Anionic Vinyl Polymerization

The concept of anionic polymerization was first developed by Ziegler and Schlenk in early 1910. Their pioneering work on the polymerization of diene initiated with sodium metal set the stage for the use of alkali metal containing aromatic hydrocarbon complexes as initiators for various α-olefins. In 1939, Scott and coworkers used for the first time the alkali metal complexes of aromatic hydrocarbon as initiators for the polymerization of styrene and diene. However, in 1956, it was Michael Szwarc who demonstrated unambiguously the mechanism of anionic polymerization of styrene, which drew significant and unprecedented attention to the field of anionic polymerization of vinyl monomers [1, 2]. Michael Szwarc used sodium naphthalenide as an initiator for the polymerization of styrene in tetrahydrofuran (THF). Upon contact with styrene, the green color of the radical anions immediately turned into red indicating formation of styryl anions. He suggested that the initiation occurs via electron transfer from the sodium naphthalenide radical anion to styrene monomer. The styryl radical anion forms upon addition of an electron from the sodium naphthalenide and dimerizes to form a dianion (Scheme 1.1). After the incorporation of all the monomer, the red color of the reaction mixture persists, indicating that the chain ends remain intact and active for further propagation. This was demonstrated by the resumption of propagation with a fresh addition of another portion of styrene. After determining the relative viscosity of the first polymerized solution at its full conversion, another portion of styrene monomer was added and polymerization was continued. Thus, Szwarc characterized this behavior of the polymerization as living polymerization and called the polymers as living polymers [2]. Here, the term living refers to the ability of the chain ends of these polymers retaining