Two TiCl 4 /Di/MgCl 2 type supported Ziegler-Natta catalysts were prepared by loading dibutylphthalate or dicyclopentyldimethoxysilane (DCPDMS) (internal donor, Di) and TiCl 4 on activated d-MgCl 2 in sequence, and a blank catalyst was prepared by loading TiCl 4 on the same d-MgCl 2 without adding Di. These catalysts have similar specific surface area and pore size distribution, thus form a suitable base for comparative studies. Propylene polymerization with the catalysts was conducted in n-heptane slurry using triethylaluminum (TEA) as cocatalyst, and the effects of Di as well as De (external donor, in this work it was DCPDMS) on the number of active centers, the distribution of active centers among three polypropylene (PP) fractions (isotactic, medium isotactic, and atactic PP chains), and chain propagation rate constants of the PP fractions were studied by counting the number of active centers in the PP fractions using a method based on selective quench-labeling of the propagation chains by 2-thiophenecarbonyl chloride. When De was not added in the polymerization, introducing a phthalate type Di in the catalyst evidently changed the active center distribution by enhancing the proportion of active centers producing isotactic PP (iPP) (C Ã i ), but scarcely changed reactivities of the three groups of active centers forming the three fractions. When the De was added in the polymerization system with TiCl 4 / phthalate/MgCl 2 catalyst, further shifting of active center distribution in favor of C Ã i took place, meanwhile reactivities of the three groups of active centers also remarkably changed in favor of C Ã i . Mutual effects of these changes led to overwhelming dominance of iPP production in the TiCl 4 /Di/MgCl 2 -TEA/De system (Di 5 phthalate, De 5 alkoxysilane). In contrast, though using alkoxysilane as Di also caused shifting of active center distribution in favor of C Ã i when De was not added, addition of alkoxysilane De caused reverse shifting of active center distribution in favor of those producing PP of lower stereoregularity. This unfavorable change largely counteracted the reactivity changes in favor of C Ã i caused by the De, rendering the catalytic system rather poor isospecificity. V C 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46605.
INTRODUCTIONIn recent years, there has been an explosion of research committed to the olefin block copolymers with diverse microstructures. Melting temperatures, glass transition temperatures, and mechanical properties of materials can be altered, when alkyl branches were introduced into carbon hydrogen backbone. 1 This was attributed to the reduction of main chain crystallinity by the alkyl branches. Brookhart et al. demonstrated that some late transition metal catalysts bearing bulky a-diimine ligand exhibited high activities in ethylene polymerization and copolymerization with comonomers, resulting in formation of different branches in the polymer chain. 2-4 Later, more palladium and nickel catalysts were developed and applied to a variety of applications areas. 5-7 These days, some researchers concentrated on new kinds of Ni(II) and Pd(II) complexes, such as dinuclear catalysts, and made astounding advances, 8,9 suggesting a tremendous potential in polymerizing materials with diverse microstructures. With the advantage of the unique chain walking feature of Brookhart-type catalysts, block polymer bearing both plasticity and elasticity can be designed, as the main point is to introduce crystalline or semicrystalline blocks into the amorphous chain segments. Xiao et al. reported a new multiblock polyethylene via chain shuttling polymerization of ethylene between a Brookhart-type catalyst and a bridged zirconocene, where the bridged zirconocene produced linear polyethylene segments. 10 Chen et al. addressed a series of a-diimine Ni(II) and Pd(II) catalyst with different substituents in ethylene (co)polymerization to control the polymer topology. 11-15 Some other researchers concentrated on utilizing the highly x,1-enchainment behavior (chain walking of
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.