Tuesday, June 25, 2013

Polymetallocenes. Not Metallocene Polymers, but Polymetallocenes.

I written in the past about metallocenes (both here and elsewhere), but always in the context of their use as a polymerization catalyst. By judicious choice of the ringed ligands, access to the active, central-core metal can be strictly limited. The result is greater control over the reaction kinetics, stereochemistry, branching, etc. Being catalysts, these materials are present in the reactor in very small amounts and very little of it leaves with the final product.

Today's topic takes this whole concept and turns it on its head. In this case, I am talking about polymers that use metallocenes as the monomers. (Why not? They are multifunctional after all.) The end result is a polymetallocene which is quite different from a "metallocene polymer" (the common term for what was discussed above). Examples of this would be this set:
The monomers have the silicon atom bridging the two ring ligands, so that the polymerization proceeds from a ring-opening mechanism. (Can we call this a 3-ring circus?)

These polymers are hardly new having been first discovered over 10 years ago. They are mostly a laboratory curiosity, although a very curious one at that. The latest research has found that unsymmetrically substituted AND atactic polymers are somehow semicrystalline. Plenty curious indeed.

Polymers being atactic and semicrystalline at the same time is not a totally new discovery either. Polyvinyl alcohol for instance, is known to be atactic and semicrystalline. The pendant -OH groups are not large enough to disrupt the crystal, so it doesn't matter where they occur along the backbone. I expect that a similar phenomenon is occurring here. The researchers don't work out all the details of the crystal cell, but given that you have sharp peaks in both the the WAXS and DSC scans, as well as typical spherultic structures under crossed polars, I'm convinced the crystallinity is present.

All is all, these are fun polymers with some unexpected results. I'm sure the student doing the work had a fun time.

1 comment:

Unknown said...

thanks for your shared

about me