| Supported Ti/Mg based Ziegler-Natta catalysts have played a major role in in-dustrial production of polypropylene for years. Electron donating complexes (External Donor) are generally introduced into this kind of polymerization system to increase its stereospecificity. In the synthesis of PP/EPR (EPR:ethylene propylene rubber) in-situ reactor alloy via sequential polymerization process, external donors not only affect chain structure of PP, but also the composition and chain structure of EPR. Therefore, it is of great significance to thoroughly study the effects and mechanism of external donor in Ti/Mg catalyst systems, and of course, novel external donors that can regu-late and optimize the structure and performance of PP and PP/EPR in situ reactor al-loy will find important applications in the industrial production of polyolefins.In this paper, effects of methyl ethers (ROCH3) and its mixture with siloxane (R2'Si(OCH3)2) as external donors in TiCl4/DIBP/MgCl2 (DQcat)-Al(C2H5)3 (TEA) catalyzed Propylene Polymerization and Its Copolymerization with Ethylene propyl-ene were systematically investigated, a mechanism model of external donor effects was proposed.Four ethers were synthesized:n-BuOCH3 (ED-1), PhCH2OCH3 (ED-2), i-BuOCH3 (ED-3), CH3(CH2)3CH(C2H5)CH2OCH3 (ED-4), which were then intro-duced into DQcat-TEA catalyzed propylene polymerization system as external donor respectively. The ethers exerted slight influences on the isotacticity of PP, but in-creased the tacticity of boiling n-heptane soluble fractions. Combination of those four ethers with Ph2Si (OMe)2(DDS) as composite external donors resulted in an increase or slightly reduction of catalytic activity, comparing to systems with only DDS as ex-ternal donor. In the presence of H2, more sterically hindered ED-3 and ED-4 in com-bination with DDS improved the isotacticity of PP and hydrogen response of the catalyst as compared to merely increased the DDS/Ti molar ratio from 5 to 10. PP synthesized with the composite external donors indicated an increase of the content of PP chains with extremely high isotacticity. Using of (cyclo-C5H9)2Si(OCH3)2 (DCPDMS) as external donor were also studied. It greatly improved the content of PP chains with extremely high isotacticity, and the molecular weight of PP was markedly increased.Based on the analysis of the distribution of the active centers, we proposed a mechanism model for the special effects of DCPDMS. Mixtures of methyl ethers and DCPDMS showed similar results, but the ether played exterted samller influences on the regulation effects of DCPDMS as the ether/DDS system in words of polymeriza-tion activity and polymer chain structure. These results indicated that the composite external donors showed some advantages over the single componet external do-nor(R2'Si(OCH3)2) system, such as higher activity and isotacticity, better hydrogen response, and new features in the PP chain structure.Effects of different composite external donors formed by ED-1, ED-2 and ED-4 mixed respectively with DDS or DCPDMS on DQcat-TEA catalyzed ethyl-ene-propylene copolymerization system were also studied. Comparing with the sys-tems with only DDS or DCPDMS, composite external donors performed apparent differences in catalytic activity, copolymer composition and chain structure. Crystal-line blocky copolymers in the product were increased by using ED-2/DDS system composite donors, while the molecular weight of copolymer were raised by using ED-2/DDS, ED-2/DCPDMS, ED-4/DCPDMS. With the use of composite external donors, the average sequence length of PP segments in the blocky copolymer fraction was increased, and randomness of the random copolymer fraction was improved. These changes in copolymer structure were expected to improve the compatibility between EPR and PP, and improve the toughening efficiency of EPR in PP/EPR in-reactor alloy. Based on the active center multiplicity of the supported Ti/Mg cata-lyst and mechanism model of external donor coordination with Mg in the vicinity of active center, a mechanissmic model of the effects of composite external donor in the catalytic system was proposed.Main novelties of the sissertaion:1. For the first time, effects of composite external donors formed by four ethers and R2'Si(OCH3)2 in TiCl4/DIBP/MgCl2-TEA catalyzed propylene polymerization were investigated. The composite external donors showed beneficial features like increas-ing activity, improving polymer isotacticity and improving hydrogen response of the catalyst, etc., and the mechanism of the effects were also studied.2. For the first time, effect of composite external donors formed by three ethers and R2'Si(OCH3)2 on TiCl4/DIBP/MgCl2-TEA catalyzed ethylene-propylene copolymeri-zation were investigated. The composite external donors showed beneficial features like increasing activity, changing the content of block copolymer fraction, copolymer composition and sequential distribution, etc., and the mechanism of the effects were also studied.3. For the first time, molecular weight and molecular weight distribution, chain struc-ture and active center distribution of PP catalyzed by TiCl4/DIBP/MgCl2-TEA/DCPDMS system were studied, which was compared to those of systems using DDS as external donor or no external donor. Mechanism for DCPDMS's strong influences on catalyst sterospecificity and molecular weight in-creasing during polymerization was proposed as well.
|
PDF Full Text Request