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Article Dans Une Revue Polymer Chemistry Année : 2016

Cobalt(III) and copper(II) hydrides at the crossroad of catalysed chain transfer and catalysed radical termination: a DFT study

Résumé

Metal complexes that mediate radical polymn. may also lead to catalyzed chain transfer (CCT) or to catalyzed radical termination (CRT), both processes occurring via the same type of hydride intermediate. What leads these intermediates to prefer reacting with the monomer, leading to CCT, or with radicals, leading to CRT, was unclear. We report here a DFT investigation of the comparative reactivity of two different hydride complexes, [(TMP)CoIII(H)] (TMP = tetramesitylporphyrin) and [(TPMA)CuII(H)]+ (TPMA = tris(2-pyridylmethyl)amine), generated from [CoII(TMP)] and [CuI(TPMA)]+, vs. the monomer and radical, using thė CH(CH3)(COOCH3) anḋ C(CH3)2(COOCH3) radicals as models for the growing PMA and PMMA radical chains. The unsubstituted porphyrin was used as a model for full quantum mech. (QM) calcns., but selected calcns. on the full TMP system were also carried out by the hybrid QM/MM approach, treating the mesityl substituents at the mol. mechanics (MM) level. The calcns. provide a basis for rationalizing the exptl. obsd. strong activity of the cobalt system in catalyzed chain transfer (CCT) polymn. without a reported activity so far for catalyzed radical termination (CRT), whereas the copper system leads to CRT but does not promote CCT. In essence, the key factors in favor of CCT for the cobalt system are a very low barrier for H transfer to the monomer and the much greater concn. of the monomer relative to the radical, yielding vCCT > vCRT. For the copper system, on the other hand, the greater barrier for H transfer to the monomer makes the CCT rate much slower, while the CRT quenching pathway favorably takes place through an electronically barrierless pathway with incipient stabilization at long C···H distances. The different spin states of the two systems (spin quenching along the CCT pathway for the Co system and along the CRT pathway for the Cu system) rationalize the obsd. behavior. The new acquired understanding should help design more efficient systems.
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Dates et versions

hal-01929756 , version 1 (01-03-2021)

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S. M. Wahidur Rahaman, K. Matyjaszewski, Rinaldo Poli. Cobalt(III) and copper(II) hydrides at the crossroad of catalysed chain transfer and catalysed radical termination: a DFT study. Polymer Chemistry, 2016, 7 (5), pp.1079-1087. ⟨10.1039/C5PY01837D⟩. ⟨hal-01929756⟩
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