Chain growth polymerization transfer

Elsewhere in this chapter we shall see that other reactions-notably, chain transfer and chain inhibition-also need to be considered to give a more fully developed picture of chain-growth polymerization, but we shall omit these for the time being. Much of the argumentation of this chapter is based on the kinetics of these three mechanistic steps. We shall describe the rates of the three general kinds of reactions by the notation Rj, Rp, and R for initiation, propagation, and termination, respectively. 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

Figure 2,3 Chain growth polymerization exemplified by free radical polymerization of polyethylene a) initiation, b) propagation, c) chain transfer, and d) termination...

In conclusion, chain-growth polymerizations are typical chain-reactions involving a start-up step (initiation) followed by many identical chain-reaction steps (propagation) - stimulated by the product of the first start-up reactions. Transfer processes may continue until, finally, the active center disappears in a termination step. 

Unlike ordinary chain reactions, chain-growth polymerization need not involve free radicals. The reactive center may instead be a carbanion or carbocation generated by intermolecular transfer of a proton or electron. Depending on the sign of the ionic charge on the chain carriers, the overall reaction is called anionic or cationic polymerization. As in free-radical polymerization, initiation is required. 

Chain-growth polymerizations that are carried out in the absence of a diluent are characterized by high viscosity and poor heat transfer. High-molecular-weight... 

Despite passage of more than 57 years since the basic discoveries, the mechanism of Ziegler-Natta polymerization is still not fully understood. As in all chain-growth polymerizations (12), the basic steps are initiation, propagation and termination (chain transfer). 

Chain-growth polymerization, as well as all other typical chain reactions, are fast reactions typified by three normally distinguishable processes, viz., (i) initiation of the chain, (ii) propagation or growth of the chain, and (iii) termination of the chain. (A fourth process, chain transfer, may also be involved.)... 

FIGURE 2.1 Variation of molecular weight with % conversion for (a) step-growth polymerization, (b) chain-growth polymerization, and (c) living chain-growth polymerization with no chain transfer and no chain termination. 

Consequently, each repeat unit of the polymer contains one double bond. ROMP is, thus, a chain-growth polymerization and belongs - together with vinyl insertion-type, pseudo anionic, and group-transfer polymerizations - to the family of polyinsertions. The basic ROMP process is shown in Scheme 19.1. 

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