Why are alkenes used to make polymers?
Well, it looks to me like since alkenes have to break one
One popular method used to make controlled-length polymers, first introduced in 1955, is the Zeigler-Natta catalyst (which has since been updated to incorporate
This is commonly used as
So, we have this happening to form a titanium alkyl complex first:
Next, this activated complex can react with alkenes. Let's use ethene. The ethene ligand first binds dihapto (
Then, a 1,2-insertion occurs (keep your eyes peeled for this one!), changing ethene into a monohapto ligand (
In these two steps, note that we have broken one
Breaking net-weaker bonds and forming net-stronger bonds is thermodynamically favorable. Hence, alkenes are a good choice for polymerization.
SIDENOTE: The above process is cyclic until it is terminated.
It allows for further polymerization by coordinating more alkenes and performing more 1,2-insertions to lengthen the alkyl chain:
and so on.
This is known as the Cossee-Arlman Mechanism, and experiments by Robert H. Grubbs have supported this mechanism as "the likely pathway for polymerization in most cases" (Inorganic Chemistry, Miessler et al., Ch. 14.4.1, pg. 571).