Chain breaking, living polymerization

Under certain conditions, irreversible chain-breaking reactions are absent and cationic ROPs of cyclic ethers proceed as living polymerizations. These conditions are found for polymerizations initiated with acylium and l,3-dioxolan-2-ylium salts containing very stable counterions such as AsFg, PFg, and SbClg or with very strong acids (fluorosulfonic and... 

Living polymerizations are useful for producing block copolymers and functionalized polymers. Facile chain-breaking reactions such as [3-hydride transfer greatly limit the possibility of living polymerization for most of the polymerizations described in this chapter, but there are significant differences between the different types of initiators ... 

Table III shows the increase of molecular weight of BCMO polymerization with conversion, although the polymer tends to precipitate. The monomer reactivity ratios of DOL-BCMO copolymerization were previously determined as rx (DOL) = 0.65 0.05, r2 (BCMO) = 1.5 0.1 at 0°C. by BF3 Et20 (8). Table IV shows a preparation of block copolymer of DOL, St, and BCMO. In the first step we polymerized DOL and St in the second step we added BCMO to this living system. The copolymer obtained showed an increase of molecular weight, and considerable BCMO was incorporated in the copolymer still remaining soluble in ethylene dichloride. The solubility behavior together with the increase of molecular weight with addition of BCMO shows that this polymer consists of block sequences of DOL-St and (St)-DOL-BCMO. This we call block and random copolymer of DOL-St—BCMO. We can deny the presence of BCMO, St, or DOL homopolymers in this system, but some chain-breaking reactions are unavoidable, leading to copolymer mixtures. Thus, the principle of formation of block copolymers by cationic system is partly substantiated.

Since the 1960s [20], much of the research in polymer synthesis has been directed at establishing living conditions for chain polymerizations. The only requirements for a polymerization to be considered living are that no chain-breaking reactions occur during the polymerization. That is, the rate constants of both chain transfer and termination should be equal to zero (Atr = 0, k, = 0). 

In practice, linear semilogarithmic kinetic plots and linear dependencies of molecular weight on monomer conversion require only that the rate constants of chain transfer and termination are much less than that of propagation klr kp, k,< kp). This is therefore the practical requirement for the synthesis of well-defined polymers, such that complete monomer conversion can be reached and the chain ends can be functionalized quantitatively. However, because chain-breaking reactions are actually present, we prefer to call such systems controlled polymerizations rather than living polymerizations. 

The term living polymerization was originally used to describe a chain polymerization in which chain-breaking reactions are absent [I], In such an ideal system, after initiation is completed, growing or propagating chains would only propagate and would not participate in transferor termination. Each chain should infinitely retain its ability to react with monomer. 

Because in many cases the quantitative information on the contribution of chain-breaking reactions is not yet available, we decided to use the term controlled/living polymerization to describe the still existing ambiguity and uncertainty in these systems. 

The term controlled is preferred to apparently living or living (with quotation marks) used to indicate synthesis of well-defined polymers under conditions in which chain breaking reactions undoubtedly occur, like in radical polymerization. 

This mechanism represents a novel type of living polymerization in that the polymer "dies after each addition of a monomer tmit. The chain "lives and grows only as long as monomer is present to be activated by the catalyst. However, block copolymers derived from the addition of other monomers (e.g., isoprene) via this mechanism have not yet been rqwrted. Breaks in the fused ring structure may occur by cationic 1 2 or 1,4 initiation. 

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