Polymerization, frontal

Frontal polymerization (FP) is a polymerization process in which polymerization occurs directionally. There are three types of FP. The first is isothermal FP, which is discussed in Chapter 5. The second is photofrontal polymerization in which the front is driven by the continuous flux of radiation, usually UV light [1-7]. The last type is thermal FP, which we will henceforth refer to ns frontal polymerization, and it results from the coupling of thermal transport and the Arrhenius dependence of the reaction rate of an exothermic polymerization. 

F P was discovered at the Institute of Chemical Physics in Chemogolovka, Russia. Chechilo and Enikolopyan studied methyl methacrylate polymerization under 3500atm pressure [8-llj. The work from that Institute was reviewed in 1984 [12]. 

The essential criterion for FP is that the system must have an extremely low rate of reaction at the initial temperature but a high rate at the front temperature such that the rate of heat production exceeds the rate of heat loss. In other words, the system must react slowly or not at all at room temperature, have a large heat release, and have a high energy of activation. For free-radical polymerization, the peroxide or nitrile initiator provides the large activation energy. It is not possible to create a system that has a long pot life at room temperature and a rapid reaction at any arbitrary temperature if the system follows Arrhenius kinetics. 

Frontal polyurethane polymerization [13-15], frontal atom transfer radical polymerization [16], and frontal ring-opening metathesis polymerization (ROMP) [17] all suffer from short pot lives that is, bulk polymerization occurs in less than an hour. In some cases, the only way to avoid even faster bulk polymerization is to cool the reagents. For example, with frontal ring-opening metathesis polymerization of dicyclopentadiene with a Grubbs catalyst, the starting materials had to be frozen to prevent rapid bulk polymerization [17]. 

Nonlinear Dynamics wi Polymers Fundamentals, Methods and Applications. Edited by John A. Pojman and Qui Tran-Cong-Miyata Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-32529-0 

We will describe a simple and inexpensive experiment with propagating fronts of addition polymerization. The method can be used to determine the front velocity dependence on the initiator concentration and to determine the effects of heat loss and gravity on a chemical reaction. This experiment is also described in Pojman, et al. (1997b). 

The test tubes containing the traveling front will be very hot—above 200 °C, so they should be allowed to cool for at least an hour before handling. 

Any unused monomer-initiator solution can be stabilized by adding a few grams of hydroquinone and disposed of as any other organic waste. 

Three 100 mL solutions of BPO in TGDMA are prepared with concentrations between 1 g per 100 mL and 4 g per 100 mL. The solutions are stable at room temperature for a few hours and should be prepared and used in the same lab period. (Initiator-monomer solutions may be refrigerated for a day or two if necessary.) Benzoyl peroxide does not dissolve quickly in TGDMA above 4 g per 100 mL. (NB The solution should not be heated to speed dissolution, because polymerization will occur.) 

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A typical example of frontal polymerization is the polymerization of methyl methacrylate (or an oligomer), placed inside a long aluminum tube 249 these tubes continuously dip into a bath with a liquid heated up to temperature of 70 - 80°C. The part at the tubes above the bath are cooled so that the reactive material does not polymerize. Polymerization shrinkage is compensated by continuous injection of a monomer or oligomer into the reaction zone. The appropriate combination of injection rate, velocity of tube movement through the reaction zone, and tube diameter are chosen according to experimental studies of the process. 

Frontal polymerization carried out as described above can be turned into a continuous process. In order to do this, it is necessary to move the newly formed polymer and the reactive mixture in the direction opposite to the direction of spreading of a thermal front at a velocity equal to the velocity of the front development to feed the reactor with a fresh reactive mass.254 Control of the process, choice of process parameters and proper design of the equipment require solving the system of equations modelling the main physical and chemical processes characteristic of frontal reactions. 

The theoretical approach to modelling frontal polymerization is based on the well developed theory of the combustion of condensed materials.255 "6 The main assumptions made in this approach are the following the temperature distribution is one-dimensional die development of the reaction front is described by the energy balance equation, including inherent heat sources, with appropriate boundary and initial conditions. Wave processes in stationary and cyclical phenomena which can be treated by this method, have been investigated in great detail. These include flame spreading, diffusion processes, and other physical systems with various inherent sources. 

Several CCT catalysts are soluble in MMA but insoluble in the resulting polymer.318 Interestingly, when these CCT catalysts are added to frontal polymerizations, they are carried in the polymerization front, decreasing the molecular weight throughout the resulting polymer. 

DBTDL was used as a catalyst in the frontal polymerization of 1,6-hexanediisocyanate with ethylene glycol. In frontal polymerization the polymerization is locally initiated and the exotherm of the reaction propagates the polymerization throughout the system. Pyrocatechol was used to avoid spontaneous polymerization. Pyrocatechol chelates tin and depresses the catalytic activity at room temperature without affecting catalysis at the higher temperature. To achieve a uniform advancing reactive front, and to avoid fingering, the viscosity of the blend was increased with colloidal silica. 

In recent decades, production of uniform polymers and cross-linked networks has been achieved by frontal polymerization (FP). FP is a method in which a monomer is converted into a polymer via a localized reaction zone that propagates through the... 

Results of studies on the FP of oxirane and oxetanes demonstrated that some monofunctional and difunctional 3,3-disubstituted oxetane, arylalkyl, and alkyl glycidyl ether monomers display frontal polymerization characteristics [157]. 

J.V. Crivello, R. Falk, and M.R. Zonca, Photoinduced cationic ring-opening frontal polymerizations of oxetanes and oxiranes. J. Polym. Sci. A Polym. Chemi. 2004. 42(7), 1630-1646. 

J.A. Pojman, et al., Binary frontal polymerization a new method to produce simultaneous interpenetrating polymer networks (SINs). J. Polym. Sci. A Polym. Chem. 1997, 35(2), 227-230. 

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

Mathematical models of frontal polymerization have also been developed. They are discussed below. These works study the velocity of the reaction front... 

We now return to the base model (2,17)-(2.21) of frontal polymerization. We want to find uiuformly propagating FP waves and perform linear and nonlinear stability analyses, as we did in the case of the gasless combustion model. Before we study the model, we would like to reformulate it using the reaction front approximation. 

It is interesting to compare the reaction front formulations of the gasless combustion and frontal polymerization models. First of all, there is an additional differential equation in the FP model, namely, equation (4.132). The equation, however, can be easily solved yielding... 

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. 

Some other works that we would like to mention studied the kinetics effects in FP [85], the influence of the gel effect on the propagation of thermal frontal polymerization waves [28], the use of complex initiators as a means to increase the degree of conversion of the monomer [30], and FP of metal-containing monomers [3]. 

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. 

Frontal polymerization can be used to synthesize valuable products, e.g. nanocomposites and liquid crystals, and theoretical studies represent an important tool for understanding the process. 

Y. Chekanov and J. Pojman, Preparation of functionally gradient materials via frontal polymerization, J. Appl. Polym. Sci., 78 (2000), pp. 2398-2404. 

S. Chen, J. Sui, L. Chen, and J. A. PotMAN, Polyurethane-nanosilica hybrid nanocomposites synthesized by frontal polymerization, J. Polym. Sci. Part A - Polym. Chem., 43 (2005), pp. 1670-1680. 

D. M. G. Comissiong, L. K. Gross, and V. A. Volpert, Nonlinear dynamics of frontal polymerization with autoacceleration. Journal of Engineering Mathematics, (2004). 

Enhancement of the frontal polymerization process, Journal of Engineering... 

Frontal polymerization in the presence of an inert material, Journal of Engi-... 

P. Goldeeder and V. Yoepert, A model of frontal polymerization including the gel effect. Math. Problems in Engin., 4 (1998), pp. 377-391. 

A model of frontal polymerization using complex initiation. Math. Problems in... 

J. Masere, F. Stewart, T. Meehan, and J. Pojman, Period-doubling behavior in frontal polymerization of multifunctional acrylates. Chaos, 9 (1999), pp. 315-322. 

M. F. Perry and V. A. Yoipekt, Linear stability analysis of two monomer systems of frontal polymerization. Chemical Engineering Science, in press, 59 (2004), pp. 3451-3460. 

J. Pojman, S. Popwell, V. Volpert, and V. Yolpeki, Nonlinear dynamics in frontal polymerization, in Nonlinear Dynamics in Polymeric Systems, J. Pojman and... 

J. A. PojMAN, B. Varisli, A. Perryman, C. Edwards, and C. Hoyle, Frontal polymerization with thiol-ene systems. Macromolecules, 37 (2004), pp. 691-693. 

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

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