Formation of Block Copolymers Starting from Living Anionic Polymers

3 Formation of Block Copolymers Starting from Living Anionic Polymers 

Anionic polymerization frequently has been used to prepare well-defined living polymers such as polystyrene, poly(a-methylstyrene), polydienes, which may be transformed by two methods into block copolymers with cationically polymerizable monomers. When a living anionic polymer is mixed with a stoichiometric amount of a living cationic polymer the cationic and anionic species may couple. For example, anionic living polystyrene (St) or poly (a-methylstyrene) (MSt) were reacted with living cationic polytetrahydrofuran (THF). In the latter system the coupling efficiency was low, probably because of proton or hydride transfer 132)  

On the other hand, coupling efficiency between living polySt and living polyTHF was nearly quantitative 133). This reaction was originally used by Vofsi and Levy 134) but the products obtained were insoluble. Later, Richards et al. under the same conditions prepared soluble multiblock copolymers, having the approximate structure 133)  

Yamashita et al. modified the polystyryl end-groups by introducing carboxylate anions (by reaction with C02) 135). After coupling this modified polySt with a living polyTHF and removing polyTHF by extraction with hot isopropanol, a solid copolymer, soluble in common organic solvents and having the structure  

Modification of the polystyryl anion was also necessary for the preparation of block copolymers with living cationic poly(l -/-butylaziridine). In the absence of modification only a mixture of homopolymers was obtained. Thus, the carbanion was first converted with propylene sulfide to the thiolate anion 126) which was then coupled with the living aziridinium cation  

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