Chain transfer to initiator

Chain transfer to initiator or monomer cannot always be ignored. It may be possible, however, to evaluate the transfer constants to these substances by investigating a polymerization without added solvent or in the presence of a solvent for which Cgj is known to be negligibly small. In this case the transfer constants Cjj and Cj determined from experiments in which (via... 

This reaction is termed chain transfer to initiator and is considered further in Sec. 3-6b. The induced decomposition of initiator does not change the radical concentration during the polymerization, since the newly formed radical (polymer chain. However, the reaction does result in a wastage of initiator. A molecule of initiator is decomposed without an increase in the number of propagating radicals or the amount of monomer being converted to polymer. 

Fig. 3-5 Dependence of the degree of polymerization of styrene on the polymerization rate. The effect of chain transfer to initiator is shown for t-butyl hydroperoxide (o), cumyl hydroperoxide ( ). benzoyl peroxide ( ), and azobisisobutyronitrile ( ) at 60°C. After Baysal and Tobolsky [1952] (by permission of Wiley-Interscience, New York).

The typical effect of initiator chain transfer [Baysal and Tobolsky, 1952] can be seen graphically in Fig. 3-6. The decrease of polymer size due to chain transfer to initiator is much less than indicated from the Ci values because it is the quantity Ci[I]/[M], which affects Xn (Eq. 3-109b). The initiator concentrations are quite low (10 4 I0 2 M) in polymerization, and the ratio [I]/[M] is typically in the range 10 3-10 s. 

The case of chain transfer to initiator may be exploited in special forms of controlled free-radical polymerization, e.g. with sulfur-containing initiators that are discussed later within the topic of living Ifee-radical polymerizations. 

Chain transfer to initiator was earlier referred to as induced initiator decomposition. 

Initiators under ideal conditions would contribute only to chain initiation by dissociation [Eq. (6.3)] into primary radicals (R ). But in certain systems they also contribute to chain termination, partly or exclusively, giving rise to significant deviations from the ideal kinetics (see Problem 6.31). The degradative chain transfer to initiator (I) may be written as... 

Derive suitable expressions for the rate of polymerization considering (a) an extreme case where chain termination occurs exclusively by the degradative initiator transfer and (b) a more general case where chain termination takes place by simultaneous occurrence of the degradative initiator transfer and the usual bimolecular mechanism. In both cases, assume that the radical I- formed by the chain transfer to initiator I is too inactive to reinitiate polymerization. 

As shown in Scheme la (which supplements Scheme 1 and helps to visualize the inifering process), chain transfer to initiator [1,43,44] involves a growing chain plus unionized initiator (RX) ... 

This irreversible process affects the polymerization only if the rate of ionization of RX (i.e., ion-generation) is slow relative to that of propagation if initiation is instantaneous or very rapid the effect of chain transfer to initiator is negligible [1, 24, 45]. If the rate of ionization of HX is high relative to that of propagation, chain transfer to HX may be neglected. Since HM X and RMnX (see Scheme la) can be reactivated by the quasiliving equilibrium (see Sect. 2.1, and Scheme 1), chain transfer to initiator is by no means a termination reaction [see also 43]. 

Figure 9 shows Nj vs time at different inifer constants (c , i = ktIi, /kp). As shown, chain transfer to initiator accelerates ionization of initiator. We can get the Nj = f (conversion) function, after dividing Eq. (10) by Eq. (11), and separating the variables ... 

For example, the rate of initiation may be slow relative to that of propagation, in which case the MWD will tend to become broader. If ion-generation is slow, the effects of chain transfer to initiator and slow initiation may overlap and may not even be distinguishable. Chain transfer to monomer, initiation by protic impurity ( H20 ), etc., may be present which will further complicate the synthesis of uniform predetermined molecular weight polymers. 

A/fp enthalpy of polymerization ktr rate constant for chain transfer to initiator... 

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