Kinetics of short‐duration ethylene–propylene copolymerization with MgCl₂‐supported Ziegler–Natta catalyst: Differentiation of active centers on the external and internal surfaces of the catalyst particles

Abstract

Ethylene-propylene copolymerization with a TiCl 4 /MgCl 2 type ZN catalyst was conducted for different durations from 30 to 600 s, and changes of polymerization rate, concentration of active centers ([C*]) and copolymer chain structure with time were traced. The copolymerization rate decayed with time, but [C*]/[Ti] increased in the same period. This was attributed to release of more active sites through disintegration of catalyst particles by the growing polymer phase. Ethylene content of the copolymer quickly decreased in the period of 30-90 s, meaning that the active centers activated in the reaction process have stronger ability of incorporating propylene than those activated at the very beginning. The copolymer samples were fractionated into two parts, namely n-heptane soluble fraction (random copolymer) and insoluble fraction (segmented copolymer with high ethylene content). With continuation of the copolymerization, active centers producing the random copolymer chains increased much faster than active centers producing the segmented copolymer chains, and became the dominant centers after 120 s. Consequently, proportion of the soluble fraction sharply increased with time. All these results indicate that the active centers located on the external surface of catalyst particles are highly different from those buried inside the particles.

Trans‐1,4‐stereospecific copolymerization of ethylene and isoprene catalyzed by MgCl2‐supported Ziegler–Natta catalyst

“…Because of the multisite nature of heterogeneous Ziegler–Natta catalysts, olefin copolymers synthesized with them have rather broad chemical composition distribution (CCD), and isoprene–butadiene copolymers synthesized with TiCl 4 /MgCl 2 ‐supported catalysts also show very broad CCD . To make brief characterization on CCD of the ethylene–isoprene copolymer synthesized in this work, each copolymer sample in Table was fractionated into two parts by extraction with boiling n ‐heptane, and the insoluble fraction (C7 ins ) was characterized by 1 H NMR, GPC, and DSC.…”

Comparative Study on Kinetics of Ethylene and Propylene Polymerizations with Supported Ziegler–Natta Catalyst: Catalyst Fragmentation Promoted by Polymer Crystalline Lamellae

“…In our previous study on ethylene polymerization kinetics with an industrial supported Z–N catalyst designed for polyethylene production, similar growth of [C*]/[Ti] with polymerization time was observed, and the growth of [C*]/[Ti] was correlated with gradual disintegration of the catalyst particles in the reaction process [21,22,33]. Hydraulic forces of the growing polymer phase were considered as the dominant factor in the particle fragmentation.…”

“…Despite extensive fundamental studies in this field over the past decades, there are still many unsolved problems concerning the polymerization mechanism and relationships between catalyst structure and polymerization behaviors. The kinetics and mechanism of ethylene and propylene polymerizations with Z–N catalysts have been studied in a broad span of reaction durations ranging from less than 1 s to more than one hour [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. When ethylene and propylene polymerizations with the same catalyst are compared, several peculiarities in the reaction kinetics have been reported.…”

“…The activity of ethylene polymerization was found to be markedly enhanced by introducing a small amount of propylene before ethylene (so-called prepolymerization) [ 25 , 27 , 28 ]. Because the intrinsic reactivity of ethylene polymerization is evidently higher than that of propylene on the same catalyst [ 22 , 25 ], it is hard to attribute such a phenomenon to certain kinds of chemical activation effect.…”

“…The occurrence of diffusion limitation in ethylene polymerization was proposed as the reason for its sensitivity to catalyst porosity. In our previous works, significant enhancement of [C*]/[Ti] with reaction time in the initial stage of ethylene (co)polymerization was observed, and fragmentation of the catalyst particles by hydraulic force of the growing polymer chains was considered the reason for the enhancement of active center numbers [ 21 , 22 ]. According to these results, fragmentation of the catalyst particles significantly influences the [C*]/[Ti] fraction in the initial stage and its changes with time, and thus determines the polymerization kinetics.…”