppm

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3442 cm−1 a

3450 cm−1 924 cm−1 4000

3500

3000

2500

2000

1500

1000 500 Wavenumber, cm−1

Fig. 2. The FTIR spectra of (a) Bz, and (b) PBz.

of phenyl ring. In the spectrum of Bz, the signal at 6.90 ppm was also assigned to the hydrogen atoms of phenyl ring. Alkyl protons of the ethoxyethanol group resonated at 3.0 ppm (triplet, –N–CH2–, 2H), 3.6 ppm (triplet, HO–CH2, 2H), 3.7 ppm (broad trip let, O–CH2, 4H). The two signals at 4.9 and 4.0 ppm corresponded to –CH2– protons of benzoxazine ring, indicating the existence of oxazine ring in Bz. While in the spectrum of PBz, the signal at 4.9 ppm, corre sponded to –CH2– protons of oxazine ring disap peared indicating the completion of ringopening reaction. The signal at 3.0–4.0 ppm related to the hydrogen atoms of ethoxyethyl can be obviously observed. Thus, Bz and PBz were synthesized success fully. The FTIR spectra of PBz and Bz are shown in Fig. 2. As is seen, the intensity of the band at 924 cm–1 fell down, which confirms that the oxazine ring open ing reaction occurred [14, 15]. The average molecular weight of PBz determined by GPC is suitable for PBz application as a photoini tiator. Photochemical Behaviors UV–Vis spectra of Bz and PBz in acetonitrile are shown in Fig. 3. The maximum absorption (λmax) and the logarithmic values of molar extinction coefficient at λmax (logε) are summarized in table. The maximum absorption wavelength of benzophenone was 250 nm, which could be ascribed to the main benzenoid π–π* type transition, while n–π*type transitions are usu ally found between 300 and 350 nm. Compared with

benzophenone, both Bz and PBz exhibited signifi cantly redshifted π–π* absorption, and their maxima were 291 and 273 nm, respectively. Moreover, the most differences were the maximum absorption and the molar extinction coefficient of Bz and PBz, which indicated that the macromolecular structure had great influence on the UV–Vis absorption of benzophenone moieties in the macrophotoinitiator. Therefore, simi lar to the polymerizable photoinitiator Bz, the copol ymer PBz was also attractive as a photoinitiator. Photopolymerization of TPGDA In the presence of coinitiator the photolysis of BP leads to the formation of two types of radicals: one of them is ketyl radical and the other one is amino radi cal; the latter radical is capable of initiating polymer ization (Scheme 2). Because the ketyl radicals are not reactive towards vinyl monomers due to the steric hin drance and the delocalization of unpaired electron, the polymerization of vinyl monomers is almost usu ally initiated by amino radicals [4]. Absorption properties of photoinitiators Photoinitiator

λmax , nm

ε, mol–1 cm–1 L

BP Bz

250.40 233.80 291.60 245.80 273.40

1.76 × 104 2.69 × 104 3.06 × 104 1.07 × 105 8.12 × 104

PBz

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O

O

hν

3

3

*

O

isc

O

*

O

*

635

O

OH •

O

O +

+

N

N • monomer polymer

Scheme 2. Photoinitiated free radical polymerization using EDAB as coinitiator.

The photopolymerization of TPGDA initiated by macrophotoinitiator as well as low molecular weight analogues was studied through FTIR (Fig. 4). The photopolymerization of TPGDA systems using BP/EDAB, Bz, PBz cured at room temperature by UV light with an intensity of 40 mW/cm2. The polymer ization behavior of TPGDA was similar to other mul tifunctional monomers [16–19]. According to Fig. 4, PBz and Bz were more efficient photoinitiator for the photopolymerization of TPGDA compared to BP/EDAB system. This result may be caused by two reasons. Firstly, redshifted absorption of π–π* transi tion at wavelengths 273 and 291 nm (as shown in Fig. 3) leads to PBz and Bz absorbing more energies

than that of BP. Secondly, chromophore groups of benzophenone and amine incorporated in one molec ular may be contributed to improving energy migra tion [11]. Compared with the BP/EDAB system, PBz and Bz revealed slightly higher concentration of radi cals generated if added at the same mass fraction to monomer. This may be ascribed to the efficient energy migration between the excited state of the photosensi tive moieties (BP moieties) and coinitiator amine along the polymer chain. Moreover, compared with Bz, the higher efficiency of PBz may come from better compatibility of macrophotoinitiator in the TPGDA system as well as formation of more reactive species

Abs 1

3

2 3 2

1

0 250

300

350

400 Wavelength, nm

Fig. 3. UV–Vis absorption spectra of (1) BP, (2) Bz, and (3) PBz (concentration = 10–5 g mL–1). POLYMER SCIENCE

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80

60

1 2 3

40

20

0

1

2

3

4

5

6 Time, min

Fig. 4. Conversion vs. time profiles for polymerization of TPGDA initiated by (1) BP, (2) Bz and (3) PBz at room temperature under UV light with an intensity of 40 mW/cm2. The photoinitiator concentration is 0.02 wt % in term of BP moieties and EDAB concentration is 0.02 wt %.

Conversion, % 100

80

60

0.02% 0.05% 0.10% 0.50%

40

20

0

1

2

3

4

5

6 Time, min

Fig. 5. Conversion vs. time profiles for polymerization of TPGDA at different PBz concentrations at room temperature under UV light with an intensity of 40 mW/cm2.

due to energy migration along the polymer chain or intramolecular reactions [9]. As shown in Fig. 5, the maximum photopolymer ization rate increased with the increase of PBz con centration due to rise of amount of free radicals that can be produced during irradiation, thus leading to the requirement of a short period of time to form the gel structure which produces a maximum in the polymer

ization rate. At the same time, the induction period was shortened with increase of PBz concentration, which is because when photopolymerization carries out in the presence of air. We also can see that the final conversion increased as the PBz concentration decreased. In fact, at high conversions due to high viscosity the photopolymer ization is completely controlled by diffusion, and the POLYMER SCIENCE

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60

40 1 2 3

20

0

1

2

3

4

5

6 Time, min

Fig. 6. Conversion vs. time profiles for polymerization of TMPTA initiated by (1) BP, (2) Bz and (3) PBz at room temperature under UV light with an intensity of 40 mW/cm2. The photoinitiator concentration is 0.02 wt % in term of BP moieties and EDAB concentration is 0.02 wt %.

Conversion, % 80

60

40 0.02% 0.05% 0.10% 0.50%

20

0

1

2

3

4

5

6 Time, min

Fig. 7. Conversion vs time profiles for polymerization of TMPTA at different PBz concentrations at room temperature under UV light with an intensity of 40 mW/cm2.

possible reason of the observed effect might be the free volume effect, which was caused by the volume shrinkage [20, 21]. Photopolymerization of TMPTA The RTIR profiles of the polymerization of TMPTA caused by three photoinitiators are shown in POLYMER SCIENCE

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Fig. 6. The kinetics of photopolymerization of TMPTA initiated by macrophotoinitiator was similar to that of TPGDA. PBz was slightly efficient than Bz and BP/EDAB systems, while the final conversion was lower comparing to TPGDA polymerization. As we know, the double bond content of TMPTA is much higher compared with bifunctional monomer TPGDA. This leads to very high crosslinking density

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in the whole polymerization process of TMPTA. Under this environment, the mobility of radicals was the most important factor to the polymerization of TMPTA: the mobility of macromolecular chain decreased, resulting in the final conversion decreased. Moreover, comparison of Figs. 5 and 7 reveals that the highest final conversion related to the PBz content is 0.05%, not 0.02%. That was because the crosslink ing density of polymeric TMPTA was so high that the mobility of radicals was low, which resulting in the decrease of the probability of radicals quenching. However, when the content of PBz surpassed 0.05%, the photopolymerization of TMPTA and TPGDA express the same behavior. This result ascribed to the more radicals increased the probability of quenching. As for the polymerization rate of TMPTA, the maxi mum photopolymerization rate increased with the increase of PBz concentration, which was similar to that of TPGDA. CONCLUSIONS In this article, we report the synthesis of a novel polymeric photoinitiator by ringopening polymeriza tion of 4hydroxy benzophenone, diglycolamine and paraformaldehyde, and then investigated their photo chemical behavior in comparison with BP, which showed significant redshifted maximal π–π* absorp tion of PBz and Bz compared with BP. The photoini tiator PBz possessed the highest absorption among PBz, Bz and BP. Photopolymerization of difunctional monomer TPGDA and trifunctional monomer TMPTA initiated by PBz, Bz and BP was studied through RTIR. PBz has a extremely high efficiency for initiating the photopolymerization of TPGDA. The results indicate that macrophotoinitiator is more effi cient than low molecular weight analogs, particularly in the system with low viscosity. The photoinitiator PBz has a very high efficiency and it will be widely applied in the field of UVcuring. ACKNOWLEDGMENTS The author would like to thank the National Natu ral Science Foundation of China (21304011) and a project funded by the priority academic program

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