Polymerization course methyl methacrylate

Autoacceleration (gel effect, Trommsdorff–Norrish effect) is a dangerous reaction behavior that can occur in free-radical polymerization systems. It is due to the localized increases in

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

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

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s produced.Alger, Mark. Polymer Science Dictionary. New York: Elsevier Applied Science, 1989. 28.

Background

Autoacceleration of the overall rate of a free-radical polymerization system has been noted in many

bulk polymerization Bulk polymerization or mass polymerization is carried out by adding a soluble radical initiator to pure monomer in liquid state. The initiator should dissolve in the monomer. The reaction is initiated by heating or exposing to radiation. As the re ...

systems. The polymerization of

methyl methacrylate Methyl methacrylate (MMA) is an organic compound with the formula CH2=C(CH3)COOCH3. This colorless liquid, the methyl ester of methacrylic acid (MAA), is a monomer produced on a large scale for the production of poly(methyl methacrylate) (PMMA ...

, for example, deviates strongly from classical mechanism behavior around 20% conversion; in this region the conversion and molecular mass 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.Chekal, Brian P. Understanding the Roles of Chemically-Controlled and Diffusion-Limited Processes in Determining the Severity of Autoacceleration Behavior in Free Radical Polymerization. Diss. Northwestern, 2002. 2002. To avoid the occurrence of thermal runaway due to autoacceleration, suspension polymerization techniques are employed to make polymers such as

polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...

. The droplets dispersed in the

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

are small reaction vessels, but the

heat capacity Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). Heat capacity ...

of the water lowers the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

rise, thus moderating the reaction.

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, ''k_t'', is caused by the raised

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.Flory, P. J. Principles of Polymer Chemistry. Ithaca: Cornell UP, 1953. 124–129. The

Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended in a medium (a liquid or a gas). This pattern of motion typically consists of random fluctuations in a particle's position insi ...

of the larger molecules in the polymer "soup" is 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

monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...

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: :

\left( \frac \right)^{1/2}.

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 mass-average molecular mass dramatically. However, the number-average molecular mass only increases slightly, leading to broadening of the molecular mass distribution (high

dispersity In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsi ...

, very polydispersed product).

References

Bibliography

* Dvornic, Petar R., and Jacovic S. Milhailo. "The Viscosity Effect on Autoacceleration of the Rate of Free Radical Polymerization". Wiley InterScience. 6 December 2007. Polymer chemistry Reaction mechanisms