Questions and Answers



Session 1:

  1. What are some of the polymers that you encounter every day? Describe their physical properties.

    Answer:

    Sandwich bags, carpets, nylon stockings, stackable chains, milk cartons, etc.

  2. Why do different polymers have different properties?

    Answer:

    They have different chemical compositions (different monomer units), different structures, different ways of being fabricated, etc.

Session 2:

  1. Why are olefins (alkenes) good monomers for polymerization reactions?

    Answer:

    The electrons in the weak p-bonds can be used to form strong s bonds to other monomer units.

  2. What kinds of structural changes accompany bond-breaking and bond-forming in olefin polymerization?

    Answer:

    The olefin monomers are flat (two-dimensional) molecules with sp2-hybridized carbon atoms. The polymers are three-dimensional molecules in which the carbon atoms are sp3 hybridized.

  3. Examine samples of LDPE (sandwich bag, squeeze bottle) and HDPE (milk jug, grocery bag). What are some of the differences in the physical properties of these substances?

    Answer:

    LDPE – more transparent, flexible, waxy

    HDPE – more opaque, rigid, non-waxy

  4. How does the molecular-level structure of these polymers influence their physical properties?

    Answer:

    The structure (e.g., extent of branching) determines how the individual polymer molecules can orient (or "pack") in the solid state. This, in turn, influences physical properties such as density, crystallinity, melting point, and strength.

  5. How can chemists control which type of polyethylene (LDPE vs. HDPE) is generated?

    Answer:

    Through the choice of appropriate catalysts and reaction conditions.

  6. Besides the extent of branching, can you think of any other structural parameters that might lead to differences in physical properties?

    Answer:

    The average value of n (the number of monomer units in the polymer) and the range in individual values of n.

Session 3:

  1. Does ethylene polymerize under mild conditions in the absence of a catalyst?

    Answer:

    No, in the absence of a catalyst, ethylene molecules would need to collide at very high energy in order to react with each other.

  2. What is the role of a catalyst?

    Answer:

    A catalyst reduces the energy of activation for a reaction by providing an alternative pathway. In this way, it speeds up the reaction and allows it to proceed under milder conditions.

  3. Why are metals often good catalysts?

    Answer:

    They provide a site where organic molecules can come together and react.

  4. What is the difference between a heterogeneous and a homogeneous catalyst? What are some of the advantages of homogeneous catalysts?

    Answer:

    Heterogeneous catalysts are insoluble in the reaction medium, while homogeneous catalysts are soluble. Since homogeneous catalysts are generally molecular species, they are more amenable to study using the spectroscopic tools of chemistry. In addition, they can be chemically modified or "tailored" to produce polymers with a particular kind of structure.

  5. How would you describe the orientation of the ligands around the Zr center in the homogeneous zirconium catalyst?

    Answer:

    The ligands – the two cp's, the alkyl group, and the olefin (or open site) – are oriented in a tetrahedral fashion around Zr.

  6. What is the nature of the bonding interaction between a metal and an olefin?

    Answer:

    The olefin uses the electrons in its p-bond to interact with the metal.

  7. Polymer chain growth can be terminated by b-hydride elimination or by reaction with H2. What is one advantage of the H2 reaction?

    Answer:

    It allows the chemist to stop chain growth at a desired stage, rather than relying on the "natural" process of b-hydride elimination. Hence, it gives the chemist some control over the value of n.

  8. Write out the first few steps of polymer growth, using cmpd. 9 as catalyst.

Session 4:

  1. In Session 2, we learned that chemists can control the properties of polyethylene by choice of appropriate catalysts and reaction conditions. Based on what you have learned in Session 4, what are some other ways in which chemists can manipulate the properties of polymers?

    Answer:

    1. By using different monomers. For example, the incorporation of a phenyl (C6H5) unit into the monomer leads to polystyrene, while the incorporation of a chloro (Cl) group leads to PVC, a polymer with very different properties.
    2. By using different fabrication techniques. For example, polystyrene can be glassy or foamy depending on how it is fabricated.
  2. "Teflon" is the polymer that results from the polymerization of tetrafluoroethylene. Write a chemical formula for this reaction. What are some of the properties of Teflon?

    Answer:

    n(CF2 = CF2) (—CF2–CF2—)n. Teflon is highly resistant to chemical attack and has a very low coefficient of friction (it is slippery). In addition, it can be used over a very wide temperature range (-73 oC 260 oC).

  3. "Co-polymers" consist of two different monomers ("A" and "B") joined in an alternating fashion (ABABAB...). Block co-polymers also consist of two different monomers, but in this case blocks of polymer containing only A units are joined to blocks of polymer containing only B units (AAAAABBBBB...). How might block co-polymers be synthesized?

    Answer:

    After the polymerization has been allowed to proceed with monomer A, the olefin feedstock is changed to B and the polymerization continues.

Session 5:

  1. A polymer's structure influences its physical properties. Describe two structural variations that are possible for polypropylene but not for polyethylene.

    Answer:

    The orientation of the monomer units along the chain (head-to-tail, head-to-head, random) and the orientation of the methyl groups with respect to the polymer backbone (tacticity).

  2. Consider the polymerization of vinylidene chloride, CH2=CCl2. What structural variations are possible in poly(vinylidene chloride)?

    Answer:

    Orientation of the monomer units along the chain.

  3. Consider the polymerization of 1,2-dichloroethylene, H(Cl)C=C(Cl)H. What structural variations are possible in poly(1,2-dichloroethylene)?

    Answer:

    Tacticity.

Session 6:

  1. Head-to-tail polymerization of propylene is observed with the [cp2Zr(R)]+ catalyst. Explain this result on the basis of molecular-level interactions.

    Answer:

    Each incoming propylene molecular orients with its methyl group in toward R, rather than out toward cp, in order to avoid unfavorable contacts with the bulky cp's. When the R group migrates to propylene, it migrates to the closer olefinic carbon, which is always the one bearing the methyl group (the "b carbon").

  2. Atactic polypropylene is always produced with the [cp2Zr(R)]+ catalyst. Explain this on the basis of molecular-level interactions.

    Answer:

    There is no preference for the methyl group on propylene to be oriented up or down, because in each case it has exactly the same interaction with a cp group. Since there is no up/down preference, a random (atactic) orientation of methyls along the chain results.

Session 7:

  1. What does it mean for a molecule or a ligand to be "chiral"? What properties does chirality impart to a molecule?

    Answer:

    Molecules that are not superimposible on their mirror images are chiral. Mirror image isomers are called enantiomers. Enantiomers have identical physical properties except that they rotate plane polarized light in opposite directions.

  2. Explain what is meant by "C2 symmetry" and "mirror plane symmetry". Can a molecule with "mirror plane symmetry" ever be chiral?

    Answer:

    "C2 symmetry" means that 180o rotation about an axis through the molecule results in a geometry equivalent to the starting geometry. "Mirror plane symmetry" means that one half of the molecule can be perfectly reflected into the other half through a symmetry plane. Molecules with mirror plane symmetry cannot be chiral.

  3. Consider a homogeneous zirconium catalyst in which two cyclopentadienyl ligands are connected by a —CH2–CH2 bridge. Which symmetry elements does the (bridged-cp2)Zr moiety possess? What is the expected tacticity of the polypropylene produced using this type of catalyst? Why?

    Answer:

    The (bridged-cp2)Zr moiety possesses both C2 and mirror plane symmetry. Like the parent unbridged cp2Zr catalyst discussed in Session 5, it would be expected to produce atactic polypropylene, since the methyl group on the propylene would have no up/down preference.

Session 8:

As you think about designing your own polyolefin polymer and a catalyst to direct its synthesis, consider the following questions:

  1. Which properties do you want your polymer to possess?
  2. Which olefin monomers might lead to polymers with the desired properties?
  3. What structural or symmetry features must be built into the catalyst in order to insure that a polymer of the desired structure is produced?
  4. What would be ideal reaction conditions for the synthesis and fabrication of your polymer?


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