13C NMR studies of zirconocene-catalyzed propylene/1-hexene copolymers: in-depth investigation of the effect of solvent polarity

Propylene/1-hexene copolymers were prepared with the isospecific Et(Ind)2ZrCl2 /MAO catalyst by using a wide range of solvents. In the order of increasing dielectric constant (ε) they are: heptane (1.92), methylcyclohexane (2.02), toluene (2.38), chlorobenzene (CB) (5.68), methylene chloride (8.93) and o-dichlorobenzene (o-DCB) (10.12). The effect of solvent structure and polarity on yield, composition, and microstructure of copolymers was investigated. The data show that solvent polarity, which has an enormous effect on the activity, has a negligible effect on the copolymer composition. Very close 1-hexene contents are obtained with all the solvents used except for toluene, in which the comonomer content is remarkably lower than that in the other solvents for all compositions. Detailed 13C NMR analysis of the copolymers obtained was performed: the reactivity ratios obtained by statistical elaboration of the experimental values of triad and tetrad comonomer compositions confirm the actual lower accessibility of the active site to 1-hexene in toluene than in the other solvents. This fact can be explained by the competition of the nucleophilic toluene with 1-hexene in the coordination to the active site.