Aryloxy Alkyl Magnesium versus Dialkyl Magnesium in the Lanthanidocene-Catalyzed Coordinative Chain Transfer Polymerization of Ethylene

Complexes
[(1,2,4-Ph3C5H2)2NdCl2K­(THF)2]2 (Nd1), {[1,2-Ph2-4-(4-MeOC6H4)­C5H2]2NdCl2K­(THF)2}2 (Nd2), {[1,2-Ph2-4-(2-MeOC6H4)­C5H2]2NdCl2[K­(THF)4]}­(THF)0.5 (Nd3), and [(1,2,4-Ph3C5H2)2TbCl2K]2 (Tb1) have been synthesized,
studied by X-ray diffraction analysis, and used in coordinative chain
transfer polymerization (CCTP) of ethylene upon activation by alkyl
magnesium derivatives. The complexes Nd1 and Tb1 exhibiting similar molecular structures and the same core type have
demonstrated similar catalytic activities. Two types of alkylating/chain
transfer agents, namely, di-n-butyl magnesium and
heteroleptic complex (BHT)­Mg­(THF)2nBu Mg1 (BHT = 2,6-di-tert-butyl-4-methylphenoxide),
have been studied in this reaction. We have found that (BHT)­Mg­(PE)
products (PE is an oligoethylene chain) are being formed at a relatively
high rate while using Mg1 at 40 °C in the solution
polymerization of ethylene; the oligomeric products comprise more
than 40 ethylene fragments, unlike Mg­(PE)2 derivatives,
which are obtained from MgnBu2 and contain about 20 ethylene fragments. Luminescence spectroscopy
study of the reaction mixtures, while initiating the complex Tb1 by MgnBu2 or Mg1, confirmed the structural proximity and high symmetry
of the catalytic complexes for both types of Mg reagents. These experimental
results reaffirmed the hypothesis about the CCTP mechanism, suggesting
the formation of trinuclear LnMg2 catalytic species. Within
this mechanism, we can explain the increase in the polymerization
degree (Pn) when Mg1 is used by growing a single oligoethylene chain (PE) per
a Mg atom to form (μ-BHT)2Mg2(PE)2 species, whereas application of MgnBu2 provides the growth of two PE chains to form
the Mg2(PE)4 product with lower solubility.