Redox Intitiated Cationic Polymerization

Two novel redox initiator systems have been developed for carrying out the cationic polymerizations of vinyl and heterocyclic monomers. The initiators are based on various onium salts as the oxidant together with an alkylborane or an organosilane as the reducing agent. Using both redox cationic initiator systems, the polymerizations of a wide variety of monomers can be carried out at or below room temperature in the presence or absence of unreactive solvents. Also described in this communication is the novel use of a two-component redox system in which the reducing agent, a silane, is delivered to the monomer sample in the vapor state. Optical pyrometry (infrared thermography) was employed as a convenient method with which to monitor the polymerizations in real-time. A study of the effects of variations in the structures of the onium salt, the silane and the type of noble metal catalyst were carried out. The use of these initiator systems for carrying out commercially attractive crosslinking polymerizations for coatings, composites and electronic encapsulations is discussed.     

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 of oxetanes

3,3‐Disubstituted oxetane monomers were found to undergo rapid, exothermic redox initiated cationic ring‐opening polymerization in the presence of a diaryliodonium or triarylsulfonium salt oxidizing agent and a hydrosilane reducing agent. The redox reaction requires a noble metal complex as a catalyst and several potential catalysts were evaluated. The palladium complex, Cl₂(COD)Pd^II, was observed to provide good shelf life stability while, at the same time, affording high reactivity in the presence of a variety of hydrosilane reducing agents. A range of structurally diverse oxetane monomers undergo polymerization under redox cationic conditions. When a small amount of an alkylated epoxide was added as a “kick‐start” accelerator to these same oxetanes, the redox initiated cationic polymerizations were extraordinarily rapid owing to the marked reduction in the induction period. A mechanistic interpretation of these results is offered. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1854–1861     

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     

Redox initiated cationic polymerization: Silane‐N‐aryl heteroaromatic onium salt redox couples

In this article, a new route for the synthesis of N‐aryl heteroaromatic onium salts by the direct copper catalyzed arylation of pyridine, substituted pyridines, isoquinoline, and acridine with diaryliodonium salts is described. It was demonstrated that these N‐aryl heteroaromatic onium salts undergo facile platinum or rhodium‐catalyzed reduction by silanes bearing Si? H groups. The reduction of N‐aryl heteroaromatic onium salts generates Brønsted acids. When this redox reaction was carried out in situ in the presence of an appropriate monomer, cationic polymerization was observed. Using this approach, the cationic polymerizations of epoxides, oxetanes, 1,3,5‐trioxane, styrene, and vinyl ethers were carried out. The use of optical pyrometry to monitor the redox initiated cationic polymerizations of some representative multifunctional monomers is described. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Ketene acetal monomers: Cationic photopolymerization

A study of the photoinduced cationic polymerization of a novel series of cyclic and acyclic ketene acetal monomers was carried out. It was observed that the cyclic monomers underwent facile cationic polymerization to give high molecular weight polymers while the acyclic ketene acetals did not. Thermally induced polymerization was also observed on standing catalyzed by glass surfaces. Photoinduced cationic polymerizations employing both diaryliodonium and triarylsulfonium salt photoinitiators were studied. It was observed that the position and type of substitutents in the cyclic ketene acetals was the major factor in determining the proportion of vinyl and ring-opening polymerizations that take place. © 1996 John Wiley & Sons, Inc.     

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     

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     

Synthesis of silicon-containing vinyl ether monomers and oligomers and their photoinitiated polymerization

Silicon-containing divinyl ether monomers were synthesized by the addition reaction of glycidyl vinyl ether ( 1 ) with various silyl dichlorides using tetra-n-butylammonium bromide (TBAB) as a catalyst. The reaction of 1 with diphenyl dichlorosilane gave bis-[1-(chloromethyl)-2-(vinyloxy)-ethyl]diphenyl silane ( 3a ) in 89% yield. Polycondensations of 3a with terephthalic acid were also carried out using 1,8-Diazabicyclo[5.4.0]-7-undecene (DBU) to afford silicon-containing polyfunctional vinyl ether oligomers ( 5 ). A multifunctional Si-monomer with both vinyl ether and methacrylate groups ( 7 ) was prepared by the reaction of 3a with potassium methacrylate using TBAB as a phase transfer catalyst. Photoinitiated cationic polymerizations of these vinyl ether compounds proceeded rapidly using the sulfonium salt, bis-[4-(diphenyl-sulfonio)phenyl]sulfide-bis-hexafluorophoshate (DPSP), as the cationic photoinitiator in neat mixtures upon UV irradiation. Multifunctional monomer 7 with both vinyl ether and methacrylate groups showed “hybrid curing properties” using both DPSP and the radical photoinitiator, 2,4,6-trimethylbenzoyl diphenylphoshine oxide (TPO). © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3217–3225, 1997     

Visible and long‐wavelength photoinitiated cationic polymerization

A new system for efficiently carrying out cationic photopolymerizations with visible and long‐wavelength UV light is described. This system is based on the principle that certain onium salt cationic photoinitiators can be reduced by free radicals produced by the hydrogen abstraction reactions of photoexcited ketones. Thus, when camphorquinone, benzil, 2‐isopropylthioxanthone, and 2‐ethylanthraquinone are irradiated in the presence of a monomer that can serve as a hydrogen donor, the resulting monomer‐bound radical rapidly reduces a diaryliodonium salt or a dialkylphenacylsulfonium salt, and the resulting monomer‐centered cations initiate the polymerizations of epoxides, vinyl ethers, and heterocyclic compounds. Onium salts with high reduction potentials, such as triarylsulfonium salts, do not undergo sensitization by this new system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 343–356, 2001     

Photoinitiated cationic polymerization of limonene 1,2‐oxide and α‐pinene oxide

Limonene 1,2‐oxide (LMO) and α‐pinene oxide (α‐PO) are two high reactivity biorenewable monomers that undergo facile photoinitiated cationic ring‐opening polymerizations using both diaryliodonium salt and triarylsufonium salt photoinitiators. Comparative studies showed that α‐PO is more reactive than LMO, and this is because it undergoes a simultaneous double ring‐opening reaction involving both the epoxide group and the cyclobutane ring. It was also observed that α‐PO also undergoes more undesirable side reactions than LMO. The greatest utility of these two monomers is projected to be as reactive diluents in crosslinking photocopolymerizations with multifunctional epoxide and oxetane monomers. Prototype copolymerization studies with several difunctional monomers showed that LMO and α‐PO were effective in increasing the reaction rates and shortening the induction periods of photopolymerizations of these monomers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Relation between the reactivities of vinyl monomers in ionic polymerizations and their 1H NMR spectra

¹H NMR chemical shifts of the protons in the vinyl groups of monomers are correlated with their reactivities in anionic, coordinated anionic, and cationic polymerizations. The relative reactivities of styrenes in anionic addition reactions with living polystyrene increase linearly with the chemical shift of the proton trans to the substituent (δ_H1). Only the plot for 2,4,6-trimethylstyrene deviates very much from the linear relation because of the large steric hindrance. The relative reactivities of methacrylates in anionic copolymerizations increase with increasing chemical shifts of protons attached to the β-carbon of methacrylates. In cationic polymerizations of styrenes, the relative reactivities decrease with increasing δ_H1. The relative reactivities in coordinated anionic polymerizations with Ti-containing Ziegler initiators show a typical feature of cationic polymerization, and those with V-containing initiators show a typical feature of anionic polymerization, indicating the importance of the coordination process in the propagation reaction with Ti-containing initiator systems. From the results, it can be concluded that the chemical shifts of the protons attached to the β-carbon of vinyl monomers can be used as a practical measure of the reactivity of vinyl monomers in ionic polymerizations and also as a tool for understanding the mechanism of polymerization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2134–2147, 2002     

Free radical accelerated cationic polymerizations

The simultaneous photoinitiated cationic polymerizations of epoxides and vinyl ethers in the presence of diaryliodonium salt photoinitiators results in an acceleration of the ring‐opening epoxide polymerization and a deceleration of the vinyl ether polymerization. These effects are seen both in mixtures of the two monofunctional monomers as well as in hybrid monomers which bear vinyl ether and epoxide groups in the same molecule. A combination of two mechanisms have been proposed to account for these effects. The reversible conversion of alkoxycarbenium to oxiranium ions results in a two‐stage reaction in which first, the epoxide, then the vinyl ether polymerization takes place. Free radical chain induced decomposition of the diaryliodonium salt produces a large incremental number of carbenium ion species which results in the acceleration effect.     

Supramolecular diaryliodonium salt‐crown ether complexes as cationic photoinitiators

Diaryliodonium salts spontaneously form crystalline 1:1 supramolecular complexes at room temperature in good to excellent yields with 18‐crown‐6 ether and its cyclohexano‐ and benzo‐substituted analogs. The complexes were characterized using IR, UV, MS, ¹H, and ¹³C‐NMR spectroscopy and by single crystal X‐ray crystallography. The analytical data obtained were consistent with a structure in which the positively charged iodine atom of diaryliodonium cation is positioned above and over the center of the crown ether ring with the positively charged iodine atom coordinated to the crown ether oxygen atoms. The diaryliodonium salt‐crown ether complexes are photosensitive and were used to carry out the photoinitiated cationic polymerizations of a number of mono‐ and difunctional monomers. During irradiation with UV light, the supramolecular complexes undergo photolysis with the generation of a Brønsted acid and with the concomitant release of the crown ether. When used as photoinitiators, the crown ether that is released markedly influences the kinetics of the subsequent cationic polymerization of the monomer. Further studies demonstrated that the photolysis of diaryliodonium salt‐crown ether supramolecular complexes can be photosensitized using typical‐electron transfer photosensitizers. Free radical‐promoted photosensitization using typical unimolecular free radical photoinitiators such as 2,2‐dimethoxy‐2‐phenylacetophenone also takes place readily. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

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 diaryliodonium salt/benzoin redox couple

A new redox couple based on the copper-catalyzed reduction of diaryliodonium salts with benzoins has been used to initiate cationic polymerizations of cyclic ethers and esters. A proposed mechanism for initiation by this redox couple is based on its stoichiometry and on the nature of the products. It was concluded that initiation of polymerization occurs both by direct arylation of the monomer and by protonation by strong Brønsted acids. The polymerization of several typical cationically polymerizable monomers using this new redox initiator were studied.     

A new visible light sensitive photoinitiator system for the cationic polymerization of epoxides

This communication reports the development of an efficient three‐component visible light sensitive photoinitiator system for the cationic ring‐opening photopolymerization of epoxide monomers and epoxide functional oligomers. The photoinitiator system consists of camphorquinone in combination with a benzyl alcohol to generate free radicals by the absorption of visible light. Subsequently, the radicals participate in the free radical chain induced decomposition of a diaryliodonium salt. The resulting strong Brønsted acid derived from this process catalyzes the cationic ring‐opening polymerization of a variety of epoxide substrates. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 866–875, 2009     

Curcumin: A naturally occurring long‐wavelength photosensitizer for diaryliodonium salts

Curcumin, a naturally occurring, intensely yellow dye extracted from the spice turmeric, is an efficient photosensitizer for diaryliodonium salt photoinitiators at wavelengths ranging from 340 to 535 nm. With curcumin as a photosensitizer, it is possible to carry out the cationic photopolymerization of a wide variety of epoxide, oxetane, and vinyl monomers with long‐wavelength UV and visible light. An example of the photopolymerization of an epoxide monomer with ambient solar irradiation is provided. Several other curcumin analogues were synthesized, and their use as photosensitizers is examined. With such photosensitizers, the range of spectral sensitivity can be extended well into the visible region of the electromagnetic spectrum. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5217–5231, 2005     

A quinoxaline derivative as a long wavelength photosensitizer for diaryliodonium salts

This article reports the use of a quinoxaline derivative as a photosensitizer for diaryliodonium salt photoinitiators. 2,3‐bis(3,4‐bis(decyloxy)phenyl)‐5,8‐bis(2,3‐dihydrothieno[3,4‐b][1,4]dioxin‐5‐yl)quinoxaline (DOPEQ), is a highly conjugated compound with strong absorption bands at wavelengths ranging from 300 to 550 nm and is shown to facilitate photoinitiated cationic polymerization of heterocyclic monomers such as oxiranes and oxetanes. The polymerizations are initiated at room temperature by using long wavelength UV light in the presence of diphenyliodonium hexafluorophosphate (Ph₂I⁺PF). The polymerizations are monitored by optical pyrometry (OP). It is also possible to initiate photopolymerizations with ambient solar irradiation in the presence of this photosensitizer dye. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 209–213, 2010     

Copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms

Butyl vinyl ether (BVE) and methyl methacrylate (MMA) mixtures were polymerized by using free radical initiators in conjunction with a cationic initiator such as diphenyl iodonium salt. Polymerization mechanism involves free radical polymerization of MMA which is switched to cationic polymerization of BVE by addition of growing poly(MMA) radicals to BVE and subsequent oxidation of electron donating polymeric radicals to the corresponding cations by iodonium ions. Two representative bifunctional monomers, ethylene glycol divinyl ether (EGDVE) and ethylene glycol dimethacrylate (EGDMA) were also used together with MMA and BVE, respectively, in photo and thermal crosslinking polymerizations. Vinyl ether and methacrylate type monomers can successfully be copolymerized by this double-mode polymerization under photochemical conditions.