Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Molecular Origin of the Induction Period in Photoinitiated Cationic Polymerization of Epoxies and Oxetanes Sungmin Park,† Landon J. Kilgallon,† Zheqin Yang,† Du Yeol Ryu,‡ and Chang Y. Ryu*,† †
Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York 12180, United States Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-Ro, Seodaemun-Gu, Seoul 03722, Korea
‡
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ABSTRACT: We investigated how the reactivity of epoxide and oxetane monomers in photoinitiated cationic polymerization is strongly influenced by their ability to form intramolecular hydrogen-bonding complexes with the Brønsted acids produced by the photoinitiator. In this paper, we highlight the importance of the thermal stability of the hydrogen-bonded complexes of the protonated monomers. We observed that the glycidyl ether structural motif (and its oxetane analog) is key for the formation of the complexes. Using temperature-controlled Fourier transform infrared (FTIR) with in situ ultraviolet irradiation, we performed a series of real-time FTIR experiments which support the hypothesis that the induction period is due to the thermal stability of the hydrogen-bonded complex. In particular, we focused on the photoinitiated cationic polymerization of bis(1ethyl(3-oxetanil)methyl) ether (“di-oxetane”, DOX) for the thermal stability study because it exhibits a prolonged induction period. Furthermore, it was possible to delay the cationic polymerization of DOX without causing autocatalytic polymerization if the temperature is kept lower than 30 °C. This is because the protonated monomer is stable and propagates very slowly at temperatures below 30 °C. On subsequent heating, the hydrogen-bonded complex of DOX loses its thermal stability, and the autocatalytic cationic polymerization of DOX occurs.
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INTRODUCTION Ultraviolet (UV)-initiated photopolymerization systems can provide many advantages over the traditional thermal curing systems because the polymerizations can be carried out solvent-free at room temperature.1−5 Thiol−ene click chemistry is one of the well-known photopolymerization methods that is effective and high yielding when used to graft thiols onto the CC double bonds.6−11 Another usage of UV light photopolymerization is the polymerization of di- or multifunctional acrylates for applications in coatings and adhesives.12−14 Both of these systems utilize radical-based photopolymerization. In this study, we focused on cationic photopolymerization because it offers advantages over radical photopolymerization. The biggest advantage of cationic photopolymerization is that it does not exhibit oxygen inhibition like radical-based systems do. Cationic polymerization is currently employed in a variety of applications including coatings, nanocomposites, adhesives, and printing inks.15−22 It also serves as a key technological platform in many imaging applications, such as © XXXX American Chemical Society
photolithography, stereolithography, and additive manufacturing.23−25 The development of onium salt-based photoinitiators was very important in the development of these applications of cationic photopolymerization. The photolysis of certain onium salts upon UV irradiation generates a Brønsted acid, which can act as an initiator for the cationic ring-opening polymerization of epoxide and oxetane monomers.26−28 Depending on the structure of the epoxide or oxetane monomer, however, the polymerization has drastically different kinetics. There have been limited studies to elucidate the molecular structure origins of the different reactivities of epoxide and oxetane monomers in cationic photopolymerization. In this study, we conducted fundamental research designed to elucidate the origins for the aforementioned reactivity differences of epoxide Received: November 21, 2018 Revised: January 11, 2019
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DOI: 10.1021/acs.macromol.8b02486 Macromolecules XXXX, XXX, XXX−XXX
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In this report, we thoroughly investigated the photoinitiated cationic polymerization of DOX using real-time (RT)-FTIR with a temperature-controlled attenuated total reflection (ATR) sample stage. The polymerization results of DOX characterized by optical pyrometry (OP) and RT-FTIR will be compared to suggest the heat-dissipation issues associated with the sample environment. It was found that the thermal stability of the hydrogen-bonded complex of DOX holds the key to manipulate the induction period of its cationic ring-opening polymerization. For the first time, the thermal trigger nature of the protonated DOX intermediates during the photoinitiated cationic polymerization has been demonstrated using RTFTIR with temperature-controlled ATR stage, to provide new insights into the molecular origin of the induction period that had been observed in OP experiments.
and oxetane monomers in the photoinitiated cationic polymerizations. One of the main issues with many epoxide and oxetane monomers is the time required for the onset of autocatalytic polymerization: the “induction period”.29−32 Monomers exhibiting a long induction period are problematic for many commercial applications, such as industrial coating and adhesive applications, because they require fast curing. In this study, we focused on the photoinitiated cationic polymerization of bis(1-ethyl(3-oxetanil)methyl) ether (“dioxetane”, DOX). This monomer exhibits a prolonged induction period, sometimes as long as minutes.33,34 Cycloaliphatic epoxide monomers containing the 3,4epoxycyclohexanecarboxylate group (Figure 1b) are the most
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EXPERIMENTAL SECTION
Materials. Bis(1-ethyl(3-oxetanil)methyl) ether (DOX) was purchased from Toagosei Chemical Co. Trimethylolpropane triglycidyl ether (TTE), 1,4-butanediol diglycidyl ether (BDG), and 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate (ERL4221E, abbreviated ERL) were purchased from Sigma-Aldrich Co. 1,2,8,9-Limonene dioxide (LDO) was provided by Arkema, Inc. All monomers were purified by freeze−thaw degassing or fractional vacuum distillation before use. Figure 1 shows the chemical structures of each of these monomers. The diaryliodonium salt photoinitiator, (4-n-octyloxyphenyl) phenyliodonium hexafluoroantimonate (IOC8), was prepared as described in a previous report.39 The chemical structure of the photoinitiator is shown in Figure 2.
Figure 1. Chemical structures of the epoxide and oxetane monomers used in this study. Monomers in column (a) contain glycidyl ether structural motifs (or the oxetane analog). Monomers in column (b) do not contain these structural motifs.
widely utilized in industrial UV light curing applications because of their fast polymerization kinetics. The “additional” ring strain associated with the cyclohexene epoxide contributes more thermodynamic driving force to open and polymerize the epoxide ring. These monomers exhibit no induction period.35,36 On the other hand, epoxide and oxetane monomers containing glycidyl ether structural motifs (or its oxetane analog) exhibit lengthy induction periods (Figure 1a).37 There have been attempts to eliminate the induction period of these monomers by copolymerization with reactive monomers that have no induction period. These reactive monomer additives are called “kick-starting agents”. This “kickstart” effect has been investigated using limonene dioxide as a kick-starting agent for the polymerization of DOX.38 Another curious property of DOX that has been studied is its frontal cationic polymerization. To observe the frontal polymerization of DOX, a resin was first irradiated with UV light at room temperature, and a hot piece of metal was touched to the surface of the resin. The DOX resin polymerizes out from the hot piece of metal in a “frontal” fashion because of the highly exothermic nature of the ringopening polymerization (19−20 kJ/mol). The occurrence of this frontal polymerization suggests that there is a metastable species formed when a DOX resin is irradiated with UV.34 While this study acknowledges the presence of stabilized monomer intermediate via protonation, this study has further exploited this concept by providing solid evidence to elucidate how the thermal stability of the protonated intermediate serves as a key mechanism of the induction period in the photoinitiated cationic polymerization of epoxies and oxetanes.
Figure 2. Chemical structures of the photoinitiator IOC8. Optical Pyrometry (OP). A Raytek RAYMMLTSSF1L infrared temperature sensor was used to monitor sample temperature as a function of UV irradiation time.40 A small sample of resin was pipetted onto a polyester mesh spacer (70 μm) sandwiched between two 12.5-μm-thick DuPont FEP thermoplastic films (fluorinated poly(ethylene-co-propylene)). The sandwiched samples were inserted in 2 × 2 cm slide frames and then placed onto the sample holder for analysis. The UV source used was an OmniCure S2000 mercury lamp (Excelitas Technonogies. Co.), with a fiber light guide. The UV irradiation intensity was measured using a Control Cure Radiometer (UV Process Supply, Inc.). Real-Time Fourier Transform Infrared Spectroscopy (RTFTIR). All RT-FTIR spectra were obtained using a Nicolet-iS-50 FTIR spectrometer (ThermoFisher Scientific Co.) with a Gladi-ATR attenuated total reflection (ATR) accessory (Pike Technologies. Inc.). A liquid-nitrogen-cooled mercury cadmium telluride (MCT-A) detector was used. IR spectra were collected every 5 s with 0.482 cm−1 interval and optical velocity of 0.4747 cm/s. Resin samples were pipetted directly onto the ATR diamond crystal. Samples were covered with a quartz plate, and a 70-μm-thick polyester mesh was used as a spacer between the ATR diamond crystal and the quartz plate. The temperature of the ATR stage was controlled by a temperature controller (Pike Technologies) equipped with the TempPro control software. An OmniCure S2000 mercury lamp (Excelitas Technologies Co.) equipped with a fiber light guide was used to irradiate the samples. Differential Scanning Calorimetry (DSC). A DSC Q2000 (TA Instruments) was used to collect all DSC data. Samples were prepared in the aluminum pan (TA Instruments). For samples irradiated with UV light before analysis with DSC, the sample was prepared in an B
DOI: 10.1021/acs.macromol.8b02486 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules open pan and then placed on a metal surface before UV irradiation. The irradiated sample was then sealed and analyzed with DSC.
membered instead of 3. Many oxetane monomers contain this structural motif. We hypothesize that glycidyl ethers and their oxetane analogs allow for the protonated monomer to form a stabilizing hydrogen-bonded complex that stabilizes the secondary oxonium ion and severely slows down the polymerization kinetics. The induction period exists because of the intramolecular stabilization of the protonated monomers. The hypothesized metastable intermediate structure of secondary oxonium ion for DOX, TTE, and BDG are shown in Figure 4. Each of these monomers contain at least one of the key structural motifs. In contrast, LDO and ERL are incapable of forming these stabilizing complexes. This is because the epoxide rings have no nearby oxygen atoms to stabilize the secondary oxonium ion via a hydrogen-bonded complex. Additionally, epoxides on cyclohexane rings have very limited flexibility. Another key experiment was conducted to investigate the effect of increased UV light intensity on the polymerization kinetics of monomers that exhibit an induction period and ones that do not. DOX was used as the monomer that exhibits an induction period, and ERL was used as the monomer that does not. Figure 5 summarizes the results of this experiment. For DOX, the induction period was significantly reduced from ∼250 to ∼ 50 s, when the UV light intensity was increased from 16 to 78 mW/cm2 (Figure 5a). For ERL, however, only the rate of propagation was affected without exhibiting an induction period by the increase in UV light intensity (Figure 5b). Continuous UV irradiation during photoinitiated cationic polymerization has two main effects on the sample. The first effect is a gradual increase in temperature because of the finite quantum efficiency of the photoacid generator.1 Second, the more intense the UV irradiation, the faster the photoinitiator undergoes photolysis to protonate monomers. For DOX, both of these effects decrease the amount of time that it takes for the sample to undergo autocatalytic polymerization. The increase in sample temperature destabilizes the hydrogen-bonded complex of protonated DOX, making propagation more likely. Interestingly, the temperature profiles of the OP experiments suggest that there is an onset temperature around 40−50 °C
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RESULTS AND DISCUSSION The photoinitiated cationic polymerization starts with the photon irradiation of a solution of IOC8 in a monomer or Scheme 1. Reaction Scheme for the Polymerization of a Generic Oxetane Monomer
mixture of monomers. The photoinitiator undergoes photolysis, producing Brønsted superacid species. The superacid quickly protonates a monomer in a fast acid−base reaction, producing a secondary oxonium ion. This protonated monomer then reacts with another monomer, forming a dimer containing a tertiary oxonium ion. This reaction proceeds, polymerizing the monomer by ring-opening polymerization (Scheme 1). Figure 3a,b shows the distinct reactivity differences between monomers with and without induction periods. The increase of sample temperature is indicative of the exothermic polymerization reaction. Only the monomers containing the glycidyl ether motif (or the oxetane analog) exhibit induction periods. It has been noted that epoxide monomers containing glycidyl ether structural motifs can form a metastable intermediate via a hydrogen-bonded complex.31 Though DOX does not contain a glycidyl ether group, it still exhibits a lengthy induction period because it contains a structural motif very similar to a glycidyl ether. The only difference is that the cyclic ether is 4-
Figure 3. OP study of the polymerization of epoxide and oxetane monomers (a) with and (b) without glycidyl ether structural motifs, which exhibit and do not exhibit an induction period, respectively. 2.0 wt % IOC8 was used as photoinitiator and the UV light intensity for each experiment was 16.3 mW/cm2. C
DOI: 10.1021/acs.macromol.8b02486 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. Hypothesized H-bonded supramolecular structure of DOX, TTE, and BDG, inducing the induction period.
Figure 5. Effect of UV light intensity on the induction period and reactivity for (a) DOX and (b) ERL, with 2 wt % IOC8 photoinitiator.
Figure 6. Correlation between the amount of DOX sample and the induction period for the optical pyrometry experiment. Figure 8. Temperature profile of the ATR plate in the temperaturecontrolled RT-FTIR experiments. (a) UV light was irradiated at room temperature (25 °C) and then (b) the temperature was increased from 25 to 70 °C with a heating rate of 5 °C/min without UV light irradiation.
for the autocatalytic polymerization of DOX at all UV light intensities (Figure 5a). For ERL, however, these two effects simply increase the propagation rate of its polymerization. ERL cannot form the metastable intermediate, so its secondary oxonium ion immediately propagates without an induction period. Consequently, the only limit on the rate of polymerization of ERL is the photolysis of the photoinitiator, so an increase in the UV light intensity simply increases the rate of the polymerization. The more rapid increase in sample temperature is indicative of faster polymerization at higher UV light intensities (Figure 5b). There are many variables that affect the polymerization kinetics of these types of ring-opening cationic polymerizations. Temperature, intrinsic monomer reactivity, and
Figure 7. Effect of the amount of DOX sample on the conversion to polymer for RT-FTIR experiments using the same UV irradiation and IOC8 concentration as in the OP experiment (Figure 6). D
DOI: 10.1021/acs.macromol.8b02486 Macromolecules XXXX, XXX, XXX−XXX
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Figure 9. Temperature-controlled RT-FTIR spectra of DOX and 2 wt % IOC8 for (a) continuous UV light irradiation for 360 s at room temperature (25 °C) and (b) sequent temperature ramp from 25 to 70 °C with a 5 °C/min heating rate without UV light irradiation. The intensity of UV light was 16.3 mW/cm2. (c) Postulated resonance structures of stabilized secondary oxonium ion of DOX with hydrogen-bonded complex.
Figure 10. RT-FTIR spectra of DOX and 2 wt % IOC8 mixture upon the application of the same temperature ramp profile from 25 to 70 °C with 5 °C/min heating rate. The sample was (a) not pretreated with UV light irradiation before the ramp, while the sample was (b) pretreated with UV light irradiation at 25 °C for 360 s with intensity of 16.3 mW/cm2 before the temperature ramp.
photoinitiator concentration are a few. Furthermore, the heatdissipation properties of the sample environment are very important for the temperature management of photoreactive samples under UV irradiation. To investigate the effect of heat dissipation on the polymerization kinetics, we conducted two different experiments in two different heat-dissipation environments. In one experiment, the polymerization of DOX was monitored using OP. For the second experiment, the same polymerization was monitored using real-time FTIR. In both experiments, the UV light intensity was fixed, but the amount of DOX resin analyte was varied. As shown in Figure 6, the induction period of the DOX polymerization greatly depends on the amount of analyte used in OP. As the amount of analyte was increased, the induction period was decreased. This is because of the gradual temperature increase of the sample. The
more analyte, the more effectively it can trap the heat generated by the absorbed UV irradiation and the small amount of polymerization without effective dissipation into the environment. Consequently, samples containing a larger amount of analyte can more rapidly reach the temperature at which it undergoes autocatalytic polymerization. The same photopolymerization experiment was repeated, except the real-time FTIR instrument (instead of OP) was used. RT-TIR allows for quantitative measurement of the polymerization by monitoring the intensity of the peak corresponding to the oxetane ring of DOX: ca. 975−980 cm−1. The formula ij (Aoxe,t /A ref,t ) yzz zz × 100 conversion (%) = jjjj1 − (Aoxe,0 /A ref,0) z{ k
E
DOI: 10.1021/acs.macromol.8b02486 Macromolecules XXXX, XXX, XXX−XXX
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destabilizes the protonated monomer. Finally, an onset of autocatalytic polymerization is triggered once the sample reaches a temperature high enough to destabilize the protonated monomers. For the RT-FTIR experiment, however, the DOX resin was pipetted directly onto the ATR diamond crystal embedded in a stainless steel base slab (k = 2200 and 16 W/m·K for diamond and stainless steel, respectively). Then the sample was covered with fused quartz plate (1000-μm-thick, k = 13 W/m·K) to allow transmission of UV light (Figure S1b). Because of the environment, the heat generated by the sample is quickly dissipated into the surroundings. This rapid heat dissipation maintains the temperature of the sample near ambient temperature. Because the sample remains isothermal at ambient temperature during RT-FTIR, the protonated monomers stay “dormant” without undergoing significant polymerization. The stark difference in the two experimental results demonstrates the importance of temperature management of samples to either suppress or trigger the cationic photopolymerization. To highlight the importance of thermal stability for the polymerization of protonated DOX, a UV-irradiated sample of DOX resin was slowly heated using the temperature-controlled sample stage of the ATR instrument. The experimental design is shown in Figure 8. The DOX resin was irradiated with UV light isothermally at room temperature (25 °C) for 6 min. Then the UV light was turned off and the temperature of the ATR sample stage was ramped from 25 to 70 °C at a heating rate of 5 °C/min. The results of the temperature-controlled RT-FTIR experiments are summarized below. Figure 9a corresponds to the UV irradiation period, isothermal at 25 °C, as shown in Figure 8a. Figure 9b shows the FTIR spectra change throughout the temperature ramp without UV irradiation (as shown in Figure 8b). During the isothermal UV light irradiation period, there is no significant spectral change around 975 cm−1 (oxetane group) as shown in Figure 9a. In contrast, the IR spectra changed dramatically upon the subsequent temperature ramp without UV light irradiation (Figure 9b). The postulated resonance structures of protonated DOX in its stabilized hydrogen-bonded complex are shown in Figure 9c. Figure 10 shows the spectral change of DOX resin upon heating (a) without and (b) with UV light pretreatment. There is no remarkable spectral change during the temperature ramp
Figure 11. Conversion plots of a DOX and 2 wt % IOC8 resin during a temperature ramp from 25 to 70 °C at a 5 °C/min heating rate, with and without prior UV light irradiation.
was used to calculate the conversion of DOX at any point during the polymerization. Aoxe represents the absorbance of the peak corresponding to the oxetane ring of DOX (975 cm−1 − 980 cm−1). The absorbance of the C−H stretching peak at 2950 cm−1 was used as an internal reference (Aref). The subscript “0” indicates that it is the initial value, and the subscript t indicates that it is a value measured at time t. Figure 7 shows that the DOX resin had undergone only
