Anionic Polymerization

Anionic polymerization was carried out on a commercial scale for many years before the nature of the polymerization was recognized, in the production of the buna-type synthetic rubbers in Germany and Russia by the polymerization of butadiene with sodium or potassium as the catalyst. The first anionic chain reaction to be so identified was the polymerization of methacrylonitrile by sodium in liquid ammonia at -75^oC (Beaman 1948). In modern times, the growth of commercial products of anionic polymerization has been phenomenal (Hsich 1981). Most of these polymerizations are based on the use of organometal catalyst system, which allow unprecedented control over polymer structure.

Mechanism
The conventional method of initiation of ionic chains involves the addition of a negative ion to the monomer, with the opening of a bond or ring and growth at one end:

NH₂^- + CH₂=CHX ──> H₂NCH₂CHX

CH₃O^- + CH₂CH₂=O ──> CH₃OCH₂CH₂O^-

Simultaneous growth from more than one center can be obtained from polyvalent ions such as those derived from:

C [RLi]₄ or N(CH₂CH₂O^-Na⁺)_3-

The more basic the ion, the better it serves to initiate chains. Thus OH^- will not initiate the anionic polymerization of styrene, NH₂^- initiates fairly well, but OCH₂^- is powerful. Similarly, more acid monomers require less basic ions, with the initiation and the stability of the resulting ion.

Initiation can also occur by the transfer of an electron to a monomer of high electron affinity. If D or D^- is an electron donor,

or

D + M ----> D⁺ + M^-
D^- + M ----> D + M^-