Homopropagation coefficient

Reactivity ratios rj are defined by r, = ka/kij, the ratio of homopropagation, ka, to cross-propagation, rate coefficients, where fcy refers to the addition of monomer j to a free-radical chain-end terminating in species i. Under ideal polymerization conditions the mole fraction of monomer units i contained in the copolymer Fj is given by eq (4.6-14), which holds for both the terminal and the implicit penultimate unit models (see Section 4.6.4.3) [45]. 

As described in Section 4.6.4.1, homopropagation data of the comonomers is required to derive copolymerization rate coefficients such as ea and ae- It goes without saying that, in addition to propagation, extended information about termination and chain-transfer for homopolymerization of the comonomers is needed to model copolymerization kinetics. 

For the mathematical description of the average propagation rate coefficient as a function of comonomer feed distribution, the homopropagation rate constants of the two comonomers are required additionally. A convenient description of the average propagation rate constant as a function of comonomer feed composition is given in a review by Fukuda et al. ° and shown here in eqn [13] ... 

In contrast to the free-radical copolymerization, the lack of termination in living ionic copol5mierization enables the direct determination of the rate coefficients of the cross-propagation step. The reaction rate of monomer B with the living chain end a is directly accessible via the concentration decrease of a , which may be traced via a suitable spectroscopic method. Alternatively, the concentration of B can be determined as a function of time with the concentration of a being held constant. The latter experimental technique requires extrapolation of time toward zero, because the cross-propagation step is immediately followed by homopropagation of B. 

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