Interaction of Propagating Radicals with Copper(I) and Copper(II) Species

Abstract

The effects of copper(I) and copper(II) metal centers on the atom transfer radical polymerization (ATRP) of styrene and methyl acrylate were investigated. The free-radical polymerizations were initiated by AIBN in the presence of copper(I) and copper(II) complexes. For methyl acrylate, the rate of the polymerization was reduced in the presence of CuIBr/dNbpy and CuIOTf/dTbpy but was unaffected by the presence of CuII(OTf)2/dTbpy. For styrene, under conditions which yield relatively low molecular weight polymer (16 000), no effect was observed in the presence of CuII(OTf)2/dNbpy; however, under conditions which yield high molecular weight polystyrene (50 000-100 000), the polymerization was limited in the molecular weight attainable and stopped at partial conversion. No effect was observed for the free-radical polymerization of styrene in the presence of copper(I) complexes. These results indicate that control in ATRP does not originate in interactions of growing radicals with copper complexes but in the reversible halogen atom transfer.

Electron transfer reactions relevant to atom transfer radical polymerization

“…The reactions of organic radicals with Cu I and Cu II compounds may involve the formation of organocopper species [151,152]. While there is currently no experimental evidence which suggests organometallic Cu II -R species are generated during Cu mediated ATRP, [153] recent studies have detailed the contribution of certain organomolybdenum species in mediating polymerization initiated by alkyl halides under ATRP conditions (Scheme 3) [20,154].…”

How the interplay of different control mechanisms affects the initiator efficiency factor in controlled radical polymerization: An investigation using organometallic MoIII-based catalysts

“…An ATRP catalyst must be a complex capable to increase both its formal oxidation state and its number of valence electrons by one unit and has, in principle, the potential to act also as an OMRP spin trap. However, studies on the Cu(I)-catalysed ATRP, specifically carried out to address this question, showed no evidence for the formation of Cu II -terminated dormant chains [20].…”

Competitive Equilibria in Atom Transfer Radical Polymerization

“…The relative bond dissociation energies of the the Mt-R, Mt-X, and R-X bonds would ultimately dictate whether polymerization would be inhibited by the formation of a Mt-R bond, whether initiation efficiency might just be reduced, or whether the entire polymerization could be mediated through the reversible formation of such a Mt-R bond (as in stable free radical polymerization, or SFRP). [75] While there is currently no experimental evidence that suggests any organometallic Cu II -R species are formed during Cu-mediated ATRP, [76] several recent studies have suggested that certain Mo III complexes initiate polymerization from alkyl halide ATRP initiators but then proceed to mediate polymerization through the reversible formation of a Mo IV -R species as in SFRP. [77] This is a particularly important mechanistic observation in light of the new ICAR and ARGET processes.…”