Kim, Jung Dae2006-07-282006-07-2819991999http://hdl.handle.net/10012/384The mechanical and physical properties of polyolefins are closely correlated to their chemical composition distributions (CCD) and molecular weight distributions (MWD). Until recently, control of these distributions was difficult due to the limitations involved with conventional polyolefin catalyst such as Ziegler-Natta catalysts. However, with the aid of the new metallocene catalysts, these microstructural distributions can now be customized to fit the requirements of several polymer applications. In this thesis, the MWD of polyethylene and the CCD-MWD of poly(ethylene-co-1-hexene) were customized through the selective combination of matallocene catalysts immobilized on a single silica support. For the case of homopolymers, the MWDs of polyethylenes produced with combined catalysts were represented as the superposition of the MWDs of polymers produced with individually supported catalysts. It was shown that the biomodal MWDs could be deconvoluted into two Flory's most probably distributions with polydispersity indexes of two for each peak. It was found that the molecular weight of polyethylene produced with Et[Ind]2ZrCl2 did not change with increasing ethylene pressure or hydrogen concentration in the reactor when ethylene pressure was higher than approximately 100 psi at polymerization temperatures of 40 and 50 C. When lower ethylene pressures were used, the molecular weight of polyethylene produced with Et[Ind]2ZrCl2 decreased with increasing hydrogen concentration. Surprisingly, the molecular weights also decreased with increasing ethylene pressure up to 100 psi in the absence of hydrogen. This behavior provided easy ways of controlling MWD of polyethylene produced with bimetallic supported catalysts, when Et[Ind]2ZrCl2 was combined with other metallocene catalysts. The supported catalyst obtained by the combination of Et[Ind]2ZrCl2 and Cp2HfCl2 was able to produce polyethylene with MWDs ranging from broad and bimodal to narrow and unimodal by simply changing ethylene pressure or hydrogen concentration. For the case of copolymers, it was shown that some supported metallocenes could produce polymers with broad and/or bimodal CCDs depending on the method involved in the treatment of the inert carrier. Before this research, the effect of support treatment on polyolefin microstructure was mainly concentrated on MWD. The trends observed in homopolymerization for the influence of polymerization conditions on the MWD were also observed in copolymerization, i.e. the MWD of copolymers produced with Et[Ind]2ZrCl2 showed the least sensitivity toward polymerization conditions. It was demonstrated that control of CCD and MWD could be simultaneously achieved to produce the kind of copolymers that are only made by reactor cascade technology when Ziegler-Natta catalysts were used. Finally, a mathematical model was developed to provide useful insights on phenomena happening at microparticle levels, some of which cannot be observed directly. According to the model, the broadening of MWD or CCD seemed to be caused by the presence of multiple active site types rather than mass or heat transfer resistances. However, if the polymerization time is too short or the ratio of polymerization rate to diffusion rate of monomer in the catalyst particle is very large, mass transfer resistances can further broaden the distributions.application/pdf11383020 bytesapplication/pdfenCopyright: 1999, Kim, Jung Dae. All rights reserved.Harvested from Collections CanadaDoctoral Thesis