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

Compound CpMoI2(iPr2dad) (iPr2dad = iPrNCHCHNiPr), obtained by halide exchange from CpMoCl2(iPr2dad) and NaI, has been isolated and characterized by EPR spectroscopy, cyclic voltammetry, and X-ray crystallography. Its action as a catalyst in atom transfer radical polymerization (ATRP) and as a spin trap in organometallic radical polymerization (OMRP) of styrene and methyl acrylate (MA) monomers has been investigated and compared with that of the dichloro analogue. Compound CpMoCl2(iPr2dad) catalyzes the ATRP of styrene and MA with low efficiency factors f (as low as 0.37 for MA and ethyl 2-chloropropionate as initiator), while it irreversibly traps the corresponding growing radical chains under OMRP conditions. On the other hand, compound CpMoI2(iPr2dad) has a greater ATRP catalytic activity than the dichloro analogue and yields f = 1 for MA and ethyl 2-iodopropionate as initiator. Under OMRP conditions, it does not irreversibly trap the growing radical chains. This comparison serves to illustrate the general principle that low initiator efficiency factors, sometimes observed in ATRP, may result from the interplay of the ATRP and OMRP mechanisms, when the latter ones involves an irreversible radical trapping process.

The interplay of two different mechanisms in controlled radical polymerization, both involving one-electron reactivity on a transition metal complex (one involving halogen atom transfer to a transition metal complex, ATRP; the other one involving the reversible formation of a metal–carbon bond, OMRP) may lead, in certain cases, to the irreversible trapping of the radical growing chains and to a consequent decrease of the “initiator efficiency factor”. This contribution highlights this phenomenon for this first time, on the basis of results obtained with a 17-electron half-sandwich MoIII complex.Figure optionsDownload as PowerPoint slide