Chain copolymerization depropagation

Ivin and Hazell (7) extended Lowry s treatment to include a fourth case where any propagating chain ending in M2 could depropagate. Such a mechanism has been shown to apply in olefin-sulfur dioxide copolymerizations. 

Yamashita et al. [157] have derived a copolymer composition equation that includes the depropj ation reaction such as might be expected in the cationic copolymerization of BCMO and THF. They consider two models. For the first one it is assumed that monomer M2 adds reversibly to both active chain ends mf and m and that depropagation by detachment of an M1 unit is neglected. The elementary reactions are then... 

Copolymerization with a properly chosen comonomer, however, may be a way to introduce into the polymer chain monomers that are otherwise unable to polymerize, or to enhance the conversion of monomers showing limited polymerizability. The previous section describes one of the possibilities related to retardation of depropagation. An analogous system is the radical copolymerization of S02 with olefins. 

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... 

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. 

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