Polymer Engineering Interview Questions and Answers
Question - 11 : - What Is The Difference Between A Heterogeneous And A Homogeneous Catalyst? What Are Some Of The Advantages Of Homogeneous Catalysts?
Answer - 11 : -
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.
Question - 12 : - How Would You Describe The Orientation Of The Ligands Around The Zr Center In The Homogeneous Zirconium Catalyst?
Answer - 12 : -
The ligands – the two cp's, the alkyl group, and the olefin (or open site) – are oriented in a tetrahedral fashion around Zr.
Question - 13 : - What Is The Nature Of The Bonding Interaction Between A Metal And An Olefin?
Answer - 13 : -
The olefin uses the electrons in its p-bond to interact with the metal.
Question - 14 : - Polymer Chain Growth Can Be Terminated By B-hydride Elimination Or By Reaction With H2. What Is One Advantage Of The H2 Reaction?
Answer - 14 : -
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 - 15 : - What Are The Other Ways In Which Chemists Can Manipulate The Properties Of Polymers?
Answer - 15 : -
- 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.
- By using different fabrication techniques. For example, polystyrene can be glassy or foamy depending on how it is fabricated.
Question - 16 : - A Polymer's Structure Influences Its Physical Properties. Describe Two Structural Variations That Are Possible For Polypropylene But Not For Polyethylene.
Answer - 16 : -
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).
Question - 17 : - Consider The Polymerization Of Vinylidene Chloride, Ch2=ccl2. What Structural Variations Are Possible In Poly(vinylidene Chloride)?
Answer - 17 : -
Orientation of the monomer units along the chain.
Question - 18 : - Head-to-tail Polymerization Of Propylene Is Observed With The [cp2zr(r)]+ Catalyst. Explain This Result On The Basis Of Molecular-level Interactions.
Answer - 18 : -
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").
Question - 19 : - Atactic Polypropylene Is Always Produced With The [cp2zr(r)]+ Catalyst. Explain This On The Basis Of Molecular-level Interactions.
Answer - 19 : -
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.
Question - 20 : - What Does It Mean For A Molecule Or A Ligand To Be "chiral"? What Properties Does Chirality Impart To A Molecule?
Answer - 20 : -
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.