OBJECTIVES

In this labortaory you will become familiar with the classifications of polymers by synthesizing and examining several of the following:

1. a linear condensation copolymer (GlyptalTM resin)
2. a branched addition polymer (polymethylmethacrylate)
3. a cross-link a natural linear polymer (cellulose)
4. a loosely cross-linked silicon-based condensation polymer (a polymethylsiloxane)
5. a cross-linked polyvinyl alcohol

INTRODUCTION

Approximately 50 % of the industrial chemists in the United States work in some area of polymer chemistry, a fact that illustrates just how important polymers are to our economy and standard of living. These polymers are essential to the production of goods ranging from toys to roofing materials. So what exactly are polymers? Polymers are substances composed of extremely large molecules termed macromolecules, with molecular masses ranging from 104 to 108 amu. The macromolecules consist of many smaller molecular units, monomers joined together through covalent bonds. The molar mass of the polymer is quoted as an average molar mass.

Both natural and synthetic polymers are ubiquitous in our lives: elastomers (polymers with elastic, rubber-like properties), plastics (the first plastic was used in 1843 to make buttons), textile fibers, resins, and adhesives. The more common polymers include acrylics, alkyds, cellulosics, epoxy resins, phenolics, polycarbonates, polyamides, polyesters, polyfluorocarbons, polyolefins, polystyrenes, silicones,and vinyl plastics, to name but a few.

Naturally occurring macromolecules are obviously derived from living things ñ wood, wool, paper, cotton, starch, silk, rubber ñ and have provided us for centuries with materials for clothing, food, and housing. Starch, glycogen, and cellulose are all polymeric versions of the monomer glucose. Proteins are macromolecules composed of monomeric units of alpha amino acids; nucleic acids are composed of subunits (nucleotides) containing a nitrogeneous base, sugar and phosphate groups. Natural rubber is a latex exudate of certain trees and composed of monomers called isoprene units. The usefulness of latex was first discovered by Lord Mackintosh in Malayasia in the last century and provided the foundation of his waterproof rainwear empire.

The temptation to improve upon nature has always been great and has rarely been resisted. When scientists linked the special properties of these substances (physical properties such as tensile strength and flexibility) to the sizes of their molecules the next logical step involved chemical modifications of naturally occurring polymers.

Synthetic celluloid derives from natural cellulose and stems from an accident that Christian Schoenbein, a chemistry professor had in 1846 - the age of plastic had begun although, initially the interest in cellulose nitrate was more for their explosive properties. When cellulose (from wood chips or fiber) is treated with a mixture of nitric acid, camphor, and alcohol, the resultant product is called CelluloidTM and bears very little resemblance to the starting material. CelluloidTM possess the ability to be molded into hard, smooth billiard balls (replacing the original, very expensive ivory balls) and into thin sheets for making movie pictures. CelluloidTM is highly flammable and today has been replaced by greatly improved synthetic polymers such as bakelite discovered in 1907 by the Belgian-American Chemist, Leo H. Baekeland.

When cellulose is treated with sodium hydroxide and carbon disulfide (CS2), cellulose xanthate is formed. A viscous (thick) solution of cellulose.xanthate, forced

through fine holes into dilute sulfuric acid, regenerates the cellulose as fine, continuous cylindrical threads called rayon. If the solution is forced instead through a narrow slit, a thin transparent film or sheet is obtained called cellophane.

THERMOPLASTS VERSUS THERMOSETTING

Polymers generally are classified into two broad groups in accordance with their behavior upon heating. Polymers that can be repeatedly melted and solidified (without damage) are said to be thermoplasts; those that solidify once but will not melt again with damage are said to be thermosets. Technically, though only thermoplasts are true plastics, even though the term "plastic" is commonly applied to all synthetic polymers.

The thermoplastic substance contains long, thin molecules which form tangled chains and is rigid at lower temperatures but gradually softens upon the application of heat and after it passes a characteristic temperature known as its glass transition temperature (Tg). Below Tg, the substance is brittle, having the characteristic properties of a glass; above Tg, the substance becomes flexible and soft. Chewing gum is a thermoplast that becomes extremely brittle when the outside temperatures drop below its glass transition temperature - this is an useful property to use in order to remove chewing gum from your clothes. Once warmed above Tg, however, the gum quickly softens and regains its flexibility. Some thermoplasts, such as polystyrene, melt before reaching their glass transition temperatures and remain rigid materials up to their melting points. Thermoplastic polymers are used frequently for injection molding of such items as food storage containers and toys that are not exposed to high temperatures. Additionally, thermoplastic polymers can be molded, pressed and extruded.

Thermosetting substances contain large, cross-linked molecules and are also rigid at lower temperatures, undergo irreversible chemical and physical changes (including decomposition) upon heating. Such substances remain solids at higher temperatures than do thermoplastic materials, and they do not melt. Thermosets are often employed in high-temperature environments, such as for electrical insulation in electric motors and gasoline engines.

Thermoplastic polymers are composed of small monomers covalently bonded end-to-end in a long chain but without covalent bonds joining adjacent chains. Such macromolecules constitute the linear polymers. Shorter side groups attached to the long chains at periodic intervals, cause the polymers to be termed branched polymers. The chains, having average molecular masses up to one million amu, may be independent of each other (as in polyethylene) or loosely lined through hydrogen bonding (as in nylon). If the long chains are linked by covalent bonds, the polymeric network becomes two- or three-dimensional, resulting in an infusible (nonmelting) and insoluble material. Such macromolecules make up the cross-linked polymers, and are found in thermosetting materials. Polymers of all types in which the long chains are produced by joining two or more different kinds of monomers are termed copolymers.

The process by which the polymerization reaction occurs permits classification of polymers into two categories: addition and condensation polymers. Addition polymers

are those in which the monomers join at unsaturated carbon atoms; several are summarized in theTable. During polymerisation, the double bonds between the pairs of carbon atoms 'open up' and the carbon atoms of separate ethylene molecules join together to form a molecule of polyethylene. The first polymersiation of ethylene was accomplished in 1933 by the use of very high pressure (1000 atm) and oxygen as a catalyst. Nowadays, with the development of the use of powerful catalysts, addition can occur at atmospheric pressure. Polymethyl methacrylate, also called LuciteTM or PlexiglasTM (originally developed as an unbreakable substitute for glass in airplane canopies), belongs to this group of addition polymers. The polymerization is initiated by a variety of substances (such as benzoyl peroxide) that can form a free radical with the unsaturated carbon atom. The resulting addition polymer is described as a branched polymer.

The second way to make a polymer is by condensation polymerisation. In this process, two compounds with reactive atoms at the end of their molecules react together, usually with the release of a small molecular unit such as water or hydrogen chloride. The presence of two or more functional groups in the monomer usually leads to the production of a cross-linked polymer. GlyptalTM resin, formed by the reaction between phthalic acid and glycerol, is a condensation copolymer, a copolymer since 2 different types of monomers combine to form the chain.

Monomers are linked through ester ( -C(=O)-O-CH₂-) bonds; the resin is termed a polyester. The more reactive phthalic anhydride is often used in place of phthalic acid in this reaction.

Paper is composed of naturally occurring cellulose, the polymeric structural material of plants. The cellulose chains are composed of linear glucose units. Parchment paper is made by cross-linking the linear cellulose polymer to form a sheet-like structure. Adjacent chains are cross-linked by ether (-CH₂-O-CH₂-) bonds resulting from dehydration of the alcohol groups (through the use of sulfuric acid as the dehydrating agent). Resulting reactions that form the macromolecules required for polymerization by either addition or condensation processes must be capable of proceeding indefinitely.

By far the most interesting uses of polymers involves replacement of diseased, worn out, or missing parts of the human body including flexible replacements for major blood vessels, replacement valves for hearts, temporary skin, and artificial joints. Artificial ball-and-socket pelvic (hip) joints made of steel (ball) and plastic (socket) are installed at the rate of 25,000 per year. People with crippling arthritis, debilitating coronary and circulatory problems, and burns all benefit from the development of biomedical polymers. Preventive and cosmetic dentistry, as well, benefit from polymers used to seal porous teeth and reconstruct missing enamel.

Linear silicones, or polysiloxanes, are comparatively new polymers based upon silicon-oxygen-silicon linkages. These polymers may be cross-linked to various degrees by additional -Si-O-Si- bonding between adjacent chains:

The R group is generally a hydrocarbon group such as -CH₃ (methyl), -CH₂CH₃ (ethyl), or -C₆H₅ (phenyl). Silicones are stable at much higher temperatures than carbon-based polymers, yet they remain flexible even at exceedingly low temperatures. Among such silicones is Silly PuttyTM , the cross-linked polymerization product of dimethyldichlorosilane, (CH₃)₂Si(OH)₂ with the release of hydrogen chloride (HCl):

The unstable dimethyldihydroxysilane condenses rapidly to form a low molecular mass linear polymethylsiloxane polymer with the elimination of water:

 

This material is an oily liquid. Additional heating continues the polymerization to form longer chains. The addition of boron trioxide allows three such chains to be joined to form Silly PuttyTM :

This medium molecular mass polymer has physical properties between those of a fluid and an elastomer. Although it is resistant to rapid deformation, it flows easily with slowly applied stress.

EXPERIMENTAL PROCEDURE

Your TA will assign you 3 out of the 5 sections to perform.

PART A Cross-Linked Condensation Polymer

CAUTION WEAR EYE PROTECTION

CAUTION Steps 1-4 result in corrosive HCl fumes being given off, and should be carried out in a hood.

1. Fill a 25 mL buret with deionized water.

CAUTION The (CH₃)₂ SiCl₂ solution used in step 2 is corrosive. Avoid contact with skin or clothing.

2. Transfer about 60 mL of (CH₃)₂ SiCl₂ (dimethyldichlorosilane) petroleum-ether solution to a clean and dry 250 mL Erlenmeyer flask.

CAUTION A too rapid addition of water during step 3 results in a too rapid reaction, the contents spurting out of the flask.

3. Add dropwise about 40 mL deionized water from the buret to the Erlenmeyer

flask. Mix the contents of the flask during this addition by swirling the flask,

using a glass stirring rod, or a magnetic stirrer. The hydrogen chloride will

bubble out of solution at the interface of the upper organic and lower

aqueous levels.

4. When the reaction if complete (bubbling diminishes), transfer the contents

of the beaker into a 125 mL separatory funnel and place a stopper in the

top of the funnel. Note the two layers.

5. Open the stopcock of the separatory funnel and drain off the unwanted lower

aqueous layer. Close the stopcock immediately after a few drops of the upper

organic layer have drained out.

6. Transfer about 50 mL of saturated NaHCO₃ (sodium hydrogen carbonate)

solution.

7. Slowly add about 20 mL of the saturated NaHCO₃ solution to the organic

Layer remaining in the separatory funnel. Shake the funnel, vent the funnel

To release excess pressure due to the formation of CO₂ and allow the layers to separate. Drain off the aqueous layer into a 100 mL beaker.

8. Repeat with a second 20 mL portion of NaHCO₃ solution and test the aqueous

layer with blue litmus paper. If the layer is still acidic, repeat the procedure

until the aqueous layer no longer tests acidic.

9. Add 10 mL deionized water to the remaining organic layer, shake, and let the layers separate. Drain off the entire aqueous layer. Wash the organic layer a second time with a fresh 10 mL portion of water.

10. Transfer the remaining organic layer to a clean and dry 125 mL Erlenmeyer flask.

11. Add 1 to 2 g anhydrous Na₂SO₄ (sodium sulfate), shake the contents, cover, and allow the flask to stand for about 30 minutes.

12. Decant the solution into a 15 mL beaker, being careful to leave the hydrated Na₂SO₄ behind.

CAUTION The petroleum ether evaporated in step 13 is a volatile and flammable liquid. Do not use an open flame to effect evaporation.

13. Place the 150 mL beaker into a 400 mL beaker half-filled with hot water obtained from the hot water-water tap, and evaporate the petroleum ether. You may have to add additional hot water.

14. Add about 1.5 b B₂O₃ (boron trioxide) and stir well. Note and record the appearance.

15. Use a spatula to remove the sticky material from the beaker, and then mold the substance into a ball. See if it bounces.

16. Place the ball in a labeled 50 mL beaker, and then heat it in a drying oven (200 ° C) for about 2 hours.

Part B GlyptalTM Resin (linear condensation copolymer)

A small aluminum dish is used to mold the polymer. A coin may be placed on the bottom of the mold if you wish to make a souvenir of this experiment.

1. In a large test tube, mix 4.0 mL glycerol, 0.5 g sodium acetate, and 10.0 g phthalic anhydride.

CAUTION Excessive heating during step 2 can cause the hot contents to spurt out. If the hot liquid contacts the skin, severe burns result.

2. Carefully heat the mixture with a low flame, starting at the top of the contents and moving down toward the bottom as the mixture melts.

3. Continue heating until the melt appears to boil, and then continue heating for 5 minutes. Sufficient heating is required to produce a nonsticky product

but excessive heating turns the product into a brittle amber material.

4. If desired, place a clean and dry coin into the aluminum dish used for a mold.

CAUTION Avoid skin contact with the hot, sticky liquid transferred in step 5, as contact causes severe burns.

5. Carefully pour the hot liquid into the mold.

6. Allow the material to cool until the end of the laboratory period. (Meanwhile, proceed with Part C.)

7. After the material has cooled, peel the mold from the cooled polymer and describe its appearance.

PART C Polymethylmethacrylate (addition polymer)

1. Half-fill a 400 mL beaker with deionized water and set it aside.

2. Transfer 5.0 mL water into a clean 6-inch (15 cm) test tube, and make a 5.0 mL calibration mark on the tube with a piece of tape or a glass-marking pencil.

3. Drain and thoroughly dry the test tube.

4. Carefully fill the dry test tube up to the 5.0 mL calibration mark with methylmethacrylate.

CAUTION Although the benzoyl peroxide in step 5 is used dermatologically for the treatment of acne, it can explode when heated. Do not exceed the stated amount of benzoyl peroxide. Wear eye protection!

5. Add about 20 mg benzoyl peroxide to the test tube using a small spatula. This amount is approximately equal in volume to that of a pea.

6. Use a ring stand and clamp the test tube in a boiling water bath

7. Stir the contents of the test tube with a clean, dry glass rod until the benzoyl peroxide is dissolved.

CAUTION Avoid breathing the fumes generated in the test tube during step 8. The monomeric methyl methacrylate is toxic.

8. Continue to heat the test tube in the water bath; do not stir until the contents thicken and stop foaming.

9. Remove the test tube from the water bath and slowly pour the thick, warm liquid into the water in the 400 mL beaker.

10. Examine the film that forms on the surface of the water, and describe its appearance.

PART D Parchment Paper (cross-linked condensation polymer)

CAUTION The concentrated sulfuric acid used in step 1 is extremely corrosive. If spilled on skin or clothing, wash the area immediately with large amounts of water.

1. Carefully pour about 30 mL 12 M H₂SO₄ (sulfuric acid) into a 400 mL beaker.

2. Fill a 250 mL beaker with deionized water.

3. Pour about 20 mL 6 M NH₃ into a second 250 mL beaker.

4. Using tongs, immerse a piece of filter paper (cellulose) in the sulfuric acid solution for 15 to 20 second.

5. Remove the filter paper from the sulfuric acid and dip it into the beaker of deionized water to rinse off the excess acid.

6. Dip the paper into the beaker containing the 6 M NH₃.

7. Rinse the filter paper under running water and set it on a watch glass to dry completely.

8. When dry, compare the texture, appearance, and strength of the treated filter paper with that of an untreated piece. Write on both pieces with a ballpoint pen and note any differences.

PART E Cross-linked polyvinyl alcohol

CAUTION The poly(vinyl alcohol) is a fine dust. Avoid inhaling it.

1. Measure 50 mL of poly(vinyl alcohol) solution into a paper cup or small beaker and observe its properties. Vinyl alcohol does not exist. Poly(vinyl alcohol) is prepared by first forming poly(vinyl acetate) from vinyl acetate following by hydrolysis to the alcohol.

2. Measure 7-8 mL of sodium tetraborate solution into another cup or beaker and observe its properties.

3. Pour sodium tetraborate into the poly(vinyl alcohol) solution while stirring vigorously with a wooden stick. The borate forms a complex structure called tetraborate, B₄O₅(OH)₄^2- , that links the poly(vinyl alcohol) polymer strands together by hydrogen bonds.

4. Examine the properties of the cross-linked polymer. See how far the polymer will flow from your hand. Is the flowing endothermic or exothermic?