anioonpolümerisatsioonimehhanismide

Anioonpolümerisatsioonimehhanismid refer to the processes by which polymers are formed through a chain-growth mechanism involving anionic active centers. This type of polymerization is typically initiated by species that can readily donate an electron or abstract a proton, generating a carbanion, alkoxide, or amide ion as the propagating species. The key characteristic is the presence of a negatively charged center at the growing chain end.

The mechanism involves three main stages: initiation, propagation, and termination. Initiation occurs when an initiator molecule reacts with a monomer to create the anionic active center. For example, an organolithium compound can react with a diene monomer to form a lithiated carbanion. Propagation involves the sequential addition of monomer molecules to the anionic active center. The negative charge on the growing chain end attracts the electrophilic part of the incoming monomer, leading to chain elongation. Termination in anionic polymerization is often not spontaneous and can be difficult to achieve. It typically requires the addition of a terminating agent, such as a proton source (like water or an alcohol) or an electrophile, which neutralizes the anionic active center. In some cases, especially with certain monomers and conditions, termination can occur through side reactions or combination of growing chains.

Anionic polymerization is particularly effective for monomers with electron-withdrawing groups, such as styrene, butadiene, isoprene, and acrylates. It allows for precise control over molecular weight and polymer architecture, leading to the formation of block copolymers and polymers with narrow molecular weight distributions. This control is facilitated by the living nature of many anionic polymerization systems, where termination is absent or can be deliberately controlled, allowing for sequential monomer addition.