Step growing, living polymerization

This type of polymerization process is called living polymerization because the polymer molecules grow forever until aU monomer is consumed. Ideally, there is no step that terminates growth. One type of this polymer is called ionic because the active species... 

There is no inherent termination step in organolithium polymerizations of hydrocarbon monomers, and this method of initiation yields living polymers. Living polymerizations are defined as those in which there is no inherent termination reaction (as described in Section 6.3.3 for free-radical polymerizalions) and in which the macrospecies continue to grow as long as monomer is supplied. 

Figure 1. Polymer molecular weight as a function of monomer conversion in chain-growth, step-growing and living polymerizations [4].

If the initiation reaction is very mnch faster than the propagation reaction, then all chains start to grow at the same time, and since there is no inherent termination step (living polymerization), the statistical distribution of chain lengths is very narrow. The average molecnlar weight is calcnlated from the mole ratio of monomer-to-initiator sites. Chain termination is nsnally accomplished hy adding proton donors, eg, water or alcohols, or electrophiles such as carbon dioxide. 

Vinyl copolymers contain mers from two or more vinyl monomers. Most common are random copolymers that are formed when the monomers polymerize simultaneously. They can be made by most polymerization mechanisms. Block copolymers are formed by reacting one monomer to completion and then replacing it with a different monomer that continues to add to the same polymer chain. The polymerization of a diblock copolymer stops at this point. Triblock and multiblock polymers continue the polymerization with additional monomer depletion and replenishment steps. The polymer chain must retain its ability to grow throughout the process. This is possible for a few polymerization mechanisms that give living polymers. 

As was stated above, the interpretation that the field affects the dis-sodation state of the growing chain ends was not uniquely substantiated by the experimental data, except those on copolymerizations. Thus it is interesting to investigate the field influence on much simpler systems than cationic homopolymerizations. For this purpose we have chosen living anionic systems in which only propagation steps are involved. The system first studied was a living anionic polymerization of styrene with n-butyllithium in the binary mixtures of benzene and tetrahydrofuran (17,24) and in the binary mixtures of benzene and dimethoxyethane (15). 

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