Depropagation chain scission

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. 

Irradiation of PMMA at 20 C results In chain scission and the production of terminally unsaturated polymer molecules, but no large production of volatile products. At 150°C the increased mobility of the polymer molecules allows the primarily formed radicals to depropagate and monomer production is quantitative. Irradiation of PP results in chain side radicals which combine to form cross links at 20 C, but undergo scission to form terminally unsaturated molecules at ISO C. VHien blends are irradiated at these temperatures, there appears to be no significant interaction of the radicals formed in the two phases because there is no evidence of block or graft formation. Thus the two constituents appear to decompose In Isolation from one another. 

A review and some new results on the thermal degradation of poly-p-xylylene have been presented by Jellinek and Lipovac [303]. Little volatile material is formed but appreciable amounts of dimer, trimer, tetramer and pentamer were isolated. Typical vacuum volatilization curves are given in Fig. 73. It has been proposed that the mechanism consists of random chain scission at abnormal structures in the chain, followed by a depropagation reaction resulting in low molecular weight polymer but very little monomer. 

At 280°C there are two fundamental changes in the nature of the reaction. First, random chain scission to depropagating radicals occurs. Secondly, depropagation can pass through the acrylonitrile units which are then liberated as monomer. Thus, the stabilization effect of acrylonitrile observed at 220°C is lost at 280°C because acrylonitrile units are liberated in the depropagation process. 

There are basically three types of thermal degradation reactions for vinyl polymers [36,37] (1) nonchain scission (2) random chain scission and (2) depropagation. In practice, mechanisms 2 and 3 blend into one another, with many polymers showing evidence of both processes. 

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.. 

The secondary mechanisms discussed in Section 3.2.1.3 also occur in systems with multiple monomers. The complexity increases, as depropagation, chain transfer to polymer and chain scission are influenced by the penultimate unit on the polymer radical, as well as the identity of the monomer and terminal radical involved in the reaction. As an example, consider a copolymerization in which one of the two monomers undergoes depropagation, shown in Scheme 3.12. Depropagation of radical-1 is a competitive process with addition of... 

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. 

Chain Scission with Depropagation. Many carbon-chain polymers and other simple chains, such as acetal resins (polyethers), are produced by chain reaction polymerization, either via double bonds or by ring opening. Such polymerizations involve repeated addition of a monomer molecule to an active center, which may be a radical, an ion, or a coordination complex. 

If a polymer is heated to the point where chemical bonds begin to break, radicals will normally be formed, rather than ions. Thus if chain scission takes place to produce free radicals at temperatures above T, rapid radical depolymerization, with production of monomer, is expected. This rapid depropagation is often termed unzipping. Table 1 shows some typical values for Tc for common polymers. They range widely from quite close to room temperature for polyacetal to close to 600°C for polytetrafluoroethylene. In reality, side reactions, which are discussed... 

This reaction can be blocked by acetylation of the chain ends, leading to a polymer which can survive above because there is no mechanism of chain scission with a low enough activation energy to initiate depropagation. Another approach is to copolymerize a small amoimt of a second monomer, such as styrene. The polymer is heated to initiate depropagation, which occurs imtil it encoimters the first comonomer unit, at which point further depropagation is blocked. The evolved monomer is recycled. 

In the ideal case, where only depropagation occurs, the mechanism can be deduced from the dependence of the observed first-order rate constant for weight loss on the initial degree of polymerization of the polymer. If depolymerization is both chain-end and chain-scission initiated and termination is first-order then application of the steady-state assumption to the concentration of depolymerizing radicals leads to the following relation (14,15) ... 

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