Bulk Photopolymerization of Methyl Methacrylate: A Test Tube Demonstration INTRODUCTION
he initiation of polymerization by ultraviolet radiation has been of particular interest in the study of free radical processes [1,2]. The test tube demonstration described here is simple and may be used to evaluate the polymerizability of new monomers or to study some of the physical properties of a polymer. Although the method is particularly effective for acrylic and methacryhc monomers, it may also be apphed to the polymerization of a wide range of "vinyl"type monomers. Generally, the method depends on the sensitization of the monomer to ultraviolet radiation with reagents such as biacetyl or benzoin [2,3]. With sensitizers, initiation stops when the source of radiation is turned off, which is followed by a rapid decay of the polymerization process. When a conventional initiator, such as dibenzoyl peroxide, is also present, the process is more rapid than when the sensitizer is used by itself. It also seems to continue after the radiation source has been discontinued. It is presumed that ultraviolet (UV)-induced decomposition of the peroxide becomes involved in the process. By this method, polymerizations may be carried out at temperatures well below those normally used with thermal initiators such as organic peroxides.
SAFETY PRECAUTIONS Before this experiment is carried out, the student must read the material safety data sheets (MSDS) for all the chemicals used as well as for the products. The instructor must approve that you have read and understood the MSDS for the safe handling of these materials. Please be advised that all chemicals should be considered hazardous and should be handled in a hood and with proper personal protective equipment (lab coat, proper gloves, approved safety glasses, and/or goggles). Avoid inhaling vapors and/or aerosolized materials. Avoid skin/eye contact with all chemicals at all times. Wash hands frequently. See the instructor if you have any questions or concerns.
I. Polymer Synthesis
APPARATUS 1. 2. 3. 4. 5. 6. 7.
A No. 9800 Pyrex-brand test tube, 12.5 cm long, i.d. - 1 2 mm A No. 0 neoprene stopper Polyethylene film, ~ 5 X 5 cm Balance, weights, weighing paper, or dishes Graduated cylinder, 25-ml capacity Test tube rack (or tall form beaker) Miscellaneous laboratory equipment such as beakers, paper towels, aluminum foil, and stirring rods 8. Laboratory hood 9. Space on a sunny window sill or some other secure, sunny location
REAGENTS AND MATERIALS 1. 2. 3. 4.
20 ml of a 0.5% solution of sodium stearate (soap) in isopropanol 10 ml of methyl methacrylate (commercial grade) 0.5 g of benzoin 0.5 g of dibenzoyl peroxide
PROCEDURE 1. Fill the 12 cm X 12 mm test tube with some of the 0.5% solution of sodium stearate in isopropanol. In a hood, pour the solution off into a suitable container. Allow the inside of the test tube to dry thoroughly (see Note 1). 2. While the test tube dries, prepare a solution of 0.5 g of benzoin in 10 ml of methyl methacrylate. Then add 0.5 g of dibenzoyl peroxide (see Note 2). 3. When the test tube is ready for use, place the methyl methacrylateinitiator solution in the test tube. Cover the neoprene stopper with the polyethylene film and stopper the test tube. 4. Place the test tube in a test tube rack or a beaker. Note the time and expose the test tube to sunlight. From time to time, note the time and observe the progress of the polymerization (see Note 3).
NOTES 1. In this experiment, coat the inside of the test tube with a thin layer of sodium stearate (or a plain, nonperfumed soap). This will assist in removing the product from the apparatus. 2. In the laboratory, the source of radiation may be a high-intensity mercury lamp. However, for simple test purposes, sunlight is quite suitable. Even a lightly overcast sky furnishes sufficient UV radiation for photo-induced polymerizations. Naturally, bright sunlight is more effective. 3. As the polymerization of methyl methacrylate is somewhat inhibited by atmospheric oxygen, the upper layer of the polymerizing monomer may still be
5. Bulk Photopolymerization of Methyl Methocrylate
fluid, even though most of the material has already been converted. Therefore, when checking on the progress of the experiment, do not simply shake the test tube. Instead, either invert the test tube or insert a probe (a long pin, a paper clip, etc.) to check on the process.
REFERENCES 1. P. J. Flory, "Principles of Polymer Chemistry," p. 149ff. Cornell Univ. Press, Ithaca, NY, 1953. 2. C. E. Schildknecht, "Vinyl and Related Polymers," p. 207ff. Wiley, New York, 1952. 3. W. E. F. Gates, British Patent 566,795, 1945.