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29 September 2008 - metathesis

A recent addition to the ever growing family of olefin metathesis concepts is ring expansion metathesis polymerization or REMP for short. Discovered in 2002 in the Grubbs (who else) group (Bielawski et al. 2007 DOI), this polymerization requires a cyclic monomer cyclooctaene and a type of Hoveyda-Grubbs Catalyst fitted with a molecular tether. This tether makes sure that new monomer joins the ring and prevents ordinary linear polymerization.

The catalyst is regenerated in an intramolecular chain transfer step. The end result is a cyclic macromolecule and simply hydrogenation affords cyclic polyethylene. COD also does the trick (Bielawski et al. 2007 DOI) but a tenacious contaminant (4-vinylcyclohexene) spoils the fun by causing linear polymerization. The triene cyclododecatriene (CDT) can be obtained sufficiently pure to go cyclic all the way.

The catalyst itself was first prepared by the Fürstner group (Fürstner et al. 2001DOI) by metathesis of the ligands:

Cyclic polymers are nothing new but this particular method for making them is appealing. One tried and tested way is to have a suitable telechelic polymer to join ends but this requires extremely dilute conditions. Another method is back-biting in polycondensation reaction.

Cyclic polymers can have physical properties very different from their linear counterparts as they do not have interfering end-groups among other characteristics. In the case of cyclic polyethylene it is found thin films have a larger contact angle for water. Cyclic PE also melts at a higher temperature (attributed to lower entropic disorder) and in fact a mix of both tend to phase separate. Cyclic PE also has a lower hydrodynamic volume which can be deduced from simple SEC. The case of cyclic CDT is an illustration that cyclic polymers in general have lower viscosity as well.

The methodology has since then been employed with other monomers but not necessarily with a tethered Grubbs catalyst, again forcing dilute reaction conditions. It was used in the synthesis of a special type of salen ligand (Zheng et al. 2007 DOI) and it was investigated as alkane metathesis (Ahuja et al. 2008DOI) with cyclooctane (forming a wild bunch of products).

A study on cyclic polytetrahydrofuran (Tezuka et al. 2008DOI) synthesised from THF reveals a distinct spherulite pattern and a slow spherulite growth rate.

In a most recent contribution, the Grubbs group have spent some effort optimizing their tethered Ru catalyst ((Boydston et al. 2008DOI): saturate the NHC ligand and increase tether size.