Chain transfer, irreversible kinetics

If both addition and fragmentation arc irreversible the kinetics differ little from conventional chain transfer. In the more general case, the rate constant for chain transfer is defined in terms of the rate constant for addition and a partition coefficient which defines how the adduct is partitioned between products and starting materials (eq. 19). 

Living polymerization is defined as chain polymerization in which chain termination and irreversible chain transfer are absent. The rate of chain initiation is usually larger than the rate of chain propagation with the result that the number of kinetic-chain carriers is essentially constant throughout the reaction. Reversible (temporary) deactivation of active centers can take place in a living polymerization, and all the macromolecules formed possess the potential for further growth. The term controlled polymerization, on the other hand, indicates control of a certain kinetic feature of a polymerization or structural aspect of the polymer. ... 

To identify the mechanism(s) responsible for loss of surface-tethered radicals in SI-PMP, several irreversible termination reactions that can result into the permanent loss of surface-tethered radicals, including (a) bimolecular termination, (b) chain transfer to monomer, (c) chain transfer to dithiocarbamyl radical, (d) chain transfer to an adjacent polymer chain, and (e) chain transfer to solvent, were considered by Rahane et al. in their development of a kinetic model to describe SI-IMP [81]. The decrease in the concentration of surface-tethered radicals by chain transfer to monomer and by bimolecular termination reactions is captured by Equation 12.2 ... 

To determine the prevailing termination mechanism in SI-PMP over a broad range of relevant reaction conditions, experimental data on film thickness evolution such as that shown in Figure 12.4 were fit in the brush regime (transitions marked by arrows in Figure 12.4) by the kinetic models that incorporate one or more termination mechanisms. For example, Rahane et al. combined Equation 12.1 with expressions for STR based on either bimolecular termination or chain transfer to monomer to develop models for how layer thickness should evolve as a function of exposure time. These models, shown as Equations 12.3 and 12.4, respectively, can be compared to experimental data of polymer layer thickness as a function of time to deduce which irreversible termination mechanisms are prevalent. 

To study the living nature of this surface initiated polymerization, several groups have performed kinetic studies. " They reported that the nonlinear growth of the polymer brushes as a funetion of irradiation time was mainly attributed to bimolecular termination reaetions, rather than chain transfer to monomer. To avoid irreversible termination reaetions, a strategy to increase the amount of deactivating species by adding tetraethylthiuram disulfide to the polymerization mixture, which is mandatory to provide a controlled radical polymerization behavior, was introduced. ... 

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