Explain why a polymer such as polybutadiene can be cross-linked, however a polymer such as polyethylene cannot be cross-linked. Make reference to what it is about the chemistry of the repeating units that allows for the formation of cross-links.
butadiene is left with a second double bond when it polymerizes, making cross chains possible. Study these two images.
http://upload.wikimedia.org/wikipedia/commons/c/c6/1,3-Butadiene_Polymerization.PNG
http://www.google.com/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/f/f7/Polyethylene-repeat-2D-flat.png&imgrefurl=http://en.wikipedia.org/wiki/File:Polyethylene-repeat-2D-flat.png&h=1052&w=1100&sz=21&tbnid=QtnQAgr3rIA1rM:&tbnh=93&tbnw=97&zoom=1&usg=__1PiU_qffpsFCTFUX8ulotS86Aik=&docid=rLW8JQHZ90znBM&sa=X&ei=giPfUIm1DOXg2gXymYHQBg&ved=0CGoQ9QEwBw&dur=2596
Polymer cross-linking refers to the formation of covalent bonds between polymer chains, which leads to the creation of a three-dimensional network structure. This network provides the polymer with unique properties, such as increased mechanical strength and thermal stability.
Polybutadiene can be cross-linked because it contains unsaturated carbon-carbon double bonds in its repeating units. These double bonds can undergo a special type of chemical reaction called addition polymerization. In this process, the polymer chains react with each other, forming new covalent bonds at the double bond sites. These new bonds connect the chains together, creating a network structure.
On the other hand, polyethylene cannot be cross-linked as it lacks reactive sites for the formation of covalent bonds. Polyethylene is composed of repeating units containing only single carbon-carbon bonds. This absence of double bonds means that polyethylene cannot undergo addition polymerization like polybutadiene. Consequently, it does not possess the necessary chemistry to form covalent cross-links between its polymer chains.
To summarize, the presence of double bonds in the repeating units of a polymer enables it to cross-link, as these double bonds can undergo addition polymerization reactions. In the case of polybutadiene, the double bonds allow for the formation of covalent bonds between chains, resulting in a cross-linked structure. However, polyethylene, lacking these double bonds, cannot form covalent cross-links.