"Psst! Hey Buddy, you want some double bonds moved around?"
In a nutshell, that is metathesis [*]. Since olefins are involved, polymerization can soon follow. The most common example is ROMP - Ring Opening Metathsis Polymerization, but I discovered yesterday REMP - Ring Expanding Metathsis Polymerization. This discovery comes courtesy of a blog devoted to metathesis: "All Things Metathesis".
Making a ring of a polymerizing chain is always a challenge as the two free ends aren't necessarily close to each other most of the time. It's more likely that the chain will extend itself by reacting with another linear chain (if there are 100 chains in the mix, that means there are 200 free ends, 198 of which are not the ones you are trying to react. Not good odds.) To steal an old slogan: "Dilution is the solution". If the mix is diluted enough, then each chain will be so far from any other chain that it will only be able to react with itself - eventually.
In the case of REMP, the catalysis is able to confine the two ends of the chain to a small space all the while allowing the ring to grow. Since the ends are close together, it is easier for achieve ring closure without having to go to high dilution.
[*] The term also has uses outside of chemistry. In linguistics, metathesis is the rearrangement of the letters in a word when it is pronounced - such as when President Bush (W) would say "nuke-you-lure".
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