Anionic polymerization systems

Sequential addition of different monomer charges to a living anionic polymerization system is useful for producing well-defined block copolymers. Thermoplastic elastomers of the triblock type are the most important commercial application. For example, a styrene-isoprene-styrene triblock copolymer is synthesized by the sequence... 

Carbon-13 NMR Studies of Model Anionic Polymerization Systems... 

Studies on benzyl alkali metal compounds, models of styrene anionic polymerization systems, showed coupling constants to the enriched a carbon in the above ranges, showing it to be sp hybridized (ll). Little effect of the charge could be detected in the Jg c values, although it should be noted that the charge is extensively delocalized into the benzene ring. 

This assumption is questionable in the light of experimental data obtained most recently for living anionic polymerization systems (15, 17). The relevant problem will be discussed later. 

Most interesting from the standpoint of commercial development is the formation of block copolymers by the living polymer method. Sequential addition of monomers to a living anionic polymerization system is at present the most useful method of synthesizing well-defined block copolymers. Depending on whether monofunctional or difunctional initiators are used, one or both chain ends remain active after monomer A has completely reacted. Monomer B is then added, and its polymerization is initiated by the living polymeric carbanion of polymer A. This method of sequential monomer addition can be used to produce block copolymers of several different types. 

In conclusion, although a number of anionic polymerization systems involving alkali metal compounds in ionizing solvents have lent themselves to a satisfactory elucidation, much remains to be learned about such systems in nonpolar media such as hydrocarbons. Furthermore, the systems discussed herein have involved only the alkali metals and a very small group of hydrocarbon monomers, for... 

The third major breakthrough was by Szwarc and coworkers who reported in 1956 (30. 31) that certain anionic polymerization systems gave "living polymers" to which a second monomer could be added without termination, so that block polymers could be formed. Interest of research groups in this system was dissipated somewhat by the finding that the sodium naphthalene diinitiator used by Szwarc resulted in undesirable 1,2- or 3,4-addition structures in diene polymers. 

For these reasons, we feel that the discovery of A-B-A thermoplastic rubbers and the domain theory gave the block polymer field a paradigm (22) that greatly accelerated research on a worldwide basis. The content as well as the volume of literature support this view (Figure 3). The usefulness of the paradigm was based on the well-characterized block polymers that can be produced from the anionic polymerization system as shown in the box and on the readily understood basics of the domain theory. 

Functionalized block copolymers by sequential monomer addition. Sequential addition of monomers to a living anionic polymerizing system is at present the most useful method for the synthesis of well-defined block copolymers.An AB diblock copolymer is produced by first completely polymerization of one monomer (A) using an initiator such as butyllithium. The second monomer (B) is then added to the living anions. When the second monomer has reacted completely, a terminating agent is introduced into the reaction mixture and the block copolymer is isolated.This method can be used to synthesize any of the various types of block copolymers (di-, tri-, tetra-... 

The chemistries utilized in gas-phase technologies employ the same Ziegler-Natta (314,315) and single-site (metallocene) catalysts (313) described in the processes included below. In gas-phase systems, however, the catalysts are generally solid-supported, but produce the same range of polybutadiene microstructures inherent to the nonsupported catalyst. Several patents also include anionic polymerization systems as useful in gas-phase processes (378). Kinetic modeling work has also been done to better predict the gas-phase polymerization behavior of 1,3-butadiene (379). 

For this reason, the focus of this chapter will be on the recent developments (since 2000) in p-star polymers synthesized by the above living anionic polymerization systems, with emphasis on the control of synthetic factors necessary to achieve well-defined structures of p-star polymers, that is, molecular weight, molecular-weight distribution, arm number, and composition. In the last 20 years, rapid progress in living/controlled radical polymerization systems as well as the application of click makes possible the synthesis of several new p-star polymers. Therefore, representative examples will also be described. The syntheses of p-star polymers before 2000 are beyond the scope of this chapter, although they will be briefly described in Section 4.2, since such subjects have been covered elsewhere by several excellent reviews (Hadjichristidis, 1999 Hadjichristidis et al, 2001). 

In certain anionic polymerization systems there can be a total absence of any formal termination reaction if all impurities are excluded. The exact details of each system depend upon the nature and type of initiator, monomer and solvent that are used. Propagation will continue until all the monomer is consumed and the carbanions remain active and are not terminated. If... 

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