Because
active sites are on the surface of a crystal there are several different
environments, with different electron distributions. This causes
Ziegler catalysts to produce non-uniform poly(ethene). Some sites
give linear poly(ethene) of differing chain lengths, some incorporate
co-monomers readily to give branched poly(ethene).
Despite many hundreds of research-years
there are still questions about details of the mechanism.
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for more on how co- monomers are incorporated to give branches
