Systematic Investigation of the Photopolymerization of Imidazolium-Based Ionic Liquid Styrene and Vinyl Monomers

The use of ionic liquids (ILs) as media in radical polymerizations has demonstrated the ability of these unique solvents to improve both reaction kinetics and polymer product properties. However, the bulk of these studies have examined the polymerization behavior of common organic monomers (e.g., methyl methacrylate, styrene) dissolved in conventional ILs. There is increasing interest in polymerized ILs (poly(ILs)), which are ionomers produced from the direct polymerization of styrene-, vinyl-, and acrylate-functionalized ILs. Here, the photopolymerization kinetics of IL monomers are investigated for systems in which styrene or vinyl functionalities are pendant from the imidazolium cation. Styrene-functionalized IL monomers typically polymerized rapidly (full conversion ≤1 min) in both neat compositions or when diluted with a nonpolymerizable IL, [C₂mim][Tf₂N]. However, monomer conversion in vinyl-functionalized IL monomers is much more dependent on the nature of the nonpolymerizable group. ATR-FTIR analysis and molecular simulations of these monomers and monomer mixtures identified the presence of multiple intermolecular interactions (e.g., π–π stacking, IL aggregation) that contribute to the polymerization behaviors of these systems. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2364–2375     

Photopolymerization kinetics in and of self‐assembling lyotropic liquid crystal templates

Photopolymerization in and of lyotropic liquid crystal (LLC) template phases shows great promise for generating nanostructure in organic polymers. Interestingly, the order imposed on the polymerization system in LLCs significantly alters polymerization kinetics. The rate of polymerization of hydrophilic monomers increases with increasing LLC order, primarily due to monomer/polymer association with surfactant and the resulting decrease of growing polymer chain diffusion. Conversely, as LLC order increases, hydrophobic monomers become less segregated as nonpolar volume increases, which decreases polymerization rate. The efficiency of initiators is also dependent on LLC template order, further contributing to polymerization rate changes. When reactive surfactants are used, LLC mesophase, location of reactive group, and aliphatic tail length also affect polymerization kinetics. Overall, these photopolymerization kinetics directly relate to the segregation behavior and local order of reactive groups and thus can be used to probe nanostructure evolution, facilitating understanding and control of ultimate polymer nanostructure. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 471–489     

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     

Kinetic investigations on the photopolymerization of di- and tetrafunctional (meth)acrylic monomers in polymeric matrices. ESR and calorimetric studies. I. Reactions under irradiation

The photopolymerization of several di- and tetrafunctional (meth)acrylic monomers in the presence of a styrene–butadiene–styrene polymeric matrix (SBS) has been studied. Electron spin resonance spectroscopy (ESR) and differential scanning photocalorimetry (photo-DSC) were used as monitoring techniques to identify the photogenerated radicals and analyze photopolymerization profiles, radical environments, and radical secondary reactions. The study of the photopolymerization and/or photocrosslinking reactions of these monomers in the solid media was carried out by taking into consideration different factors, such as the influence of both monomer and photoinitiator structures on the hydrogen abstraction in the binder with formation of benzylic and allylic radicals, the polymerization of the monomers itself and the hydrogen abstraction reaction in the polymerized acrylic chains. Finally, irradiation of the system SBS/photoinitiator in the absence of monomer was also accomplished. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2775–2783, 1998     

Solvent‐free inkjet printing process for the fabrication of conductive,transparent, and flexible ionic liquid‐polymer gel structures

A new and solvent‐free process for the fabrication of inkjet printed ionic liquid‐polymer gel microstructures with high‐resolution (line widths of ～40 μm), good electrical conductivity (5–30 mS cm^?1), optical transparency, and mechanical flexibility is presented. Carrying out the printing and polymerization process in nitrogen atmosphere eliminates the inhibiting influence of oxygen and guarantees homogeneously gelled structures. Careful selection and combination of ionic liquids (ILs) and unsaturated monomers makes it possible to achieve low viscosities which are printable with commercially available inkjet printers and printheads without adding extra solvents. By using different types and amounts of ILs and monomers the resulting properties of the printed IL‐polymer gels can be controlled in terms of ionic conductivity, optical transmission, and mechanical flexibility. Higher conductivities are possible by using a bifunctional instead of a monofunctional monomer, which allows one to lower the amount of monomer without loss in mechanical strength. Cast samples make it possible to obtain data of transmission (～90% for 170‐μm thick films) and mechanical flexibility (E = 0.02–0.23 MPa) of bulk material. Comparing electrical conductivity of printed and cast samples, the higher values of printed samples indicate the conductivity enhancing influence of moisture absorbed from the surrounding atmosphere after the fabrication process. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

The influence of N‐vinyl pyrrolidone on polymerization kinetics and thermo‐mechanical properties of crosslinked acrylate polymers

Polymerization of multifunctional acrylate monomers generates crosslinked polymers that are noted for their mechanical strength, thermal stability, and chemical resistance. A common reactive diluent to photopolymerizable formulations is N‐vinyl pyrrolidone (NVP), which is known to reduce the inhibition of free radical photopolymerization by atmospheric oxygen. In this work, the copolymerization behavior of NVP was examined in acrylate monomers with two to five functional groups. At concentrations as low as 2 wt %, NVP increases the polymerization rate in copolymerization with multifunctional acrylate monomer. The relative rate enhancement associated with adding NVP increases dramatically as the number of acrylate double bonds changes from two to five. The influence of NVP on polymerization kinetics is related to synergistic cross‐propagation between NVP and acrylate monomer, which becomes increasingly favorable with diffusion limitations. This synergy extends bimolecular termination into higher double bond conversion through reaction diffusion controlled termination. Copolymerizing concentrations of 5–30 DB% NVP with diacrylate or pentaacrylate monomer also increases Young's modulus and the glass transition temperature (T_g) in comparison to neat acrylate polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4062–4073, 2007     

Photopolymerization of coordinated ionic liquid monomers: Realizing the benefits of structured media using only common reagents

Here, we provide a detailed report on a new type of structured media for improving photopolymerizations: coordinated ionic liquids (ILs). Coordinated ILs are readily formed from the bistriflimide ([Tf₂N]^?) anion and coordination complexes composed of Li⁺ cations with polar organic monomers without an additional cosolvent. Photopolymerization kinetics and monomer conversion were monitored in real time using attenuated total reflectance Fourier transform infrared spectroscopy and the material properties of the products were examined using gel permeation chromatography and differential scanning calorimetry. Generally, coordinated IL monomers displayed improved reaction kinetics at both high and low salt concentrations as well as distinct product properties. The noncovalent (and reversible) interactions between monomer and salt in coordinated ILs hold promise as an efficient and versatile form of structured media for photopolymerizations. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2004–2014     

Photoinitiated cationic polymerization of epoxy alcohol monomers

A series of epoxy alcohols were prepared by simple, straightforward methods. These compounds were very reactive monomers that polymerized rapidly on UV irradiation in the presence of cationic photoinitiators. The kinetics of the cationic photopolymerization of these monomers were studied with diaryliodonium salt photoinitiators and real‐time IR spectroscopy. The rate of epoxide ring‐opening polymerization was enhanced markedly by the presence of the hydroxy group. Using model compounds, the monomers were shown to polymerize via an activated monomer mechanism. Simple epoxy alcohols polymerized to give polymers with a hyperbranched structure. The novel monomers also were observed to accelerate the rate of the photopolymerization of mono‐ and multifunctional epoxides. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 389–401, 2000     

Spectroscopic and photopolymerization studies of benzyl methacrylate/poly(benzyl methacrylate) two‐component system

The influence of the viscosity of a two‐component system on its molecular dynamics (on the basis of hypersonic wave velocity and attenuation coefficient) and photopolymerization kinetics was studied. The system investigated represented the solution of poly(benzyl methacrylate), PBzMA (MW = 70000) in its monomer, benzyl methacrylate (BzMA). The viscosity of the system was varied by adding various amounts of the polymer to the monomer (10–50 wt %). The molecular dynamics in the neat BzMA was studied by the proton Nuclear Magnetic Resonance (NMR) spin‐lattice relaxation time measurements and the wide‐line ¹H NMR spectroscopy in a wide range of temperature. Information on the local dynamics in liquid BzMA above its melting temperature was gained from the high‐resolution ¹H and ¹³C NMR spectra. The hypersonic wave velocity and the attenuation coefficient were investigated in the appropriate temperature range related to a viscoelastic relaxation process by the Brillouin light scattering method. The kinetic measurements have demonstrated that the photopolymerization rate rapidly increases and the monomer conversion decreases with increasing polymer‐to‐monomer ratio; this effect has been noted in the whole range of polymer concentration and reaction temperature studied. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1336–1348, 2010     

Photopolymerization kinetics of tributylmethylammonium‐based (meth)acrylate ionic liquids and the effect of water

Polymerizable ionic liquids were synthesized from the neutralization reaction between tributylmethylammonium hydroxide and methacrylic or acrylic acid, and their photopolymerization kinetics have been determined. The acrylate monomer polymerization rate exhibited a profound dependence on the water content as follows: the monomer viscosity was a strong function of the water content, increasing substantially as the water content increased from less than 5 to 30%. For the tributylmethylammonium acrylate with less than 5% water, the viscosity was 2000 times greater than that of butyl acrylate. The high viscosity is proposed to reduce both the propagation and termination steps. Both monomers exhibited an increase in rate with temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3766–3773, 2008     

2′ Substituted sergeants influencing the deoxy soldiers in folded oligonucleotides

We demonstrate that conformational‐biased monomers can alter neighboring monomer conformation leading to globe changes in polymer topology. This work is compared to the “sergeants and soldiers” effect observed when a small percentage of optically active monomers control a polymer's helical handedness. Specifically, we show that one biased monomer in a sequence of flexible monomers yields an oligonucleotide block copolymer that folds into two topologies with competitive free energies and activation barriers (from an unfolded state). Double substitutions, on the other hand, have varying degrees of influence depending on the position of the second substitution. Substitutions within the same guanosine block have a lesser effect relative to substitutions in different blocks. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3271–3278, 2006     

Effect of introducing a cationic system into a thiol‐ene photopolymerizable formulation

Hybrid materials derived from a thiol‐ene and cationic polymerization were obtained from concomitant polymerization. The hybrid materials were cured by both photopolymerization and thermally induced polymerization. The kinetics of the photopolymerization were measured using time resolved‐IR and optical pyrometry. The nucleophilic character of the polysulfide obtained initially in the thiol‐ene polymerization inhibited the development of the cationic photoinitiated polymerization of epoxy monomers. Besides, the epoxide groups underwent a proton catalyzed addition reaction with the thiols to form new sulfides groups in the reaction mixture. It is proposed that the formed sulfides can terminate the growing polyether chains forming dormant species like trialkylsulfonium salts. These salts promote the thermal polymerization of the epoxy monomer in a post treatment, producing hard and transparent materials. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4829–4843, 2007     

Atom transfer radical polymerization of ionic liquid monomer: The influence of salt/counterion on polymerization

Understanding the influence of salt/counterion on atom transfer radical polymerization (ATRP) is important to optimize the conditions for ATRP of ionic monomers, such as ionic liquid monomer. This article reports the results of a systematical investigation of the variables associated with ATRP in the presence of different types and amounts of salts, solvents, ligands, and monomers. A series of control ATRP experiments were conducted under various polymerization conditions. The kinetics of the polymerizations, the molecular weight, and molecular weight distribution of the formed polymers were studied by nuclear magnetic resonance and gel permeation chromatography. The results indicated that all of the studied variables influenced the ATRP process to different degrees. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2175–2184     

On the origin of ionic liquid‐induced variations in hydroxypropyl methacrylate propagation rate coefficients

Pulsed laser polymerizations were used to study the propagation kinetics of hydroxypropyl methacrylate (HPMA) in ionic liquids (ILs) and common organic solvents. The functional monomer was chosen to investigate the complex interplay of all interactions between monomer molecules and between monomer and solvent molecules and to obtain a deeper understanding of the impact of these interactions. The solvent effect on the HPMA propagation rate coefficient (k_p) was examined using a linear solvation energy relationship (LSER) based on Kamlet‐Taft solvatochromic parameters π*, α, and β. The results suggest that dipolarity/polarizability, associated with π*, and hydrogen bond–donating ability of the solvents, accounted for by α, majorly contribute to variations in k_p. Hydrogen bond–accepting (electron pair donating) ability of the solvents (β parameter) is of much lesser importance. In addition, LSER enables the prediction of HPMA k_p based on solvatochromic parameters of the solvents. The results suggest that interactions between the hydroxyl group of the monomer and the anion are dominant compared with classical hydrogen bonding between carbonyl and hydroxyl groups of the monomer units. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3188–3199, 2010     

Influence of the secondary functionality on the radical‐vinyl chemistry of highly reactive monoacrylates

The impact of secondary functionalities on the radical‐vinyl chemistry of monoacrylates characterized by secondary functionalities that dramatically enhance their polymerization rate was elucidated utilizing experimental and computational techniques. Firstly, bulk interactions affecting the acrylate reactivity towards photopolymerization were removed by polymerizing at 5 wt % monomer in 1,4‐dioxane. Following deconvolution of bulk interactions impacting reactivity towards photopolymerization, a linear correlation between average polymerization rates and Michael addition reaction rate constants was observed on a logarithmic scale. This result indicates that the presence of the secondary functionality intramolecularly alters the monomer chemistry in a manner which impacts both of these distinct reaction types in a similar manner. These monomers exhibited reduced activation energies in both Michael addition and photopolymerization reactions as compared to hexyl acrylate. Reduction up to 20 ± 8 kJ mole^?1was observed for Michael addition reactions and 12 ± 1 kJ mole^?1 for photopolymerization reactions, thereby explaining the higher reactivity of the acrylates characterized by the secondary functionalities. Cyclic voltammetry experiments conducted to investigate the nature of the acrylic double bonds indicated that the rapidly polymerizing acrylates are more readily reduced as compared to traditional acrylates. Further, a distinct monotonic correlation of the irreversible cathodic peak potentials of the (meth)acrylates to photopolymerization and Michael addition reactivity was observed. The computationally estimated acrylic LUMO energies characterized by the secondary functionalities (?2.3 eV to ?2.7 eV) were also found to be lower relative to hexyl acrylate (?2.2 eV). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4859–4870, 2009     

Near‐infrared spectroscopy investigation of water effects on the cationic photopolymerization of vinyl ether systems

Real‐time Fourier transform near‐infrared spectroscopy has been used to monitor monomer and water concentrations simultaneously during cationic vinyl ether photopolymerization. The use of near‐infrared peak area methods allows the water content to be conveniently and nondestructively determined in any monomer or polymer for which the water peak has previously been calibrated by gravimetric analysis. Although the shape of the absorption band due to absorbed water in a monomer changes with the quantity of water, the integrated intensity from about 5350 to 4900 cm^?1 can be correlated directly to the water concentration, and this region is well removed from the vinyl‐based absorption at approximately 6190 cm^?1. This approach provides a highly informative, dynamic technique for examining the influence of moisture on polymerization reactions. Significant differences have been observed in the effects of absorbed water on the cationic photopolymerization kinetics of vinyl ether monomers with or without an ? OH group. Along with the rapid consumption of water coupled to vinyl ether polymerization, acid‐catalyzed hydrolysis reactions have also been spectroscopically observed, giving rise to the formation of aldehyde groups. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1985–1998, 2004     

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     

Thiol‐ene photopolymerization of polymer‐derived ceramic precursors

The liquid, ceramic precursor monomer VL20 was copolymerized with a thiol monomer in a traditional radical thiol‐ene photopolymerization. Polymerization occurred via addition of the thiol functional group to the vinyl silazane functional group in a 1:1 ratio consistent with a step‐growth polymerization. Gelation occurred at a high conversion of functional groups (70%) consistent with an average molecular weight and functionality of 560 and 1.7, respectively, for VL20 monomers. Initiatorless photopolymerization of the thiol‐VL20 system also occurred upon irradiation at either 365 or 254 nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1752–1757, 2004     

Ionic IPNs as novel candidates for highly conductive solid polymer electrolytes

Five ionic imidazolium based monomers, namely 1‐vinyl‐3‐ethylimidazolium bis(trifluoromethylsulfonyl)imide (ILM1), 1‐vinyl‐3‐(diethoxyphosphinyl)‐propylimidazolium bis(trifluoromethylsulfonyl)imide (ILM2), 1‐[2‐(2‐methyl‐acryloyloxy)‐propyl]‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (ILM3), 1‐[2‐(2‐methyl‐acryloyloxy)‐undecyl]‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (ILM4), 1‐vinyl‐3‐ethylimidazolium dicyanamide (ILM5) were prepared and used for the synthesis of linear polymeric ionic liquids (PILs), crosslinked networks with polyethyleneglycol dimethacrylate (PEGDM) and interpenetrating polymer networks (IPNs) based on polybutadiene (PB). The ionic conductivities of IPNs prepared using an in situ strategy were found to depend on the ILM nature, T_g and the ratio of the other components. Novel ionic IPNs are characterized by increased flexibility, small swelling ability in ionic liquids (ILs) along with high conductivity and preservation of mechanical stability even in a swollen state. The maximum conductivity for a pure IPN was equal to 3.6 × 10^?5 S/cm at 20 °C while for IPN swollen in [1‐Me‐3‐Etim] (CN)₂N σ reached 8.5 × 10^?3 S/cm at 20 °C or 1.4 × 10^?2 S/cm at 50 °C. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4245–4266, 2009     

Nanostructured ion-conductive films: Layered assembly of a side-chain liquid-crystalline polymer with an imidazolium ionic moiety

An ion-conductive mesogenic monomer with an imidazolium ionic moiety has been designed to obtain self-assembled materials forming ionic layers. Self-standing polymer films are prepared by in situ photopolymerization of the monomer that forms homeotropic monodomain on a normal glass substrate in the smectic A phase. Macroscopically oriented, layered nanostructures are formed in the film. The ionic conductivity parallel to the smectic layer has been measured for the oriented film. In the smectic A phase at 150 °C, the magnitude of conductivity is about 10⁻² S cm⁻¹. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3486–3492, 2003