Question

summarize the causes and ramifications of the gel( Trommsdorff or Norris-Smith) effect

Answer 1

Trommsdorff or Norris Smith, or acceleration effect, Is a dangerous reaction behavior that can occur in free radical polymerization systems. It is due to the localized increases in viscosity of the polymerizing system that slow termination reactions. The removal of reaction obstacles therefore causes a rapid increase in the overall rate of reaction, leading to possible reaction runaway and altering the characteristics of the polymers produced.

Autoacceleration must be caused by a totally different polymerization mechanism. A decrease in the termination rate is the basis of the phenomenon. This decrease in termination rate, k_t, is caused by the raised viscosity of the polymerization region when the concentration of previously formed polymer molecules increases. Before autoacceleration, chain termination by combination of two free radical chains is a very rapid reaction that occurs at very high frequency (about one in 10⁴ collisions). However, when the growing polymer molecules - with active free radical ends - are surrounded in the highly viscous mixture consisting of a growing concentration of "dead" polymer, the rate of termination becomes limited by diffusion. The Brownian motion of the larger molecules in the polymer "soup" are restricted, therefore limiting the frequency of their effective (termination) collisions.

In other words, Trommsdorff of Norris-Smitch effect is causes for reduced rate of termination of chain-end radicals due to their low mobility in a highly viscous system.

Effects: With termination collisions restricted, the concentration of active polymerizing chains and simultaneously the consumption of monomer rises rapidly. Assuming abundant unreacted monomer, viscosity changes affect the macromolecules but do not prove high enough to prevent smaller molecules - such as the monomer - from moving relatively freely. Therefore, the propagation reaction of the free radical polymerization process is relatively insensitive to changes in viscosity.

Approximately, as the termination decreases by a factor of 4, the overall rate of reaction will double. The decrease of termination reactions also allows radical chains to add monomer for longer time periods, raising the weight average molecular weight dramatically. However, the number average molecular weight only increases slightly, leading to broadening of the molecular weight distribution (high polydispersity index, very polydispersed product)