Methyl methacrylate termination rate constant

Which mechanism of termination will be preferably applied depends largely on the monomer used. Thus, methyl methacrylate chains terminate to a large extent by disproportionation, whereas styrene chains tend to termination by combination. The ratios of termination rate constants 8 = ktJkic (for disproportionation, td, combination,, c) are 5 == 0 and 5 = 2 for styrene [95] and methyl methacrylate [96], respectively. In the case of styrene, however, the values of 8 reported in the literature are at variance. Berger and Meyerhoff [97] found 8 = 0.2, at 52°C. Therefore, it is possible that a fraction of styrene terminates by disproportionation. 

Figure 2 shows the strong dependence of the termination rate constant on the viscosity q of the solvent. For methyl methacrylate, kt is... 

The explanation for this effect (known variously as the gel effect, Tromsdorff effect or auto-acceleration effect) is that the chain termination reaction slows down during conversion and, as can be seen by reference to equations (2.5) and (2.6), a decrease in the termination rate constant leads to an increase in both overall rate and molecular weight. The reason for the drop in termination rate is that as the reaction mixture becomes more viscous the radical ends of the polymer chains find increased difficulty in diffusing towards each other, leading to the important mutual termination reaction. Small monomer molecules on the other hand find little difficulty in diffusion at moderate conversion so that propagation reactions are relatively little affected, until the material becomes semi-soUd, when the propagation rate constant also decreases. It is of interest to note that the gel effect may be induced by the addition of already formed poly(methyl methacrylate) or even another polymer such as cellulose tripropionate because such additions increase the viscosity of the system. 

Schulz et al.12,13 estimated the elementary rate constants for methyl methacrylate in solvents whose viscosities varied by a factor of 170, indicating that the termination rate constants were inversely proportional to the viscosity of the solvents. The variation of propagation rate constants was much less than that of termination rate constants. They have also obtained similar results in the free radical polymerization of benzyl methacrylate. 

It was recently shown12) that in radical polymerization the chain termination rate constant is observed to decrease with the introduction of a polyfunctional complex-ing agent into the system. An especially sharp decrease of the termination rate, up to the formation, under certain conditions, of living radical polymerization centers, was noted in the methyl methacrylate-orthophosphoric acid system. 

In the bulk polymerization of methyl methacrylate (MMA), the Trommsdorff effect sets in after an unperturbed period of about 15-20 % conversion. This enables a separation to be made between unperturbed and autoaccelerated kinetics. Benson and North, O Driscoll, Ito and others have attempted to describe the kinetics of autoacceleration or gelation by introducing a chain-length-dependent value of the termination rate constant... 

Terephthaldoxime, //,// -[4,4-diphenyl(methane)] bismaleimide reaction, 514 Terephthalic acid, polyesters use, 494 Termination rate constant, MA-methyl methacrylate copolymerization, 281 Terpenes... 

Combination and disproportionation are competitive processes and do not occur to the same extent for all polymers. For example, at 60°C termination is virtually 100% by combination for polyacrylonitrile and 100% by disproportionation for poly (vinyl acetate). For polystyrene and poly (methyl methacrylate), both reactions contribute to termination, although each in different proportions. Each of the rate constants for termination individually follows the Arrhenius equation, so the relative amounts of termination by the two modes is given by... 

The above explanation of autoacceleration phenomena is supported by the manifold increase in the initial polymerization rate for methyl methacrylate which may be brought about by the addition of poly-(methyl methacrylate) or other polymers to the monomer.It finds further support in the suppression, or virtual elimination, of autoacceleration which has been observed when the molecular weight of the polymer is reduced by incorporating a chain transfer agent (see Sec. 2f), such as butyl mercaptan, with the monomer.Not only are the much shorter radical chains intrinsically more mobile, but the lower molecular weight of the polymer formed results in a viscosity at a given conversion which is lower by as much as several orders of magnitude. Both factors facilitate diffusion of the active centers and, hence, tend to eliminate the autoacceleration. Final and conclusive proof of the correctness of this explanation comes from measurements of the absolute values of individual rate constants (see p. 160), which show that the termination constant does indeed decrease a hundredfold or more in the autoacceleration phase of the polymerization, whereas kp remains constant within experimental error. 

Baxendale, Evans and coworkers reported in 1946 that the polymerization of methyl methacrylate (MMA) in aqueous solution was characterized by homogeneous solution kinetics, i.e. where mutual termination of free radicals occurred, in spite of the fact that the polymer precipitated as a separate phase. Increases in the rates of polymerization upon the addition of the surfactant cetyl trimethyl ammonium bromide (CTAB) were attributed to the retardation of the rate of coagulation of particles, which was manifested in a reduction in the effective rate constant for mutual termination,... 

By means of the spatially intermittent reactor, Ito and O Driscoll determined, for example, the absolute rate constant of termination, fct, for methyl methacrylate copolymerization with butyl or dodecyl methacrylate. The value of /ct is a function of the monomer mixture composition [89]. 

Most recently the polymerization of methyl methacrylate with Ba counterion has been reported. In THF at —70 °C the rate constant for propagation is independent of the active centre concentration and growth seems to occur via ion pair species only. The kinetic data obtained compare favourably with those for polymethylmethacryl sodium and caesium.As before, active centres are terminated by side-reactions involving the ester group. 

Figure 13. Arrhenius plots of the rate constant for the termination reaction by the monomer (methyl methacryl cesium in THF) (19)...

Tip 16 Polymerization of methyl methacrylate, styrene, and vinyl acetate. MMA, when polymerized, exhibits termination by both combination and disproportionation (in fact, disproportionation is promoted at higher temperatures). Termination by disproportionation leads to the formation of radicals and, eventually, polymer molecules with a TDB. We also know that TDBs will become competitive with the monomer vinyl bonds for radicals as conversion increases. TDB polymerization (characterized by rate constants close (in value) to propagation rate constants) leads to trifunctional LCB. Yet, upon analysis, poly(MMA) chains are linear. How come What is the explanation/reasoning for this observation We also know that styrene terminates predominantly via combination. Styrene also exhibits transfer to monomer, which is enhanced at higher temperature levels. Transfer to monomer generates chains with TDBs. Yet, polystyrene is linear. What is the explanation ... 

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