Cationic photopolymerization of alkyl glycidyl ethers

An investigation of the photoactivated cationic ring‐opening frontal polymerizations of a series of alkyl glycidyl ethers was carried out with the aid of a novel technique, optical pyrometry. With this technique, the effects of various experimental parameters, such as the photoinitiator type and concentration, as well as the effects of the monomer structure on the frontal behavior of these monomers were examined. Upon irradiation with UV light, the photopolymerizations of many alkyl glycidyl ethers displayed a prominent induction period at room temperature as the result of the formation of long‐lived, relatively stable secondary oxonium ions. The input of only a small amount of thermal activation energy was required to induce the further reaction of these species with the consequent autoaccelerated exothermic ring‐opening polymerization. Photoactivated frontal polymerizations were observed for both mono‐ and polyfunctional alkyl glycidyl ether monomers. The ability of monomers to exhibit frontal behavior was found to be related to their ability to stabilize the secondary oxonium ion intermediates through hydrogen‐bonding effects. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3036–3052, 2006     

Effect of Temperature on the Cationic Photopolymerization of Epoxides

The effects of temperature on the photoinitiated cationic ring‐opening polymerizations of a number of different epoxide monomers were studied with the aid of a modified optical pyrometer instrument. Depending on the structures of the epoxide monomer, various behaviors were observed. The results were interpreted as due to steric and electronic features inherent in the structures of the monomers that affect the stabilization of the secondary oxonium ions, which are formed as intermediates in these polymerizations. At one extreme, cycloaliphatic epoxides such as cyclohexene oxide give highly reactive oxonium intermediates that exhibit high rates of polymerization even at subambient temperatures. At the other extreme, alkyl glycidyl ethers produce oxonium ion intermediates, which are so stable that they do not spontaneously react to form polymer at room temperature. By manipulation of the structure of the epoxide monomer, novel monomers with tailored reactivities can be prepared.     

Reactivity of oxetane monomers in photoinitiated cationic polymerization

Studies of the onium salt photoinitiated cationic ring‐opening polymerizations of various 3,3‐disubstituted oxetane monomers have been conducted with real‐time infrared spectroscopy and optical pyrometry. The polymerizations of these monomers are typified by an extended induction period that has been attributed to the presence of a long‐lived tertiary oxonium ion intermediate formed by the reaction of the initially formed secondary oxonium ion with the cyclic ether monomer. Because the extended induction period in the photopolymerization of these monomers renders oxetane monomers of limited value for many applications, methods have been sought for its minimization or elimination. Three general methods have been found effective in markedly shortening the induction period: (1) carrying out the photopolymerizations at higher temperatures, (2) copolymerizing with more reactive epoxide monomers, and (3) using free‐radical photoinitiators as synergists. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3205–3220, 2005     

Redox initiated cationic polymerization: The unique behavior of alkyl glycidyl ethers

Redox systems composed of a diaryliodonium or a triarylsulfonium salt together with a silane bearing Si? H groups were used for the in situ generation of strong Brønsted acids at room temperature in the presence of alkyl glycidyl ether monomers. Secondary oxiranium intermediates are generated with lifetimes from minutes to hours at room temperature. These systems undergo rapid, exothermic cationic chain polymerization when the temperature is raised. Metastable monomer‐redox initiator systems were also observed to undergo frontal polymerizations when a localized heat source is applied to the sample. The application of these delayed cationic ring‐opening polymerization systems for the development of one‐component structural adhesives that undergo rapid thermosetting at low temperatures are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Photoinduced cationic ring‐opening frontal polymerizations of oxetanes and oxiranes

The photoinitiated cationic ring‐opening polymerizations of certain epoxides and 3,3‐disubstituted oxetanes display the characteristics of frontal polymerizations. When irradiated with UV light, these monomers display a marked induction period, during which little conversion of the monomer to the polymer takes place. The local application of heat to an irradiated monomer sample results in polymerization that occurs as a front propagating rapidly throughout the entire reaction mass. For the characterization of these frontal polymerizations, the use of a new monitoring technique, employing optical pyrometry, has been instituted. This method provides a simple, rapid means of following these fast polymerizations and quantitatively determining their frontal velocities. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1630–1646, 2004     

Redox initiated cationic polymerization: Reduction of diaryliodonium salts by 9‐BBN

Diaryliodonium salts undergo facile reduction by the dialkylborane, 9‐BBN. The combination of these two reagents constitutes a redox couple that can be employed as a convenient and versatile initiator system for the cationic polymerizations of styrenic monomers, vinyl ethers and the ring‐opening polymerizations of cyclic ethers and acetals including; epoxides, oxetanes, tetrahydrofuran, and 1,3,5‐trioxane. The polymerizations of these monomers can be carried out in either neat monomer or under solution conditions. Typically, the redox cationic polymerizations of the above monomers are rapid and exothermic. Optical pyrometry (infrared thermography) was employed as a convenient method with which to monitor and optimize the aforementioned redox initiated cationic polymerizations. Studies of the effects of variations in the structure and concentrations of the diaryliodonium salt and 9‐BBN on the polymerizations of various monomers were carried out. A mechanism for the redox cationic initiation of the polymerizations was proposed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5639–5651, 2009     

Redox initiated cationic polymerization

A novel catalytic method for carrying out the cationic polymerizations has been developed based on a redox initiator system in which the reducing component is delivered to the reaction mixture in the vapor state. The redox couple consists of a diaryliodonium salt that is dissolved in the monomer and a noble metal catalyst is added. The silane reducing agent is introduced to the reaction mixture in the vapor state using air as the carrier gas. Reduction of the diaryliodonium salt by the silane results in the liberation of a Brønsted superacid that initiates cationic polymerizations. A study of the effects of variations in the structures of the diaryliodonium salt, the silane, and the type of noble metal catalyst was carried out. In principle, the initiator system is applicable to all types of cationically polymerizable monomers and oligomers including: the ring‐opening polymerizations of such heterocyclic monomers as epoxides and oxetanes and, in addition, the polymerization of vinyl ether monomers such as vinyl ethers. The use of this initiator system for carrying out commercially attractive cross‐linking polymerizations for coatings, composites, and encapsulations is discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1825–1835, 2009     

Hybrid free radical/cationic frontal photopolymerizations

The irradiation of hybrid photopolymer systems consisting of a free radically polymerizable multifunctional acrylate monomer and a cationically polymerizable epoxide or oxetane monomer was conducted under conditions where only the free radical polymerization takes place. This results in the formation of a free‐standing polyacrylate network film containing quiescent oxonium ions along with the unreacted cyclic ether monomer. The subsequent application of a point source of heat to the film ignites a cationic ring‐opening frontal polymerization that emanates from that site and propagates to all portions of the irradiated sample. This article examines the impact of various molecular structural and experimental parameters on these novel hybrid frontal polymerizations that produce interpenetrating network polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4331–4340, 2007     

Synergistic effects in hybrid free radical/cationic photopolymerizations

Multifunctional alkyl glycidyl ether and oxetane monomers are usually deemed to be poorly reactive and are consequently of limited use for high speed photocuring applications. However, these monomers can be made to undergo exceedingly rapid exothermic photopolymerization when combined with a multifunctional acrylate monomer and a corresponding free radical photoinitiator. Under optimum UV irradiation conditions, these hybrid photopolymerizations take place rapidly and substantially without an induction period. A mechanism was proposed on the basis of thermal acceleration of the cationic ring‐opening polymerizations induced by the fast exothermic free radical acrylate photopolymerization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3759–3769, 2007     

Hyperbranched polyethers by ring‐opening polymerization: Contribution of activated monomer mechanism

Propagation in the cationic ring‐opening polymerization of cyclic ethers involves nucleophilic attack of oxygen atoms from the monomer molecules on the cationic growing species (oxonium ions). Such a mechanism is known as the active chain‐end mechanism. If hydroxyl groups containing compounds are present in the system, oxygen atoms of HO? groups may compete with cyclic ether oxygen atoms of monomer molecules in reaction with oxonium ions. At the proper conditions, this reaction may dominate, and propagation may proceed by the activated monomer mechanism, that is, by subsequent addition of protonated monomer molecules to HO? terminated macromolecules. Both mechanisms may contribute to the propagation in the cationic polymerization of monomers containing both functions (i.e., cyclic ether group and hydroxyl groups) within the same molecule. In this article, the mechanism of polymerization of three‐ and four‐membered cyclic ethers containing hydroxymethyl substituents is discussed in terms of competition between two possible mechanisms of propagation that governs the structure of the products—branched polyethers containing multiple terminal hydroxymethyl groups. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 457–468, 2003     

Novel photocurable monomers: The synthesis of difunctional vinyl ethers with a phosphonate group and difunctional 1‐propenyl ethers with a phosphonate group and their photoinitiated cationic polymerization

Phosphorus‐containing vinyl ether monomers and 1‐propenyl ether monomers were prepared by the regioselective addition reaction of glycidyl vinyl ether (GVE) or 1‐propenyl glycidyl ether with diaryl phosphonates with quaternary onium salts as catalysts. The reaction of GVE with bis(4‐chlorophenyl) phenylphosphonate gave bis[1‐(4‐chlorophenoxy methyl)‐2‐(vinyloxy)ethyl]phenylphosphonate in a 68% yield. The structures of the resulting phosphorus‐containing vinyl ether monomers and 1‐propenyl ether monomers were confirmed by IR and ¹H NMR spectra and elemental analysis. Photoinitiated cationic polymerizations of the resulting phosphorus‐containing vinyl ether monomers and 1‐propenyl ether monomers were investigated with photoacid generators. The polymerization of vinyl ether groups and 1‐propenyl ether groups of the obtained monomers proceeded very smoothly with a sulfonium‐type cationic photoinitiator, bis[4‐(diphenylsulfonio)phenyl]sulfide‐bis(hexafluorophosphate), upon UV irradiation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3105–3115, 2005     

Computational evaluation of radical ring‐opening polymerization

The tendencies of ring‐opening processes in radical ring‐opening polymerizations were evaluated by AM1 and PM3 semi‐empirical calculations and 6‐31G*‐level calculations based on the density functional theory (DFT) B3LYP models. Sixteen cyclic monomers bearing vinyl or exomethylene groups were categorized into ring‐opening and no‐ring‐opening monomers by the evaluation of the differences of the internal energies and the lengths of the cleaving bonds between the ground states of the initial radicals and the activated states in the ring‐opening processes. Although the semi‐empirical calculations not parameterized to radical reactions resulted in the moderate categorization of the ring‐opening monomers, the DFT calculation clearly distinguished the ring‐opening and no‐ring‐opening monomers. The ring‐opening tendencies were also evaluated with the changes in the internal energies throughout the ring‐opening processes, but this method could not group the ring‐opening and no‐ring‐opening monomers clearly. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2827–2834, 2007     

Expanding vinyl ether monomer repertoire for ring-expansion cationic polymerization: Various cyclic polymers with tailored pendant groups

Herein, we clarified the ring-expansion cationic polymerization with a cyclic hemiacetal ester (HAE)-based initiator was versatile in terms of applicable vinyl ether monomers. Although there was a risk that higher reactive vinyl ethers may incur β-H elimination of the HAE-based cyclic dormant species to irreversibly give linear chains, the polymerizations were controlled to give corresponding cyclic polymers from various alkyl vinyl ethers of different reactivities. Functional vinyl ether monomers were also available, and for instance a vinyl ether monomer carrying an initiator moiety for metal-catalyzed living radical polymerization in the pendant allowed construction of ring-linear graft copolymers through the grafting-from approach. Furthermore, ring-based gel was prepared via the addition of divinyl ether at the end of the ring-expansion polymerization, where multi HAE bonds cyclic polymers or fused rings were crosslinked with each other. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3082–3089     

Modification of photoinitiated cationic epoxide polymerizations by sulfides

The addition of sulfides has a marked effect on the rates of onium salt induced photoinitiated cationic ring‐opening polymerizations of epoxide monomers. Various behaviors have been observed that depend on the structure of the sulfide. Dialkyl sulfides strongly inhibit the photopolymerizations of these monomers, whereas diaryl sulfides have a retarding effect on the photopolymerizations. Real‐time infrared spectroscopy and optical pyrometry have been employed as analytical methods to probe the kinetic effects of the addition of a variety of sulfides on cationic epoxide ring‐opening photopolymerizations. A mechanism is proposed that involves the formation of sulfonium salts as intermediates. The observations made in this study have important implications for cationic photopolymerizations in general and for photoinitiated cationic ring‐opening polymerizations of epoxides in particular. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2504–2519, 2005     

Interaction of epoxy and vinyl ethers during photoinitiated cationic polymerization

A kinetic study of the independent and simultaneous photoinitiated cationic polymerization of a number of epoxide and vinyl (enol) ether monomer pairs was conducted. The results show that, although no appreciable copolymerization takes place, these monomers undergo complex interactions with one another. These interactions are highly dependent on the epoxide monomer employed. In all cases, the rate of epoxide ring-opening polymerization is accelerated, whereas that of the vinyl ether is depressed. When highly reactive cycloaliphatic epoxides are subjected to photoinitiated cationic polymerization in the presence of vinyl ethers, the two polymerizations proceed in a sequential fashion, with the vinyl ether polymerization taking place after the epoxide polymerization is essentially complete. A mechanism involving an equilibration between alkoxy-carbenium and oxonium ions has been proposed to explain the results. In addition, the free-radical-induced decomposition of the diaryliodonium salt photoinitiator also takes place, leading to a decrease in the induction period. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4007–4018, 1999     

Synthesis and cationic photopolymerization of monomers based on nopol

Starting with nopol [(R)‐(−)‐2‐(2′‐hydroxyethyl)‐6,6‐dimethyl‐8‐oxatricyclo[3.1.1.1^2,3]octane, I] as a substrate, two new, interesting monomers, allyl nopol ether epoxide III and nopol 1‐propenyl ether epoxide IV, were prepared. The photoinitiated cationic polymerizations of these two monomers as well as several other model compounds were studied using real‐time infrared spectroscopy. Surprisingly, the rates of epoxide ring‐opening polymerization of both monomers were enhanced as compared to those of the model compounds. Two different mechanisms which involve the free radical induced decomposition of the diaryliodonium salt photoinitiator were proposed to explain the rate acceleration effects. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1199–1209, 1999     

Investigations of the reactivity of “kick‐started” oxetanes in photoinitiated cationic polymerization

The ability of certain alkyl substituted epoxides to accelerate the photoinitiated cationic ring‐opening polymerizations of oxetane monomers by substantially reducing or eliminating the induction period altogether has been termed by us “kick‐starting.” In this communication, the rates of photopolymerization of several model “kick‐started” oxetane systems were quantified and compared with the analogous biscycloaliphatic epoxide monomer, 3,4‐epoxycyclohexylmethyl 3′,4′‐epoxycyclohexanecarboxylate (ERL). It has been found that the “kick‐started” systems undergo photopolymerization at rates that are at least two‐fold faster than ERL. These results suggest that “kick‐started” oxetanes could replace ERL in many applications in which high speed ultraviolet induced crosslinking photopolymerizations are carried out. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 586–593     

Synthesis of epoxy monomers that undergo synergistic photopolymerization by a radical‐induced cationic mechanism

A series of novel, cycloaliphatic, cationically photopolymerizable epoxide monomers bearing benzyl ether groups were prepared. These monomers display a considerable enhancement in the rate of their cationic ring‐opening polymerizations in comparison with monomers that do not contain such groups. In this article, a synergistic free‐radical mechanism is proposed that accounts for this effect, and supporting evidence is offered for its verification. During UV irradiation of an onium salt cationic photoinitiator, the aryl radicals that are generated abstract labile benzyl hydrogens present in such monomers to generate the corresponding carbon‐centered radicals. Subsequently, these radicals are oxidized to benzyl carbocations by the onium salt via a nonphotochemical chain process. The observed increase in the rate and extent of the cationic ring‐opening polymerization of the epoxide monomers is due to the aforementioned mechanism, which effectively increases the number of reactive cationic species present during polymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3578–3592, 2001     

Synthesis of novel benzoxazines containing glycidyl groups: A study of the crosslinking behavior

Benzoxazines derived from aniline and 4‐hydroxybenzoic acid and from phenol and 4‐aminobenzoic acid were prepared with two different synthetic approaches. When the carboxylic group reacted with epichlorohydrin, glycidylic derivatives M‐1 and M‐2 , respectively, were obtained. The ring opening of benzoxazine and epoxy took place simultaneously with no catalyst for both monomers. Likewise, both ring‐opening polymerizations took place when boron trifluoride monoethylamine (BF₃·MEA) or 4‐(N,N‐dimethylamino)pyridine was used as a catalyst for M‐1 . However, for M‐2 , when BF₃·MEA was used as a catalyst, the epoxy and benzoxazine ring openings could be distinguished, and a polyether intermediate containing benzoxazine side chains could be obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1529–1540, 2006     

Anionic ring‐opening polymerization of N‐glycidylphthalimide: Combination of phosphazene base and activated monomer mechanism

Anionic ring‐opening polymerization of glycidyl phthalimide, initiated with alcohol–phosphazene base systems and based on monomer activation with a Lewis acid (iBu₃Al), has been studied. No propagation occurred for initiator: iBu₃Al ratios less or equal to 1:3. For larger Lewis acid amounts, the first anionic ring‐opening polymerizations of glycidyl phthalimide were observed. Polymers were carefully characterized by NMR, MALDI‐TOF mass spectrometry, and size exclusion chromatography and particular attention was given to the detection of eventual transfer or side‐reactions. However, polymer precipitation and transfer reaction to aluminum derivative were detrimental to monomer conversion, polymerization control, and limited polymer chain molar masses. The influence of reaction temperature and solvent on polymer precipitation and transfer reactions was studied and reaction conditions have been optimized leading to afford end‐capped poly(glycidyl phthalimide) with narrow molar mass distributions. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1091–1099