q & a_polimer

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Q & A : Polimer

Semester Ganjil

November 2014

8/10/2019 Q & A_polimer


1. Compounds like styrene, PhCH=CH 2, are unstable as pure liquids. Such olefins tendto explode, even in the absence of oxygen. Particles of a sticky solid, which is solublein benzene, can be found in debris resulting from the explosion.

A. Explain the origin of the explosive decomposition of styrene and of the heat whichis given off. You may find it useful to refer to the table of bond energies below.

Bond Bond dissociation energy (kcal/mol)

Olefin C-H 102.7

Alkane C-H 98.8

C-C 82.6

C=C 145.8

H-H 104

B. Write a plausible structure of the sticky solid which results from styrenedecomposition. Very briefly describe the structural possibilities .

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A. The olefin polymerization breaks a pi bond (the energy of which is 145.8 - 82.6Kcal/mol) and forms a single bond (the energy of which is 82.6 Kcal/mol) for eachreacting mol of styrene. The net enthalpy change of this reaction is computed bysubtracting the enthalpy of bonds formed from the enthalpy of bonds destroyed,because you have to put in energy to break bonds and energy is released when bondsform. Thus DH = (145.8 - 82.6 - 82.6)Kcal/mol = 19.4 Kcal/mol, which means that thereaction is highly exothermic; hence the explosion.

B. Structural possibilities: atactic, syndiotactic, isotactic

Structure: (-CH 2-CHPh-CH2-CHPh-CH2-CHPh-) Its stickiness is your clue that it’s atactic.

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2. Discuss the structural features of a polymer which would make it a usefulelastomer; specifically describe the underlying thermodynamic property whichcauses a stretched elastomer to return to its unstretched state. Mention theeffect of molecular stereoisomers. Discuss the role of inter (between) chaininteractions and mention those factors which determine the temperature rangeover which the elastomer is useful –for example what happens on a molecularlevel at low temperatures. Illustrate your discussion with an actual example,using molecular formulas.

• What makes a useful elastomer: A useful elastomer has enough crosslinking (via vulcanization, hydrogenbonding, dipole-dipole interaction,etc.) between polymer chains toprovide a restoring force, but not so much crosslinking that it iscrystalline. A useful elastomer also has a low Tg (glass transitiontemperature: the temperature below which it goes into the glass state

and becomes brittle). • Thermodynamics: The underlying thermodynamic property which provides the restoringforce is entropy DS.DS > 0 if you go from stretched to unstretched

because there are more conformations available in the relaxes state, sothere is more disorder.

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• Stereoisomerism: • stereoregular/isotactic: higher intermolecular interactions, betterpacking, more crystalline. • stereoirregular/atactic: lower intermolecular interactions, lesscrystalline, more "gooey" and inefficiently packed. • double bonds: cis bonds tend to make for less efficient packingforces. The presence of such bonds cause the polymer to be less

crystalline, more rubbery. On the other hand, trans bonds enforce amore rodlike structure. • Interchain forces: The stronger the forces (covalent crosslinks, hydrogen bonds, dipole-dipole interactions, etc) the more crystalline and rigid. Polymers withvery little interchain interaction are viscous and gooey.

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• empera ure: Polymers have a glass transition temperature Tg, below which thepolymer is hard and crystalline. Above Tg, thermal energy overcomessome intermolecular forces and the polymer is either elastomeric or

gooey. Some polymers are so rigid that heating leads todecomposition before approaching Tg. Useful Elastomers: •Vulcanized rubber is a great elastomer. 0% vulcanized is gooey, 2%vulcanized is elastomeric, and 5% vulcanized is hard. • Non-useful elastomers: Nylon has too many hydrogen bonds, so its interchain forces are toostrong. Any polymer with a lot of crosslinking fails to be a good elastomer.

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3. Write in turn the stereochemical structures of isotactic, syndiotactic, and atacticpolypropylene. Indicate which form is the most stable in solution (has the lowestfree energy) and which would have the lowest melting point. Briefly explain youranswer.

On the far left is isotactic polypropylene. Notice that all of the methyl groups are onthe same side of the chain. This is the most ordered and therefore the mostcrystalline of the three polymers. It will have the highest melting point, which alsoindicates that it will have the lowest DG of solvation because it's stable to beginwith.

The atactic extreme will have the lowest melting point and highest DG of solvation.

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Give an example of any useful elastomer

Polyisoprene (natural rubber) once vulcanized (cross-linked a little bit) isa great elastomer.

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Below are structures for stereoisomers of polystyrene.

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A. Draw a structure for the monomer from which polystyrene isprepared. B. Write down the tacticity of each of the numbered polymers: C. Which structure do you expect to exhibit the highest meltingpoint and which would have the lowest melting point? D. What physical methods might you use to distinguish these

stereoisomers? Suggest two and explain. Checking the melting point and density will verify which one isisotactic; the isotactic one will have both the highest mp and thehighest density. E. Under what conditions does polystyrene exhibit elasticproperties? Why is polystyrene not used as an elastomer?

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A. Draw a structure for the monomer from which polystyrene isprepared.

B. Write down the tacticity of each of the numbered polymers:

#1 is syndiotactic, #2 is isotactic, and #3 is actactic

C. Which structure do you expect to exhibit the highest melting point

and which would have the lowest melting point?

The isotactic polymer will have the highest melting point due to itspacking efficiency and high interchain forces.

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D. What physical methods might you use to distinguish these stereoisomers? Suggesttwo and explain.

X-ray crystallography can be used to elucidate the structures and look for differentdegrees of crystallinity. 1H or 13C NMR can be used to assign tacticity. Checking the melting point and density will verify which one is isotactic; the isotacticone will have both the highest mp and the highest density.

E. Under what conditions does polystyrene exhibit elastic properties? Why ispolystyrene not used as an elastomer?

Polystyrene exhibits elastic properties above its glass transition temperature (Tg isabout 100 oC.) It is not used as an elastomer because its glass transition temperature istoo high above room temperature to be practical.

Polystyrene is stretchy above 100 0C but brittle below that temperature. Why?

Polystyrene exhibits elastomeric properties above room temperature. Morespecifically, the glass transition temperature of polystyrene is above 100 oC, so it

does not exhibit elastomeric properties until this temperature is reached.

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A. What is the monomer from which polyisobutylene is made?

B. Draw a structure for polyisobutylene.

C. What is vulcanization? Why can’t polyisobutylene be vulcanized? Vulcanization is a process in which sulfur forms sulfide or disulfide bridgesbetween polymer chains by reacting with double bonds. Polyisobutylene has no

double bonds, so it cannot be vulcanized or hydrogenated.

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Consider a polymer X with empirical formula (CH)n. It is a gummy polymer thatbecomes a more brittle polymer (Y) when hydrogenated. Deduce the structureof X and Y from these data.

X's empirical formula leads us to conclude that each carbon is bonded to one Hand 2 other carbons. Because X can be hydrogenated, it must have double bonds.Each carbon in X, therefore, has one C=C bond, one C-C bond, and one C-H bond.

X can have double bonds that are cis or trans . Because X is gummy, it can be deducedthat it's the cis isomer, which is likely to form coils as opposed to the rigid rods formedby the trans isomer.

Y is the hydrogenated form of X. Each carbon is bonded to 2 hydrogens and twocarbons.

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