Anioonpolümerisatsioonis

Anioonpolümerisatsioonis is a type of chain-growth polymerization where the propagating species is an anion. This process is initiated by a species that can generate a carbanion or another anionic center. Common initiators include alkali metal alkyls, Grignard reagents, or organolithium compounds. The anionic initiator abstracts a proton from a monomer with an acidic hydrogen or adds directly to a polar monomer, creating the initial carbanion.

The polymerization proceeds by the addition of monomer units to the growing anionic chain end. The anionic chain end attacks the electrophilic part of the incoming monomer, typically a double or triple bond that is polarized by electron-withdrawing groups. The polarity of the monomer is crucial for anionic polymerization to occur efficiently. Electron-withdrawing groups, such as nitrile (-CN) or ester (-COOR), enhance the electrophilicity of the double bond, making it susceptible to nucleophilic attack by the carbanion.

Anionic polymerization is known for its ability to produce polymers with very narrow molecular weight distributions, often approaching values characteristic of controlled polymerization. This is because the growing anionic chains typically do not terminate spontaneously. Termination can occur if impurities like water or oxygen are present, which can protonate the carbanion. However, if the reaction is carried out under strictly anhydrous and anaerobic conditions, the polymer chains can remain "living" after all monomer has been consumed. This living nature allows for controlled chain growth, sequential monomer addition to create block copolymers, and precise control over molecular weight. Common monomers polymerized via anionic mechanisms include styrene, butadiene, isoprene, and methacrylates.