In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar (i.e. they have a very low dispersity index, Đ). Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.
Living polymerization is desirable because it offers precision and control in macromolecular synthesis. This is important since many of the novel/useful properties of polymers result from their microstructure and molecular weight. Since molecular weight and dispersity are less controlled in non-living polymerizations, this method is more desirable for materials design In many cases, living polymerization reactions are confused or thought to be synonymous with controlled polymerizations. While these polymerization reactions are very similar, there is a distinction between the definitions of these two reactions. While living polymerizations are defined as polymerization reactions where termination or chain transfer is eliminated, controlled polymerization reactions are reactions where termination is suppressed, but not eliminated, through the introduction of a dormant state of the polymer. However, this distinction is still up for debate in the literature. The main living polymerization techniques are:
Living anionic polymerization Living cationic polymerization Living ring-opening metathesis polymerization Living free radical polymerization Living chain-growth polycondensations
History Living polymerization was demonstrated by Michael Szwarc in 1956 in the anionic polymerization of styrene with an alkali metal / naphthalene system in tetrahydrofuran (THF). Szwarc showed that electron transfer occurred from radical anion of naphthalene to styrene. The initial radical anion of styrene converts to a dianion (or equivalently disodio-) species, which rapidly added styrene to form a "two – ended living polymer." An important aspect of his work, Szwarc employed the aprotic solvent tetrahydrofuran, which dissolves but is otherwise unreactive toward the organometallic intermediates. After initial addition of monomer to the initiator system, the viscosity increased (due to increased polymer chain growth), but eventually cease after depletion of monomer concentration. However, addition of more monomer caused an increase in viscosity, indicating growth of the polymer chain, and Szwarc concluded that the polymer chains had never been terminated. This was a major step in polymer chemistry, since control over when the polymer was quenched, or terminated, was generally not a controlled step. With this discovery, the list of potential applications expanded dramatically. Today, living polymerizations are used widely in the production of many types of polymers or plastics. For instance, poly(phthalaldehyde) polymer, first developed in 1967, can be synthesized via both living cationic and living anionic polymerization reactions producing both the cyclic or linear form of the polymer respectively. The approach offers control of the chemical makeup of the polymer and, thus, the structural and electronic properties of the material. This level of control rarely exists in non-living polymerization reactions.
Fast rate of initiation: low dispersity
One of the key characteristics of a living polymerization is that the chain termination and transfer reactions are essentially eliminated from the four elementary reactions of chain-growth polymerization leaving only initiation and (chain) propagation reactions. A key characteristic of living polymerization is that the rate of initiation (meaning the dormant chemical species generates the active chain propagating species) is much faster than the rate of chain propagation. Thus all of the chains grow at the same rate (the rate of propagation). The high rate of initiation (together with absence of termination) results in low (or narrow) dispersity index (Đ), an indication of the broadness in the distribution of polymer chains. The extended lifetime of the propagating chain allowing for co-block polymer formation and end group functionalization to be performed on the living chain. These factors also allow predictable molecular weights, expressed as the number average molecular weight (Mn). For an ideal living system, assuming efficiency for generating active species is 100%, where each initiator generates only one active species the Kinetic chain length (average number of monomers the active species reacts with during its lifetime) at a given time can be estimated by knowing the concentration of monomer remaining. The number average molecular weight, Mn, increases linearly with percent conversion during a living polymerization
v = [ M ] 0 − [ M ] [ I ] 0 {\displaystyle \ v={\frac {[M]_{0}-[M]}{[I]_{0}}}}
Techniques
Living anionic polymerization
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