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     

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     

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.     

Spherically propagating thermal polymerization fronts

We demonstrate for the first time spherically propagating frontal polymerization that also exhibits spin modes. We have developed an interesting system using the amine‐catalyzed Michael addition of a trithiol to a triacrylate to create a rubbery gel. The gel suppresses convection and bubble formation during front propagation. A peroxide is also present to act as a thermal initiator. The front propagates via free‐radical polymerization of the remaining triacrylate after being initiated photochemically in the center of the reactor. It is possible to prepare the rubbery gel in any shape and then initiate thermal frontal polymerization. So‐called spin modes have been observed for the first time in spherically propagating fronts in which waves of polymerization propagate on the expanding spherical front. A system using a diacrylate dissolved in dimethyl sulfoxide with added silica gel and with persulfate as the initiator supports spherical fronts but does not exhibit spin modes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1387–1395, 2006     

Facile synthesis of amphiphilic gels by frontal free‐radical polymerization

We report a new facile strategy for quickly synthesizing poly(2‐hydroxyethyl acrylate‐co‐vinyl versatate) amphiphilic gels with excellent physicochemical properties by frontal free‐radical polymerization. The appropriate amounts of 2‐hydroxyethyl acrylate, vinyl versatate (VeoVa 9) and ammonium persulfate initiator were mixed together at ambient temperature in the presence of N‐methyl‐2‐pyrrolidone as the solvent medium. Frontal polymerization (FP) was initiated by heating the wall of the tube with a soldering iron. Once 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 132 and 157 °C, depending on the initiator concentration. The morphology, swelling rate, and swelling behavior of amphiphilic gels prepared via FP were comparatively investigated on the basis of scanning electron microscopy, water contact angle, and swelling measurements. Results show that the amphiphilic gels prepared via FP behave with good swelling capacity both in water and organic solvents. The FP can be exploited as an alternative means for synthesis of amphiphilic gels with additional advantages of fast and efficient way. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 823–831, 2010     

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     

Synthesis of poly(<Emphasis Type="Italic">N</Emphasis>-methylolacrylamide)/polymethylacrylamide hybrids via frontal free-radical polymerization

In this study, poly(N-methylolacrylamide)/polymethylacrylamide (PNMA/PMAA) hybrids were produced successfully by frontal free-radical polymerization at ambient pressure. In a typical run, the appropriate amounts of reactants (N-methylolacrylamide, NMA; methylacrylamide, MAA) and initiator (ammonium persulfate) were dissolved in dimethyl sulfoxide at ambient temperature. Frontal polymerization (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. The dependences of the front velocity and front temperature on the initiator concentration, reactant dilution, and NMA/MAA components were thoroughly investigated. The front temperatures were between 69 and 116 °C, depending on the persulfate concentration. We have also investigated the FP of PNMA/PMAA hybrids with N-methyl-2-pyrrolidone as solvent. Results show that FP can be exploited as a means for the preparation of PNMA/PMAA hybrids with the potential advantage of higher throughput compared to the traditional mode.     

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     

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     

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     

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     

Modeling isothermal free-radical frontal polymerization with gel effect using free volume theory, with and without inhibition

Frontal Polymerization is a process that converts monomers into polymers by means of a propagating spatially localized reaction front. Such fronts exist with free-radical polymerization, where in the simplest case, a mixture of monomers and initiator is placed into a test tube and upon initiation of the reaction at one end of the tube, a self-sustained wave develops and propagates through the tube. Isothermal Frontal Polymerization (IFP), often referred to as interfacial gel polymerization, occurs due to the coupling of mass diffusion of the species and the gel effect. Utilizing the free volume theory of Vrentas and Duda for describing the self-diffusive behavior of the gel effect, we mathematically model and study this IFP process. We determine, both numerically and analytically, characteristics of the process including the propagation velocity of the reaction zone, the structure of the wave, and the distance traveled by the front before it breaks down due to reactions ahead of the front     

A facile pathway for the fast synthesis of colloidal crystal‐loaded hydrogels via frontal polymerization

In this work, a dually sensitive colloidal crystal (CC)‐loaded hydrogel has been synthesized via frontal polymerization (FP) in a facile and rapid way. First, a polystyrene CC film was fabricated by vertical deposition on the inner wall of a test tube. Then, a mixture of acrylic acid (AAc), 2‐hydroxyethyl methacrylate (HEMA), and glycerol along with the initiator and crosslinker was added to this test tube to carry out FP, resulting in the formation of CC‐loaded hydrogel. The influence of the mass ratios of HEMA/AAc on front velocity and temperatures were studied. The swelling behavior, the morphology, and the stimuli‐responsive behavior of the CC‐loaded hydrogels prepared via FP were thoroughly investigated on the basis of swelling measurement, scanning electron microscopy, and reflection spectra. Results show that the as‐prepared CC‐loaded hydrogels exhibit excellent dual sensitivity to both methanol concentrations and pH values with very short response time, which can be observed visually without the aid of instruments. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

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     

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     

Kinetic study of the nitroxide‐mediated controlled free‐radical polymerization of n‐butyl acrylate in aqueous miniemulsions

The controlled free‐radical homopolymerization of n‐butyl acrylate was studied in aqueous miniemulsions at 112 and 125 °C with a low molar mass alkoxyamine unimolecular initiator and an acyclic β‐phosphonylated nitroxide mediator, N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl) nitroxide, also called SG1. The polymerizations led to stable latices with 20 wt % solids and were obtained with neither coagulation during synthesis nor destabilization over time. However, in contrast to latices obtained via classical free‐radical polymerization, the average particle size of the final latices was large, with broad particle size distributions. The initial [SG1]₀/[alkoxyamine]₀ molar ratio was shown to control the rate of polymerization. The fraction of SG1 released upon macroradical self‐termination was small with respect to the initial alkoxyamine concentration, indicating a very low fraction of dead chains. Average molar masses were controlled by the initial concentration of alkoxyamine and increased linearly with monomer conversion. The molar mass distribution was narrow, depending on the initial concentration of free nitroxide in the system. The initiator efficiency was lower than 1 at 112 °C but was very significantly improved when either a macroinitiator was used at 112 °C or the polymerization temperature was raised to 125 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4410–4420, 2002     

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     

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     

Frontal cationic curing of epoxy resins

We studied the frontal curing of trimethylolpropane triglycidyl ether (TMPTGE) using two BF₃‐amine initiators and two fillers, kaolin and fumed silica. In the case of kaolin, the range of concentrations allowing for frontal polymerization to propagate was dependent on its heat absorption effect whereas in the case of silica it was a consequence of the rheological features of this additive. However, for both systems the velocity and front temperature show the same trends; in all cases front velocities were on the order of 1 cm/min with front temperatures about 200 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2000–2005, 2010     

Ring-opening polymerization of 1,5-dioxepan-2-one initiated by a cyclic tin–alkoxide initiator in different solvents

Ring-opening polymerization of 1,5-dioxepan-2-one initiated by 1,1,6,6-tetra-n-butyl-1,6-distanna-2,5,7,10-tetraoxacyclodecane was carried out in chloroform, dichloromethane, or 1,2-dichloroethane. Effects of reaction temperature, solvent, and monomer-to-initiator ratio were investigated. Polymerization kinetics showed a first-order dependence on the monomer for polymerization in chloroform and dichloromethane at 40°C. The kinetic order with respect to the initiator were a first order when dichloromethane was used as the solvent, the order in initiator changed, depending on the initiator concentration when chloroform was used. A maximum in molecular weight was observed at 40°C when chloroform was used as the solvent. The change of solvent did not markedly alter the polymerization rate or the molecular weight of the polymers prepared, as expected from the coordination insertion mechanism. Depolymerization of the polymers formed was observed when the reaction was allowed to continue after complete monomer conversion in chloroform as reaction medium at 40°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3407–3417, 1999