Synthesis of Supported Metallocene Catalysts and Application to Copolymerization of Ethylene with 4-Methyl-1-pentene

In the family of commodity polyolefins, linear low density polyethylene (LLDPE)a copolymer of ethylene and -olefinis the latest addition. LLDPEs have wide applications. Therefore, all the nine (9) Saudi petrochemical industries produce LLDPEs. The total production volume is about 4 million metric tons per annum which places Saudi Arabia as the largest LLDPE producer in the Middle East and within top five in theworld. LLDPEs are mostly produced by low pressure conventional supported catalysts. In this regard, the versatility of catalyst type and polymer synthesis methodology creates new knowledge. This eventually makes new polymeric materials for advanced applications. Metallocenes are a modern innovation in polyolefin catalysis, and have several advantages over the Ziegler-Natta catalysts. Hence, the overall objective of the proposed study is to synthesize various supported metallocene catalysts with varying anions, and apply them to prepare ethylene-1-hexene and ethylene-4-methyl-1-pentene copolymers. In this work, the supported metallocene catalysts were prepared using the following order: dehydroxylated silica (alone or functionalized with ClSiMe2Cl), methylaluminoxane (MAO) cocatalyst and (nBuCp)2ZrCl2). These catalyst systems were applied to homo- or copolymerize ethylene with 4-methyl-1-pentene as well as its isomer 1-hexene, using solution, in-situ, and slurry polymerization trials. In the latter two modes of polymerizations, no MAO was fed separately into the reactor. The variations in the polymerization processes varied the MAO cocatalyst anions. The effects of the resulting cocatalyst anions on polymerization performance and the product properties were elaborately studied by determining the reactor operability and catalyst productivity, and by characterizing the polymers using gel permeation chromatography, Fourier Transform infra-red spectroscopy, 13C NMR spectroscopy, differential scanning calorimetry (DSC), successive self-nucleation and annealing (SSA) thermal fractionation, and crystallization fractionation technique (Crystaf). This study has advanced the fundamental understanding of supported metallocene catalysis and development of ethylene--olefin copolymers as follows. The functionalizing linker ClSiMe2Cl did not affect the catalyst productivity. The supported systems showed similar productivity as the solution polymerization which is hardly reported in the literature. The distributive as well as the bulk properties of the polymers varied with the MAO anion design. This understanding produced selected application-specific polymers. The copolymerization mechanism was found to be well predicted by the first order Markovian (terminal) model. ClSiMe2Cl suppressed chain transfer reactions in the supported catalyst. Microstructural properties, determined using 13C NMR/SSA-DSC, elaborated how supported MAO anions made copolymer backbones with compositional heterogeneity. The nonisothermal Avrami-Erofeev crystallization model well matched the experimental data notably for Avrami-Erofeev index of 2 to 3. This shows bi- and tri-dimensional crystal growth. The apparent crystallization activation energy, predicted by the Avrami-Erofeev crystallization model, did not depend on the cooling rates. This finding, therefore, refutes the concept of instantaneous variable crystallization activation energy, articulated in the literature. Furthermore, the crystallization model successfully elaborated the influence of compositional heterogeneity on copolymer crystallization behavior. So far as product development is concerned, this study shows how to synthesize LLDPEs that are blocky and more degradation-resistant, have improved processability (fairly low melting point and crystallinity), and that can be used for load-bearing applications