Radical Polymerization

EXPERIMENT 2

Free Radical Polymerization

2.0

Objective:

To synthesize polystyrene via free-radical polymerization and to examine the physical properties of the polymer.

2.1

Introduction:

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:

H H

│ │ C=C

│ │ H X

Examples of the many commercially important vinyl polymers include polyethylene (X=H), polypropylene (X=CH3), polystyrene (X = C6H5), polyvinyl chloride (X=Cl), polyvinyl alcohol (X=OH) or its acetate (X=OCCH ), as well as Teflon (the monomer is 3 CF2 = CF2). Individual units may be linked in a head-to-tail, head-to-head or random fashion.

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 O-O bond rupture to give two benzoyloxy radicals.

2.2

Materials:

Styrene benzoyl peroxide xylene

2.3

Apparatus:

Test tube glass slide

2.4

Safety Precaution:

The initiator is an organic peroxide that is hazardous. It may irritates the skin, eyes and mucous membrane. It SHOULD NOT be ground or subjected to heat or shock, since explosive decomposition may occur. Do not breathe the vapors given off by the pure chemicals or the mixture involved in this experiment. Keeps all beakers covered with watch glasses.

2.5

Procedure:

a) In a fume hood, put 4.55g (5 mL) of styrene in a medium sized test tube andadd 0.2g benzoyl peroxide. [Care: do not CRUSH this solid]. b)

Place the test tube in a water bath (≈ 100ºC) for 1 ½ hours.

c) Cool the beaker, and examine the physical properties ofthe polymer (bulk). d) The residual polystyrene in the beaker can beremoved by rinsing the beaker with few ml of xylene and warming it again in a water bath untill the polymer dissolved. e) Pour a few drop of the warm xylene solution on a glass slide andlet the solvent evaporate under the hood. A thin film of polystyrene will be left on theslide. This is a techniques referred to as solvent casting that is used to make films from bulk polystyrene. f)

Discard the remaining xylene mixture into an appropriate waste container and washed the beaker with soap and water.

g) Weigh your product of polystyrene. h) Observe and compare the structure of the polystyrene (bulk) and the polystyrene film under a miscroscope

2.6

Data analysis:

a)

Describe the physical properties of the polymer obtained : (colour, clear/opaque, stiff/flexible, brittle, solid/pellets/powder, etc.).

b)

Compare the appearance of the solvent cast films with that of the bulk polystyrene.

c)

Compare the structure of the the polystyrene (bulk) and the polystyrene film under a miscroscope.

d)

2.7

Determine the molecular weight of your polystyrene.

Questions:

a) Draw the structure of benzoyl peroxide and show how benzoyl peroxide decomposes to form two radicals. b) Write the overall initiation stepof the polymerization of styrene. c) Show the propagation step of styrene via: i)

head-tail

ii)

tail-tail

iii)

head-head

d) Write the termination step of the reaction via: i) ii)

chain coupling disproportionation

iii)

chain transfer to another polymer

CHM 572 EXPERIMENT 3 REPORT SHEETS

Name :

Group Members :

1. 2. 3.

Date of Experiment :

3.0

Objective: Write the objective of the experiment:

3.1

Procedure: Summarized your procedure in a form of flow chart.

3.2

Results and Discussions: a)

Weight of polystyrene obtained = ______ g

b)

Describe the physical properties of the polystyrene.

c)

Compare the appearance of the solvent cast polystyrene film with that of the polystyrene (bulk).

d)

Compare the structure of the solvent cast polystyrene film with that of the bulk polystyrene. Paste the optical micrographs of both nylon thread and nylon film.

e) Determine the molecular weight of your polystyrene.

3.3

Conclusions: Conclude or give any comments on the results obtained from the experiment.

3.4

Questions:

a) Draw the structure of benzoyl peroxide and show how benzoyl peroxide decomposes to form two radicals.

b) Write the overall initiation stepof the polymerization of styrene.

c) Show the propagation step of styrene via: i)

head-tail

ii)

tail-tail

iii)

head-head

d) Write the termination step of the reaction via: i)

chain coupling

ii)

disproportionation

iii)

chain transfer to another polymer