ARTICLE This journal isThree tailored ansa-type Zr (IV)-and Hf (IV)-complexes, Zr-1, Hf-1 and Zr-2, all bridged by a dimethylsilylene group and bearing both a 6-[2-(BR2)phenyl]pyrid-2-yl motif (R = ethyl, mesityl) and an η 1 /η 5 -bound ligand with different Lewis base character and steric demand, have been synthesized. Their structures have been determined by single-crystal X-ray diffraction analysis. Upon activation with methylalumoxane (MAO), Zr-1, Hf-1 and Zr-2 are capable of polymerizing norborn-2-ene (NBE) via ring-opening metathesis polymerization (ROMP). In the MAO-activated copolymerization of ethylene (E) with NBE; however, pure vinyl insertion polymerization (VIP)-derived poly(NBE)-co-poly(E) is obtained by Zr-1, Hf-1 and the model pre-catalyst (Zr-3), which does not contain the borylamino motif. In contrast, Zr-2 yields copolymers containing both ROMP-and VIP-derived poly(NBE) ROMP -copoly(NBE) VIP -co-poly(E) units via an α-H elimination process. Variable-temperature 11 B NMR measurements allow for identifying tri-and tetracoordinated borane species. The propensity of catalysts Zr-1, Hf-1, Zr-2 and Zr-3 to switch from VIP to ROMP is discussed on the basis of their structural differences. A detailed NMR study of the reaction of Zr-1, Zr-2, Hf-1 and in particular of dimethylsilylene-bis(6-[2-(diethylboryl)phenyl]pyrid-2-ylamido)ZrCl 2 (Zr-4) with MAO and with MAO/NBE clarifies the reaction cascade from VIP to ROMP and allows for establishing a general concept for a tailored switch from VIP to ROMP within the same polymer chain, allowing for tandem VIP -ROMP copolymerization of E with NBE. ARTICLEThis journal is

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