Question

Electron Withdrawing (EWD) and Electron Donating (ED) groups on a monomer have a significant bearing on the strength of initiator required for polymerisation to occur. Using a cyanoacrylate based monomer illustrate how the EWD functionality on a monomer and anionic initiator strength can be tailored for use in a commercial application

Answer 1

Alkene monomers with Electron withdrawing groups like CN, C₆H₅ etc. Undrgoes polymerization in the presence of strong bases such as KNH₂, LiNH₂ or organometallic compounds such as n- butyllithium as initiators. In chain initiation step, base adds to double bond of alkene monomer to form a carbanion. In the chain propagation step, the carbanion adds to another monomer molecule to form a bigger carbanion and the process is repeated to form a polymeric carbanion. In chain termination, the chain reaction is terminated by the addition of proton. Electron withdrawing groups stabilize the carbanion by dispersing its negative charge.

Cyanoacrylates can be polymerized by a weak base like NaOH because of the presence of two electron withdrawing groups

- CN and - COOCH₃. Cyanoacrylates polymerize quickly even in presence of basic impurities like metal oxides present on the surface of glass to form super glue or crazy glue. It has a high tensile strength and one drop of it can support a weight of about 2000lbs.