Sequence and branching distribution of ethylene/1-butene copolymers prepared with a soluble vanadium-based Ziegler-Natta catalyst

The theoretical relations between ¹³C NMR line intensities and the triad distributions of ethylene/1-butene copolymer containing 1-butene units incorporated by primary and secondary insertion (B, B*) were derived. On the basis of these relations, the triad distributions of ethylene/1-butene copolymer prepared with a soluble vanadium-based Ziegler-Natta catalytic system VOCl₃/Al₂(C₂H₅)₂Cl₃ were determined by ¹³C NMR measurements. Conditional probabilities (P_EE, P_EE, P_EB+, P_BE, P_BB, P_BB-, P_B,E, P_B,B,)₁ mole fractions of comonomers (E, B, B*), and products of reactivity ratios (r_EBr_EB, r_EB+rB+E) were calculated from the triad distributions observed. The analysis of these values suggests that the copolymer has an alternating-type sequence distribution and 80% of 1-butene units in copolymer follow the primary insertion. It is also suggested that this catalyst has one type of active site. The latter result was confirmed by comparison of the branching distribution of copolymer determined by temperature-rising elution fractionation (TREF) with that of copolymer model calculated theoretically on the assumption that the catalyst has only one type of active site. These results are distinct from those of a heterogeneous titanium-based SiO₂-supported catalytic system of TiCl₂/Al(C₂H₅)₃. The latter catalyst has multiple active sites which form copolymers having 1-butene units incorporated by primary insertion and a nearly Bernoullian sequence distribution. It is suggested that the simultaneous use of ¹²C NMR and TREF can enlighten the copolymer structure and the nature of the active sites.