Responses of a Supported Ziegler–Natta Catalyst to Comonomer Feed Ratios in Ethylene–Propylene Copolymerization: Differentiation of Active Centers with Different Catalytic Features

Modification of the Acyl Chloride Quench-Labeling Method for Counting Active Sites in Catalytic Olefin Polymerization

The reliable and efficient counting of active sites in catalytic olefin polymerization has been realized by using acyl chloride as the quench-labeling agent. However, the molar ratio of acyl chloride to the alkylaluminum cocatalyst must be larger than 1 in order to completely depress side reactions between the quencher and Al-polymeryl that is formed via chain transfer reaction. In this work, a tetrahydrofuran/thiophene-2-carbonyl chloride (THF/TPCC) mixture was used as the quenching agent when counting the active sites of propylene polymerization catalyzed by MgCl2/Di/TiCl4 (Di = internal electron donor)-type Ziegler–Natta catalyst activated with triethylaluminum (TEA). When the THF/TEA molar ratio was 1 and the TPCC/TEA molar ratio was smaller than 1, the [S]/[Ti] ratio of the polymer quenched with the THF/TPCC mixture was the same as that quenched with only TPCC at TPCC/TEA > 1, indicating quench-labeling of all active sites bearing a propagation chain. The replacement of a part of the TPCC with THF did not influence the precision of active site counting by the acyl chloride quench-labeling method, but it effectively reduced the amount of acyl chloride. This modification to the acyl chloride quench-labeling method significantly reduced the amount of precious acyl chloride quencher and brought the benefit of simplifying polymer purification procedures after the quenching step.