Depropagation temperature dependence

The ability to determine which copolymerization model best describes the behavior of a particular comonomer pair depends on the quality of the experimental data. There are many reports in the literature where different workers conclude that a different model describes the same comonomer pair. This occurs when the accuracy and precision of the composition data are insufficient to easily discriminate between the different models or composition data are not obtained over a wide range of experimental conditions (feed composition, monomer concentration, temperature). There are comonomer pairs where the behavior is not sufficiently extreme in terms of depropagation or complex participation or penultimate effect such that even with the best composition data it may not be possible to conclude that only one model fits the composition data [Hill et al., 1985 Moad et al., 1989]. 

Thermal degradation of polystyrene involves depropagation and first order termination [37]. Initial decomposition temperature is 300-330° [38]. The kinetics of the decomposition follows the general rule da/dt = k (1 - a)[1-(1 - where a is the conversion (and equal to the ratio of the volatilized mass to the initial mass), k is the overall rate constant, and b is a parameter depending on the pyrolysis temperature. 

A ceiling temperature [16] (temperature at which rate of propagation and depropagation are equal) is independent of the kind of initiator, but depends on the given olefin. The polymerization is not always accelerated by heat and in fact in some cases slows as it approaches the ceiling temperature. 

When depropagation takes place at an elevated temperature, at a rate that is equal to the propagation in a free-radical polymerization, then the temperature of the reaction is a ceiling temperature (see Chap. 3). Termination can take place by disproportionation. Secmidary reactions, however, may occur in the degradation process depending upon the chemical structure of the polymer. Such side reactions can, for instance, be successive eliminations of hydrochloric acid, as in poly(vinyl chrolide), or acetic acid as in poly(vinyl acetate). 

We assumed that poly-BFl-AP is terminated at both chain ends. The phenyl group of the anionic initiator was believed to be fixed, whereas a relatively acidic proton linked to the indenyl moiety is present at the other end. If the propagation/ depropagation reaction requires the presence of a negative charge in the terminal indenyl moiety, the reaction kinetics will depend on the concentration of the depro-tonated chains and ultimately on the acid-base equilibrium. In deuterated nitrobenzene, the depolymerization rate is negligible at room temperature probably because... 

---