Initiators For Cationic Polymerization Research Articles

Cationic Polymerized Epoxy and Radiation Cured Acrylate Blend Nanocomposites Based on WS2 Nanoparticles—Part A: Curing Processes and Kinetics

Cationic photo-initiated and polymerized epoxies are characterized by good adhesion, high modulus, zero volatiles, low shrinkage and living polymerization characteristics. Radiation—cured acrylate resins are characterized by rapid initial curing with increased initial strength. The combination of radiation-cured acrylates and epoxies may present advantageous attributes. Thus, the system investigated is a hybrid epoxy/methyl acrylate and three different initiators for cationic polymerization of epoxies, the radical reaction of acrylates and the thermal initiator. When incorporating additives like opaque WS2 nanoparticles (NPs), absorption of the photo radiation takes place, which may lead to low photo activity. Curing kinetics measurements revealed that the absorbing/masking effect of WS2 was insignificant, and surprisingly, the level of curing was enhanced when the WS2 NPs were incorporated. FTIR results demonstrated that covalent bonds were formed between the inorganic fullerenes (IF-WS2) and the crosslinked matrix. Viscosity measurements showed a surprising reduction of five to ten times in the low-shear viscosity upon NPs incorporation compared to neat resins. It was concluded that the decrease of viscosity by the inorganic NPs, in addition to the enhanced level of conversion, has profound advantages for structural adhesives and 3D printing resins. To the best of our knowledge, this investigation is the first to report on a radiation-induced curing system containing opaque WS2 NPs that leads to an enhanced degree of curing and reduced shear viscosity.

Effect of UV-curing conditions on the cationic polymerization kinetics of vinyl ether monomers

In this study, the performance of a new triarylsulfonium photoinitiator in the initiation of cationic polymerization of vinyl monomers was investigated by infrared spectroscopy (IR). The effect of the emission spectrum of light sources on the kinetics of the curing process was examined. The results indicate that the light source characterized by a total overlap of its emission spectrum with the photoinitiator absorption spectrum is the best system for effective cure. In the case of smaller overlap of spectrums, the conversion degree and the rate are lower. Nevertheless, it is easy to regulate by extending the energy of the emitted light. At the UVB light source, the maximal conversion degree is 71.8%, with the polymerization rate of 1.68 s-1, which is faster than the corresponding free-radical photopolymerization of acrylic monomers.

Effect of Cationic Photoinitiator on the Polymerization Kinetics of Vinyl Ether Monomers

In this study, the performance of a new triarylsulfonium photoinitiators in the initiation of cationic polymerization of vinyl monomers was investigated by infrared spectroscopy (IR). The photoinitiator exhibit absorption characteristics compatible with the emission characteristics of medium pressure mercury lamps, which are the main sources of UV light in the industry. The experimental results indicate that the photoinitiator is most effective at a concentration of 1%. At the investigated conditions, the maximal conversion degree is 77.8% and the polymerization rate is 0.56 s-1. The curing process is even faster than the corresponding free-radical photopolymerization of acrylic monomers

New High-Power LEDs Open Photochemistry for Near-Infrared-Sensitized Radical and Cationic Photopolymerization.

Cyanines derived from heptamethines were investigated in combination with iodonium salts as initiators of the radical polymerization of tripropylene glycol diacrylate and epoxides derived from bisphenol-A-diglycidylether. A new near-infrared (NIR) LED prototype emitting at 805 nm with an exposure intensity of 1.2 W cm-2 facilitated initiation of both radical and cationic polymerization using sensitizers derived from cyanines. This new light-emitting device has brought new insight into the photochemistry of cyanines with the general structure 1 because a combination of photonic and thermal processes strongly influences reaction pathways. In particular, cationic cyanines comprising a cyclopentene moiety and diphenylamino group in the center initiated the cationic polymerization of epoxides. Selective oxidation of this unit explains why specifically these derivatives may function as initiators for cationic polymerization. In contrast, when the diphenylamino group was replaced by a barbital group at the meso-position cationic polymerization of epoxides was not initiated.

Alkylsulfonyl Halides as Initiators of Cationic Polymerization of Oxazolines

The example of methanesulfonyl halides has shown the possibility of initiating cationic polymerization of 2-oxazolines by alkylsulfonic acid halides. Chain propagation rate constants and activation energy of the processes have been determined. The kinetic mechanisms suggest a slow initiation of polymerization followed by fast chain propagation. It has been shown that the polymerization of 2-ethyl-2-oxazoline initiated by aliphatic sulfonyl halides proceeds predominantly at the active centers of the ionic type. The synthesis of oligo(3-sulfonyl)methacrylate-graft-poly(2-isopropyl-2-oxazoline) is described as an example of using alkylsulfonyl halides for graft copolymers.

The use of sulfonyl halides as initiators of cationic polymerization of oxazolines

The kinetic investigations of the cationic polymerization of 2-ethyl-2-oxazoline are performed with the use of aromatic sulfonic acid halides as initiators. The rate constants of chain propagation and the activation energy are calculated. It is found that the polymerization predominantly proceeds via the livingchain, rather than the pseudoliving, polymerization mechanism.

Frontal cationic curing of epoxy resins in the presence of defoaming or expanding compounds

ABSTRACTThermal frontal polymerization was carried out with trimethylol propane triglycidyl ether using two different BF3‐amine complexes, B‐950 and B‐110 from Leepoxy, as initiators for cationic polymerization. The amounts of filler (kaolin or fumed silica), defoaming, or expansion agents were varied to study how the compositions affected the front velocity, expansion, and flexural modulus of the resulting epoxy resins. The polymer produced with B‐950 initiator showed higher modulus than the polymers produced with B‐110. Moreover, fumed silica created stronger materials than kaolin. The presence of BYK as a defoamer or an expansion agent such as the Expancel #80 was also able to affect significantly the mechanical properties. differential scanning calorimetry studies indicated that the conversion was complete and that kaolin and silica increased the rate of reaction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40339.

Interlaminar fracture toughness behavior of electron-beam cured carbon-fiber reinforced epoxy-resin composites

The work describes the preparation and physical-mechanical characterization of unidirectional CFRP panels manufactured by an electron beam curing technique. Delamination fracture toughness in Mode I and II is investigated in order to evaluate the influence of fiber–matrix adhesion strength, matrix toughness and matrix crosslinking density as determined by the radiation curing process. A matrix system comprising a DGEBA epoxy monomer and an initiator of cationic polymerization have been used, with one batch of resin mixed with a PES monomer in order to enhance matrix toughness. Curing was achieved with a pulsed 10 MeV Electron Beam accelerator. Thermally cured composite systems have also been manufactured and tested for comparison. Results from double cantilever beam and end notched flexure delamination tests have been analyzed and correlated with results from short beam shear, dynamic mechanical thermal analysis tests and SEM micrographs of delaminated surfaces. POLYM. COMPOS., 35:1529–1542, 2014. © 2013 Society of Plastics Engineers

Greener, cleaner polymerization of isobutene

Methylaluminoxane (MAO)/silyl halide-based initiator systems greatly improve the sustainability of isobutene (IB) polymerizations by eliminating halogenated solvents, reducing energy consumption and removing the need for postpolymerization purification. These solvent-free systems give rise to moderate yields of high-molecular-weight (MW) polymers at convenient temperatures without conventional toxic solvents. In addition, facile separation of MAO and polyisobutene (PIB) was accomplished with supported MAOs. The resulting system gave enhanced activity and high yields of medium-to-high-MW polymer at temperatures up to ambient. The MW-temperature profile for MAO-based systems approximates that for γ-radiation and perfluoroarylated Lewis acid–based systems; however, the former does not require high-purity monomer and is more economical and easily scalable. Modeling experiments involving 2,4,4-trimethyl-1-pentene indicate that silyl halides undergo ligand-exchange type reactions with MAO where halogen attached to silicon is replaced with a methyl substituent from MAO with concomitant production of chloraluminoxane and initiation of cationic polymerization in an ill-defined manner.

Control of cationic epoxy polymerization by supramolecular initiation

We show that BF4−·4-chloroanilinium·18-crown-6 complex can serve as an efficient thermoresponsive initiator in cationic polymerization of epoxy showing a high apparent activation energy of 100 kJ mol−1. Remarkably the conversion at the gel point and consequently the structure of the networks does not significantly depend on the initiation temperature. This bodes well for applications of supramolecular initiation to control the polymerization of epoxy thermosets.

Triphenylcyclopropenium hydroxytris(pentafluorophenyl)borate salt for solvent free epoxy curing

A triphenylcyclopropenium salt paired with hydroxytris(pentafluorophenyl)borate counter ion was synthesized as an initiator for cationic polymerization and its activity was compared with triphenylcyclopropenium hexafluoroantimonate. Triphenylcyclopropenium hydroxytris (pentafluorophenyl)borate exhibits excellent activity as a room temperature thermal initiator for epoxide polymerization under solvent free conditions, due to excellent solubility in epoxy resins.

Photoinduced Electron Transfer Reactions of Highly Conjugated Thiophenes for Initiation of Cationic Polymerization and Conjugated Polymer Formation

Photoinduced electron transfer reactions of highly conjugated thiophene derivatives, 4,7-di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[1,2,5]thiadiazole (DTDT) and 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2,3-di(thiophen-2-yl)quinoxaline (DTDQ), with diphenyliodonium hexafluorophosphate (Ph2I+PF6–) and triphenylsulfonium hexafluorophosphate (Ph3S+PF6–) were investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and polymerization studies. High fluorescence quantum yields, long fluorescence lifetimes (∼10 ns in aprotic solvents), and absence of detectable phosphorescence at 77 K for both compounds indicate inefficient intersystem crossing into the triplet state and dominant role of singlet excited state. Photolysis of DTDT or DTDQ in the presence of iodonium salt with visible light results in the formation of radical cations of DTDT and DTDQ as detected by laser flash photolysis. Sulfonium salts do not undergo such electron transfer reactions due to the unfavorable redox potentials. The importance of the described photoinduced electron transfer process with respect to the initiation of cationic polymerization and formation of conjugated polymers was demonstrated.

New photoinitiators for cationic polymerization

Four new diaryliodonium photoinitiators have been developed and their performance in initiation of cationic polymerization of example vinyl and epoxy monomers has been tested by Fluorescence Probe Technology (FPT). The photoinitiators exhibit absorption characteristics compatible with the emission characteristics of medium pressure mercury lamps, which are main sources of UV light in the industry. This solves the technological problem related to poor match between the absorption characteristics of commercial photoinitiators and the emission characteristics of industrial UV light sources.

Synthesis and characterization of a double photochromic initiator for cationic polymerization

A novel cationic photoinitiator namely, 2-benzyl-2-(N,N-dimethyl-2-oxo-2-phenylethyl) ammonium hexafluoroantimonate-1-(4-morpholinophenyl)-butane-1-one (BDMPP+SbF6-), carrying two photochromophoric groups was synthesized and characterized. Theoretical absorption characteristics of the salt were studied and compared with those obtained experimentally. Photoinitiation activity of this salt was demonstrated by polymerization of various monomers at λ=350nm. Upon irradiation by UV light, cationic species formed from homolytic dissociation followed by electron transfer or directly by heterolytic scission initiate cationic polymerization.

Ab initio MODPOT/VRDDO/MERGE calculations on energetic compounds. I. Mechanism of initiation of cationic polymerization from electrostatic molecular potential contour maps

Abstract : Experimentally it had been observed in 1969 for cyclic ethers with conventional substituents that their propensity to polymerize was influenced by their basicities and ring strain. For energetic new polymers it was desired to predict their propensity to polymerize of cyclic ethers substituted with energetic groups prior even to synthesis of monomers themselves. We carried out ab-initio quantum chemical calculations on these energetic cyclic ethers and generated the electrostatic molecular potential contour maps. These maps indicate vividly the basicities of the cyclic ethers and permit the relative ranking of their tendencies to polymerize. The maps also allow optimal choice of good copolymer candidates. (Author)

Novel dibenzocycloheptenyl phosphonium salts as thermolatent initiator in cationic polymerization

AbstractIn this study, novel thermolatent cationic initiators based on dibenzocycloheptenyl phosphonium salts (1a, 1b, 1c, 1d, 2a, and 3a) were synthesized and their efficiency was examined in bulk polymerization of glycidyl phenyl ether (GPE). The polymerization of GPE was performed with 1 mol % of dibenzocycloheptenyl triphenylphosphonium salts (1a–1d) with different counter anions (SbF6−, PF6−, AsF6−, and BF4−) at 25–200°C for 1 h. The order of initiator activity was found as 1a > 1d > 1b > 1c. To examine the effect of phosphine moiety, the activity of 1a was compared with dibenzocycloheptenyl‐tri‐n‐butylphosphonium hexafluoroantimonate (2a). The order of initiator activity was observed as 1a > 2a. The initiator activity of 1a was compared with that of 10,11‐dihydro‐dibenzocycloheptenyltriphenylphosphonium hexafluoroantimonate (3a) to understand the effect of extended conjugation in dibenzocycloheptenyl ring. In general, with the increase in the polymerization temperature, conversion (%) also increases. The solubility of initiators in various solvents and epoxy monomers was also examined. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Novel piperidinium salts as latent initiators for cationic polymerization of epoxide and vinyl ether

AbstractNovel dibenzylpiperidinium salts with nonnucleophilic anions (DBPi‐SbF6, DBPi‐PF6) have been prepared as latent cationic initiators. Utility of these salts in the photo and thermal‐induced cationic polymerizations of epoxide and vinyl ether monomer systems has been studied. The new initiator, DBPi‐SbF6 showed good solubility, high reactivity, and high thermal latency for polymerizations of epoxide and vinyl ether monomers with only 1 wt % of concentration. Cationic polymerization of vinyl ether monomer was significantly faster than epoxide monomer by the synthesized initiators. This article describes the synthesis, characterization, and activity of novel initiators. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

A visible light photochemical route to silver–epoxy nanocomposites by simultaneous polymerization–reduction approach

The in situ synthesis of silver–epoxy nanocomposite by visible light photoinduced processes was described. The visible light photoinitiator, camphorquinone (CQ) (2,3-bornanedione), was used to generate electron-donating radicals upon photolysis. Subsequent oxidation of these radicals to the corresponding cations in the presence of silver hexafluoroantimonate (AgSbF6) leads to the simultaneous formation of silver nanoparticles and initiation of cationic polymerization of epoxy resin, 1,3-bis(3,4-epoxycyclohexylethyl)tetramethyldisiloxane (EPOX) present in the formulation. The cured nanocomposites were characterized by FT-IR, DSC and TEM measurements.

In Situ Synthesis of Silver−Epoxy Nanocomposites by Photoinduced Electron Transfer and Cationic Polymerization Processes

The in situ synthesis of silver-epoxy nano composite was described. A radical photoinitiator was used to generate electron donating radicals upon photolysis. Subsequent oxidation of these radicals to the corresponding cations in the presence of silver hexafluoroantimonate (AgSbF6) leads to the simultaneous formation of silver nanoparticles and initiation of cationic polymerization of epoxy resin, 1,3-bis(3,4-epoxycyclohexylethyl)tetramethyldisiloxane (EPOX) present in the formulation. The cured nanocomposites were characterized by FT-IR, DSC and TEM measurements

Organoborate Acids as Initiators for Cationic Polymerization of Styrene in an Ionic Liquid Medium

A controlled cationic polymerization of styrene has been achieved using bisoxalatoboric acid (HBOB) as an initiator in a room temperature ionic liquid (IL) under mild reaction conditions to obtain polymers of low molecular weights and narrow polydispersity. HBOB represents a relatively easily handled cationic initiator as compared to those often used in cationic polymerization reactions. The concentration of HBOB was optimized, and it was found that the molecular weights of these polymers increased with decreasing HBOB concentration. Studies were performed with different water contents in the reaction mixture to gauge the water tolerance above which the polymerization is inhibited. Experiments were also carried out with other organoborate acids, and it was found that the trend in yields followed trends in Bronsted acidity. The efficiency of different ILs as reaction solvents is also discussed.