Plastics Lab Polyamide (Nylon) Reaction of a dicarboxylic acid (or a derivative) with a diamine forms a linear polyamide through a condensation reaction. The commercial product is called nylon 6–6 (each monomer has six carbons) and is made from adipic acid and hexamethylenediamine. In this lab the acid chloride will be used instead of adipic acid. O
O
Cl C
CH2CH2CH2CH2C
H +
Cl
H N
H CH2CH2CH2CH2CH2CH2
H
Hexamethylenediamine
Adipoyl Chloride
O C
N
CH2CH2CH2CH2
O
H
H
C
NCH2CH2CH2CH2CH2CH2
N
Nylon 6-6
The acid chloride is dissolved in cyclohexane and this is added carefully to the hexamethylenediamine dissolved in water. These liquids are immiscible and two layers will form. At the point of contact between the layers, the nylon will form. The nylon can be pulled out continuously to form a long strand of nylon. Procedure: Pour 20 ml of a 5% aqueous solution of hexamethylenediamine (1,6–hexanediamine) into a 100 ml beaker. Add 20 drops of 20% NaOH solution. Carefully add 20 ml of a 5% solution of adipoyl chloride in cyclohexane to the solution by pouring it down the wall of the slightly tilted beaker. Two layers will form, and there will be an immediate formation of a polymer film at the liquid-liquid interface. Using a long-nosed tweezers grab the mass at the center, and slowly raise the strand so that the polyamide forms continuously, producing a string which can be pulled out for many feet. The strand may break if you pull it out too fast. Rinse the strand with water, and lay it on a paper towel to dry. Stir the remainder of the contents of the beaker to make a mass of polymer. Pour off the liquid and wash the polymer with water. Do not put the polymer residue in the sink! Put the residue in the trash.
Polystyrene An addition polymer, polystyrene, is made in this lab. The reaction may be made by free radical, cationic, or anionic catalysts (free radical is most common). In the lab, polystyrene is made by a free radical-catalyzed polymerization.
2 The reaction is initiated by a free radical source. The initiator used will be benzoyl peroxide. Benzoyl peroxide is somewhat unstable and from 80ºC to 90ºC, decomposes to a benzoyl radical. O C
O O
O
O heat
C
2
Benzoyl Peroxide
C
O
Benzoyl Radical
If an unsaturated monomer is present, the catalyst radical adds to it, initiating a chain reaction by producing a new free-radical. In the following reaction, R stands for the catalyst radical, and the reaction with styrene is as follows. R + CH2
CH
R
CH2
CH
The chain continues to grow: R
CH2
CH
+
CH2
CH
R
CH2
CH
CH2
CH
etc.
Procedure: Place 25 to 30 ml of styrene monomer in a 150 ml beaker and add 0.7 grams of benzoyl peroxide. Heat the mixture on a hot plate (in the hood) until the mixture develops a yellow color. When the color disappears and bubbles begin to appear, take the breaker of styrene off the hot plate (the reaction is exothermic). After the reaction subsides, put the beaker of styrene back on the hot plate and continue heating until the liquid becomes very syrupy. With a stirring rod, pull out a long filament of material from the beaker. If this filament can be cleanly snapped after a few seconds of cooling, the polystyrene is ready to be poured. If the filament does not break, continue heating the mixture and repeat the above process until the filament breaks easily. Pour the syrupy liquid on a watch glass (put some waxed weighing paper on it first to ease its removal). After cooling, the polystyrene can be separated from the weighing paper by peeling the paper off.
3
Plexiglas In this lab, you will make Plexiglas (Lucite) by the free radical addition polymerization of methyl methacrylate with benzoyl peroxide. CH3 n
C
CH2
CH3 CH2
benzoyl peroxide
C
O
O
CH3
CH3
CH3
C
CH2
C
CH2
C
C
O
C
O
C
O
O
CH3
O
CH3
O
CH3
etc.
Benzoyl peroxide is the initiator for the reaction. With mild heating from 80 to 90ºC, the weak oxygen-oxygen bond breaks to form a pair of benzoyl free radicals. These radicals may either initiate the reaction or may decompose to give CO2 and phenyl free radicals, which may also start the reaction. C
O
O
O
C
heat
C
2
O
benzoyl peroxide
O
2
O
benzoyl free radical
+
2 CO2
phenyl free radical
Polymerization begins by the initiating free radical adding to the double bond of the alkene to form a bond and the generation of a new free radical. This new radical then adds to another molecule of alkene to generate a still larger radical, and so on. Besides free radicals, the reaction can also be initiated by acids, bases, metals, metal complexes, light, and heat. The methyl methacrylate prepared commercially comes from acetone as shown below. CH3 CH3
C
HCN
CH3 CH3
C
C
O
OH
acetone
acetone cyanohydrin
CH3OH H2SO4
CH3 O CH2
C
C
O
CH3
The first reaction involves nucleophilic addition of HCN to acetone to form acetone cyanohydrin. The second reaction involves hydrolysis of the cyano group to yield a carboxylic acid, esterification by the alcohol, and dehydration. The Plexiglas has outstanding stability, excellent optical properties, and a unbreakable nature. It has replaced glass for use in eyeglasses and prisms. Its only drawback it its tendency to be more easily scratched. It is also used as the plastic in screwdriver handles and combs, and is a
4 major ingredient in paints. It can be drilled and sawed and weathers well. Procedure: Place about 10 ml of methyl methacrylate in a 6 inch test tube and add 10 to 20 mg of benzoyl peroxide. If you want, place a small inert object like a coin or a bug in the test tube also. Loosely stopper the test tube and heat in a boiling water bath (in the hood, please!) for 20 minutes or until the liquid becomes quite thick. Let the contents in the test tube solidify, wrap the test tube in paper towel and break it with a hammer or iron ring to examine the material.
Thiokol Rubber The story of rubber is an old one that goes back at least 2600 years to ancient Egypt where is was used to make rubber balls for games. This use remained unchanged for hundreds of years until 1770 when Joseph Priestly found that rubber could also be used to erase pencil mistakes. People during this time also discovered the waterproofing qualities of rubber and began to use it for manufacturing and covering shoes, boots, raincoats, mailbags, and life preservers. This type of natural rubber however, has a serious flaw. It is only useable at mild temperatures. It becomes hard and brittle in winter and sticky and messy in the summer. A few people were convinced that the negative features of natural rubber could be overcome. Charles Goodyear found the necessary breakthrough in 1839 by accidentally dropping a large piece of rubber mixed with sulfur onto a hot stove. When the mixture was scraped off, instead of being sticky and gummy, it had charred like leather. Goodyear's discovery later became known as "vulcanization". We know today that natural rubber is actually a polymer, consisting of many isoprene units linked together in the cis-configuration. CH3 CH2
C
CH
CH2
Isoprene
CH2 C CH3
CH2 C
CH3 CH2
H
H C
C CH2
CH2
CH2 C
CH3
C H
Natural rubber - all cis -configuration
Vulcanization permits the formation of sulfur bridges between different chains of the rubber molecules at both the double and allylic
5 hydrogens. These bridges or cross-links make the rubber much harder and stronger, and able to withstand heat and cold. CH3 CH2
C
CH3 CH
CH2CH2
C
CH
CH2 sulfur, heat
CH3 CH2
C
CH
CH2CH2
CH3 CH S CH
C
catalysts
CH3 C
CH
CH2
CH
CH2
CH3 CH
CH2CH2
C H
CH3 C
S
CH3 CH
CH2CH2
C
CH
CH2
H
With vulcanized rubber, more dependable products can be made. Later developments allowed the rubber to have all the desirable qualities of being long wearing, elastic, pliable, water resistant, air-tight, and shock absorbing. During the last 50 years, a number of synthetic rubber compounds have been developed including: Neoprene, Butyl rubber, Styrene-butadiene (SBR), Buna N nitrile rubber, and silicone rubbers. In this lab, you will make Thiokol rubber by the nucleophilic aliphatic substitution reaction of sodium polysulfide with ethylene dichloride. S Na2S4 Sodium polysulfide
+
CH2
CH2
Cl
Cl
Ethylene dichloride
CH2CH2
S
S S
CH2CH2
S
S
S S
Thiokol rubber Polyethylene polysulfide
The sodium polysulfide used in the lab is made from the reaction of dilute NaOH with elemental powdered sulfur. Procedure: Place 40 ml of 10% NaOH solution into a 400 ml beaker and dilute to 100 with water. Heat this solution to boiling and then add 8 grams of powdered sulfur. Stir until all of the sulfur has dissolved. As the polysulfide is formed, the solution will change from a light yellow to a dark brown. Cool the solution to 70ºC and add 20 ml of ethylene dichloride (1,2–dichloroethane) and 5 to 10 drops of a liquid dishwashing
6 detergent to promote mixing of the two insoluble layers. Stir the mixture well, maintaining the temperature at 70ºC until a color change has occurred and a large, spongy lump of rubber has formed. After the reaction is finished, remove the white to yellow product from the solution, wash it with water, and squeeze between paper towels to dry. Test the rubber like properties of the Thiokol produced by stretching, squeezing, and bouncing it (when dry).
7 Name Date
Report for Experiment: Plastics 1.
Nylon 6–8 has the formula H N
(CH2)6
H
O
N
C
O (CH2)6
C n
In the names, Nylon 6–6 and Nylon 6–8, what do the numbers refer to?
2.
CCl2 , is polymerized with vinyl Vinylidene chloride, CH2 CHCl , to make Saran wrap. Draw at least two chloride, CH2 units of the copolymer.
3.
Kel-F is an addition polymer with the following partial structure. Draw the monomer used to make it. F
F
F
F
F
F
C
C
C
C
C
C
F
Cl
F
Cl
F
Cl
