Polypropylene Chain Structure Regulation by Alkoxysilane and Ether Type External Donors in TiCl_4/DIBP/MgCl_2-AlEt_3 Ziegler-Natta Catalyst

摘要

Two different monoethers, CH_3CH_2CH_2CH_2OCH_3 (NBME) and (CH_3)_2CHCH_2-OCH_3 (IBME), and Ph_2Si(OCH_3)_2 were used as external donors in propylene polymerization using MgCI_2/TiCI_4/DIBP catalyst (DIBP: diisobutyl phthalate) activated by triethylaluminium (TEA). The monoethers alone showed no improvement in the isotacticity index of polypropylene (PP). When one of the ethers and Ph_2Si(OCH_3)_2 were added together in the polymerization system as composite external donors, the propylene polymerization activity and isotacticity index were increased as compared to those of the systems with only silane as external donor, though the melting temperature and melting enthalpy of PP decreased. The effects of the external donors on the active center distribution of the catalyst system are demonstrated by deconvolution analysis of the PP molecular weight distribution into multiple Flory components, and the mechanism of external donor regulation effects is discussed. All the results show that adding monoether in MgCI_2/TiCI_4/DIBP-TEA/silane catalyst system can reduce the stereoregularity of the isotactic chains of the polymer while, it maintains the atactic PP fractions at a very low level.