1.* (1997 F 1) This problem involves stereochemistry, catalysis, and polymer chemistry. Consider the stereoisomers of two coordination compounds shown below. These are "precatalysts"; when each complex is activated by converting one chloride into a methyl group and the other chloride ion is replaced by a weakly coordinating anion, the resulting complex will catalyze the polymerization of propene. The two different catalysts produce very different types of polypropylene. One catalyst produces high-melting polypropylene that can be made into strong fibers which are used to manufacture carpets. The other catalyst produces a sticky, low melting polypropylene which has no important application. Explain the different behavior of the two catalysts using stereochemical arguments and illustrate the structures of the two types (high-melting and low-melting) of polypropylene. Indicate which catalyst leads to which polymer and why they behave differently. Note: for the precatalyst on the right the CH2CH2 bridge connects the rings behind the Zr atom and is difficult to visualize.
2.* (1997 3 1) Compounds like styrene, PhCH=CH2, are unstable as pure liquids. Such olefins tend to explode, even in the absence of oxygen. Particles of a sticky solid, which is soluble in benzene, can be found in debris resulting from the explosion. A. Explain the origin of the explosive decomposition of styrene and of the heat which is given off. You may find it useful to refer to the table of bond energies below.
B. Write a plausible structure of the sticky solid which results from styrene decomposition. Very briefly describe the structural possibilities.
3.* (1996 3 1) Discuss the structural features of a polymer which would make it a useful elastomer; specifically describe the underlying thermodynamic property which causes a stretched elastomer to return to its unstretched state. Mention the effect of molecular stereoisomers. Discuss the role of inter (between) chain interactions and mention those factors which determine the temperature range over which the elastomer is useful–for example what happens on a molecular level at low temperatures. Illustrate your discussion with an actual example, using molecular formulas.
4.* (1996 3 2) Consider the thermodynamic factors underlying the polymerization of a simple olefin: propylene. 
A. Estimate the value of the enthalpy change DH for this polymerization. The bond energies of a carbon-carbon sigma bond and a carbon-carbon pi bond are approximately 86 and 63 Kcal per mole respectively. B. Deduce the sign of the enthalpy change for the polymerization. C. Then deduce the sign of the entropy change for the reaction. D. What is the sign of the Gibbs free energy change for this reaction? Explain your reasoning and show your calculation.
5.* (1996 3 3) Write in turn the stereochemical structures of isotactic, syndiotactic, and atactic polypropylene. Indicate which form is the most stable in solution (has the lowest free energy) and which would have the lowest melting point. Briefly explain your answer.
6.* (1995 F 16) Professor Waymouth at Stanford has invented a remarkable homogeneous catalyst which, when activated by an aluminum alkyl derivative, produces a block copolymer from propylene. This copolymer has alternating isotactic and atactic regions; it is an elastomer even though there are no covalent cross links between individual polymer chains. The active catalyst exists in two conformations (shown below); these slowly interconvert during the catalytic reaction.
A. Briefly discuss the stereochemical properties of these two conformers. Explain why one conformer produces isotactic polymer whereas the other produces atactic polymer. B. Discuss the structural features of this copolymer which make it a useful elastomer.
7.* (1994 F 3F) Give an example of any useful elastomer.
8.* (1994 F 11H) A method of choice for making iso-octane from isobutylene and isobutane is to react the latter in the presence of an acidic catalyst such as concentrated sulfuric (H2SO4) or hydrofluoric (HF) acid. This catalyzed process is typically carried out at 00C - 100C. Explain how this catalyst affects the value of the equilibrium constant for this reaction.
9.* (1994 F 12) We return to Prof Waymouth’s research on soluble catalysts which produce stereoregular polypropylene. Two stereoisomeric precatalysts are shown below. When these Zr(IV) metallocene stereoisomers are activated with aluminum alkyl, one isomer produces isotactic polypropylene; the other isomer produces atactic polypropylene. You are not told which catalyst produces which polymer. Recall that isotactic polypropylene is a high-melting, stereoregular polymer whose microstructure is a polymeric coil. By contrast, atactic polypropylene has a disordered, stereorandom microstructure; the atactic form is lower melting. Given the above facts and the structures depicted below for the two precatalysts predict which catalyst forms isotactic and which forms atactic polypropylene. (Hint: the polymerization process which determines stereoregularity occurs at the zirconium center.)
10.* (1994 2 3B) When natural rubber, polyisoprene (shown below) is treated with a small amount of elemental sulfur, the resulting product ("vulcanized rubber") is a useful elastomer; before treatment with sulfur, polyisoprene is a gooey material. Upon stretching, such unvulcanized polyisoprene flows and exhibits little restoring force; it is not a useful elastomer. In general terms, explain what the sulfur is doing to make vulcanized rubber useful.
Natural rubber (cis-polyisoprene) before vulcanizing with sulfur. C. Professor Robert Waymouth (Stanford Chemistry) recently invented and patented a useful elastomer (a rubber) made entirely from propylene (1-propene). This material, which should compete with the elastomer used in "Air Jordan" basketball shoes, has no olefinic bonds, does not react with sulfur but nevertheless is a useful rubber; Waymouth’s new elastomer exhibits a strong restoring force when stretched. Waymouth’s compound is a block copolymer comprised of alternative sections of isotactic and atactic polypropylene. Pure isotactic polypropylene is not useful as an elastomer; it is high melting and is employed as a fiber for carpets, shirts, etc. Pure isotactic polypropylene does not stretch very much before breaking at room temperature. By contrast, pure atactic polypropylene is lower melting; when stretched this polymer flows. Atactic polypropylene shows little restoring force upon stretching, it flows–much like unvulcanized polyisoprene (or well chewed chewing gum). In two or three sentences, propose a molecular explanation for the useful elastomeric properties of Waymouth’s new block copolymer. D. Why can't one use sulfur to vulcanize atactic polypropylene?
11.* (1993 2 2) Below are structures for stereoisomers of polystyrene.
A. Draw a structure for the monomer from which polystyrene is prepared. B. Write down the tacticity of each of the numbered polymers: C. Which structure do you expect to exhibit the highest melting point 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 is isotactic; the isotactic one will have both the highest mp and the highest density. E. Under what conditions does polystyrene exhibit elastic properties? Why is polystyrene not used as an elastomer?
12.* (1992 2 1) Polystyrene is stretchy above 1000C but brittle below that temperature. Why?
13.* (1992 2 2) 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?
14.* (1991 2 6) Consider a polymer X with empirical formula (CH)n. It is a gummy polymer that becomes a more brittle polymer (Y) when hydrogenated. Deduce the structure of X and Y from these data.
|
