Терминалдалмайтын тізбектік өсу полимерлеуі

Chain growth polymerization without the ability to terminate

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 polydispersity index). 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 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. 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

The naphthalene anion initiates polymerization by reducing styrene to its radical anion, which dimerizes to the dilithiodiphenylbutane, which then initiates the polymerization. These experiments relied on Szwarc's ability to control the levels of impurities which would destroy the highly reactive organometallic intermediates.

Monomers for living cationic polymerization are electron rich alkenes such as vinyl ethers, isobutylene, styrene, and N vinylcarbazole. The initiators are binary systems consisting of an electrophile and a Lewis acid. The method was developed around 1980 with contributions from Higashimura, Sawamoto and Kennedy. Typically, generating a stable carbocation for a prolonged period of time is difficult, due to the possibility for the cation to be quenched by a β protons attached to another monomer in the backbone, or in a free monomer. Therefore, a different approach is taken However, it is important to note that by definition, the polymers described in this example are not technically living due to the introduction of a dormant state, as termination has only been decreased, not eliminated (though this topic is still up for debate). But, they do operate similarly, and are used in similar applications to those of true living polymerizations.

Living polymerizations are used in the commercial synthesis of many polymers.

Copolymers are polymers consisting of multiple different monomer species, and can be arranged in various orders, three of which are seen in the figure below. While there exist others (alternating copolymers, graft copolymers, and stereoblock copolymers), these three are more common in the scientific literature. The unique property of this material is that the size of the pores (or the size of the PMMA cylinders) can be easily tuned by the ratio of PS to PMMA in the synthesis of the copolymer. This can be easily tuned due to the easy control given by living polymerization reactions, thus making this technique highly desired for various nanoscale patterning of different materials for applications to catalysis, electronics, etc.