Anionic polymerization termination with electrophilic

The addition of the anion takes place at the unsubstituted carbon atom, which, in this case, carries a partial positive charge. Since the growing chain end is a genuine anion, chain termination can occur by addition of a reactive cation. As in cationic polymerization, combination of two growing ends is not possible. Chain transfer with electrophiles can also occur. 

The need for well defined polymer species of low polydls-perelty and of known structure arises from the Increasing Interest In structure-properties relationship In dilute solution as well as In the bulk. A great variety of methods have been attempted, to synthesize so-called model macromolecules or tailor made polymers-over the past 20 years. The techniques based on anionic polymerization, when carried out In aprotic solvents, have proved best suited for such synthesis, because of the absence of spontaneous transfer and termination reactions that characterize such systems. The "living 1 polymers obtained are fitted at chain end with carbanionic sites, which can either Initiate further polymerization, or react with various electrophilic compounds, intentionally added to achieve functionalizations. Another advantage of anionic polymerizations is that di-functlonal Initiators are available, yielding linear polymers fitted at both chain ends with carbanionic sites. In this paper we shall review the various utility of anionic polymerization to the synthesis of tailor made well defined macromolecules of various types. 

For example, termination of a living anionic polymerization with a tetrafunctional electrophile such as silicon tetrachloride will produce a four-armed star polymer as shown in equation 7. Given that PLi is a well-defined living polymer, a branched polymer with a predictable, well-defined structure will be formed from the linking reaction. 

Anionic polymerization is one of the best methods for the synthesis of well-defined macromonomers. Fimctional initiation or termination by a suitable electrophilic reagents are the best ways for the incorporation of the reactive end groups (50). According to this methodology living polystyryllithium initially reacts with ethylene oxide to form the less reactive alkoxide, followed by the reaction with methacryloyl chloride for the synthesis of the macromonomer (eq. 14) (51). 

MALDI-tof-ms was used to examine the products from some anionic polymerizations with styrene initiated by sec-butyl-Li or t-C4H90(CH2)3 Li and extended by isoprene (98). Several electrophilic reagents affected termination. The results showed unusual end group rearrangements to yield some end groups not normally expected. From this observation it was possible to suggest new mechanisms for the termination reaction of the polymer. 

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