Mechanistic Study On Electron Donor Effects In Propylene Polymerization With MgCl₂-Supported Ziegler-Natta Model Catalysts

Electron donors are playing important roles in MgCl2-supported Ziegler-Natta catalysts.However,mechanism of the electron donor effects is still not clear.In this work,three TiCl4/Di/MgCl2 type MgCl2-supported Ziegler-Natta catalysts(Di =dibutylphthalate,dicyclopentyldimethoxysilane and cyclohexylmethyldimethoxysilane)were prepared,meanwhile a blank catalyst was also prepared by loading TiCl4 on ?-MgCl2 without Di.According to the results of nitrogen adsorption analysis,these four catalysts have similar pore size distribution and specific surface area,which can be used for conducting meaningful comparative studies.Propylene slurry polymerization with the model catalysts was carried out in n-heptane either with or without adding dicyclopentyldimethoxysilane(De)as an external donor in the polymerization,and triethylaluminum(TEA)was used as cocatalyst.To investigate the effect of Di as well as De on the number of active centers and active center distribution,each PP sample was fractionated into three fractions(isotactic PP,medium-isotactic PP,atactic PP)by two-step solvent extractions.2-Thiophenecarbonyl chloride(TPCC)was used as a quenching agent to quench-label the propagation chains,and the concentration of active centers in the polypropylene(PP)fractions([C*i],[C*Mi],[C*a])was quantitatively measured.Chain propagation rate constants of the different PP fractions(kp)were calculated based on the data of polymerization rate and active center concentration.Effects of the internal donor were first studied by investigating propylene polymerizations in the absence of De.Comparion of propylene polymerization with TiCl4/DBP/MgCl2 catalyst with the blank catalyst showed that,the distribution of active center was evidently changed by introducing DBP in the catalyst,as the proportion of active centers producing isotactic polypropylene(C*i)was markedly increased.However,intrinsic reactivities(kp)of the three groups of active centers were hardly changed.Thus introducing a phthalate type Di in the catalyst moderately enhanced the isotacticity of PP product.Using alkoxysilane as Di in the catalyst also led to shift of active canter distribution in favour of C*i.While chain propagation rate constant(kp)of the active center producing isotactic PP fractions decreased slightly,resulting in little difference of isotacticity of PP between TiCl4/alkoxysilane/MgCl2 catalysts and the blank catalyst.It is worth to note that introducing alkoxysilane with bulky alkyls as Di evidently raised the total number of active centers([C*]/[Ti]),but using phthalate as Di caused marked decrease of[C*]/[Ti].These two kinds of Di exterted much different effects on the structure of titanium species adsorbed on the catalyst.The mutual effects of Di and De on the active center distribution and polymerization micro kinetics were studied by investigating propylene polymerization with the model catalysts.When De was added in the propylene polymerization system catalyzed with TiCl4/DBP/MgCl2,it was found that the active center distribution was further shifted in favour of C*i.Moreover,the reactivities of the three groups active centers were also significantly changed in favor of C*i.Mutual effects of these changes caused an overwhelming dominance of isotactic PP production with TiCl4/Di/MgCl2-TEA/De catalyst system(Di = phthalate,De= alkoxysilane).On the contrary,adding De in polymerization system with TiCl4/DCPDMS/MgCl2 catalysts led to relative increase of the active centers producing PP of lower stereoregularity.This unfavorable change in active center distribution largely offset the reactivity changes in favour of C*i,resulting in quite poor isotacticity in this catalyst system.In summary,this work has presented more detailed and direct expeimental evidences that can help comprehensive understanding of the electron donor effects in propylene polymerization with supported Ziegler-Natta catalyst.