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Sandwich bags, carpets, nylon stockings, stackable chains, milk cartons, etc.
Question 2. Why Do Different Polymers Have Different Properties?
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They have different chemical compositions (different monomer units), different structures, different ways of being fabricated, etc.
Question 3. Why Are Olefins (alkenes) Good Monomers For Polymerization Reactions?
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The electrons in the weak p-bonds can be used to form strong s bonds to other monomer units.
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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.
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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.
Question 7. How Can Chemists Control Which Type Of Polyethylene (ldpe Vs. Hdpe) Is Generated?
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Through the choice of appropriate catalysts and reaction conditions.
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The average value of n (the number of monomer units in the polymer) and the range in individual values of n.
Question 9. Does Ethylene Polymerize Under Mild Conditions In The Absence Of A Catalyst?
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No, in the absence of a catalyst, ethylene molecules would need to collide at very high energy in order to react with each other.
Question 10. What Is The Role Of A Catalyst?
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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.
Question 11. Why Are Metals Often Good Catalysts?
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They provide a site where organic molecules can come together and react.
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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.
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The ligands – the two cp's, the alkyl group, and the olefin (or open site) – are oriented in a tetrahedral fashion around Zr.
Question 14. What Is The Nature Of The Bonding Interaction Between A Metal And An Olefin?
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The olefin uses the electrons in its p-bond to interact with the metal.
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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.
Question 16. What Are The Other Ways In Which Chemists Can Manipulate The Properties Of Polymers?
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After the polymerization has been allowed to proceed with monomer A, the olefin feedstock is changed to B and the polymerization continues.
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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).
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Orientation of the monomer units along the chain.
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Tacticity.
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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").
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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.
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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.
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"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.
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The (bridged-cp2)Zr moiety possesses both C2Zr 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.
Polymer Chemistry Practice Test
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