Tuesday, August 07, 2012

Self-Immolative Polymers

Polymer degradation usually means that the polymer chains are being broken somewhere in the middle. This is the most common degradation mechanism for polymers and occurs with biodegradable polymers that are composted, most cases of thermal degradation, oxidation, exposure to UV light, etc. Much more rare is degradation of a polymer proceeding from the ends towards the middle. People are first exposed to this idea when learning about the ceiling temperature for a polymerization - the temperature at which the forward reaction (polymerization) is in equilibrium with the reverse reaction (depolymerization).

Both of these mechanisms usually take a lot of energy to completely degrade a polymer, needing a continual stream of energy input - lots of heat or UV or whatever. In neither case can you initiate the reaction and just let it go, there are too many obstacles (side reactions) in the way.

But all that is changing. A recent report ($) [1] tells of polymers ("self-immolative") that degrade from the ends after UV light (or 2-photon near-IR (NIR) light) removes the endcap of the polymer. The monomers of the polymer have a strong tendency to depolymerize but only the endcaps prevent it. Remove them and the whole polymer unzips all on it's own as the monomers fall off via a cyclization mechanism. The researchers are most interested in using it for an approach to controlled release within the body, hence the interest in the NIR mechanism [2].

But what if this approach could be applied to more mainstream plastics? The plastics would only need a brief exposure to sunlight before starting to degrade. While that may be appealing, there would be a certain element of the game "hot potato" to it as well, as the plastic would be continually degrading and you could never be sure when it will have degraded too much until it fails.

[1] There are a couple of articles with free access about this technology, one be a brief review similar polymers, the other being a report on dendrimers.

[2] NIR is able to penetrate the body without significant absorption, but it the photons are not energetic enough to break off the endcaps. If 2 NIR photons are absorbed at the same location however, the energy is sufficient. An extra advantage of this method is that the absorbing site can be extremely localized if the 2 photons arrive from different directions. No tissue along either pathway absorbs enough light for damage, but the polymer located at their intersection does.

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