Brian E. Woodworth

Controlled Radical Polymerization in the Presence of Oxygen

Living polymerizations occur without termination or transfer reactions and have the advantage of being able to form well-defined polymers with predictable molecular weights and narrow polydispersities. The first examples of this were living anionic polymerizations,1 which require the exclusion of moisture and oxygen and are run at low temperatures. Radical polymerization methods have the advantage of being insensitive to the presence of water and have even been carried out in aqueous media. This allows for less rigorous reaction conditions and is convenient for industrial application. Free radical polymerizations typically have slow initiation and form a high molecular weight polymer limited by transfer and termination reactions leading to poorly controlled molecular weights and broad molecular weight distributions.2 Also, in contrast to living ionic polymerization, it is very difficult to prepare well-defined homopolymers and block copolymers. In recent years, radical polymerizations have been developed into controlled/“living” polymerizations yielding well-defined polymers. Currently, nitroxide-mediated,3 metal-mediated,4 and either rutheniumor coppercatalyzed atom transfer radical polymerization (ATRP)5 are at the forefront of controlled radical polymerizations. Improvements to these processes have been aimed toward application to new monomers, new initiators and new architectures, compositions, and functionalities.6 In ATRP, recent advances have also been in the direction of new ligands7 and new metals8 which affect the activity and selectivity of the ATRP catalysts for various monomers. Also, improvements have been made in ATRP by the addition of small amounts of zerovalent metal.9 Up to this point, radical polymerizations need to be carried out in an oxygen-free environment. ATRP, in fact, requires less stringent conditions since O2 can react with the catalyst as opposed to reacting with the free organic radicals which should be present in a much lower concentration. However, oxidation reduces the active catalyst concentration. For example, the Cu(I) catalyst is oxidized to a Cu(II) species which is not an active ATRP catalyst and can even be a deactivating species, if a halogen ligand is present, and further slow the polymerization.7b In this communication, we report that controlled radical polymerizations with polymers having low polydispersities (Mw/Mn < 1.2) can be prepared without any removal of oxygen or inhibitor and does not require purging with inert gas, if a sufficient amount of zerovalent metal is present. If Cu(I)Br/dNbpy complex is added (in excess), the polymerization occurs but at a slow rate. This is due to two factors: first, the amount of Cu(I) is reduced by oxidation to Cu(II), second, the concentration of Cu(II), which is a deactivator, is increased further slowing the polymerization.7b Adding Cu(0) to the system, reduces the Cu(II) to Cu(I) and allows for a smaller concentration of catalyst to be added initially.