The effect of a trithiol and inorganic fillers on the photo‐induced thermal frontal polymerization of a triacrylate

Thermal frontal polymerization is a process in which a localized reaction propagates through an unstirred system by the coupling of the thermal diffusion and the Arrhenius kinetics of an exothermic polymerization. A trithiol was found to affect the front velocity and the time for inducing a front upon exposure to UV light for trimethylolpropane triacrylate polymerization fronts with either kaolin or calcium carbonate filler present. The addition of trithiol and filler both decreased the front velocity. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8091–8096, 2008     

Frontal free‐radical copolymerization of urethane–acrylates

We report the first synthesis of urethane–acrylate copolymers via free‐radical frontal polymerization. In a typical run, the appropriate amounts of the reactants (urethane–acrylate macromonomer and 2‐hydroxyethyl acrylate) and initiator (ammonium persulfate) were dissolved in dimethyl sulfoxide. Frontal polymerization was initiated by the heating of the wall of the tube with a soldering iron, and the resultant hot fronts were allowed to self‐propagate throughout the reaction vessel. Once it was initiated, no further energy was required for the polymerization to occur. The dependence of the front velocity and front temperature on the initiator concentration was investigated. The front temperatures were between 55 and 65 °C, depending on the persulfate concentration. Thermogravimetric analysis indicated that the urethane–acrylate copolymers had higher thermal stability than pure frontally prepared polyurethane. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3018–3024, 2006     

Thermal frontal polymerization with a thermally released redox catalyst

We studied thermal frontal polymerization using a redox system in an attempt to lower the temperature of the frontally polymerizable system while increasing the front velocity so as to obtain a self‐sustaining front in a thinner layer than without the redox components. A cobalt‐containing polymer with a melting point of 63 °C (Intelimer 6050X11) and cumene hydroperoxide were used with a triacrylate. The use of the Intelimer decreased the front velocity but allowed fronts to propagate in thinner layers and with more filler while still having a pot life of days. Nonplanar modes of propagation occurred. Fronts propagated faster when 6‐O‐palmitoyl‐L ‐ascorbic acid was used as a reductant. Interestingly, fronts were also faster with the reductant even without the Intelimer if kaolin clay was the filler; however, the pot life was significantly reduced. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Isothermal frontal polymerization: Confirmation of the isothermal nature of the process and the effect of oxygen and polymer seed molecular weight on front propagation

Isothermal frontal polymerization is a directional polymerization that utilizes the Norish‐Trommsdorff (gel) effect to produce optical gradient materials. When a solution of methyl methacrylate and thermal initiator contacts a polymer seed (a small piece of poly(methyl methacrylate), a viscous region is formed in which the polymerization rate is faster than in the bulk solution. We obtained definitive evidence of the isothermal nature of the process by placing thermocouples above the propagating front. Using the optical technique of laser line deflection (Weiner's method), we studied the front propagation to determine the induction period, and the maximum distance propagated as a function of the molecular weight of the seed. We determined that the polymer seed must have a minimum molecular weight to initiate a front. We also determined that oxygen would act as a bulk polymerization inhibitor and increase the front propagation distance, but after purging the monomer–initiator solution with oxygen for several hours, the distance was shortened. We ascribed this behavior to the formation of peroxy radicals from the slow decomposition of the initiator and subsequent reaction with oxygen. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3601–3608, 2006     

A study of the effects of thiols on the frontal polymerization and pot life of multifunctional acrylate systems with cumene hydroperoxide

In an effort to create frontal polymerization systems with a “fail‐safe” curing mechanism, we studied the effects of thiols on the thermal frontal polymerization velocity and pot life of a mixture of a multifunctional acrylate, kaolin clay (filler), and cumene hydroperoxide with either trimethylolpropane tris(3‐mercaptopropionate) or 1‐dodecanethiol (DDT). The acrylates were trimethylolpropane triacrylate, trimethylolpropane ethoxylate triacrylate, 1,6‐hexanediol diacrylate, and di(ethylene glycol) diacrylate. Without a thiol, frontal polymerization did not occur. The front velocity increased with the concentration of either thiol, which has not been observed with peroxide initiators. The use of DDT yielded longer pot lives than the trithiol. The front velocities were inversely related to the pot lives. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3850–3855     

Effects of thiols,lithium chloride,and ethoxylated monomers on the frontal polymerization of a triacrylate

Thermal frontal polymerization is a process in which a localized reaction propagates through an unstirred system by the coupling of the thermal diffusion and the Arrhenius kinetics of an exothermic polymerization. With multifunctional acrylates, such as trimethylolpropane triacrylate (TMPTA‐n), front temperatures can reach 250 °C, resulting in smoke from unreacted peroxide. Addition of a thiol lowers the front temperature and the front velocity due the copolymerization between the thiol and the acrylate, with some formulations not sufficiently reactive to sustain frontal polymerization. The effects of molecular weight per thiol and functionality of thiol on front temperature and velocity were studied in the frontal copolymerization of TMPTA‐n/trimethylolpropane ethoxylate triacrylate and different thiols. We also investigated the front temperature and velocity for a system containing triacrylate and dodecyl acrylate. Finally, the effects of lithium chloride in the presence of thiol on the front velocity and front temperature were studied. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Facile synthesis of poly(hydroxyethyl acrylate) by frontal free‐radical polymerization

We report the first synthesis of poly(hydroxyethyl acrylate) (PHEA) without solvent by free‐radical frontal polymerization (FP) at ambient pressure. In a typical run, the appropriate amounts of reactant (hydroxyethyl acrylate) and initiator (1,1‐di(tert‐butylperoxy)‐3,3,5‐trimethylcyclohexane) (Luperox 231) were mixed together at ambient pressure. FP was initiated by heating the wall of the tube with a soldering iron, and the resultant hot fronts were allowed to self‐propagate throughout the reaction vessel. Once initiated, no further energy was required for polymerization to occur. To study the macrokinetics, we also produced PHEA frontally with ammonium persulfate as initiator and dimethyl sulfoxide as the solvent. The dependences of the front velocity and front temperature on the initiator concentration and reactant dilution were investigated. The front temperatures were between 124 and 157 °C, depending on the ammonium persulfate concentration. Thermogravimetric analysis indicates that PHEA prepared by FP with ammonium persulfate as initiator had higher thermal stability than solvent‐free frontally prepared PHEA with Luperox 231 as initiator. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 873–881, 2007     

Isothermal frontal polymerization: Confirmation of the mechanism and determination of factors affecting the front velocity,front shape,and propagation distance with comparison to mathematical modeling

Isothermal frontal polymerization (IFP) is a directional polymerization that uses the Trommsdorff, or gel, effect to produce gradient materials for optical applications. When a solution of methyl methacrylate and a thermal initiator contacts a polymer seed (a small piece of polymer), a viscous region is formed in which the polymerization rate is faster because of the Trommsdorff effect. Using the optical techniques of laser line deflection (Weiner's method) and shadowgraphy along with controls, we obtained definitive experimental evidence of IFP. Moreover, we were able to measure accurately and precisely the front position and front concentration profile as a function of time by monitoring IFP systems and controls of various initiator concentrations and cure temperatures. The experimental data were compared with theoretical predictions from a model using mass‐diffusion and radical polymerization kinetics. The model reproduced the decrease of the propagation time and showed an increase in the propagation velocity for an increase in the initiator concentration and/or cure temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5774–5786, 2005     

Effect of orientation on thermoset frontal polymerization

We observed that the velocities of descending thermoset polymerization fronts were strongly affected by the orientation of the tube. The front remained approximately perpendicular to the gravitational vector but propagated almost 1.8 times as fast at 75° along the axis of the tube. We performed a study of the velocity and front‐shape dependence on orientation with propagating fronts of triethylene glycol dimethacrylate with peroxide initiator and acrylamide/bisacrylamide polymerization in dimethyl sulfoxide with persulfate initiator. The percentage increase of velocity was independent of the initiator concentration but strongly dependent on the viscosity. Convection under the front flowed away from the tube wall nearest the vertical axis and was stronger as the angle increased. The front shape also changed, becoming significantly distorted near the wall from which the convection originated. We applied a simple geometric argument to explain the angular dependence for small angles on the basis of the assumption that convection did not affect the velocity of propagation normal to the front. The increase in velocity along the tube axis could be explained by a projection of the normal velocity onto the tube axis, following a 1/cosθ dependence. For higher angles, the convection was not sufficiently strong to maintain a level front. When the difference from a 180° orientation was considered, the velocity dependence exactly followed the geometric relationship. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3504–3508, 2002     

Synthesis of crosslinked poly(butyl methacrylate‐co‐pentaerythritol triacrylate) gel by single electron transfer‐living radical polymerization and its oil‐absorbing properties

A novel high oil‐absorbing crosslinked gel was synthesized by copolymerization of butyl methacrylate (BMA) with a small amount of pentaerythritol triacrylate (PETA) crosslinker using single electron transfer‐living radical polymerization (SET–LRP) initiated with carbon tetrachloride (CCl₄) and catalyzed by Cu(0)/hexamethylenetetramine (HMTA) in N, N‐dimethylformamide (DMF). The polymerization followed first‐order kinetics as indicated by linear increase of monomer concentration with reaction time. Effects of reaction temperature, crosslinker, initiator, and catalyst on the oil‐absorbing properties of the crosslinked gel were investigated in detail. The oil absorptions of the crosslinked gel to chloroform, toluene could reach 51.9, 34.5 g/g, respectively. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Gas‐free initiators for high‐temperature free‐radical polymerization

Quaternary ammonium persulfates as free‐radical initiators for high‐temperature polymerization are synthesized and their shelf‐life stability investigated. These initiators do not have gaseous byproducts and are therefore ideal for frontal polymerization, a process in which polymeric materials are produced via a thermal front that propagates through the unreacted monomer/initiator solution. Quaternary ammonium persulfate initiators offer additional qualities such as high solubility in organic media and low volatility, which are desirable for frontal polymerization. The initiators are synthesized using two procedures, and the initiating efficacy of the respective products is compared to a peroxide initiator in the frontal polymerization of 1,6‐hexanediol diacrylate. Of all the quartenary ammonium persulfates synthesized, tricaprylmethylammonium (Aliquat) persulfate (APSO) is the best initiator for frontal polymerization because it is soluble in organic media, is very reactive, and does not produce volatile byproducts under decomposition. A study of the decomposition kinetics of APSO is performed, and frontal polymerization is proposed as a quicker analytical technique to assay the purity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3984–3990, 2000     

Solvent‐free free‐radical frontal polymerization: A new approach to quickly synthesize poly(N‐vinylpyrrolidone)

The first synthesis of poly(N‐vinylpyrrolidone) without solvent by free‐radical frontal polymerization at ambient pressure is reported. The appropriate amounts of two reactants N‐vinyl‐2‐pyrrolidone (NVP) and initiator 2,2′‐azobis‐isobutyronitrile (AIBN) without solvent were mixed together at ambient temperature. Frontal polymerization was initiated by heating the wall of the tube with a soldering iron, and the resultant hot fronts were allowed to self‐propagate throughout the reaction vessel. Once initiated, no further energy was required for polymerization to occur. To suppress the fingers of molten monomer, a small amount of nanosilica was added. The dependence of the front velocity and front temperature on the AIBN concentration was thoroughly investigated. The as‐prepared polymers were characterized by gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). Results show that without postpolymerization solvent removal, waste production can be reduced. Solvent‐free FP could be exploited as a means for preparation of PVP with the potential advantage of higher throughput than solvent‐based methods. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2177–2185, 2008     

The effect of acrylate functionality on frontal polymerization velocity and temperature

Frontal polymerization is a method of converting monomer(s) to polymer via a localized reaction zone that propagates from the coupling of thermal diffusion with the Arrhenius kinetics of an exothermic reaction. Several factors affect front velocity and temperature with the role of monomer functionality being of particular interest in this study. Polymerizing a di and triacrylate of equal molecular weight per acrylate revealed that as the proportion of triacrylate was increased the velocity and temperature increased. This is attributed to increased crosslinking and autoacceleration. Comparing several different acrylate monomers, both neat and diluted with dimethyl sulfoxide (DMSO) so as to maintain constant acrylate group concentration, shows that velocity increases with increased functionality from mono to difunctional monomers. This trend breaks when applied to tri‐ and tetraacrylates, with fronts containing trifunctional monomer being the fastest. Acrylates containing hydroxyl functionality, as in the case of pentaerythritol‐based triacrylates, are slower than acrylates without. This is attributed to a chain‐transfer event and was tested using octanol and a hydroxyl‐free acrylate. It has also been shown that small amounts of water cause a lowering of front velocity due to energy lost via vaporization, which lowers the front temperature. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 982–988     

Factors affecting propagating fronts of addition polymerization: Velocity,front curvature,temperatue profile,conversion, and molecular weight distribution

Several properties of propagating fronts of addition polymerization were studied. A power function could be fit to the velocity dependence on initiator concentration, but not with the exponents predicted by current models or in agreement with other published work. Bubbles from the volatile by-products of initiator decomposition were found to affect the front velocity and curvature. The front velocity for triethylene glycol dimethacrylate polymerization was found to depend linearly on temperature over a moderate range. The conversion of methacrylic acid in fronts varied greatly with initiator type and concentration. Benzoyl peroxide produced much lower conversion than t-butyl peroxide, but fronts with tBPO propagated slower. A dual initiator system of BPO and tBPO produced rapidly propagating fronts with good conversion but the contribution of each initiator to the velocity was not additive. The possibility of chain branching was considered. The apparent molecular weight distributions were very broad, often trimodal, and found to depend on initiator type and concentration as well as the tube diameter. The temperature profiles were measured and found to be very sharp for BPO and broader for tBPO but both had front temperatures in excess of 200°C, indicating a high ceiling temperature. © 1995 John Wiley & Sons, Inc.     

Photo‐triggered redox frontal polymerization: A new tool for synthesizing thermally sensitive materials

A novel approach of photo‐triggered redox frontal polymerization (FP) by integrating photocaged superbase (QA‐DBU) with a peroxide initiator (dibenzoyl peroxide, BPO) is presented for the synthesis of thermally sensitive materials. Under photo‐irradiation at a localized region, the regenerated superbase can diffuse into unirradiated regions and effectively actuate redox FP in a diffusion‐controlled manner. Moreover, the redox FP can be conducted at a much lower front temperature with enhanced front velocity. Astonishingly, the front temperature can be well‐modulated by changing the concentration of latent superbase. The prepared thermally sensitive fluorescent polymer composites exhibit enhanced fluorescence emission intensity compared to that of conventional thermal FP. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4515–4521     

Magnetic resonance imaging of spiral patterns in crosslinked polymer gels produced via frontal polymerization

Frontal polymerization is a process in which a localized reaction zone propagates through a monomer reactant mixture, leaving a polymer product in its wake, and is the result of the coupling of the thermal transport and Arrhenius dependence of the exothermic polymerization. Under most conditions, a planar front is stable. However, for multifunctional acrylates at room temperature, fronts may propagate in a helical fashion along the axis of the reactor. This front propagation is typical of what is called a spin mode, in which the subsequent polymer sample has alternating spiral patterns of low and high monomer conversion evident on the sample surface. For the first time, we demonstrate that magnetic resonance imaging on a submillimeter scale can be used to show that the spiral patterns are not restricted to the sample surface but are distributed throughout the volume. Samples were soaked in water, and the transverse proton relaxation times were imaged. The results suggest proton mobility is smaller in the high‐conversion region in which the hot spot propagated than in the low‐conversion region. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1075–1080, 2001     

Propagation velocity of the reaction front in addition polymerization systems

Traveling polymerization fronts in unstirred solutions of methylmethacrylate, methacrylic acid, or acrylamide with some free radicals initiators (through thermal decomposition) have been observed experimentally. A local heating of the initial reactant mixture, under suitable conditions, leads to a reaction front that propagates along the space coordinate with a constant velocity. In this article, a physical interpretation of this phenomenon is provided through a mathematical model that accounts for the depolimerization reaction and is based on the constant pattern approach. Moreover, an approximate explicit analytic expression for the velocity of propagation of the polymerization front is proposed. The theoretical values are compared with those measured experimentally as a function of the initiator concentration for different addition polymerization systems. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35:1047–1059, 1997     

First solvent‐free synthesis of poly(N‐methylolacrylamide) via frontal free‐radical polymerization

We report the first synthesis of poly (N‐methylolacrylamide) (PNMA) via free‐radical frontal polymerization (FP) with solid monomers at ambient pressure. The appropriate amounts of reactants (N‐methylolacrylamide) (NMA) and initiator (ammonium persulfate) were mixed together at ambient temperature without solvent. FP was initiated by heating the wall of the tube with a soldering iron, and the resultant hot fronts were allowed to self‐propagate throughout the reaction vessel. Once initiated, no further energy was required for polymerization to occur. To suppress the fingers of molten monomer, a small amount of nanosilica was added. We also produced PNMA with dimethyl sulfoxide (DMSO) or N‐methyl‐2‐pyrrolidone, as solvent by FP, to study the macrokinetics in FP of PNMA without fillers. The front velocity and front temperature dependence on the ammonium persulfate and N‐methyl‐2‐pyrrolidone concentration were investigated. The polymer was analyzed by thermogravimetric analysis. Results show that without postpolymerization solvent removal, waste production can be reduced. Solvent‐free FP could be exploited as a means for preparation of PNMA with the potential advantage of higher throughput than solvent‐based methods. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4322–4330, 2007     

Preparation and optical characterization of two photoactive poly(bisphenol a ethoxylate diacrylate) copolymers containing designed amino‐nitro‐substituted azobenzene units,obtained via classical and frontal polymerization,using novel ionic liquids as initiators

The frontal polymerization (FP) of bisphenol A ethoxylate diacrylate (BPAEDA) was carried with and without the presence of two different azobenzene comonomers by means of an external heating source. The first azomonomer (MDR‐1) is a derivative of disperse red‐1, N‐ethyl‐N‐(2‐hydroxyethyl)‐4‐(4‐nitrophenylazo)aniline, whereas the second (E)‐2‐(4‐((4‐nitrophenyl)diazenyl)phenyl)‐5,8,11‐trioxa‐2‐azatridecan‐13‐yl methacrylate (4PEGMAN) comes from the azo‐dye N‐methyl‐N‐{4‐[(E)‐(4‐nitrophenyl)diazenyl]phenyl}‐N‐(11‐hydroxy‐3,6,9‐trioxaundecas‐1‐yl) amine. In this work, an ionic liquid trihexyltetradecylphosphonium persulfate was used as initiator. This compound produced stable propagating polymerization fronts with good velocities and moderate maximum temperature values. Moreover, this initiator prevented bubble formation and was found to be the most efficient when it was used in lower amounts with respect to other initiators, such as benzoyl peroxide, 2,2′‐azobisisobutyronitrile, aliquat persulfate®, and tetrabutylphosphonium persulfate. The thermal properties of the obtained polymers and copolymers were determined by thermogravimetric analysis and differential scanning calorimetry. The nonlinear optical (NLO) characterizations of the developed BPAEDA/MDR‐1 and BPAEDA/4PEGMAN copolymers were performed according to the Z‐Scan technique in film samples prepared by classical polymerization. It has been proven that samples with higher 4PEGMAN content (0.26 mol %) exhibited outstanding cubic NLO‐activity with positive NLO‐refractive coefficients in the promising range of n₂ = +3.2 × 10^?4 esu. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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