Free Radical Polymerization of Styrene using Solution Process and Bulk Process
This lab was designed to demonstrate the free radical polymerization of styrene using solution process and bulk process. It demonstrates specifically with the polymerization of styrene monomer to form polystyrene (PS). This lab was also intended to show the difference between solution process and bulk process and how it affects the yield and appearance of the final product.
Polystyrene is a major polymer used for a wide variety of commercial applications, and a vast majority of this polymer is synthesized via free-radical polymerization. The polymerization of styrene is often initiated by thermal generation of radical species. For example, azo-bis-isobutyronitrile (AIBN) thermally decomposes to form 2-cyano-2-propyl radicals that initiate the polymerization of styrene to polystyrene (Scheme 1).
AIBN decomposes into 2-cyano-2-propyl radicals
2-cyano-2-propyl radicals react with styrene monomer to form polystyrene
The free radical polymerization is usually terminated by two mechanisms, coupling or disproportionation. Either mechanism involves the reaction between two growing chain ends. In this lab, the reaction is terminated by adding small amount of methanol to the reaction mixture.
Styrene polymerizations were performed in a round bottom flask with one open arm. 10 ml (96.015 mmol) of dry, oxygen free styrene and 0.2 g (1.218 mmol) of AIBN was added to a 100 ml round bottom flask. The mixtures were then degassed by bubbling through dry N2(g) for 5 minutes. The experimental setup is shown as figure below. The setup of running water is for the purpose of condensing the vapor of reaction mixture back to the reactor. (The nitrogen purge is not shown)
The polymerization was performed in a 100ml round bottom flask merged in a warmed water bath at 80 °C for 1 hour. The mixture was stirred constantly by a magnetic stirring bar during the reaction. For terminating the polymerization, 5ml of methanol was added to the mixture to deactivate any radical species that might present. Reaction mixture was then poured into 250 ml chilled methanol for filtering process. The polymer solution is filtered and washed with methanol for 3 times and final product were dried to constant weight. The conversion and total PS content in the final product were given by:
The solution polymerization follows the same procedure as bulk process except the addition of 30ml of dry, degassed toluene to the reactor before the reaction started.
Results and Discussions:
The reaction mixture was transparent during the polymerization. The stirring bar vibrating at the end of polymerization indicated the increasing fluid viscosity. Heat transfer and mixing become difficult as the viscosity of reaction mixture increases. The final product of bulk polymerization is in the form of large translucent castings. The castings are soft and sticky which indicate that the molecular weight polymer obtained was low. The polymerization is obtained with a broad molecular weight distribution due to the high viscosity and lack of good heat transfer. As a result, the reaction temperature increases significantly above 80°C which lead to low molecular weight product.
The amount of PS in the reaction product is determined to be 3.249g. Assume the density of styrene monomer is 1g/ml. The mass of styrene monomer is 10g.
Percent conversion = 3.249g/10g = 32.49%
The solution in the reactor became cloudy as reaction proceeds. It is because the solvent was saturated with polymer and the polymer starts precipitating in the solution. As a result of solution polymerization, the product was in the form of fine white...