Synthesis and Characterization of Polyethylenes by Nickeldiimine and Metallocene Catalysts

Abstract

<p>Polyethylene (PE) synthesis has shown tremendous progress towards the production of polymers with innovative chemical and physical properties as a consequence of the development of new classes of single-site catalysts, including metallocene and the more recently developed nickel-diimine catalysts. This thesis describes the synthesis and characterization of a series of PE materials using a variety of metallocene and nickel-diimine catalysts. The main objective of this thesis work is to develop a better understanding of the influences of polymerization conditions and catalyst systems on polymer properties. A detailed investigation of the polymerization of ethylene by (adiimine) nickelCII) catalysts was first carned out. The catalysts used were ((ArN=C(An)-C(An)=NAr)NiBr2 and (ArN=C(H)- C(H)=NAr)NiBr2; where An = acenaphthene and Ar = 2,6-(i-PrhC6H3). Changes in the a-diimine backbone structure showed remarkable effects on the polymer microstructure as well as catalyst activity. For all the three catalysts, increasing ethylene pressure or reducing polymerization temperature led to a reduction in the chain walking rates, and thus reduced the melting temperature and crystallinity of the polymer produced. A Nila-diimine catalyst, 1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine nickel(II) dibromide was then supported on MMAO-treated silica to produce short chain branched PEs by ethylene polymerization. The supported catalyst gave far lower activity than the homogeneous catalyst. Depending on polymerization conditions, two active site populations were observed during polymerization using supported catalyst; one remained fixed on the surface of the support, and the other was extracted from the support exhibiting the same polymerization behavior as the homogeneous catalyst. A tandem homogeneous catalytic system was also used for the synthesis of ethylene-l-hexene copolymers from ethylene as the sole monomer. The catalytic system employed the tandem action between an ethylene trimerization catalyst, (11 5- CsH4CMe2C6Hs)TiCh(1)/MMAO, and a copolymerization metallocene catalyst, [(115-sMe4)SiMe2CBuN)]TiCh(2)/MMAO. During the reaction, VMMAO in situ generated I-hexene with high activity and high selectivity and simultaneously 2/MMAO copolymerized ethylene with the produced I-hexene to generate butyl branched PE. By simple manipulation of the catalyst molar ratio and polymerization condition, a series of branched PE samples were efficiently produced. A binary catalyst system, consisting of a Nila-diimine catalyst, 1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine nickel(lI) dibromide(l) and a zirconocene catalyst, rac-ethylenebis(indenyl) zirconium dichloride](2), was developed to synthesize a series of reactor blends of linear and branched PEs. The nickel diimine catalyst generated branched PE, while the zirconium catalyst produced linear PE. At various levels oftemperature, ethylene pressure, and Catalyst 2 fraction, PE blends with different melting behaviors were produced. GPC-V analysis of the PE samples showed monomodal molecular weight distributions with narrow polydispersities.</p>