Polymerization free-radical, autoacceleration

From these experimental and modeling studies, the mechanism of the living free radical polymerizations initiated by a combination of TED and DMPA have been elucidated. The TED produces DTC radicals that preferentially cross-terminate with the propagating carbon radicals. By this cross-termination reaction, the carbon radical concentration is kept low (as was shown in figure 6) and the rate of polymerization is decreased, as is the autoacceleration effect. This suppression of the autoacceleration peak in HEM A polymerizations and, interestingly, in DEGDMA polymerization has been observed to increase as the TED concentrations are increased. This behavior has been predicted successfully by the model as well. 

By using the free-volume theory, one can successfully monitor the whole course of free-radical polymerization. It is commonly accepted that the presence of autoacceleration and limiting conversion in the polymerization reaction are due to the diffusion-controlled kt and kp, respectively. The initiation efficiency, f, will behave in a way similar to kp. 

In free-radical polymerization, the autoacceleration (or gel effect, Trommsdorff effect) has been known for a long time 161 168>. 

Polystyrene and poly(methyl methacrylate) polymerizations are typical of homogeneous bulk chain-growth reactions. The molecular weight distributions of the products made in these reactions are broader than predicted from consideration of classical, homogeneous phase free-radical polymerization kinetics because of autoacceleration (Section 6.13.2) and temperature rises at higher conversions. 

If the initiator concentration used in a free-radical polymerization system is low and insufficient, leading to a large depletion or complete consumption of the initiator before maximum conversion of monomer to polymer is accomplished, it is quite likely to observe a limiting conversion poo which is less than the maximum possible conversion pc, as shown in Fig. 6.2. This is known as the dead-end effect and it occurs when the initiator concentration decreases to such a low value that the half-life of the kinetic chains approximates that of the initiator. However, if there is autoacceleration effect or gel effect (described later) leading to a sharp rise in rate of polymerization, viscosity of medium, and degree of polymerization, pure dead-end effect cannot be observed. 

Chain polymerizations which do not terminate by bimolecular chain coupling generally result in polymers with the most probable molecular weight distribution of 2.0 free radical polymerizations which terminate by bimolecular chain coupling result in pdi = 1.5. However, chain transfer to polymer, autoacceleration, slow initiation, and slow exchange between active species of different reactivities result in much higher polydispersities. 

Free-radical polymerizations of certain monomers exhibit autoacceleration at high conversion via an additional mechanism, the isothermal gel effect or Trommsdorff effect (23-26). These reactions occur by the creation of a radical that attacks an unsaturated monomer, converting it to a radical, which can add to another monomer, propagating the chain. The chain growth terminates when two radical chains ends encounter each other, forming a stable... 

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). 

Both the Trommsdorff effect and thennal autocatalysis can lead to the autoacceleration in free-radical polymerization. The results indicated that the autoacceleration observed in the smallest test tube was mostly due to the Trommsdorff effect, while both the Trommsdorff effect and thermal autocatalysis strongly affected the onset of autoacceleration in the larger polymerization system. When an exothermic reaction is performed in a larger system, more heat tends to be accumulated in a reactor, since a sur ce-to-volume ratio is decreased as the size of the system increases. There was a critical size in the inner diameter of the test tube at which the behavior of the autoacceleration of the polymerization changes. 

Polymerization of vinyl or methacrylic monomers (especially in conjunction with crosslinking monomers) within the wood often results in an autoacceleration during the latter phase of the polymerization this phenomenon is known as the Trommsdorff or gel effect in homopolymerization reactions (Duran and Meyer, 1972 Trommsdorff et a/., 1948). The gel effect arises from a decrease in the termination rate of the free radical polymerization, caused in turn by the effect of the local viscosity on the diffusion rates of the growing polymer chains. Since the heat of polymerization cannot be removed rapidly enough to maintain isothermal conditions, autoacceleration is characterized by a strong exotherm the intensity of the exotherm depends on the catalyst level, as illustrated in Figures 11.4 and 11.5 (Siau et al., 1968). 

When vinyl chloride is polymerized in solution, there is no autoacceleration. Also, a major feature of vinyl chloride free-radical polymerization is chain transferring to monomer [2%]. This is supported by experimental evidence [297,298]. In addition, the growing radical chains can terminate by chain transferring to dead polymer molecules. The propagations then proceed from the polymer backbone [297]. Such new growth radicals, however, are probably short lived as they are destroyed by transfer to monomer [299]. 

The vast majority of photopolymerizations used in industry are free-radical polymerizations, which have been studied extensively (for reviews, see references 1-5 as well as Photopolymerization, Free Radical (qv)). By far the most widely used classes of monomers for UV-initiated free-radical photopolymerizations are multiftinctional acrylates and methacrylates. Several investigations have demonstrated that free-radical polymerizations of these monomers exhibit imusual kinetic behavior, including immediate onset of autoacceleration, the formation of heterogeneous polymers (2,6-11), and the attainment of a maximum conversion... 

Autoacceleration, where the rate of polymerization increases with conversion in isothermal conditions, is observed in both thermal- and photoinitiated free-radical polymerizations because the termination mechanisms are the same for both. As the chains grow longer, it becomes more difficult for the active centers to diffuse and imdergo bimolecular termination thus, termination frequency decreases and active centers at the chain ends can become trapped. In cases where termination is controlled by diffusion, the pseudo-steady-state assumption is no longer valid and chain length dependent termination (CLDT) may occur (67). As is discussed for chain cross-linking photopolymerizations below, more complicated kinetic treatments must then be considered, including unsteady-state kinetics. 

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. 

O Neil, G.A., Wisnudel, M.B., and Torkelson, J.M. (1996) A critical experimental examination of the gel effect in free radical polymerization do entanglements cause autoacceleration Macromolecules, 29, 7477-7490. 

Free-radical polymerizations are exothermic and so energy is evolved at an increasing rate as autoacceleration begins. If the dissipation of energy is poor there may well be an explosion. The simplest ways of avoiding autoacceleration are to stop the reaction before chain diffusion becomes difficult or to use dilute solutions of monomer, though the latter is complicated by the occurrence of chain transfer reactions. 

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