Depropagation step polymerization

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

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 active species in the large majority of cationic polymerization of heterocyclics are onium ions. Therefore, the depropagation step is always accompanied by the formation of a strained onium ion from the penultimate unit. Thus, the presence of a penultimate unit that arose of a strained monomer, will retard depropagation... 

A few brief comments are merited about the thermodynamics of polymerization reactions [42,43]. In principle, all polymerization reactions are reversible. However, the reversibility of the propagation step is very dependent on there being a reaction mechanism available for the reverse process. In the majority of polymerization reactions, the depropagation step is either not possible or other side reactions occur which dominate under conditions where reversibility might be expected. Thus, the ability to study thermodynamic equilibria in a polymerization process is restricted to relatively few polymerization systems even though thermodynamic behaviour is not a function of the precise nature of the propagating species in, say, chain polymerization processes. 

Since depropagation is the exact reverse of propagation, the activation energy of the depropagation step Ei is expected to equal the sum of the activation energy of the propagation in polymerization E plus the heat of poljunerization AHp, as shown in Figure 4. 

During polymerization in some systems, the reverse step of depolymerization is favored as the propagation steps. The temperature above which the reverse depropagation reactions are favored over the forward propagation reactions is called the... 

Initiation is very likely owing to several causes, such as thermal effects, catalyst fragments, chemically incorporated oxygen, and weak bonds produced in the terminal step of the polymerization reaction. Althou initiation is prohahly a composite of all these factors, treatment here is limited to random and terminal initiation. Depropagation is taken to he the reverse of growth, and transfer is presumed to consist in the abstraction of hydrogen from another molecule. It is also likely that a radical can react with a hydrogen from its own chain the most probable position of attack is from the third to the seventh carbon atom adjacent to the radical end. In order to facilitate the mathematical treatment, disproportionation has been assumed to be the only termination step (28). 

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