Free radical polymerization, frontal

Mathematical models of the frontal copolymerization process were developed, studied and compared with experimental data in [67, 90]. An interesting observation was that the propagation speed of the copolymerization wave was not necessarily related to the propagation speeds in the two homopolymerization processes, in which the same two monomers were polymerized separately. For example, the propagation speeds in the homopolymerization processes could be 1 cm/min in each, but in the copolymerization process, the speed could be 0.5 cm/min. Mathematical models of free-radical binary frontal polymerization were presented and studied in [66, 91]. Another model in which two different monomers were present in the system (thiol-ene polymerization) was discussed in [21]. A mathematical model that describes both free-radical binary frontal polymerization and frontal copolymerization was presented in [65]. The paper was devoted to the linear stability analysis of polymerization waves in two monomer systems. It turned out that the dispersion relation for two monomer systems was the same as the dispersion relation for homopolymerization. In fact, this dispersion relation held true for W-monomer systems provided that there is only one reaction front, and the final concentrations of the monomers could be written as a function of the reaction front temperature. 

Self-propagating free-radical binary frontal polymerization. Journal of Engi-... 

Frontal free-radical polymerization is fairly well understood. Studies on the velocity dependence on temperature and initiator concentration have been performed (7,11,23), Frontal polymerization in solution was performed (70), and initiators that do not produce gas were developed (24), The velocity can be affected by the initiator type and concentration but is on the order of a cm/min for monofunctional acrylates and as high as 20 cm/min for multifunctional acrylates (24). 

Spontaneous Frontal Polymerization Propagating Front Spontaneously Generated by Locally Autoaccelerated Free-Radical Polymerization... 

Frontal polymerization discovered in 1972 (5) could be realized in free-radical polymerization because of its nonlinear behavior. If the top of a mixture of monomer and initiator in a tube is attached to an external heat source, die initiators are locally decomposed to generate radicals. The polymerization locally initiated is autoaccelerated by the c(xnbinatithermal autocatalysis exclusively at the top of the reaction systmn. An interface between reacted and unreacted regions, called propagating front, is thus formed. Pojman et al. extensively studied the dynamics of frontal polymerization (d-P) and its applicatim in matoials syndiesis (I -I3). 

Figure 11.7 shows the temperature history at a fixed point in the reaction tube as a front passes. The temperature at this point is ambient when the front is far away and rises rapidly as the front approaches. Hence, a polymerization front has a very sharp temperature profile (Pojman et al., 1995b). Figure 11.7 shows five temperature profiles measured during frontal free-radical polymerization of methacrylic acid with various concentrations of BPO initiator. Temperature maxima increase with increasing initiator concentration. For an adiabatic system, the conversion is directly proportional to the difference between the initial temperature of the unreacted medium and the maximum temperature attained by the front. The conversion depends not only on the type of initiator and its concentration but also on the thermodynamic characteristics of the polymer (Pojman et al., 1996b). 

A number of radical polymerization reactions are highly exothermic and are able to support frontal polymerization. Free-radical polymerization with a thermal initiator can be approximately represented by a three-step mechanism. First, an... 

Lewis and Volpert continue the discussion of the isothermal form of frontal polymerization in Chapter 5. Isothermal frontal polymerization is also a localized reaction zone that propagates but because of the autoacceleration of the rate of free-radical polymerization with conversion. A seed of poly(methyl methacrylate) is placed in contact with a solution of a peroxide or nitrile initiator, and a front propagates from the seed. The monomer diffuses into the seed, creating a viscous zone in which the rate of polymerization is faster than in the bulk solution. The result is a front that propagates but not with a constant velocity because the reaction is proceeding in the bulk solution at a slower rate. This process is used to create gradient refractive index materials by adding the appropriate dopant. 

Frontal free-radical polymerization applications to materials synthesis. Polym. News, 28, 303-310. 

Tu, J., Chen, L., Fang, Y., Wang, C., and Chen, S. (2010) Facile synthesis of amphiphilic gels by frontal free-radical polymerization. J. Polym. Sci., Part A Polym. Chem., 48, 823-831. 

Thiols can be used in two ways with free-radical polymerization. Thiols react with electron-rich enes (allyl ethers) via a step-growth mechanism to create a polymer only if both ene and thiol have functionalities of at least two. The allyl ethers cannot homopolymerize. If thiols are present, the acrylate can homopolymerize and copolymerize with the thiol. Pojman et al studied frontal thiol-ene polymerization using pentaer-ythrytoltriallyl ether (PTE) and trimethylolpropanetris (3-mercaptopropionate) (95%) (TTl). Not surprisingly, the front velocity was a maximum at a 1 1 thiol ene ratio (Figure 35). ... 

Arrhenius kinetics and is highly exothermic can support localized polymerizations that propagate. Frontal polymerization has been studied with many different polymerization mechanisms but free-radical polymerization is the most studied. Most of the work has focused on the dynamics of the process, but recently applications have been studied. Hydrogels have been prepared frontally, which have superior properties to those prepared by conventional methods. 

It was thought that the particle size trends in frontal free-radical polymerization would be similar to particle size trends in SHS. In Ti + C and Ti + B systems in SHS, as particle size increases, velocity decreases (7). This trend is seen because in many SHS systems, one component must melt so that the other can diffuse into the melt and begin the reaction. The same trend was expected in frontal polymerization as seen in Figure 9. 

The most favorable conditions for reactive processing of monolithic articles are created when the frontal reaction occurs at a plane thermal front. For example, a frontal process can be used for methyl methacrylate polymerization at high pressure (up to 500 MPa) in the presence of free-radical initiators. The reaction is initiated by an initial or continuous local increase in temperature of the reactive mass in a stationary mold, or in a reactor if the monomer is moving through a reactor. The main method of controlling the reaction rate and maintaining stability is by varying the temperature of the reactive mass.252... 

Keywords Modeling, gasless combustion, frontal polymerization, free-radical polymeriza-... 

Frontal polymerization, the process of propagation of a polymerization wave, is important from both fundamental and applied viewpoints. In this chapter, we reviewed theoretical results on the base model of free-radical frontal polymerization. Based on the analogy of the gasless combustion model and using the methods developed in combustion theory, we determined uniformly propagating polymerization waves and discussed their linear and nonlinear stability. 

C. Spade and V. Volpert, On the steady state approximation in thermal free radical frontal polymerization, Chem. Eng. Sci., 55 (2000), pp. 641-654. 

Pojman and his co-workers demonstrated the feasibility of traveling fronts in solutions of thermal free-radical initiators in a variety of neat monomers at ambient pressure using liquid monomers with high boiling points (5-7) and with a solid monomer, acrylamide (8,9), Fronts in solution have also been developed (10). The macrokinetics and dynamics of frontal polymerization have been examined in detail (//). A patented process has been developed for producing functionally-gradient materials (12,13). 

Free-radical chemistry is the most widely used but not the only one. Frontal curing of epoxy resins has been studied (14-18). Begishev et al. studied anionic polymerization of e-caprolactam (19). Frontal Ring-Opening Metathesis Polymerization (FROMP) has been successfully achieved with dicyclopentadiene (20) and applied to making IPNs (21). Mariani et al. have achieved FP with urethane chemistry (22). 

A significant problem in frontal polymerization is the formation of bubbles at the front. These bubbles affect the front velocity (Pojman et al., 1996b) and can cause voids in the final product. The high front temperature can cause boiling of some monomers at ambient pressures. The main source of bubbles in free-radical systems is the initiator, because all nitriles and peroxides produce volatile byproducts. The bubbles make a porous polymer, which may have less than optimal properties. 

Pojman, J. A. Ilyashenko, V. M. Khan, A. M. 1996b. Free-Radical Frontal Polymerization Self-Propagating Thermal Reaction Waves, J. Chem. Soc. Faraday Trans. 92, 2825-2837. 

Most work has been with free-radical systems but other chemistries can be used. Begishev etal. studied frontal anionic polymerization of e-caprolactam [18, 19], and epoxy chemistry has been used as well [20-23]. Mariani ctal. demonstrated frontal ring-opening metathesis polymerization [17]. Fiori et al. produced polyacrylate-poly(dicydopentadiene) networks frontally [24], and Pojman etal. studied epoxy-acrylate binary systems [25]. Polyurethanes have been prepared frontally [13,14, 26]. Frontal atom transfer radical polymerization has been achieved [16] as well as FP with thiol-ene systems [27]. Recent work has been done using FP to prepare microporous polymers [28-30], polyurethane-nanosilica hybrid nanocomposites [31], and segmented polyurethanes [32]. 

Pojman, J.A., llyashenko, V.M., and Khan, A.M. (1996) Free-radical frontal polymerization self-propagating thermal reaction waves. J. Chem. Soc., Faraday Trans., 92, 2825-2837. 

McFarland, B., Popwell, S., and Pojman, J.A. (2004) Free-radical frontal polymerization with a microencapsulated initiator. Macromolecules, 37, 6670-6672. 

Devadoss, D. E. and Volpert, V.A. (2006) Modeling isothermal free-radical frontal polymerization with gel effect using free volume theory, with and without inhibition. J. Math. Chem., 39, 73-105. 

Poiman, J. A. Fortenberry, D. Lewis, L. L. etal. ACS Symp. Ser. (Solvent-Free Polymerization and Processes), Solvent-Free Synthesis by Free-Radical Frontal Polymerization, Vol. 713 American Chemical Society Washington, DC, 1998 140-153. 

Studies had been done in SHS to observe the effect of various parameters on wave-front velocity and product morphology. Among the parameters studied were the green (unreacted) density and particle sizes of the reactants. All the systems so far considered use liquid monomers or solid monomers in solution. Relatively little work has been done with solid monomers. Pojman et al, demonstrated frontal acrylamide polymerization with a variety of free-radical initiators (72). Savostyanov et al. studied transition metal complexes of acrylamide without initiator (73). No studies were performed on the effect of particle size and/or green density. We therefore investigated those two factors to compare to work done in intermetallic SHS systems. 

The major difference between SHS and free-radical frontal polymerization of a solid monomer is that the latter is not a stoichiometric process. As a result, acrylamide frontal velocities are not affected by the monomer particle size. The most important factor affecting the front velocity and the front temperature is the green density. 

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