J.Am.Chem.Soc.http://dx.doi.org/10.1021/ja512073m(2015)

With applications in biomedical engineering, photovoltaics and drug delivery, ring-opening metathesis polymerization (ROMP) has quickly become a standard technique for polymer synthesis. The reaction is tolerant of multiple functional groups and its 'living' nature produces polymers with low dispersity. Many transition-metal catalysts have been developed for ROMP, but now Andrew Boydston and co-workers from the University of Washington have described a metal-free version of the reaction.

Though the use of metal catalysts for these reactions has already been successful, even very small amounts of metal byproducts can be problematic if, for example, the polymers are to be applied in a biological setting. Boydston and co-workers set out to develop a ROMP method initiated by the one-electron oxidation of a vinyl ether. They envisaged that the reaction would proceed through a [2+2] cycloaddition to form a cyclobutyl radical cation and that the use of a suitable cyclic alkene monomer could allow a ring opening to compete with reduction and lead to propagation. Early experiments focussed on the polymerization of norbornene in the presence of a variety of simple vinyl ethers with the reaction initiated by bulk electrolysis. The results were promising — producing polymers with an average molecular weight of 11.8 kDa and a dispersity of 2.2 — but only in low yield. The researchers attributed this to the low solubility of the polymer in the nitromethane solvent required for electrolysis and the heterogeneous nature of the initial oxidation; they thus turned to photoredox catalysis.

Blue light irradiation of a 100:1 mixture of norbornene monomer and vinyl ether initiator, in the presence of a pyrylium photoredox mediator, produced a polymer with properties very similar to those made using the more traditional ruthenium catalysis. The change in molecular weight of the polymer during the reaction, however, did not follow the same trend as for a ruthenium-catalysed process. This was attributed to the reaction possibly featuring reversible termination events — an observation that could be exploited in a spatiotemporally controlled polymerization. SD