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|Title:||SYNTHESIS, CHARACTERIZATION AND PROPERTIES OF NOVEL POLYOLEFINS BY SINGLE-SITE CATALYSTS|
|Keywords:||Chemical Engineering;Chemical Engineering|
|Abstract:||<p>Developments in single-site catalysts for olefin polymerization have offered unprecedented freedom in designing new polyolefins, controlling polyolefin structure, and tailoring materials properties. The main theme of this sandwich-style thesis is the synthesis, characterization, and properties of a novel series of polyolefin materials that have unique chain structures and superior materials properties. These polyolefin materials were respectively prepared with a variety of recently developed single-site catalysts. High-strength polyethylene fibers were in situ synthesized Via nanoextrusion polymerization of ethylene with a mesoporous particle (MCM-41) supported titanocene catalyst, Cp2TiCh. The unique nanotube structure prevented the conventional folding of growing polymer chains inside the nanotubes and thus led to the formation of polyethylene nanofibrils with unique extended chain crystalline structure. Further aggregation of nanofibrils resulted in macro-scaled polyethylene microfibers. The morphological properties of nascent PE fibrous materials were investigated extensively using ESEM, XRD, and DSC. Tensile mechanical analyses showed that the polyethylene microfibers produced in this nanofabrication process exhibited tensile strength comparable to those of polyethylene fibers fabricated through post-reactor processing methods. A Ni/a-diimine catalyst, 1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine nickel(II) dibromide, was supported on various mesoporous particles (MCM-41 and MSF) having different nanotube structures to produce short chain branched polyethylenes by ethylene polymerization. The effects of catalyst supporting methods and nanotube structure of the mesoporous particles on the catalyst impregnation were studied. In ethylene polymerization, some active sites of the supported catalysts showed reduced chain walking ability compared to the homogeneous counterparts. The effects of nanotube structure of mesoporous particles on ethylene polymerization activity, polymer property, and polymer particle morphology were investigated. A series of branched polyethylenes with various chain topologies were prepared with chain walking Pd- and Ni-diimine catalysts, [(ArN=C(Me)C(Me )=NAr)Pd(CH3)(NCMe )]SbF 6 and (ArN=C(An)-C(An)=NAr)NiBr21 MMAO, respectively, under different reaction conditions. In addition to topological characterizations using GPC-VIS and \3C NMR techniques, a rheological study was conducted to investigate their unique rheological behaviors. The high molecular weight polyethylenes prepared under low ethylene pressures (0.2 and 1 atm at 35°C) with the Pd-diimine catalyst exhibited extremely low viscosity and typical Newtonian flow behavior, suggesting dendritic topological structures. Changing the polymer chain topology by varying reaction conditions and/or changing the catalyst metal center significantly affected polymer rheological properties. These novel dendritic polyethylenes have potential in such applications as lubricants, printing materials, and polymer processing additives. A one-step polymerization process employing binary tandem catalyst systems, (,,5 -C5H4CMe2C6H5)TiCh/MMAO coupled with [(,,5_ C5Me4)SiMelBuN) ] TiCh/MMAO or rac-Me2Si(2- eBenz[e ] Ind)2ZrChl MMAO, was applied and investigated for an efficient production of ethylene- exene copolymers with ethylene as the sole monomer. This strategy utilized a tandem action between the two single-site catalysts. During the polymerization, the trimerization catalyst, (,,5 -C5H4CMe2C6H5)TiCh/MMAO, trimerized ethylene to form I-hexene, while the other metallocene catalyst component efficiently copolymerized the in situ produced I-hexene with ethylene to form butyl branched polyethylenes. A simple adjustment of the molar ratio of catalyst components, catalyst combination, and reaction conditions effectively regulated branching density in the copolymers. The branching structure of the copolymers was confirmed and characterized using 13C NMR and DSC. This one-step process has clear advantages over the two-step procedure for linear low density polyethylene (LLDPE) production. A binary catalyst system, consisting of a bisiminepyridine iron catalyst [(2-ArN=C(Me))zCsH3N]FeCh (Ar = 2,6-C6H3(Me)z)/MMAO and a zirconocene catalyst rac-Me2Si(2-MeBenz[e]Ind)zZrCh/MMAO, was developed to synthesize isotactic polypropylenes grafted with atactic side chains. The iron catalyst in situ generated I-propenyl ended atactic polypropylene macromonomer, while the zirconium catalyst incorporated the macromonomer into the copolymer. The effects of reaction conditions, such as catalyst addition procedure and ratio of the two single-site catalysts on branching frequency were studied. Copolymer samples having branching densities up to 8.6 aPP side chains per 1000 iPP monomer units were obtained. Novel long chain branched isotactic polypropylenes were produced by propylene copolymerization with a very small amount of non-conjugated diene, 1,7 -octadiene or I,9-decadiene, with a single-site metallocene catalyst, racMe2Si(2-MeBenz[e] Ind)2ZrCh/MMAO. By controlling the amount of dienes in the polymerization system, isotactic polypropylenes with different long chain branching densities were effectively produced. Rheological studies showed that the presence of long chain branching significantly enhanced polymer melt strength and improved polymer processibility. This in-reactor method is more effective and convenient than other post-reactor processes used for producing LCBedPP.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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