Depropagation reaction

With most common monomers, the rate of the reverse reaction (depropagation) is negligible at typical polymerization temperatures. However, monomers with alkyl groups in the a-position have lower ceiling temperatures than monosubstituted monomers (Table 4.10). For MMA at temperatures <100 °C, the value of is <0.01 (Figure 4.4). AMS has a ceiling temperature of <30 °C and is not readily polymerizable by radical methods. This monomer can, however, be copolymerized successfully (Section 7.3.1.4). 

At higher temperature in free-radical polymerization systems, a reverse reaction (depropagation) takes place. For such a situation, the disappearance of monomer... 

The predicted continuous increase in Rp with temperature is based upon an assumption that the propagation reaction is irreversible. This assumption is not correct and the reverse reaction, depropagation, also can occur, i.e. the propagation step is more accurately represented by... 

Chain reaction (depropagation, unzipping, unzippering, chain depolymerization). Stepwise reaction. 

Copolymerizations of other monomers may also be subject to similar effects given sufficiently high reaction temperatures (at or near their ceiling temperatures - Section 4.5.1). The depropagation of methacrylate esters becomes measurable at temperatures >100 °C (Section 4.5.1).96 O Driseoll and Gasparro86 have reported on the copolymerization of MMA with S at 250 °C. 

The main reaction in the radiation degradation of all the polyfolefin sulfone)s is the depropagation step... 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

When the rate of the propagation reaction equals that of depropagation... 

We emphasize that the conditions subscripted with a zero (time, initiator and monomer concentration) are not the beginning of a reaction, but rather some point well advanced in the polymerization process when the remaining amount of monomer is small in absolute terms but large compared to the desired end state of the polymerization (Mg M ). The amount of initiator Ig is to be achieved by addition to any present immediately before time zero, and the final monomer concentration, M, is set by production specifications. We do not set any predetennined bounds on upper and lower temperature limits. In practice the upper limit will be detennined by either reaction variables (depropagation and initiator depletion) or by process variables (heat exchange), while the lower temperature limit will be determined by process variables (solubility, heat exchange). We do not here consider the process variables to be constraints. 

The depropagation reaction at the top is favored over that at the bottom. Note that this explanation applies only to MSM sequences. Such a corrective device is not open to MM sequences, in which we may expect the fraction of head-to-head to Increase with temperature in the "normal" manner. 

In contrast to the kinetic approach, deviations from the terminal model have also been treated from a thermodynamic viewpoint [Kruger et al., 1987 Lowry, 1960 Palmer et al., 2000, 2001]. Altered copolymer compositions in certain copolymerizations are accounted for in this treatment in terms of the tendency of one of the monomers (M2) to depropagate. An essential difference between the kinetic and thermodynamic treatments is that the latter implies that the copolymer composition can vary with the concentrations of the monomers. If the concentration of monomer M2 falls below its equilibrium value [M]c at the particular reaction temperature, terminal M2 units will be prone to depropagate. The result would be a... 

Recall the discussion in Sec. 2-3 concerning the competition between linear polymerization and cyclization in step polymerizations. Cyclization is not competitive with linear polymerization for ring sizes greater than 7 atoms. Further, even for most of the reactants, which would yield rings of 5, 6, or 7 atoms if they cyclized, linear polymerization can be made to predominate because of the interconvertibility of the cyclic and linear structures. The difference in behavior between chain and step polymerizations arises because the cyclic structures in chain polymerization do not depropagate under the reaction conditions that is, the cyclic structure does not interconvert with the linear structure. 

The depropagation reaction occurs by a similar nucleophilic attack by the penultimate oxygen atom (as shown by the dotted arrow) followed by the expulsion of monomer. 

By using the value of kp at 20° C and the value of the equilibrium monomer concentration, the rate constant calculated for the depropagation reaction, ka, was 4.67 X 10-2 sec-1 at 20° C. An activation energy of 19.4 kcal/mole and a pre-exponential factor of 1.65 x 1013 sec-1 were calculated for the depropagation reaction. 

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

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