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UV-Initiated Bubble-Free Frontal Polymerization in Aqueous Conditions Paul Michael Potzmann,† Francisco Javier Lopez Villanueva,‡ and Robert Liska*,† †

Institute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna, Austria BASFwe create chemistry, Ludwigshafen, Germany

‡

S Supporting Information *

ABSTRACT: From an industrial point of view the most interesting method of bulk curing of acrylate-based monomers is a thermal frontal reaction initiated by the application of UV-light. In waterbased systems, UV triggering of bubble-free thermal front reactions is difficult to realize as solvent boiling and decomposition of the thermal initiator lead to uncontrollable heat loss, porous polymer samples, and expansion of the formulation. Especially bubble formation in the lightexposed area causes diffuse light scattering and as a result nonreproducible investigation of the UV initiation. Within this work a new thermal initiator has been synthesized, allowing us to report a bubble-free steady-state thermal front reaction in water. The new system allowed us to achieve a well-observable front reaction and provided the basis for an accurate and reproducible, systematic analysis of UV initiation in water-based frontal polymerizations, considering the ratios of all reactive components.

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INTRODUCTION Starting from the discovery of frontal polymerization during the mid-1970s in the Soviet Union,1 numerous applications have emerged in recent days for this new method of macromolecular material preparation. For example, three new papers were published recently, such as for structure reinforcement,2 stimuli-responsive hydrogels,3 or controlled drug release.4 The main advantage beside the accessibility of new materials with enhanced properties is the low-energy input required. An excellent overview on the topic is given by Pojman in his recent review.5 In comparison to photofrontal polymerization, where a continuous light exposure is required, in UV-initiated frontal polymerization the photoreaction is only used to trigger the thermal front. To avoid confusion, it has to be mentioned that within this work the term frontal polymerization refers to thermal frontal polymerization. Generally in frontal polymerization the reaction heat generated by the polymerization is used to initiate further polymerization in the surrounding monomer by cleavage of a thermal initiator. At low heat loss, it is possible to generate a self-sustaining, hot front which propagates step by step through the reaction vessel. The most important tools for describing frontal polymerizations are the front temperature profile as well as the front velocity. Both are tunable to a large extent by selection of the monomer and thermal initiator with regard to their heat generation and reactivity as well as by adjustment of the concentrations of all reactive components. From a theoretical point of view, such a thermal front can be initiated by any process able to increase the temperature in the initiation volume up to a level at which the generated polymerization heat exceeds the heat loss of the system. By application of an © XXXX American Chemical Society

additional photoinitiator, it is possible to provide the energy needed for the thermal initiation of the front, completely by using the polymerization heat of an initial photoreaction. In the literature this method is also used to overcome the problem of a limited curing depth of photopolymerization of (meth)acrylic formulations.6−8 The need for solvent-based frontal polymerization is directly connected to the possible applications, which in most cases is the production of hydrogels.9−13 According to the widespread application areas of hydrogels, ranging from biomedical techniques to soil additives, the synthesis methods focus on customization to achieve specific improvements. Besides the easy, safe, and homogeneous initiation conditions provided by UV-triggering of the process, the application of solution-based formulations can also contribute different advantages, as it further extends the range of materials which can be produced by frontal polymerization. The first example of a front reaction in a monomer solution is described by Pojman in 1996 using high-boiling solvents like DMSO or DMF to allow comparatively high reaction temperatures.14 While frontal polymerization in general is characterized by very high reaction temperatures in the range 140−180 °C, the application of high amounts of solvent decreases the reactivity and also the front temperature to values below 150 °C due to dilution. With regard to the final properties, even minimal variations of the reaction parameters and formulation compositions can dramatically increase for example mechanical properties,13 the Received: October 28, 2015 Revised: November 30, 2015

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Macromolecules sensitivity conditions of smart hydrogels,12,15 and of course the water absorption behavior.10,16,17 While solution-based frontal reactions can provide improved materials, some inherent drawbacks have to be mentioned. Aside from the loss of reaction heat to the solvent, instabilities of the front and bubble formation due to solvent boiling or initiator decomposition are significant factors regarding the applicability of the method, especially for mild UV-based initiation methods. Regarding the production of hydrogels in the literature, besides other solvents also water is used.9,16,17 But one has to keep in mind that under the applied conditions boiling bubbles occur in all reactions. Despite the drawbacks of a high heat capacity and low viscosity, it offers the advantage that the used solvent can remain in the final product. UV-initiated and solvent, especially water-based, frontal polymerizations have been investigated in fundamental research as well as in application-oriented studies as both strategies show great promise regarding the industrial use. The goal of this work was the discussion of the combination of both methods which has not been shown in the literature. In order to meet this goal, it is necessary to investigate a system free of any influence by bubbles, caused by either boiling or gas release during initiator decomposition. Such bubble formation leads to numerous drawbacks like uncontrollable heat loss, porous polymer samples, expansion of the material, and in the case of initiation by UV-light diffuse scattering of the light beam. Therefore, the synthesis of a new thermal initiator without inherent gas formation and a suitable reactivity was required.

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the calculated amount of distilled water was added. In a following step the photoinitiator (w-BAPO) was dissolved in the formulation. To ensure premature photopolymerization, the formulations were stored in the dark. In a separate vial the exact amounts of thermal initiator were provided. To avoid pot life related influences, the thermal initiator was added to the monomer mixture a few minutes before starting the experiment. In a typical run a glass vial (i.d. 20 mm) was filled with the formulation (≈16 mL), and the initiation volume at the top was laterally exposed to UV light (3 W cm−2 at the tip of the waveguide) until a temperature of 98 °C was reached. The time of exposure was kept the same for all experiments within a measurement series. Tables 1 and 2 show the composition of an optimized

Table 1. Composition of a Bubble-Free Frontal Polymerization Formulation in Water wt %

component monomer mixture AAc:BAM 98:2 wt %

27.00 KBSPS (corresponds to 0.0045 mol per double bond) 0.60 w-BAPO 0.15 water 72.25

Table 2. Front Properties and Light Exposure Conditions of a Bubble-Free Frontal Polymerization Formulation in Water front velocity maximum temperature front starting time light intensity time of light exposure

EXPERIMENTAL SECTION

Materials and General Methods. The photoinitiator bis(2,4,6trimethylbenzoyl)phosphinic acid was provided by BASF SE, The Chemical Company, and converted to sodium bis(2,4,6-trimethylbenzoyl)phosphinate (w-BAPO) according to Mueller et al.18 Potassium peroxymonosulfate in the Trippel salt form (oxone), benzenesulfonyl chloride, acrylic acid (AAc) N,N-methylenebis(acrylamide) (BAM), and 2,2′-azobis(2-methylpropionamidine) dihydrochloride (Azo56) were purchased from Sigma-Aldrich. 2,2′Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (Azo44) was purchased from Wako Pure Chemical Industries. To trigger the frontal polymerization process, the samples were laterally irradiated with filtered UV-light (320−500 nm) from an Exfo OmniCure series 2000 for a defined duration at an intensity of 3 W cm−2 measured at the tip of 8 mm OmniCure liquid light guide. Temperature measurements were recorded with a pico data logger USB TC-08 equipped with Ktype thermocouples. Synthesis of Potassium Benzosulfonylperoxysulfate [KBSPS]. Within 5 min oxone (5 g 16.3 mmol) was dissolved in approximately 40 mL of water and neutralized with KOH (1 mol/L) to pH 7.0 in a beaker by the use of a pH electrode. Freshly distilled benzenesulfonyl chloride (2.3 mL 18 mmol) was added dropwise over a period of 2 h, while keeping the mixture neutral by the use of additional KOH. To provide a high reaction rate while preventing the decomposition of the product, the reaction temperature was kept between 9 and 13 °C. The precipitate was filtered off and washed vigorously with petroleum ether. After drying in the suction filter the product was obtained as white crystals. Yield: 1.05 g (22%); mp: decomposes at 86 °C. The structure was confirmed via X-ray crystallography, showing a reasonable bond distance of 1.4776 Å for the peroxide bond. Single-crystal data for KBSPS are available in the Supporting Information. For safety reasons the component was handled by the use of plastic tools while the maximum synthesized amount was always below 2 g. Frontal Polymerization. The preparation of the reactive formulation was carried out as follows: The desired amounts of acrylic acid and N,N-methylenebis(acrylamide) were weighed in, and

0.56 cm/min 98.5 °C 240 s 3 W/cm2 18 s

formulation, the applied light exposure conditions, and the according front properties obtained, as an example, for a frontal polymerization in water without any bubble formation. Figure 1 shows the experimental setup as used for all frontal polymerization experiments. The temperature during the reaction was

Figure 1. Example of a bubble-free frontal polymerization in our experimental setup. measured by five k-type thermocouples (RS Components) at different levels in the formulation and recorded via a Pico Technology USB TC08 Datenlogger. The maximum temperature was determined in the steady-state front region 2 cm below the initiation volume. The location of the optical front was observed as the change of the refractive index between monomer and polymer phase. To highlight the optical difference, a special diamond-shaped background was used. From the plot of front position and reaction time the front velocity was determined as the slope of the regression line of the steady-state front reaction. Values for frontal polymerization conditions presented B

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which make a homogeneous, bubble-free final product possible. The chemical structure responsible for the gas-free thermal radical formation is in both cases a persulfate modified for the application in organic solvents. Similar to ammonium or potassium persulfate, the thermal stability of such components is quite high, which only allows the application in frontal polymerizations with high reaction temperatures. For bubble-free water-based frontal polymerization a new solution had to be found. A new, gas-free thermal initiator has to show a suitable thermal stability for reasonable reaction kinetics below 100 °C and a sufficient solubility in water. Concerning the need for a bubble-free system, sulfonyl peroxides are interesting as the peroxide bond, similar to persulfates, shows no gas formation. Further sulfonyl peroxides are very promising regarding their thermal stability because they can be used in radical polymerizations at room temperature. As reported in the literature, di(methylsulfonyl) peroxide and di(phenylsulfonyl) peroxide are used in radical polymerization at low temperature.22,23 Regarding the properties of persulfates and persulfonyl components, we considered the application of an asymmetric radical initiator (Figure 2).

in the following paragraphs are given as a mean value of at least three experiments.

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RESULTS AND DISCUSSION From a scientific point of view, it is necessary to analyze every new system, which can be polymerized by a frontal reaction, methodically, considering the ratios of all reactive components. The fundament of such studies has to be a well working reproducible system. According to our goal, the investigation of a water-based frontal polymerization, initiated by light, two main criteria have to be met to achieve a reproducible system. On the one hand, the system has to be free of any boiling effects, and on the other hand, no bubble formation due to initiator decomposition should occur. To avoid boiling, during the frontal polymerization, the reacting formulation must never exceed 100 °C if water is used as solvent. The maximum temperature of a front reaction is mainly influenced by the type and dilution of a reactive monomer and the stability of the used thermal initiator. Because of their good solubility, the most common thermal initiators for water-based polymerizations are ammonium or potassium persulfates, which are used in the literature for the frontal polymerization of numerous monomers, mainly in the field of hydrogel synthesis.9,10,16,17 Persulfates are comparatively stable thermal initiators which show no gas formation during decomposition. The high heat capacity of water requires the use of a very reactive monomer, with a high heat of polymerization, which can provide the necessary energy for systems with high water content during polymerization. Therefore, our formulations were based on acrylic acid in combination with 2% N,Nmethylenebis(acrylamide) as cross-linker. In preliminary experiments we found that for our formulations it was not possible to achieve nonboiling thermal fronts in highly diluted reaction mixtures using a persulfate as thermal initiator. With potassium or ammonium persulfate the minimum monomer content needed to sustain a frontal polymerization was 31%. The achieved maximum temperatures were in all measurements slightly above 100 °C, and boiling bubbles occurred in all experiments. At higher dilutions it was not possible to sustain a front reaction over the full vial length. These observations correspond to data presented in the literature11,16,17,19 for minimum values of low reactive systems. To overcome the problem of a boiling front, we focused on the application of thermal initiators with comparatively low thermal stability and sufficient solubility in water. By the use of 2,2′-azobis[2-(2imidazolin-2-yl)propane] dihydrochloride (Azo44), an azoinitiator with a very low thermal stability (10 h half-life at 44 °C), nonboiling conditions could be achieved with high dilutions containing less than 29% monomer. To the best of our knowledge up to now, there is no boiling-free system described in the literature for water as a comparatively low boiling solvent. By the use of Azo44 such a system is possible but still shows inherent gas bubbles due to N2 formation during initiator decomposition, which does not allow the synthesis of a nonporous polymer and more importantly inhibit a reproducible investigation of the UV initiation process as diffuse light scattering is caused by the formed bubbles. The problem of bubble formation has been addressed in the literature for organic solvents and solvent-free frontal polymerizations by the application of quaternary ammonium persulfates20 as well as phosphonium-based ionic liquids,21

Figure 2. Considerations on asymmetric sulfate sulfonyl peroxides.

The solubility in water of such a component is provided by the sulfate group, while the thermal stability is a result of the combination of sulfate and sulfonyl peroxides and was expected to lie between the stability of persulfates (T1/2 = 10 h: 60 °C) and the sulfonyl peroxides (T1/2 = 10 h: ≈20 °C). The synthesis of such a component is described by Willstatter and Hauenstein in their fundamental research on Caro’s acid24 but has never been used as a thermal radical initiator. The vague description on the stability of the crystalline component and its solubility in water allowed the assumption of a possible application as a thermal radical initiator. Scheme 1. Synthesis of KBSPS

We improved the synthesis of potassium benzosulfonylperoxysulfate (KBSPS) as a potential thermal initiator by the use of oxone as a cheap and easy to handle commercial source of the dipotassium salt. As described in Figure 3, in a first reaction step oxone was neutralized by the use of KOH before the resulting dipotassium salt was allowed to react with the sulfonyl chloride to form the final product. In the literature, C

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the problem of an inhomogeneous starting volume, we applied a water-soluble bisacylphosphine oxide as photoinitiator to generate the necessary heat by a homogeneous photopolymerization to trigger the thermal front reaction while never exceeding the boiling temperature of the formulation. As the new thermal initiator KBSPS decomposes without gas formation and allows to frontal polymerize below boiling temperature, we were able to prepare a completely nonporous polymer hydrogel throughout the whole sample. Figure 3 shows the improvement by the application of the thermal initiator KBSPS and the method of photoinitiation in the synthesis of a bubble-free hydrogel sample. Generally the optical front of the reaction was observed by a sharp change of the refractive index within 1 mm between monomer and polymer phase. Figure 5a shows a typical temperature profile of a passing front, measured with a temperature sensor placed in the steady-state region of the frontal reaction. The maximum temperature is measured slightly after the optical front has passed the temperature sensor. The front velocity is calculated from the function of the optical front vs the time (Figure 5b). To guarantee that the reaction reached steady-state conditions, the slope of the regression line in the constant region of the graph indicates the velocity. Initial frontal polymerization experiments, using KBSPS as new thermal initiator, showed that the reactivity, regarding the front velocity and maximum temperature, was similar to Azo44 and significantly more reactive than ammonium persulfate. Information on the comparison of Azo44 and KBSPS regarding the reactivity in the basis formulation (Table 1) is included in the Supporting Information. It was possible to realize nonboiling systems for dilutions below 28% monomer content with front velocities in the range of 0.5 cm min−1. The correlation between front velocity and stability of the thermal initiator does not allow high reactivity and long pot life at the same time, for systems following Arrhenius kinetics.5 The described reactive formulations have to be used within the first 20 min after addition of the thermal initiator to obtain reproducible measurements. To verify the expected high double-bond conversions, the residual monomer content was evaluated by extraction of the frontal polymerized samples and analysis via HPLC (Supporting Information). Resulting from the advantageous conditions in the frontal reaction zone, all calculated values confirmed high conversions above 98%.

Figure 3. (a) Thermal-initiated nonboiling formulation (Azo44), (b) photoinitiated nonboiling formulation (Azo44), and (c) photoinitiated nonboiling formulation (KBSPS).

diluted Caro’s acid is prepared and neutralized in the first reaction step. By starting with the neutralization of oxone, the preparation of the acid is omitted. In addition, the unproblematic use of the salt provides the possibility to calculate the amount of needed KOH accurately. The resulting quick procedure leads to an increased yield, as a major limiting factor of the reaction is the decomposition of the dipotassium salt in solution. By X-ray crystallography a bond distance of 1.4776 Å was measured for the peroxide bond comparable to literature for other peroxides (1.45 ± 0.02 Å).25 The initiator does not show significant changes in reactivity after 1 week of dry storage at 4 °C. It is necessary to store the product at dry conditions, as even small amounts of water dramatically reduce the stability and lead to a brown color change and a dramatically reduced reactivity after only 48 h. It has to be mentioned that the synthesized white crystalline compound is sensitive to heat and friction. Frontal Polymerization Conditions. For hydrogel synthesis, in the literature the main method used to trigger a frontal polymerization in water is thermal initiation by a heating element. For this method it is necessary to increase the temperature in a certain volume, while the rest of the formulation should remain at the initial temperature. This is achieved by heating of an initial volume with an accordingly hotter heating element, which leads to evaporation in the starting region in contact with the heat source. To overcome

Figure 4. (a) Typical temperature profile. (b) Determination of the front starting time from a velocity plot. D

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Figure 5. (a) Position to the temperature sensors 1−5 in the experimental setup. (b) Transition of photoinitiation to steady-state thermal front reaction, visualized by the function of front velocity and the temperature profile throughout the reaction. The passing by of the temperature sensors by the optical font is illustrated by dashed lines.

Figure 6. Effect of the thermal initiator concentration on the front velocity: (left) Azo56 [2,2′-azobis(2-methylpropionamidine) dihydrochloride 10 h half-life at 56 °C]/31% monomer; (right) KBSPS/27% monomer.

intersection of the linear regression of the velocity and the starting position. The described method is not completely independent of the front velocity but allows a qualitative comparison of different systems. In Figure 6, the front formation process is visualized by the temperature profiles of five temperature sensors in combination with the optical front propagation. The sharp optical front of the thermal reaction is gradually generated in the initial volume. In the directly exposed initiation volume a sharp temperature increase can be observed before the lamp is turned off to avoid boiling. The polymerization is transformed from a homogeneous photoreaction into a sharp thermal reaction, while in the transition area photopolymerization, due to diffuse light exposure, as well as thermal polymerization occur. While the final thickness of the traveling optical front is only 1 mm, the formation of the front in the transition area extends over 5−15 mm depending on the light exposure conditions. Within this distance the reached maximum temperature and velocity of the

Generally the conditions of UV initiation do not influence the properties of the steady-state thermal front reaction, such as front velocity or maximum temperature. To characterize the influence of the photoinitiation process, the value of front starting time is introduced. In the literature, the front starting time is understood as the time needed, until an optical front is recognizable or the time until a front migrated up to a certain distance. The determination by one of these methods provides no statement on whether the recognizable front is also a constant front and highly depends on the front velocity. While these methods are applicable for common fast reaction fronts, for low reactive frontal polymerizations in water the resulting error is critical. To overcome these problems, a new method to measure the front starting time is proposed. The velocity function of a slow front reaction is characterized by a nonconstant section in the beginning due to a delay during the front formation phase. This delay can be indirectly measured as the time between the start of initiation by light and the E

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Figure 7. Effect of the thermal initiator (left) and the photoinitiatior (right) concentration on the front starting time.

nonboiling window occurs at higher dilutions while still showing the same reactivity. While the degree of water content in the reaction can be influenced by adjusting the thermal initiator concentration, the velocity of the front reaction always shows values between 0.45 and 0.6 cm/min for KBSPS as thermal initiator. Furthermore, the application of KBSPS is also possible in combination with other water mixable reactive monomers. Within this work we present only formulations based on acrylic acid as reactive monomer in the use of water-based frontal polymerization to highlight the upper limit of possible dilution with water. For other less reactive monomers the range of the nonboiling window is shifted to higher monomer concentration. To give an example, the range of the nonboiling window is shifted from around 30% monomer content during frontal polymerization, for acrylic acid as main reactive component, to about 45% monomer content for hydroxyethyl acrylate. Frontal Polymerization Experiments: UV Initiation Process. Even if our results on bubble containing systems were not as satisfactory regarding the statistical error as for bubble-free systems, it was in general possible to give a statement on the steady-state front parameters, such as front velocity and maximum temperature. Regarding the UV initiation process, on the other hand, any kind of bubblecontaining system provided unusable results for the front starting time due to diffuse light scattering and the resulting high statistical error. As described in the chapter frontal polymerization conditions and as shown in Figure 4 (right), this is an effect that is even further increased by the low reactivity of the highly diluted systems. For the presented data the chosen concentrations of thermal initiator and photoinitiator as well as the light exposure conditions were selected to allow statements on the behavior of the initiation conditions. The data presented in Figure 6 were obtained by formulations based on KBSPS and a very short light exposure of only 18 s with 3 W cm−2. With lower light intensities and longer exposure times, in the order of 35 s with 1.5 W cm−2, significantly faster front starting times with values similar to the light exposure times can be obtained. As for all reactions the statistical error of the front starting time results from the error of the velocity, which mainly depends on the temperature conditions in the formulation during experiments; it was not possible to reduce it below ±15 s. Therefore, the initiation conditions were intentionally tailored to increase from possible

front is significantly lower compared to the steady-state front conditions. In addition, the effect of the photoinitiation on the reaction temperature can be observed in this region. In Figure 6, the temperature profile of the second temperature sensor illustrates these relations between photoreaction and thermal reaction. We assume that this overlapping area of photoinitiated and thermally initiated polymerization is necessary for the formation of a thermal front. Because of the partial photoreaction, which leads to a partial gelation of the initiation volume, it is possible that the thermal reaction can be sustained at lower temperatures during the process of front formation until the steady-state front is developed. Frontal Polymerization Experiments: Steady-State Front Reaction. Beside the initiator type, the concentration of a thermal initiator is one of the most discussed parameters for frontal reactions in the literature.6,7,14,19,26 In the course of the methodical study of the new nonboiling water-based systems, the effect of the concentration of the thermal initiator had to be analyzed by a series of frontal polymerization reactions. Figure 6 underlines the improvement by the use of a bubblefree system. While in the left graph a slightly boiling system, containing a nitrogen releasing thermal azo-initiator, shows a comparatively large error bar, in the right graph, a bubble-free system allows to accurately interpret the relation between front velocity and thermal initiator concentration. As shown in the literature for frontal polymerizations without solvent, the function of velocity versus thermal initiator concentration can be fitted with a power function.26 For solvent-based frontal polymerizations at comparable concentrations, literature presents values which rather reflect a linear regression.14,19 Pojman suggests in his studies on solvent-based frontal polymerizations that this linear correlation could be caused by the use of a solvent.14 While the large error of the velocity measurement in a slightly boiling system still left room for interpretation, the precise determination of the bubble-free optical front showed that within the nonboiling window a linear relation between velocity and thermal initiator concentration exists. For formulations based on a thermal initiator without inherent gas formation, such as KBSPS, the nonboiling window is synonymous for a completely bubble-free system (Figure 6, right). As additional result of our experiments we can conclude that the nonboiling window of a formulation can be shifted within some general limitations. By decreasing the monomer content and increasing the thermal initiator concentration, the F

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Macromolecules instant front formation after light exposure to values above 200 s which made statements on the UV initiation process possible. By the use of KBSPS as thermal initiator, it was possible to separately analyze the photochemical and thermal reaction in the initiation volume, as KBSPS, unlike azo initiators or benzoyl peroxide, shows no photochemical reactivity at all. The left graph in Figure 7 shows the influence of an increased thermal initiator concentration on the front starting behavior. Because of the above-described determination method of the front starting time, we were able to exclude the error of the resulting different front velocities. Apart from the low reactivity on the border to the uncreative formulation, an increase of thermal initiator concentration leads to a decrease of the front starting time in a linear correlation. The right graph of Figure 7 shows the influence of the photoinitiator concentration on the front starting time. Although the graph represents low reactive values of a different magnitude compared to front reactions in bulk, the shape of the function shows similar behavior as for highly reactive solventfree photoinitiation of acrylate-based frontal polymerization.6 While the front starting time can be dramatically lowered by increase of the photoinitiator content up to a certain value (0.1%), it shows only low increase by application of higher contents due to saturation of the photopolymerization in the initial volume.

power function, similar to solvent-free formulations, but rather follows a linear correlation. Regarding the quality of the UV initiation, we proposed a new method for determination of the front starting time, suitable for low reactive initiations, as needed for highly diluted or filled formulations. In addition, it was possible to separately investigate the influence of thermal initiator and photoinitiator concentration on the quality of UV initiation, as the new thermal initiator showed no UV sensibility. Our experiments showed that within the boiling-free window the reactivity of initiation can be increased by use of higher concentrations of thermal initiator in a linear manner, while the concentration of photoinitiator shows an optimum value above which the photoreaction is saturated. Overall, the presented bubble-free frontal polymerization in water provides a suitable tool for the analysis of all important frontal polymerization parameters including the quality of UV initiation. Furthermore, the method shows high potential as a tool for the preparation of hydrogel moldings as the absence of bubbles promises good values in optical and mechanical properties suitable for all kinds of applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02348. Single crystal data for KBSPS; calculation of the doublebond conversion; comparison of reactivity; link to an example video of a bubble-free frontal polymerization27 (PDF) Structure of KBSPS (CIF)

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CONCLUSION For hydrogels as an emerging modern material, the economic preperation via frontal polymerization in water has been studied by different work groups in the past decade, as this technique can provide a variety of advantages. Since frontal polymerization per definition proceeds in unstirred reactors, any gas formed during the reaction leads to local bubbles, which remain in the final material. While for some application areas bubbles may not necessarily be a drawback, the resulting decrease in optical and mechanical properties is a limiting factor for a number of intended applications. Regarding the use of conventional thermal initiators, such as peroxides, persulfates, or azo initiators, for water-based frontal polymerization, low solubility, low reactivity, and gas formation during decomposition are the main limitations concerning a bubble-free final product. In addition, the common method of thermal initiation always leads to solvent and monomer evaporation in the initiation volume. To the best of our knowledge, in the literature no synthesis of hydrogels via waterbased frontal polymerization with reasonable reaction kinetics below boiling temperature has been reported, especially not concerning a bubble-free reaction and initiation process. Within this work a new thermal initiator has been synthesized and applied in UV-initiated frontal polymerization, allowing us to report the first completely bubble-free thermal front reactions in water throughout the whole sample. By the bubble-free reaction it was possible to investigate the process of UV initiation in water-based front formulations in detail. We could show that the initiation is characterized by a transition state where UV and thermal polymerizations engage to form a precise, steady-state thermal front. The front reaction itself was analyzed toward its reactivity, by means of the front velocity, with variation of the thermal initiator concentration. Besides an improvement in terms of statistical error, compared to bubblecontaining reactions, we could confirm that the resulting dependence in the bubble-free window could not be fitted to a

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by BASFwe create chemistry is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02348 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02348 Macromolecules XXXX, XXX, XXX−XXX