Experiment 5. Synthetic Polymers. References: Brown & Foote, Chapters 24 INTRODUCTION: A polymer (Greek: polys + meros = many parts) is a giant or macromolecule made up of repeating structural units. The reaction that unites small molecules called monomers (Greek: mono = one) into a polymer is called polymerization. Among the natural polymers important to life processes are starch and cellulose (monomer is glucose), proteins (monomers are amino acids), and nucleic acids (monomers are nucleotides, the phosphate ester of certain N-glycosides). In this experiment, the preparation of several polymers in each of two main classes will be illustrated. BACKGROUND: Synthetic polymers can be categorized in several ways, one of which depends on their method of synthesis. Two main classes of polymers are addition polymers and condensatiom polymers. PART A: Addition Polymers Chain growth or addition polymers are formed by a reaction in which monomer units add to another with no other by-products. The most common type are vinyl polymers: Individual units may be linked in a head-to-tail, head-to-head or random fashion. Examples of the many commercially important vinyl polymers include polyethylene (X=H), polypropylene (X=CH 3 ), polystyrene (X = C 6 H 5 ), polyvinyl chloride (X=Cl), polyvinyl alcohol (X=OH) or its acetate (X=OCCH 3 ), as well as Teflon (the monomer is CF 2 = CF 2 ), and Lucite or Plexiglas (the monomer is CH 2 = C(CH 3 )CO 2 CH 3 ). Other functional groups, such as dienes, epoxides and aldehydes, can also self-unite to give addition polymers. Copolymers are prepared from the polymerization of a mixture of two or more monomers. Since the ratios of the monomers can be widely varied, copolymers for specific uses can be tailor-made. A1. Polystyrene Polystyrene is a thermoplastic polymer, most commonly known for its use in Styrofoam. Styrene is usually polymerized by a free-radical chain mechanism. The reaction is initiated by a free radical source. In this experiment the initiator is benzoyl peroxide, a relatively unstable molecule which decomposes at 80-90ºC with 0-0 bond rupture to give two benzoyloxy radicals: Page 1
The initiator radicals, symbolized by R. add to the C=C bond of styrene monomer to produce a new, benzyl-type free radical, as shown below: This radical adds to another styrene monomer; the process repeats itself in what is called the propagation step, during which the polymer chain continues to grow: As many as 5000 monomer units may add to one another before the chain is terminated. Thus, the initiator radical contributes only a very small fraction (perhaps 0.02%) to the molecular weight of the polymer. Termination may occur by the combination of two radicals (either both polymer radicals, or one polymer radical and one initiator radical) by the abstraction of a hydrogen atom, or in other ways. The overall equation for the polymerization is: A2. Methyl Methacrylate Polymer The clear, colourless plastic Lucite or Plexiglas is made by polymerization of the vinyl ester methyl methacrylate: In this experiment, benzoyl peroxide will be used to initiate a free radical chain polymerization of the monomer, similiar to the polystyrene synthesis. A trace of a weak base is added to neutralize any acid present. A control, kept at room temperature, will be compared with a sample polymerized at boiling water temperature. Page 2
PART B: Condensation Polymers Step Growth or Condensation polymers are formed by reaction between molecules with two or more functional groups (such as glycols, diamines, and dicarboxylic acids), with each new bond created in a separate step. This method gives long chains joined by many ester or amide links along with the elimination of some small molecule such as water, ammonia, or hydrogen chloride. Examples include Nylon (a polyamide), Dacron (a polyester), Bakelite, and polyurethane foams. B1. Bakelite Bakelite is the oldest totally synthetic polymer invented by Leo Baekeland in 1907. Phenol and formaldehyde react, in the presence of acid or base, to form a three-dimensional polymer -- Bakelite -- suitable for making many molded articles. The reaction sequence for the acid catalyzed reaction is: Under carefully controlled conditions, o- or p-hydroxybenzyl alcohol may actually be isolate. The next step involves protonation of the alcohol, formation of a benxyl cation, and an electrophilic aromatic substitution: In this manner, phenol molecules become linked to one another at ortho or para positions by methylene groups; Page 3
B2. Nylon 6-6 Under appropriate conditions a variety of dicarboxylic acids will react with diamines to give polyamides and water. These polymers can hydrogen-bond with each other to give strong fibres. PRE-LAB PREPARATION: 1. Draw the structure of benzoyl peroxide and show how benzoyl peroxide decomposes to form two radicals (use curly arrows, etc.). 2. Two different radicals could be produced when an initiator radical adds to styrene. Show the structures of two radicals. Which one is preferred, and why? 3. Draw the structure of poly(methyl methacrylate). 4. Draw the structures of 1,6-hexanediamine, adipoyl chloride and nylon 6-6. EXPERIMENTAL PROCEDURE: PART A: Addition Polymers A1. Polystyrene a) In a fume hood, to 4.55g (5 ml) of styrene in a medium sized test tube, 0.2g benzoyl peroxide. [Care: do not CRUSH this solid]. b) Write your name on a little (9mm x 20mm) piece of paper and put it into the test tube. c) Place the test tube in a beaker of boiling water (on a hotplate) for over 1 hour. d) Cool the tube, and examine the polymer. You may have to break the test tube, but first wrap it in a paper towel (have your TA show you how this is done). Be careful with the broken glass. A2. Methyl Methacrylate Polymer a) In a fume hood, to 5 ml of freshly distilled methyl methacrylate in a medium test tube, add 1 drop of N,N-dimethylaniline and 0.08g of benzoyl peroxide. b) Write your name on a little (9mm x 20mm) piece of paper and put it into the test tube. c) Place the test tube in a beaker of boiling water (on a hotplate) for about 1 hour. d) Cool the tube, and describe the polymer. You may have to break the test tube, but first wrap it in a paper towel (have your TA show you how this is done). Be careful with the broken glass. PART B: Condensation Polymers B1. Bakelite a) In a fume hood, place 1.2 g of phenol, 3.5 ml of glacial acetic acid, and 1.5 ml of 37-40% aqueous formaldehyde solution in a test tube. b) Warm the tube slightly, with shaking, to melt the phenol and obtain a homogeneous solution. c) Cool the test tube in an icebath and then place the test tube in the fumehood. Add 3.5 ml of conc. HCl to the test tube while still in the fumehood. Shake until you see the solid polymer starting to form and set aside to finish. d) Cool the tube, then dislodge and examine the polymer. Page 4
B2. Nylon 6-6 NOTE: For this polymer, measure everything directly into your beaker. Don t use your graduated cylinder, as the polymer will form in your graduated cylinder and is difficult to remove. Use the graduation on your beaker. a) Pour approx. 10 ml of 5% aqueous solution of 1,6-hexanediamine into a 100 ml beaker. Add 20 drops of 10% sodium hydroxide solution. b) Tilt the beaker slightly, and carefully pour down the wall approx. 10 ml of a 5% solution of adipoyl chloride in hexane, trying to avoid mixing or turbulence. c) A polymer film will form at the interface. Grasp this with a piece of hooked metal or pair of tongs. Fasten it to a large test tube or narrow beaker, and slowly rotate it, to draw out a polymer fibre. d) Finally, rinse your fibre with water and dry on a piece of paper towel. f) Obtain and label an IR spectrum of your polymer. g) Once you have made all four polymers, show them to your TA. PART C: Check-Out All students must check out and return locker key. Any missing glassware/equipment can be obtained from Rm. 308. Page 5