Design of selective divalent chain transfer agents for coordinative chain transfer polymerization of ethylene and its copolymerization with butadiene

PhMg(CH2)5MgPh and MesMg(CH2)5MgMes – divalent bis-metalated chain transfer agents (CTA) – were designed, synthesized and implemented in the polymerization of ethylene or the copolymerization of ethylene with butadiene mediated by {(Me2Si(C13H8)2)Nd(μ-BH4)[(μ-BH4)Li(THF)]}2. The systems showed coordinative chain transfer (co)polymerization features with a selectivity towards the initiation depending on the CTA used. Whereas PhMg(CH2)5MgPh initiated chain growth both at the alkyl and aryl sides, MesMg(CH2)5MgMes led to an unprecedented selective polymer chain growth from the alcanediyl moiety while the mesityl groups remain as spectators. The mechanism of the polymerization initiation has been investigated computationnaly at the DFT level. The theoretical contribution of the study highlights the different intermediates formed upon combination of the neodymium metallocene and the magnesium CTA, and rationalize the specifity confered by the mesityl groups to induce selective initiation on an alkyl moiety.