Anionic chain polymerization living

Block copolymers have commercial potential for obtaining products that can incorporate the desirable properties of two or more homopolymers. This potential has led to an intense effort to find reaction systems that proceed as living polymerizations. Some anionic chain polymerizations proceed as living polymerizations under conditions where no viable chainbreaking reactions occur, and this has resulted in useful block copolymers (Sec. 5-4). 

Thermoplastic elastomers have the important advantage over conventional elastomers that there is no need for the additional chemical crosslinking reaction and fabrication is achieved in the same way as for thermoplastics. However, only certain polymerization methods can be used to synthesis block copolymers — primarily living anionic chain polymerization and certain step polymerizations. Triblock copolymers are produced by living anionic polymerization by sequential addition of different momomer charges to a living anionic system, for example, a styrene-isoprene-styrene is synthesized by the sequence... 

SBR is prepared by anionic polymerization initiated by lithium alkyls in cycloaliphatic media as a solvent. The main feature of this class of anionic chain polymerization is that it is a living polymerization, i.e., the polymeric chain ends are able to survive even when monomer is completely depleted and to reinitiate the polymerization reaction when monomer is newly added. Due to the absence of termination reactions, pol5mier active chain ends do not inherently terminate, continuously growing up to tiie complete depletion of monomers this in turn means that the average pol5uner molecular weight can be predicted from the amount of starting material and tiie quantity of the initiator. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Anionic polymerization is a powerful method for the synthesis of polymers with a well defined structure [222]. By careful exclusion of oxygen, water and other impurities, Szwarc and coworkers were able to demonstrate the living nature of anionic polymerization [223,224]. This discovery has found a wide range of applications in the synthesis of model macromolecules over the last 40 years [225-227]. Anionic polymerization is known to be limited to monomers with electron-withdrawing substituents, such as nitrile, carboxyl, phenyl, vinyl etc. These substituents facilitate the attack of anionic species by decreasing the electron density at the double bond and stabilizing the propagating anionic chains by resonance. 

Fig. 51. Schematic illustration of the mechanism of microgel formation in the anionic dispersion polymerization of 1,4-DVB initiated by living PBS chains in heptane. [Reprinted with permission from Ref. 247, Copyright 1995, American Chemical Society].

What would it take to achieve better control over radical polymerization so that, for example, block copolymers could be prepared Remember that the key to making block copolymers anionically is the living nature of the intermediate—chain termination does not compete with initiation and propagation. Could we design a free radical system in which we could turn off termination reactions until we wanted them ... 

The linear polymers, as shown above, can be built up by the sequential addition of monomer or by coupling the living anionic chains using compounds like dichloro dimethylsilane. Hence, the base polymer would have styrene polymerized first, followed by butadiene, and then addition of the coupling agent. If a multifunctional coupling agent such as silicon tetrachloride is used, a radial block or star-branched SBS is formed. 

PMPS-Z -PS and PMPS-/ -Pl were synthesized by the anionic ring-opening polymerization of tetramethyltetraphenylcyclotetrasilane (prepared from commercially available octaphenyltetrasilane) initiated by the living anionic chain ends of polystyrene and polyisoprene [52, 53] (Fig. 4b). 

A special kind of termination in ionic polymerizations is the mutual combination of anionic and cationic living chains (see Chap. 5, Sect. 5.8). When the two polymers consist of different monomers, block copolymers are formed. The two macroions can also consist of the same monomer. 

Telechelic polymers are defined as macromolecules with reactive sites on the polymer chain, usually as endgroups on linear polymers [106]. This macro-molecular architecture has successfully produced a wide variety of block copolymers using macroinititated polymerizations. Living anionic polymeriza-... 

K-Resin SBC synthesis is a batch anionic solution polymerization of styrene and 1,3-butadiene using an n-butyllithium (NBL) initiator in a process referred to as living polymerization . Although often referred to as a catalyst, each NBL gives rise to a distinct polymer chain. Polymer chains grow by adding monomer... 

A new synthetic route for the preparation of polyisobutylene (PIB) based block copolymers was developed by combining living carbocationic and anionic polymerizations. Living PIB chains were quantitatively end-capped with 1,1-diphenylethylene (DPE) leading to 1,1-diphenyl-l-methoxy (DPOMe) and/or 2,2-diphenyl vinyl (DPV) termini. This end-capping process is very sensitive to temperature, and retroaddition of DPE occurs in an equilibrium reaction above about -70 °C. Both the DPOMe and DPV terminated PIBs, and the mixtures of the two endgroups were quantitatively metalated with K/Na alloy, Cs metal and Li dispersion in THF at room temperature. 

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