Propagation and depolymerization

In the anionic lactam polymerization it is always the lactam anion which is incorporated into the polymer molecules either in reaction (24) 

Even without taking into account side reactions changing the concentrations of the active species, a great number of rate and equilibrium constants are required in order to describe the whole course of polymerization. So far, no data are available on equilibrium (25) from which the concentrations of lactam and polymer amide anions could be estimated. Therefore, the individual rate coefficients can be obtained only from measurements of the initial rates of the isolated reactions. In this case, the participation of the reaction products in subsequent reactions can be neglected. 

At the very beginning of polymerization, the linear monomer units are incorporated into the polymer in reaction (24) which is followed by the fast proton exchange (25) regenerating lactam anions. The proton exchange between pyrrolidone and its anion 

Was found to proceed lO times faster (at 30°C, feo = 10 1 mole sec ) than the propagation [166]. Hence, the rate determining step of polymerization is the acylation of lactam anions (24). At the beginning of polymerization and at low temperatures, the depolymerization can be neglected for most lactams (except the five- and six-membered) and the rate of polymerization is given by [94] 

Similarly, any compound consuming the acyllactam growth centres (e.g. primary and secondary amine [162, 170]) decreases the rate of polymerization. So far, the rate coefficients 21 of the addition of the first lactam anion to the growth centre have been estimated only for a few monomers. In most cases, however, the published rate coefficients were calculated from the second order rate equation 

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The ceiling temperature is therefore defined as the temperature at which the rates of propagation and depolymerization are equal. For that reason, is a threshold temperature above which a specific polymer cannot exist. Representative values of for some common monomers are given in Table 14.23. 

The addition of radicals and, in particular, propagating radicals, to unsaturated systems is potentially a reversible process (Scheme 4.46). Depropagation is cntropically favored and the extent therefore increases with increasing temperature (Figure 4.4). The temperature at which the rate of propagation and depropagalion become equal is known as the ceiling temperature (rc). Above Tc there will be net depolymerization. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

Reduction of bismuth compounds could take place by reaction with polymer radicals propagating the depolymerization of polypropylene, either by electron transfer or ligand transfer which are typical redox reactions between alkyl radicals and metal compounds 59... 

In the polymerization of dialkyldichlorosilanes with sodium in refluxing toluene, propagation and a concurrent back-biting reaction to cyclic material could give the range of products found if the products are kinetically, instead of thermodynamically, determined (12). No evidence for depolymerization has been found for the reaction in toluene solution. 

It should be noted that, whereas the preceding discussion has been cast in terms of free-radical polymerizations, the thermodynamic argument is independent of the nature of the active species. Consequently, the analysis is equally valid for ionic polymerizations. A further point to note is that for the concept to apply, an active species capable of propagation and depropagation must be present. Thus, inactive polymer can be stable above the ceiling temperamre for that monomer, but the polymer will degrade rapidly by a depolymerization reaction if main chain scission is stimulated above T.. 

All propagation reactions are irreversible, and depolymerization does not occur. To conform to this condition, no isomerization such as trans-amidation may occur either. 

Depropagation n. The sequential chain scission step during depolymerization responsible for the formation of monomer. Has a lower activation energy than propagation and hence is favored at high temperatures. 

Fig. 1. Change in energy of a polsmaerization/depolymerization equilibrium along the reaction coordinate, showing the relation of the activation energies of propagation and depropagation and the heat of polymerization.

Heterochain polymers produced by ring-opening polymerization contain the hetero-atoms in the main chain as well as in the monomer and the polymer chain competes with the monomer for the reaction with the propagating species. This competition leads to polymer transfer and back-biting reactions during the polymerization. Heterochain polymers are also susceptible to depolymerization by the ionic active species which are easily formed during processing. 

Several important assumptions are involved in the derivation of the Mayo-Lewis equation and care must be taken when it is applied to ionic copolymerization systems. In ring-opening polymerizations, depolymerization and equilibration of the heterochain copolymers may become important in some cases. In such cases, the copolymer composition is no longer determined by die four propagation reactions. 

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