Poly termination rate constant

Figure 8.13. Template polymerization of methacrylic acid along with poly(N-vinylp5nrolidone). Termination rate constant as a function of [T o/[M]o.

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. 

Furthermore, the authors pointed out that they obtained in the emulsion polymerization of styrene (monomer to water ratio 1 2) with an inisurf concentration of 5.4 X 10 mol/1 water in the presence of an alkylated poly (oxyethyl-ene) emulsifier (alkyl chain length C16 -18 and 20 oxyethylene units 4% by weight related to water) the same overall rate of polymerization as with water-soluble initiators in the concentration range 10 to 10 mol/1 water. The polymer produced in the presence of inisurf has a molecular weight of some of 10 g/mol mainly due to the lowered termination rate constant. 

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. 

For N-vinylcarbazole in methylene chloride solutions cycloheptatrienyl ion has been shown to be a very efficient initiator, reacting by a rapid and direct addition to the olefin (82). A mechanistic scheme involving virtually instantaneous and quantitative initiation, rapid propagation (and transfer) and no true termination appears to operate, enabling rate constants for propagation kp, to be determined very simply from initial slopes of conversion/time curves. Under the experimental conditions used the initiators were almost totally dissociated and there seems every reason to suppose that the propagating cations are similarly dissociated (Section II.C.2). The derived rate constants therefore refer to the reactivity of free poly-(N-vinylcarbazole) cation, kp, and relevant data are summarised in Table 7. 

The rate constant for spontaneous decomposition was reported to be 40 x i0 s at 65 °C in cyclohexane [101, 103]. The rate of decomposition of PSLi in cyclohexane at 150 °C is 0.205 min corresponding to a 3.5-min half-life [104]. In the presence of 2 equivalents of n,sec-dibutylmagnesium at 100 °C, the rate of decomposition of PSLi is 1.9 X 10 min while it is 6.4 x 10 " in the absence of additive, corresponding to half-lives of 102 and 3 h, respectively [105]. Similar decomposition reactions have been observed for poly(styryl)sodium [102]. The thermal stability of poly(a-methylstyryl)lithium is much lower than that of poly(styryl)lithium. The observed half-lives for spontaneous termination are 5 h and a few minutes at 25 and 60 °C, respectively [106]. The relative thermal stability of styryl carbanionic chain ends follows the order K Na > Li for the alkali metal counterions. 

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 ... 

The reactivity ratios for a copolymer poly(A-co-B) can be written as r = k A/kAB where Icaa and Icab are the rate constants of the chain terminating in A continuing with another A or B unit, respectively (see Figs. 3.45 and 3.46). Out of these reactivity ratios one can calculate the run number, R, which is defined as the average number of consecutive A- and B-sequences per 100 chain units ... 

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