Involvement of Supramolecular Complexes in the Capture and

Article pubs.acs.org/Macromolecules

Involvement of Supramolecular Complexes in the Capture and Release of Protonic Acids During the Cationic Ring-Opening Polymerization of Epoxides C. Y. Ryu, M. J. Spencer, and J. V. Crivello* Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States ABSTRACT: The kinetics of the cationic ring-opening polymerizations of epoxide monomers were controlled through the use of supramolecular proton complexes. α,ω-Diglycidyl oligoethylene oxides bearing multiple ethyleneoxy spacer groups form metastable supramolecular proton complexes with strong Brønsted acids generated either by the photolysis of onium salts or by their redox reactions with reducing agents. Trapping of the Brønsted acids by in situ complex formation results in a delay of the onset of cationic ring-opening. However, once polymerization begins, the epoxide groups of the monomer are very rapidly consumed resulting in characteristically highly exothermic autoaccelerated polymerization reactions. Crown ethers can also form supramolecular complexes with hydronium ions derived from the reaction of protonic acids with water and these complexes can be used to modify the kinetic behavior of the cationic ring-opening thermal and photopolymerizations of a variety of epoxide monomers. The ring size of a crown ether has a strong influence on the stability of the supramolecular complex formed, and this was shown to have a major impact on the kinetics of the cationic ring-opening polymerizations.

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INTRODUCTION In a series of publications from this laboratory, we have described the results of our investigations into the mechanisms of the cationic ring-opening polymerizations of epoxide1−3 and oxetane4,5 monomers initiated by photogenerated Brønsted acids derived from the UV irradiation of diaryliodonium and triarylsulfonium salts. When the cationic ring-opening photopolymerizations of a variety of different epoxide monomers were carried out under homogeneous conditions at room temperature, four different characteristic types of kinetic behavior were observed and were subsequently related to the respective structures of the monomers.3 The polymerizations of cycloaliphatic epoxides with highly strained oxirane rings, such as cyclohexene oxide and cyclopentene oxide, exhibit high rates of reaction with little or no induction periods. Aryl glycidyl ethers undergo rather slow polymerizations with rather short induction periods. In contrast, the cationic photopolymerizations of alkyl glycidyl ethers are typically preceded by rather long induction periods that are followed by rapid, exothermic, autoaccelerated polymerizations. A number of other epoxides, such as tri- and tetra-substituted oxiranes, while reactive, undergo inefficient photopolymerization due to rearrangements and other side reactions. A unified explanation was offered for the apparent wide range of kinetic behaviors of the cationic photopolymerizations of the above-mentioned epoxide monomers based on the generalized ring-opening mechanism depicted in Scheme 1. © 2012 American Chemical Society

Scheme 1. Mechanism of the Cationic Ring-Opening Photopolymerization of Epoxide Monomers

The photolysis of an onium salt, as depicted in Scheme 1 for diaryliodonium salts, (eq 1) generates a strong Brønsted acid Received: December 2, 2011 Revised: February 3, 2012 Published: February 22, 2012 2233

dx.doi.org/10.1021/ma202618r | Macromolecules 2012, 45, 2233−2241

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more than one monomer. As photolysis proceeds, the photogenerated Brønsted acids immediately form the corresponding supramolecular complexes, 3, and their concentration begins to builds up in the reaction mixture. At some point, the concentration of 3 reaches a sufficiently high value that even a slight increase in temperature results in its destabilization with the consequent spontaneous rapid thermally autoaccelerated polymerization of the monomer. The more heat generated from the exothermic ring-opening polymerization, the faster the protonic acid is released from the destabilized supramolecular

(HMtXn) by a complex process, in which both heterolysis and homolysis of the diaryliodonium salt photoinitiator takes place. For efficient cationic ring-opening polymerization, the diaryliodonium salt photoinitiator is selected such that the anion, MtXn−, is of low nucleophilic character such as PF6−, AsF6−, or SbF6−. The resulting Brønsted acid, HMtXn, generated protonates the epoxide monomer (eq 2) to give the secondary oxiranium ion, 1. A tertiary oxiranium ion, 2, is formed by the SN2 attack of monomer on 1 (eq 3). Once 2 is formed, polymer growth takes place by the repetitive addition of monomers to the tertiary oxiranium chain end (eq 4). The overall rate of a cationic epoxide ring-opening photopolymerization is determined by the slowest step in Scheme 1. Under conditions of high intensity UV irradiation and using diaryliodonium salts with high quantum yields for Brønsted acid production (Φ = 0.7−0.96,7), the rate controlling step in Scheme 1 has been shown to be eq 3. Ultimately, the reactivity of the secondary oxiranium ion intermediate 1 governs the rate at which the entire polymerization process proceeds. Consequently, those structural factors that increase the stability of 1 and decrease its reactivity tend to slow the polymerization and vice versa. Cycloaliphatic epoxides that possess no additional stabilizing features produce secondary oxiranium ions that are highly strained and unstable and this explains the correspondingly high rates of polymerization observed with these monomers. Aryl glycidyl ethers and terminal epoxy alkanes undergo protonation to yield less strained secondary oxiranium ions that are correspondingly less reactive and, therefore, these monomers display intermediate reactivity. Of more interest are the aforementioned alkyl glycidyl ethers that exhibit long induction periods followed by very rapid, autoaccelerated photopolymerization profiles. We3 have attributed the prominent induction period as due to the formation of stabilized secondary oxiranium ion intermediates such as 3 in which a proton is coordinated by both the ether and epoxide oxygen atoms in the monomer to form supramolecular complexes. The basicities of the oxygen atoms of open chain ethers and epoxides are similar.8 While coordination as shown in structure 3 is depicted for the sake of simplicity as involving a proton, it is recognized that other species, such as water if present, may also be involved in coordination sphere. In such cases, coordination by the epoxide can be viewed as involving a hydronium ion, H3O+, H5O2+, or similar species. Since most epoxide monomers are hygroscopic and the experiments described herein were not carried out under rigorously dry vacuum line conditions, the presence of water can be assumed in all cases. In addition, complexation of the Brønsted acid may involve the glycidyl ether groups of

complex and this escalates the rate of the polymerization with a consequent sharp rise in the sample temperature. Extension of the concept of the involvement of supramolecular complexes in the “capture and release” of Brønsted acids led to the proposal that increasing the coordinating ability of the monomer ligands from bidentate to tetradentate should result in the formation of complexes with enhanced stability. Indeed, it was observed that 1,2-ethanediol diglycidyl ether as shown in 4, which can optimally undergo tetradentate coordination with a hydronium ion and trimethylolpropane triglycidyl ether that is capable of forming a cage-like complex as shown in 5 by hexadentate coordination, give supramolecular complexes that are considerably more stable than 3. This enhanced stability is reflected in the observed lengthening of the induction periods during the photoinitiated cationic polymerization of these latter two monomers. The effectiveness of hydrogen bonding is greatly influenced by temperature. For example, the solution phase behavior of the lower critical solution temperature (LCST) has been experimentally9,10 and theoretically11 studied for poly(ethylene oxide) in aqueous solution in terms of weakened hydrogen bonding at elevated temperatures. As expected, temperature has a similar marked effect on the stability of the hydrogen bonded supramolecular complexes which, controls the overall rates of the photoinitiated cationic ring-opening polymerizations of epoxides and oxetanes. In the case of alkyl glycidyl ether monomers, raising the temperature at which a photoinitiated cationic polymerization is carried out results in an appreciable

Scheme 2. Proposed Mechanism for the Redox Initiated Cationic Polymerization of Epoxides

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Table 1. Materials Used in this Investigation

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decrease in the induction period. On the other hand, when the photopolymerizations of 1,2-epoxyalkanes are carried out at −10 to 0 °C, they display prolonged induction periods that are similar to those observed for the cationic photopolymerizations of alkyl glycidyl ethers conducted at room temperature. It was also interesting to note that, under the same temperature conditions, the polymerization of cyclohexene oxide was not appreciably slowed. This may be attributed to the high of reactivity of both the monomer and the instability of the corresponding secondary oxonium ion intermediate produced by protonation. Recently,12−15 a series of onium salt-based redox couples whereby strong Brønsted acids can be rapidly generated at and below room temperature has been developed by our group. For example, the platinum or palladium catalyzed reductions of a diaryliodonium or a triarylsulfonium salt with a silane bearing a Si−H group releases Brønsted acids such as HMtXn that function as powerful initiators for both cationic ring-opening and vinyl addition polymerizations. As shown in Scheme 2, the proposed mechanism of the initiation process involves the formation of a highly reactive silicenium cation intermediate, 6 (eq 5). This intermediate undergoes further reaction with traces of water or other protonic agents (eq 6) present in the monomer to generate the Brønsted acid, HMtXn, which subsequently initiates cationic polymerization in a manner as previously depicted in Scheme 1. The different characteristic kinetic behaviors of the various types of epoxy compounds described above for photopolymerizations was also observed when redox cationic initiation was applied to these same monomers. In this article, we describe the results of our efforts to design and use supramolecular complexes analogous to 3−5 to control the rates of cationic ring-opening epoxide polymerizations using both photo- and redox initiation.

EXPERIMENTAL SECTION

Materials. Cyclohexene oxide was obtained from the Aldrich Chemical Co. and was dried and distilled from CaH2. All glycidyl ether monomers were kindly provided as samples by CVC Specialty Chemicals, Inc., Maple Shade, NJ. 1,2-Epoxydecane was provided as a gift from the Viking Chemical Co. Blooming Prairie, MN. 3,4Epoxycyclohexylmethyl 3′,4′-epoxycyclohexane carboxylate (ERL4221E) was obtained from the Union Carbide Corp. (now Dow Chemical Co.). 1,3-Bis(2(3,4-epoxycyclohexyl)ethyl)-1,1,3,3-tetramethyldisiloxane was prepared by platinum catalyzed hydrosilylation of 4vinylcyclohexene oxide with 1,1,3,3,-tetramethyldisiloxane according to a previously described method.16 Organosilanes, 1,1,3,3-tetramethyldisiloxane, triethylsilane, and the Lamoreaux (2.0−2.5% Pt in octanol) catalyst were purchased from Gelest, Inc., Morrisville, PA. Cl2(COD)Pd(II) was purchased from Strem Chemicals, Inc., Newburyport, MA and used as a 2.0% solution in nitromethane. Hexafluoroantimonic acid (98%) was purchased from Acros Organics, Geel, Belgium. All other monomers, reagents, chemicals and crown ethers, tetraethylene glycol dimethyl ether and poly(ethylene glycol) dimethyl ether were used as purchased from the Aldrich Chemical Co., Milwaukee, WI. The onium salts, (4-n-undecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC-11 SbF 6 ), (4-n-hexadecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC-16 SbF 6 ), (4-noctyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC-8 SbF6),17 and S-(4-n-decyloxyphenyl)-S,S-diphenylsulfonium hexafluoroantimonate (SOC-10 SbF6),18 were synthesized and purified by previously described methods. Shown in Table 1 are the structures, names, and abbreviations of the photoinitiators and monomers used during the course of this investigation. Synthesis of the Hydronium Hexafluoroantimonate-18-crown-6 Complex. To 0.590 g (2.23 mmol) 18-crown-6 there were added 0.688 g (2.0 mmol) hexafluoroantimonic acid hexahydrate and 5 mL distilled water. An immediate precipitate of a white crystalline product was formed. The mixture was stirred and allowed to stand for 1.5 h. Then the product was vacuum filtered and washed with cold distilled water. Because of the solubility of the complex in water, there were considerable losses. After drying in vacuo, there were obtained 0.342 g 2235


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of the desired complex with a melting point of 185 °C. Extensive discoloration and decomposition took place at 199 °C. Anal. Calcd for C12H27F6O7Sb: C, 27.77; H, 5.24. Found: C, 28.02; H, 5.43. Addition of the hydronium hexafluoroantimonate-18-crown-6 complex to cyclohexene oxide results in spontaneous exothermic polymerization. Analogous results were obtained with n-butyl vinyl ether Synthesis of the Hydronium Hexafluoroantimonate-15-crown-5 Complex. In a similar manner, the addition of 0.4906 g (2.23 mmol) 15-crown-5 to 0.688 g (2.0 mmol) hexafluoridoantimonic acid hexahydrate resulted in the immediate formation of a white precipitate. The reaction was exothermic. There was added 0.5 mL of distilled water, and this resulted in the partial dissolution of the product. The remaining product was vacuum filtered. There were obtained 0.311 g (33%) complex as a white crystalline powder. The complex is insoluble in chloroform and an additional 0.09 g of product were recovered on standing from the filtrate. There were obtained 0.401 g (42% yield) of the complex having a melting point of 98−100 °C. Anal. Calcd for C10H23F6O6Sb: C, 25.3; H, 4.88. Found: C, 24.24; H, 5.16. The hydronium hexafluoroantimonate-15-crown-5 complex was soluble in nitromethane. Addition of this solution to 4-vinylcyclohexene-1,2 dioxide resulted in immediate exothermic polymerization. Optical Pyrometry (OP). General descriptions19−21 of the apparatus construction, sample and analytical techniques used in the optical pyrometric analysis for photo- and redox initiated polymerization reactions were given in previous publications from this laboratory. The basic optical pyrometry (OP) apparatus was modified slightly to adapt it for the study of the thermally induced cationic polymerizations of bulk monomers. The progress of an exothermic polymerization was remotely monitored by following the sample temperature using an infrared camera (optical pyrometer). The sensitivity of the OP technique is largely determined by the infrared camera that is used. In these experiments, an Omega Corp. Model OS 552-V1-6 infrared camera with a sensitivity of +0.5 °C over the range of −18 to +538 °C was employed. Reactions that produce a sample temperature change of at least 1 °C/min can be monitored with this instrument. Polymerizations were carried out in standard Fischer Scientific aluminum weighing dishes. These containers were very regular both in size and in weight (1.0−1.1 g). To the weighing dish was added 1.0 g of the liquid monomer followed by a predetermined amount of the diaryliodonium or triarylsulfonium salt initiator followed by the addition by means of a syringe of a solution of the metal-containing catalyst. The commercially available platinum-containing Lamoreaux catalyst was purchased as a 2.0%. platinum solution in n-octanol. Also employed as a catalyst was a 2.0% solution of Cl2(COD)PdII in nitromethane. An acetone washed and dried 22 mm long #1 steel paper clip was added to serve as a stirrer. The monomer−onium salt mixture was stirred until the onium salt had completely dissolved. Assuming a density of approximately 1 g/mL, the thickness of the liquid layer in the weighing dish was 0.33 mm. The container was positioned on the sample stage of the OP apparatus with focal point of the infrared camera located at the surface of the monomer film. This was achieved with the aid of a laser sighting device. After an equilibration period of 20 s, a predetermined amount of the designated silane was added using a syringe while the monomer−catalyst−onium salt solution was rapidly stirred. All polymerizations were carried out at an initial temperature of 23−25 °C. The samples were slowly heated at a rate of 7 °C/min by means of a heater positioned directly below the sample. A baseline heating curve was established by the monitoring the temperature of a prepolymerized sample of monomer using the infrared camera. During kinetic runs, the temperature of the sample was monitored by OP at a rate of one measurement per second.

Article

RESULTS AND DISCUSSION

A long-term objective of this investigation was to develop epoxide monomer-initiator systems that would both possess long shelf lives, while retaining the desirable ability to undergo rapid polymerization at low temperatures. The approach we have pursued in this work was to employ cationic photo- and redox initiator systems that generate strong Brønsted acids in situ in the presence of monomers and other agents that can reversibly sequester those acids as supramolecular proton complexes such as 3 and 4 rendering them inactive or poorly reactive for the initiation of polymerization at room temperature. It was expected that such supermolecular complexes could be easily dissociated at relatively low temperatures to trigger cationic polymerization. During this investigation, we have relied upon optical pyrometry to characterize both the photo- and redox initiated cationic ring-opening polymerizations of epoxide monomers. Optical pyrometry (OP) is a convenient, highly reliable technique that was originally developed in this laboratory for remotely monitoring the rapid kinetics of free radical and cationic photopolymerization reactions.19−21 The technique is based on the principle that bulk addition polymerizations are highly exothermic events that result in a readily measurable elevation in the temperature of the sample. Using an infrared camera (i.e., an optical pyrometer), the course of the evolution of the temperature of a monomer sample is remotely measured as a function of the time. The OP technique is especially well suited for the study of the photoinduced polymerizations of multifunctional monomers that undergo cross-linking reactions. It has been shown19 that the initial portion of the temperature versus time curve of an optical pyrometry trace is proportional to the corresponding conversion versus time curves as determined by either real-time infrared spectroscopy (RTIR) or by differential scanning photocalorimetry (DSP). As described in detail in the Experimental Section of this article, the original OP apparatus that was designed for the study of photopolymerizations was modified to equip it with the capability to study redox cationic polymerizations conducted under controlled conditions.13,14 Our initial experiments were focused on α,ω-diglycidyl ethers of oligoethylene oxides such as diethylene glycol diglycidyl ether and triethylene glycol diglycidyl ether. It was proposed that such monomers, respectively, would form crown ether-like complexes 7 and 8 by penta- and hexadentate proton coordination. As previously indicated, structures 7 and 8 are both simplified and idealized. Traces of water are nearly always present in such hydrophilic monomers and, in addition, one or more moles of water may also be involved in the coordination sphere. Figure 1 depicts a study of the thermal profile obtained by optical pyrometry of the photopolymerization of triethylene glycol diglycidyl ether c arried out using (4-nhexadecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC-16 SbF6) under relatively high intensity (1580 mJ/cm2 min) UV irradiation conditions. Unless otherwise noted, continuous UV irradiation was applied throughout the entire course of this as well as in other studies shown in the present article. Polymerization was preceded by a very long (200 s) induction period during which the temperature rises slowly from 25 to 50 °C over the course of 200 s. After this induction period, a very abrupt and highly exothermic polymerization occurs with the temperature spontaneously rising to 220 °C over the course of approximately 3−5 s. During this thermally 2236


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Figure 2. Redox cationic polymerization of a 2.0 g sample of dipropylene glycol diglycidyl ether using 1.0 wt % (4-nundecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC-11 SbF6), and 50 μL of triethylsilane in the presence of 30 ppm of Cl2(COD)PdII.

Figure 1. Photopolymerization of triethylene glycol diglycidyl ether with 2.0 mol % (4-n-hexadecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC-16 SbF6) at a UV light intensity of 1580 mJ/cm2 min.

induced autoaccelerated cationic polymerization, there is very rapid consumption of the epoxide functional groups and the final conversions are always nearly quantitative. This has been verified by real-time IR studies and is indicated in the OP curves by the very sharp peaks that are obtained. When the UV irradiation of the thin film sample was discontinued prior to the onset of the highly exothermic portion of the polymerization, there appeared to be no change in the appearance of the monomer. However, heating a film of the irradiated monomer briefly at temperatures ranging from 50 to 70 °C resulted in the rapid, exothermic polymerization of the sample. When the hot tip of a soldering iron was brought into contact with the liquid monomer film, we also observed that the polymerization quickly took place as a front that proceeded from that point outward and extending to all portions of the irradiated film. This exothermic frontal cross-linking of the epoxy monomer suggests that there would be a temperature threshold (50−70 °C), where the cationic polymerization propagates autocatalytically.

On the basis of the above experiments, it was concluded that even very powerful Brønsted acids, such as HSbF6, can be sequestered in the presence of α,ω-diglycidyl ethers of oligoethylene oxides that can form planar cyclic or threedimensional supramolecular proton complexes. With this information in hand, it was decided to investigate the possibility of employing crown ethers as additives to modify the reactivity of epoxide monomers. The complexation of protonic acids by crown ethers has considerable literature precedent. Izatt et al.22 showed that stable crystalline complexes between the hydronium ion and cis-syn-cis-cyclohexano-18-crown-6 can be prepared. Further work by Weber et al.,23 Chevenert et al.,24,25 Junk and Atwood,26 McKenna and Eyring,27 and Koltoff and Chantooni28 have also reported the formation, isolation and spectroscopic identification of similar complexes. Leis et al.29 have described the complexation of hydronium ions at the surfaces of hydrogen dodecyl sulfate micelles. We were especially interested to determine whether such crown ether complexes would preferentially form in the presence of epoxide monomers and how this would influence the character of the subsequent cationic ring-opening polymerization. The results of a study of the photopolymerization of cyclohexene oxide carried out in the presence and absence of various amounts of 18-crown-6 are shown in Figure 3. In this experiment, we have

Figure 2 depicts the redox initiated cationic ring-opening polymerization of a 2.0 g sample of dipropylene glycol diglycidyl ether carried out using 1.0 wt % (4-nundecyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC-11 SbF6), and 50 μL triethylsilane in the presence of 30 ppm of Cl2(COD)Pd(II) as a catalyst. The redox reaction rapidly generates HSbF6 at room temperature. However, polymerization takes place only very slowly on standing over the course of 1−2 h. This appears due to the capture of the redox generated acid by the monomer with the formation of a supramolecular hydronium ion complex analogous to 7. As Figure 2 shows, slowly heating this mixture at a programmed rate of 7 °C/min results in polymerization of the monomer that begins slowly as heating progresses and then sharply accelerates at approximately 70 °C with the temperature rising to a maximum of 184 °C.

Figure 3. Photopolymerization of cyclohexene oxide with 0.5 mol % of SOC-10 SbF6 in the presence and absence of 3.0, 5.0, and 7.0 mol % 18-crown-6 (light intensity 2350 mJ/cm2 min). 2237


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Scheme 3. Test for Complexation of Secondary versus Tertiary Oxiranium Ions by 18-Crown-6

employed S-(4-n-decyloxyphenyl)-S,S-diphenylsulfonium hexafluoridoantimonate (SOC-10 SbF6) as the photoinitiator. We have found that diaryliodonium salts undergo complexation with 18-crown-6 and further work in this area will be reported in a separate communication. Under similar conditions, it is important to note that triarylsulfonium salts do not complex with crown ethers. Previously,30 we have commented on the lower reactivity of triarylsulfonium salts compared to their diaryliodonium salt counterparts as cationic photoinitiators. Under the standardized irradiation conditions of this experiment,3 the photoinitiated cationic polymerization of cyclohexene oxide monomer proceeds rapidly and exothermically with a short (2−3 s) induction period that is mainly due to a delay in the response of the OP instrumentation and to the sample configuration. When 3 mol % of 18-crown-6 is added and the experiment repeated, photopolymerization is delayed and proceeds eventually after approximately a 40 s induction period. This suggests that although the crown ether is present in a small concentration, it is highly effective in sequestering the very strong Brønsted acid, HSbF6 before the onset of the exothermic autocatalytic cationic polymerization. Our postulate of the supramolecular arrested acid catalyst is also supported by the dependence of the induction period on the crown ether concentration. A further increase in the concentration of 18crown-6 on the photopolymerization of cyclohexene oxide to 5 and then 7 mol % based on the monomer results in a progressive lengthening of the induction period. It is noteworthy, that the peaks in the OP temperature profiles of the modified photopolymerizations of the cyclohexene oxide sharpen appreciably with an increase in the amount of crown ether added. We interpret this behavior as due to the buildup of the concentration of crown ether sequestered protonic acid in the reaction mixture as photolysis proceeds. As the thermally induced polymerization begins, the release of heat generated as

a result of the ring-opening polymerization of the monomer triggers the autoacceleration of the cationic polymerization. When more reactive cationic centers are created and arrested by supramolecular complexation, triggering the release of the accumulated acid gives rise to a more rapid polymerization with a consequent sharper rise in the sample temperature. The possibility that the crown ether may also complex with the reactive tertiary oxiranium chain ends was considered. Additional experiments were carried out with the objective of verifiying or discounting this postulate. A 1.26 g (0.5 mmol) sample of 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate was combined with 2.5 × 10−4 mol of SOC-10 SbF6 and 7.5 × 10−4 mol of 18-crown-6. The liquid sample was irradiated for 45 s with UV light with an intensity of 3000 mJ/ cm2 min. To the irradiated liquid sample there was added 1.0 × 10−4 mol 2,6-di-tert-butylpyridine. Subsequently, the mixture was heated to 80 °C over the course of 6 min. Under these conditions, no polymerization was observed to take place. 2,6Di-tert-butylpyridine is a “proton trap” that due to the steric hindrance about the nitrogen atom allows it to neutralize protons. However, it is not capable of trapping larger ions such as oxonium ions. These results tend to support our postulate that crown ethers bind initiating species such as protons or hydronium ions but not tertiary oxiranium ions that constitute the propagating chain ends present in epoxide ring-opening polymerizations. Scheme 3 shows the overall plan of this experiment. It is well-known that linear poly(ethylene oxides) can complex metal cations.31 It was of interest to determine whether these same linear polyethers can also effectively undergo proton complexation with Brønsted acids. In Figure 4 are compared the cationic photopolymerizations of cyclohexene oxide carried out in the presence and absence of 5 mol % of two methyl ether terminated poly(ethylene oxides); tetra2238


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in delaying the onset of the cationic photopolymerization is clearly evident, it is very less pronounced than for this less reactive monomer. The metal cation complexing ability of crown ethers has been shown to depend on the size of the rings and their associated number of coordination sites.23,32,33 Crown ethers with large ring sizes preferentially coordinate with larger metal cations, while those with smaller diameter rings form complexes that accommodate metal cations with smaller ionic radii. It was proposed that similar effects should also be observed for the formation of crown ether complexes with hydronium ions derived from Brønsted acids as well. In Figure 6 is shown a Figure 4. Comparison of the cationic photopolymerizations of cyclohexene oxide using 0.5 mol % IOC-11 SbF6 carried out in the presence and absence of 5.0 mol % tetraethylene glycol dimethyl ether and 1000 g/mol poly(ethylene oxide) dimethyl ether (light intensity 1050 mJ/cm2 min).

methylene glycol dimethyl ether, 9, and poly(ethylene glycol) dimethyl ether (Mn = 1000 g/mol), 10. Such compounds are generally referred to as “podands”. While not as effective complexing agents as 18-crown-6, both podands display the ability to significantly delay the onset of the cationic photopolymerization of cyclohexene oxide. This observation is consistent with our postulate that such polymers can also form supramolecular complexes with protonic acids.

Figure 6. Effect of the crown ether ring size on the photopolymerization of cyclohexene oxide with 0.5 mol % SOC-10 SbF6. (3 mol % crown ether, light intensity 2350 mJ/cm2 min).

comparison of the effects of the addition of 3 mol % of a series of crown ethers with different ring sizes on the photopolymerization of cyclohexene oxide in the presence of 0.5 mol % of SOC-10 SbF6. Included in this study for comparative purposes is an OP trace for the photopolymerization conducted in the absence of a crown ether. As noted previously, the photoinitiated polymerization of the epoxide monomer takes place rapidly in the absence of a crown ether. When crown ethers are added, in all cases, the onset of the photopolymerizations is delayed for appreciable periods of time. However, the length of the induction period is directly related to the size of the crown ether and increases in the order: 18-crown-6 > 15-crown-5 > 12-crown-4. This was interpreted to be indicative of the poorer coordinating ability of both 12crown-4 and 15-crown-5 for H3O+SbF6− as compared to 18crown-6. The respective sizes of the cavities of the three crown ethers have been reported to be as follows: 12-crown-4, 120− 150 pm; 15-crown-5, 170−220 pm; 18-crown-6, 260−320 pm.34 Fornili et al.35 calculated the size of the hydronium ion to be 248 pm which is midway between the radii of the Na (190 pm) and potassium (266 pm) cations. These two metal cations are respectively preferentially complexed by 15-crown-5 and 18-crown-6. Lastly, the gas-phase binding energies between the hydronium ion and 18-crown-6 and 15-crown-5 were calculated by Sharma and Kebarle36 and found to be −88.5 kcal/mol and −76.9 kcal/mol, respectively. These values are higher than the binding constants for 18-crown-6 with the alkali metal cations Na+ or K+ (−56 kcal/mol and −72 kcal/mol, respectively) and were attributed by Bühl and Wipff37 to the formation of the formation of the multiply hydrogen bonded highly symmetrical complexes, 11 and 12. It is expected that the magnitudes of the above binding energies will be somewhat different in monomer

It was of interest to explore whether this effect could be extended to include other related monomers such as 3,4epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate. Accordingly, the photopolymerization of this monomer containing 1.0 mol % IOC-11 SbF6 was carried out in the presence and absence of 3.0 mol % tetraethylene glycol dimethyl ether. This study is shown in Figure 5. While the effect of this linear ether

Figure 5. Study of the photopolymerization of 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate with 1.0 mol % IOC-11 SbF6 conducted in the presence and absence of 3 mol % tetraethylene glycol dimethyl ether (light intensity 2700 mJ/cm2 min). 2239


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solutions of varying polarity and complexing ability. As molecular modeling shows, the small cavity of 12-crown-4 does not allow the hydronium ion to form a planar complex. Rather, the hydronium ion is perched above the plane of the crown ether ring forming a weaker complex, 13. It is also worthwhile to note that as one proceeds through the series 11 to 13, the number of coordination sites of the crown ether with the hydronium ion decreases resulting in weaker, more easily thermally dissociable complexes.

Figure 7. Study of the photopolymerization of 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate with 0.5 mol % SOC-10 SbF6 conducted in the presence and absence of 3 mol % 18-crown-6 (light intensity 2700 mJ/cm2 min).

photoinitiator mixture, polymerization proceeds slowly and only after a long induction period. 1,3-Bis(2(3,4-epoxycyclohexyl)ethyl)-1,1,3,3-tetramethyldisiloxane (Figure 8) is a difunctional biscycloaliphatic epoxide that

Further evidence for the formation of the above crown etherhydronium ion supramolecular complexes was sought. The 1:1 hydronium hexafluoroantimonate-18-crown-6 supramolecular complex (11, MtXn− = SbF6−)was prepared for the first time in this laboratory as a colorless crystalline solid (mp 185 °C dec) by combining hexafluoroantimonic acid hexahydrate with 18crown-6 in a manner similar to that described by Izatt et al.22 The desired complex precipitated from the reaction mixture and was recovered by filtration. The analogous crystalline (mp 98−100 °C) H3O+SbF6−-15-crown-5 complex was also prepared in a similar manner. Attempts to prepare and isolate the analogous H3O+SbF6−-12-crown-4 complex resulted in a low melting, amorphous waxy solid. The addition of H3O+SbF6−-18-crown-6 complex to cyclohexene oxide resulted in slow exothermic polymerization at room temperature. Under the same conditions, the polymerization of the diepoxide monomer, 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexane carboxylate, did not take place. However, gelation was noted when the monomer-complex mixture was gently warmed. Research on the preparation, characterization and use of this and other crown ether-strong Brønsted acid supramolecular complexes in cationic ring-opening and vinyl polymerizations is currently in progress and will be reported in a forthcoming publications. The ability of crown ethers to modify the character of the cationic polymerizations of epoxide monomers is a general phenomenon that can be applied to epoxide monomers other than cyclohexene oxide. For example, Figure 7 depicts the results of a study of the photopolymerization of 3,4epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate, conducted in the presence and absence of 18-crown-6. Duplicate kinetic runs are shown in this figure for each case. It is interesting that based on the thermal OP profiles that were obtained; the nature of the polymerization was markedly altered in the presence of the crown ether. When no crown ether is present, the polymerization proceeds rather sluggishly under continuous UV irradiation, although without a substantial induction period and a sharp rise in sample temperature. The maximum temperature attained by the sample was only 55 °C. We have commented on the sluggish cationic ring-opening polymerization behavior of this monomer previously.38 When 18-crown-6 is added to the monomer-

Figure 8. Study of the photopolymerization of 1,3-bis(2(3,4epoxycyclohexyl)ethyl)-1,1,3,3-tetramethyldisiloxane with 0.5 mol % SOC-10 SbF6 conducted in the presence and absence of 6 mol % 18crown-6 (light intensity 2350 mJ/cm2 min).

is considerably more reactive than 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate (Figure 7) due to the fact that it possesses no nucleophilic centers other than the highly strained epoxide oxygen atoms. As shown in Figure 8, when the photopolymerization of this former monomer is carried out using 1.0 mol % SOC-10 SbF6, polymerization ensues rapidly and exothermically without a substantial induction period. In contrast, when the photopolymerization is conducted in the presence of 3 mol % 18-crown-6, initially polymerization proceeds slowly. This induction period culminates in a rapid, exothermic autoaccelerated polymerization. The sharp rise in the temperature of the sample is consistent with the arrest and accumulation of the supramolecular complex and the final thermally induced release of acid which initiates the cationic polymerization. As a final consideration, it is worthwhile to note that the photo- and redox induced cationic ring-opening polymerizations of α,ω-diglycidyl oligoethylene oxides yields threedimensional networks that are similar to cryptates.39 These cryptates are capabable of binding many cationic species 2240


Macromolecules

Article

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including; metal cations, protons, hydronium ions and reactive oxiranium chain ends. Equation 7 depicts the proposed

supramolecular intramolecular coordination of hydronium ions by a polymer produced from a monofunctional glycidyl ether. Because of the multiplicity of oxygen atoms that can serve as coordination sites, the corresponding polymerization of α,ω-diglycidyl oligoethylene oxides would be expected be even more effective as supramolecular complexing agents. In support of this concept, Dimonie and Teodorescu40 have reported that beads of α,ω-diglycidyl oligoethylene oxides cross-linked using triethylenetetramine exhibit phase transfer catalytic activity in typical SN2 reactions.

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CONCLUSIONS α,ω-Diglycidyl ether monomers in which the two epoxide groups are separated by multiple ethyleneoxy or propyleneoxy units possess the ability to form in situ supramolecular complexes with hydronium ions derived from strong protonic acids. Because of the stability of these complexes, the cationic ring-opening polymerization of the epoxide groups is suppressed. The suppression is manifested as an induction period. However, since such supramolecular complexes possess a low threshold for dissociation, once polymerization begins, it proceeds rapidly as a highly exothermic process. Similarly, crown ethers form supramolecular complexes with hydronium ions derived from Brønsted acids and we have demonstrated in this article that when these complexes are added to epoxide monomers rapid cationic polymerization ensues. Further, the addition of crown ethers to highly reactive epoxide monomers significantly modifies their reaction profiles in both thermally and photochemically initiated cationic ring-opening polymerizations.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

REFERENCES

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