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    1. Why a polymer such as polybutadiene can be cross-linked, however a polymer such as polyethylene cannot be cross-linked. Please make reference to what it is about the chemistry of the repeating units that allows for the formation of cross-links.

    2. Car tires are made of vulcanized rubber, which is a crosslinked polymer formed through the addition of sulfur and heat to form disulfide bridges between polymer chains. Car tires are actually fairly difficult to recycle when compared to other non-crosslinked polymer products such as plastic bottles. Consider why this might be the case, and give your thoughts.

    3. Shown attached is a schematic representation of the water-ethanol phase diagram.

    Making reference to the phase diagram, explain in a few sentences how one could enrich a solution of 15 percent ethanol - 85 percent water to a composition of 60 percent ethanol - 40 percent water. Be qualitative in your description, not quantitative.

    Furthermore, explain why this procedure could not be used to reach a completely pure composition of 100 percent ethanol.

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    Solution Preview

    1. Polyethylene has only methylene CH2 repeating units which are not very reactive. On the other hand, polybutadiene has -CH=CH-CH2- repeating subunits. The double bond carbons are very reactive and are good reaction sites for reactions with other monomers. You can also have cross-linking in the polymeric chain when the conformation brings two alkene units close to each other so that they can react. The allyl carbon is also a good site of reaction where reactions like halogenation can take place forming cross-links.

    2. For recycling, bonds in the polymer must be broken to ...

    Solution Summary

    Cross-linked polymers are examined.