Effect of α- and β-H/F substitution on the homolytic bond strength in dormant species of controlled radical polymerization: OMRP vs. ITP and RAFT

Abstract

The X-C bond dissociation energies (BDEs) for five series of X-CH2-nFnCH3-mFm molecules (n = 0, 1, 2; m = 0, 1, 2, 3) with X = H, I, SC(S)OEt, Co(acac)2 or Mn(CO)5 were calculated using a DFT approach, yielding results in good agreement with the few experimentally determined values. Calculations were also carried out on the simpler (CO)5Mn-CFnH3-n molecules (n = 0, 1, 2, 3), for which experimental data are available. The BDE trends as n and m vary are different for different X groups: BDE increases as n increases (particularly from 0 to 1) for X = H, I and SC(S)OEt, but decreases (particularly from 1 to 2) for X = Co(acac)2 and Mn(CO)5. The effective charge analysis suggests that the effect of the bond polarity on the ionic component of the bond energy is a major contribution to these trends. These results rationalize the limited control, for the polymerization of vinylidene fluoride (VDF), by the iodine transfer polymerization (ITP) and reversible addition-fragmentation chain-transfer (RAFT) polymerization approaches. They also predict a better controlled process for this monomer by organometallic mediated radical polymerization (OMRP), mediated by Co(acac)2. They also allow predictions for the performance of the same processes for other fluorinated monomers. The results for X = Mn(CO)5 suggest that the (CO)5Mn-CH2-nFnCH3-mFm molecules cannot be thermally activated at significant rates. Therefore, these molecules either do not form or are photochemically reactivated during the Mn2(CO)10-assisted ITP polymerization of VDF.