Autoacceleration
Encyclopedia
Autoacceleration 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 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.
systems. The polymerization of methyl methacrylate
, for example, deviates strongly from classical mechanism behavior around 20 percent conversion; in this region the conversion and molecular weight of the polymer produced increases rapidly. This increase of polymerization is usually accompanied by a large rise in temperature if heat dissipation is not adequate. Without proper precautions, autoacceleration of polymerization systems could cause metallurgic failure of the reaction vessel or, worse, explosion.
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.
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. This also implies that at the onset of autoacceleration the overall rate of reaction increases relative to the rate of unautoaccelerated reaction given by the overall rate of reaction equation for free radical polymerization:
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).
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of the polymerizing system that slow termination reactions. The removal 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.
Background
Autoacceleration of the overall rate of a free radical polymerization system has been noted in many bulk polymerizationBulk polymerization
Bulk polymerization or mass polymerization is carried out by adding a soluble initiator to pure monomer into liquid state.Bulk polymerization has several advantages over other methods, these advantages are :...
systems. The polymerization of methyl methacrylate
Methyl methacrylate
Methyl methacrylate is an organic compound with the formula CH2=CCOOCH3. This colourless liquid, the methyl ester of methacrylic acid is a monomer produced on a large scale for the production of poly .-Production:...
, for example, deviates strongly from classical mechanism behavior around 20 percent conversion; in this region the conversion and molecular weight of the polymer produced increases rapidly. This increase of polymerization is usually accompanied by a large rise in temperature if heat dissipation is not adequate. Without proper precautions, autoacceleration of polymerization systems could cause metallurgic failure of the reaction vessel or, worse, explosion.
Causes
Norrish and Smith, Trommsdorff, and later, Schultz and Harborth, concluded that autoacceleration must be caused by a totally different polymerization mechanism. They rationalized through experiment that a decrease in the termination rate was the basis of the phenomenon. This decrease in termination rate, kt, is caused by the raised viscosityViscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher 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
Brownian motion
Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...
of the larger molecules in the polymer "soup" are restricted, therefore limiting the frequency of their effective (termination) collisions.
Results
With termination collisions restricted, the concentration of active polymerizing chains and simultaneously the consumption of monomerMonomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
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. This also implies that at the onset of autoacceleration the overall rate of reaction increases relative to the rate of unautoaccelerated reaction given by the overall rate of reaction equation for free radical polymerization:
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
Polydispersity index
In physical and organic chemistry, the polydispersity index , is a measure of the distribution of molecular mass in a given polymer sample. The PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual...
, very polydispersed product).