Vanadium(III)–catalyzed copolymerization of ethylene withnorbornene : Microstructure at tetrad level and reactivity ratios

Vanadium(III) complex bearing PMePh2ligand has been synthesized, fully-characterized and investi-gated for homo- and co-polymerization of ethylene with norbornene, in combination with aluminumcompounds, i.e., methylaluminoxane (MAO) and Et2AlCl. The results are carefully compared with thoseobtained with VCl3(THF)3and VCl3in the presence and absence of ethyltrichloroacetate (ETA), and freephosphine. VCl3(PMePh2)2was likely activated through a dissociative mechanism where the dissocia-tion of the labile phosphine ligand was the first and possibly the rate-determining step. The activity ofVCl3(THF)3was slightly higher than that of VCl3(PMePh2)2likely due to a different competition of re-insertion rate of dissociated ligand compared to the insertion/coordination of (co)monomers. Copolymersobtained at low norbornene feedstock concentration from VCl3(THF)3, showed greater non-uniformityin terms of composition distribution with respect to those from VCl3(PMePh2)2because of a morepronounced compositional drift in the semi-batch polymerization process. This strongly affects thecopolymer’s thermal properties, the copolymers from VCl3(PMePh2)2exhibiting higher Tgs. The effectof PMePh2and THF ligand on reactions responsible for chain growth termination, affecting the copoly-mers molecular weight, is discussed. Control over the norbornene incorporation and molecular weightof the resultant copolymers proved to be possible by changing the polymerization temperature, and ETA loading.In addition, ad-hoc. experiments, designed to ensure the uniformity of the catalytic copolymerizationprocess even in the presence of the observed relevant (and unavoidable) compositional drift were per-formed, and copolymers microstructure and catalytic mechanisms were thoroughly investigated. Dueto the peculiar features of the catalytic systems, these studies could be performed only after appropri-ate modifications to well established methods were developed. As a by-product of these investigations,general and original computational methods are proposed, whose applicability goes beyond the casestreated here.