Bimolecular termination rate

Fig. 12 Instantaneous chain length distribution on a number and weight basis, where dead chains are formed by disproportionation termination. The value of n is increased by decreasing the bimolecular termination rate...

Two measurements of the bimolecular termination rate coefficient in the polymerization of ethylene by Ti complexes are available. For (7r-CsH5)2TiCl2/AlMe2Cl a value of 0.5 1 mole sec at 0°C has been reported [111] and for (7r-CsHs)2TiEtCl/AlEtCl2, 0.61 mole sec at0°C [202]. The rate coefficient of decomposition of the latter complex in the absence of ethylene was much lower being 5 x 10 1 mole sec at 20 C [202]. 

It is known that observing in radical polymerization processes change of chains bimolecular termination rate constant kt (reaction is controlled by diffusion) is often connected with the change of reaction solution viscosity [4, 5] which is naturally increased by the accumulation of reaction product in system - polymer. And then the contribution of viscosity factor is significant and that is why the reduction of rate constant of chains bimolecular termination kt is observed first of all. Fiowever, for a number of monomers it is necessary to consider the factor of influence of initial reaction solution viscosity on polymerization parameters. 

The obtained data (namely the non-linearity of change of Vp on [M]) may be explained by the fact that due to comparatively high values of r rei of initial solutions of monomer salts (at [M] > 1 mole l1) the constant of chains bimolecular termination rate kt even at conditions of very small conversions is turned to be sensitive to the viscosity of initial reaction solutions and consequently to monomer concentration. Mentioned change of values of relative viscosity with the rise of initial monomer concentration should lead to symbatic reduction of constant kt and thus to mentioned non-linear increase of polymerization initial rate. The suggested explanation may be checked experimentally. If we assume that in the studied system the constant kt is naturally depended on monomer solution viscosity then in accordance with North s conceptions [4, 5] we should take that k0 rf1. Then polymerization rate may be calculated by equation 1... 

Mixtures of photoinitiators have been actively studied. Michler s Ketone and benzoyl peroxide have been shown to effectively induce the photopolymerisation of methyl methacrylate through the formation of an initial complex shown in scheme 3 7, Although the exact initiating radical does not appear to be ascertained it is almost certainly the arylalkylamino radical from the Michler s Ketone. In the interaction of benzil and thioxanthone with triethylamine in the photoinduced polymerisation of acrylic monomers their is a competition between reverse electron transfer and ketyl radical formation . As the carbonyl concentration increases the bimolecular termination rates due to radical recombination increases. The same workers also studied the same system but replaced the ketone initiators with pyrene . Their inability to identify pyrene end groups indicated that the active initiating species arise from a complex between the pyrene and the triethylamine. 

Case II (1) No chain transfer to small molecules (i.e., monomers and CTAs) or these small molecules are highly water insoluble (2) Fast bimolecular termination rate (3) The polymer particles are relatively small (typically dp < 200 nm) 75 = 0.5 75 = 0.5... 

They were able to observe that inereased styrene eoneentrations reduces the polymerization rates of both methaerylate and styrene due to an inerease in the termination rate and due to flie stability of the styryl radical. Raising styrene concentrations also inereases the final methacrylate conversion, but the final styrene conversion decreases because styrene plasticizes the network, allowing methacrylate conversion to rise at higher styrene concentrations. The final concentration of radicals is reduced at higher styrene concentrations, because of an increase in the bimolecular termination rate for networks with low cross-link densities. The proportion of styryl radicals trapped in the vitrified matrix was found to be markedly higher flian the proportion predicted from the ratio of styrene monomer in the feed resin or from the copolymerization rate constants... 

Figure 4.7. Effect of double bond conversion on the product of bimolecular termination rate coefficient and absorbed light intensity multiplied by quantum yield of initiaton =h4>, calculated from the bimolecular termination model (II) and the mixed termination model (III). Monomer and conditions as in Figure 4.6 [30].

An important characteristic of ionic polymerization is that the propagation rate coefficients are several orders of magnitude higher than for free-radical polymerization. In the equation fct[X] is the bimolecular termination rate coefficient multiplied by the impurity concentration. This equation shows that the rate of polymerization is proportional to the first power of initiation rate, i.e., to the first power of dose rate. Water is a common chain breaker of cationic polymerization since it replaces the cation by a hydroxonium ion. As a proton donor it also inhibits the anionic polymerization... 

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