Let's Make an Addition Polymer
The Polymerization of Polymethylacrylate from a Methyl Acrylate Monomer
Objective: The objective of this demonstration is to produce an addition polymer using a free radical initiator.
Review of Scientific Principles:
Poly (methyl acrylate) is an addition polymer. The reaction can be represented as:
Because the high rate and heat of polymerization of acrylates make control of bulk polymerization impractical, the most important method of preparation is by emulsion polymerization. Water serves as a moderator and a solvent for the catalyst system. The catalyst is produced by the reaction of bromate ions and hydrogen sulfite ions and is known to produce hydroxyl radicals (OH.) and hydrogen bisulfite radicals (HSO3.). Other water soluble catalysts such as ammonium peroxydisulfate or potassium peroxydisulfate (often called persulfate), may be used, but the mixture must be heated to generate free radicals.
In this demonstration, methyl acrylate in an aqueous emulsion polymerizes in the presence of a free-radical catalyst. The polymer is coagulated in a concentrated solution of sodium chloride to yield a white product. An acetone solution can be used to produce a film.
Time: 20 -25 minutes are required
Materials and Supplies:
- 300 ml of 0.6 M (5%) methyl acrylate, CH2 = CHCOOCH3 (To prepare: mix 16 ml of methyl acrylate with 284 ml of water.)
- 5 ml of 0.1 M potassium bromate, KBrO3, (To prepare: dissolve 0.4 g KBrO3 in water and dilute to 25 ml. This will be enough for 5 demonstrations.)
- 5 ml of 0.45 M sodium hydrogen sulfite, NaHSO3
(freshly prepared) (To prepare: dissolve 0.47 g NaHSO3
in water and dilute to 10 ml. This will be enough for 2 demonstrations.)
- 300 ml 5 M NaCl (To prepare: dissolve 88 g of NaCl in water and dilute to 300 ml)
- 10 ml acetone, CH3COCH3
- 1 Liter Erlenmeyer flask
- 50 ml beaker
- 2 stirring rods
- polyethylene sheet, 15 cm x 15 cm
- gloves, plastic or rubber
General Safety Guidelines:
- Perform this demonstration in a hood and wear plastic or rubber gloves.
- Methyl acrylate has an acrid odor and is a lachrymator. The monomer is highly irritating to eyes, skin, and mucous membranes.
- If large concentrations of the vapor are inhaled, lethargy and convulsions may result.
- Since methyl acrylate and acetone are extremely flammable, this demonstration should be performed away from flames or other sources of heat.
- Perform this demonstration in a hood and wear gloves.
- Place 300 ml of 5% methyl acrylate in a 1-Liter Erlenmeyer flask.
- Add 5 ml of 0.1 M potassium bromate solution and 5 ml of 0.45 M sodium hydrogen sulfite solution.
- Swirl the flask to mix the contents thoroughly and allow the reaction to proceed for about 15 minutes, shaking the flask occasionally during this time. As the particles grow, the mixture quickly takes on a milky appearance.
- Pour the emulsion into 300 ml of 5 M sodium chloride solution to coagulate the polymer.
- Remove the polymer mass with tweezers.
- it thoroughly and knead it under water to remove salt and any unreacted monomer.
- Repeat once or twice with additional fresh water.
- Prepare a film by tearing or cutting the white poly (methyl acrylate) into small pieces and dissolving it in acetone (about 1 g/10 ml).
- Stir the mixture with a glass rod. The resulting liquid should be thick and viscous.
- Pour this liquid onto a piece of polyethylene film and allow it to dry for several hours.
- The film can be peeled from the polyethylene surface.
The washed polymer can be discarded in a waste container.
- This should be conducted only as a "DEMONSTRATION." Adequate hood facilities are generally not available for each member of the class to run this as a qualitative lab and student exposure is definitely a consideration. Special note should be made to the safety section of this lab.
- Addition polymerization can also be shown more expeditiously by the use of the following:
a) Squirt string (polystyrene) from an aerosol can. This can be purchased from a number of discount stores.
b) Aerosol foam insulation which is used in homes to seal electrical boxes. This can be purchased at most hardware stores.
c) Mounting in Minutes is a product which is used in model railroading. This two can mix may be purchased from most hobby shops.