Catalysis
Farabi Hossain; Md Enamul Hoque
Abstract
This mini-review addresses the burgeoning requirements for environmentally friendly processes in the polymer industry, focusing on recent progress in developing catalytic systems for sustainable olefin polymerization. Improvements in homogeneous and heterogeneous catalyst design have resulted in greater ...
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This mini-review addresses the burgeoning requirements for environmentally friendly processes in the polymer industry, focusing on recent progress in developing catalytic systems for sustainable olefin polymerization. Improvements in homogeneous and heterogeneous catalyst design have resulted in greater control over polymer properties (molecular weight, tacticity, comonomer incorporation). Particularly, Group 4 metallocene and post-metallocene catalysts have enjoyed high activity and a degree of control over polymer microstructure. Late transition metal catalysts (nickel and palladium complexes in particular) afford unique advantages in producing branched polyethylene and conducting polymerizations in polar solvents. Computational studies and novel support strategies have yielded improvements for heterogeneous Ziegler-Natta catalysts. Potential for reducing environmental impact through green catalysis approaches include enzyme-based systems, ionic liquids, and photoactivated catalysts. These catalytic advances have permitted previously unavailable control over polymer properties, including molecular weight distribution and functional group incorporation. Challenges remain regarding the stability of the catalysts, incorporation of comonomer, and economic feasibility. Future work focuses on new ligand design, extending the monomer scope to include renewable feeds tocks and improving the activation procedures. There is a critical need to integrate computational modeling, machine learning, and advanced characterization techniques to facilitate catalyst discovery and understanding of complex structure-property relationships. In general, this review demonstrates the ongoing development of olefin polymerization toward more sustainable practices and describes the important role of advanced catalytic systems in determining the future of the polymer industry).
Olefin polymerization and copolymerization
Wei Wang; Taoyi Zhang; Liping Hou
Abstract
The present paper systematically studies the homo- and copolymerization of ethylene or propylene using metallocene as catalyst and diethyl zinc as chain transfer agent to obtain the polyolefin waxes with narrow molecular weight distribution and with a high activity. The molecular weight of the resultant ...
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The present paper systematically studies the homo- and copolymerization of ethylene or propylene using metallocene as catalyst and diethyl zinc as chain transfer agent to obtain the polyolefin waxes with narrow molecular weight distribution and with a high activity. The molecular weight of the resultant polymer could be controllable by the concentration of diethyl zinc quantitatively. The introduction of a-olefin into the ethylene polymerization system would shield the chain transfer action, and the shielding effect in propylene (co) polymerization is more serious, due to the mass transfer resistance of the substituents on the monomers. Branched comonomer and long chain comonomer provide stronger shielding effect. The regression results show that the order of the chain transfer reaction of propylene polymerization is smaller than that of ethylene polymerization, and the order of the chain transfer reaction of copolymerization is less than that of homopolymerization. It indicates that the substituent on the monomer would result in the deviation of the regression data from the ideal primary reaction order.
Catalysis
Walter Kaminsky; Mercia Fernandes
Abstract
Beside Ziegler-Natta and Phillips catalysts the development of methylaluminoxane (MAO) as cocatalyst in combination with metallocenes or other transition metal complexes for the polymerization of olefins has widely increased the possibilities in controlling the polymer composition, polymer structure, ...
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Beside Ziegler-Natta and Phillips catalysts the development of methylaluminoxane (MAO) as cocatalyst in combination with metallocenes or other transition metal complexes for the polymerization of olefins has widely increased the possibilities in controlling the polymer composition, polymer structure, tacticity and special properties with high precision. These catalysts allow the synthesis of isotactic, isoblock, syndiotactic, stereoblockor atactic polymers, as well as polyolefin composite materials with superior properties such as film clarity, tensile strength and lower content of extractables. Metallocene and other single site catalysts are able to copolymerize ethene and propene with short and long chained a-olefins, cyclic olefins, or polar vinyl monomers such as ethers, alcohols or esters, especially, if the polar monomers are protected by aluminum alkyls. Different vinyl ethers such as vinyl-ethyl ether, vinyl-propyl ether, vinyl-hexyl ether, and 2,7-octadienyl methyl ether (MODE) were copolymerized with olefins using triisobutyl aluminum as protecting agents. Polar monomers could be incorporated into the polymer chain by up to 16 mol%. Such copolymers show better gas barrier and surface properties, as well as solvent resistance and they are suitable for blends of polyolefins with polyethers and other polar polymers because of an excellent adhesion of the two polymers.
Catalysis
Helmut G. Alt
Abstract
EEthylene polymerization catalysts became available in an enormous variety. The challenge in this research is to find catalysts that are able to connect ethylene molecules in such a way that not only linear chains are produced but variations like branched materials that possess very interesting mechanical ...
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EEthylene polymerization catalysts became available in an enormous variety. The challenge in this research is to find catalysts that are able to connect ethylene molecules in such a way that not only linear chains are produced but variations like branched materials that possess very interesting mechanical properties like linear low density polyethylene (LLDPE). In this contribution, three different types of catalysts are presented that are able to do not only one job at a time but three. These are “intelligent catalysts”. Catalysts of type 1 are homogeneous metallocene complexes that can be activated with methylaluminoxane (MAO).With ethylene they produce their own support and they become heterogeneous catalysts (self-immobilization) and they prevent fouling in polymerization reactors. The produced resin has evenly distributed ethyl branches (without a comonomer) with unique properties and the MAO that is necessary in the activation step can be recycled. Catalysts of type 2 are dinuclear complexes with two different active sites. One centre can oligomerize ethylene and the other one can copolymerize the in statu nascendi produced oligomers with ethylene to give branched LLDPE (a molecule as the smallest reactor for LLDPE) and/or bimodal resins.Catalysts of type 3 are MAO activated iron di (imino) pyridine complexes that are able to oligomerize ethylene to give not only oligomers with even numbered carbon atoms but also odd numbered ones. In this reaction, one catalyst does three jobs at a time: oligomerization, isomerization and metathesis of ethylene.
Catalysis
Toshiaki Taniike; Keisuke Goto; Minoru Terano
Abstract
Heterogeneous Ziegler-Natta and homogeneous metallocene catalysts exhibit greatly different active sitenature in olefin polymerization. In our previous study, it was reported that MgCl2-supported titanocenecatalysts can generate both Ziegler-Natta-type and metallocene-type active sites according to the ...
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Heterogeneous Ziegler-Natta and homogeneous metallocene catalysts exhibit greatly different active sitenature in olefin polymerization. In our previous study, it was reported that MgCl2-supported titanocenecatalysts can generate both Ziegler-Natta-type and metallocene-type active sites according to the type of activators.The dual active site nature of the supported titanocene catalysts was further explored in the present study: The influence of the ligand structure of titanocene precursors was studied on the nature of active sites when supported on MgCl2 in ethylene and propylene homopolymerization, and ethylene/1-hexene copolymerization. It was found that the reducibility of titanocene precursors by alkylaluminum is closely related to the appearance of the dual active site nature, while the kind of olefin did not affect the type of active sites formed during polymerization. The Ziegler-Natta-type active sites produced poorly isotactic polypropylene and less branched polyethylene, while the metallocene-type active sites produced atactic polypropylene and exhibited much higher incorporation efficiency for 1-hexene.
Computational chemistry & molecular modeling
Naeimeh bahri-Laleh; Laura Falivene; Luigi Cavallo
Abstract
In this study we have tested the ability of a standard DFT computational protocol to reproduce the experimentally obtained stereoselectivity of 26 different C2-symmetric zirconocene catalysts active in propylene polymerization. The catalysts were chosen for their relevance in metallocene catalyzed ...
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In this study we have tested the ability of a standard DFT computational protocol to reproduce the experimentally obtained stereoselectivity of 26 different C2-symmetric zirconocene catalysts active in propylene polymerization. The catalysts were chosen for their relevance in metallocene catalyzed polymerization of propylene. To this end, primary insertion of both si- and re-propylene enantiofaces into the Zr-CH2-CH(CH3)2 bond was considered to simulate the growing chains step. The energy difference between these two transition states, ΔEre-si, was taken as a measure of the stereoselectivity (pentad: mmmm%) of different catalysts. The results clearly indicated that there was a good agreement between ΔEre-si and the mmmm% values, so that greater ΔEre-si could correspond to higher mmmm%. A model was fitted to the experimentally obtained mmmm% against theoretical ΔEre-si. The coefficient of determination (R2) of the resultant plot was 0.9793, which indicated a good accuracy of the model. Finally, to quantify the steric role of the studied ligands in the observed stereoselectivity, the analysis of the buried volume (VBur) and of the steric maps was performed for two representative complexes. The images revealed that a greater asymmetric localization of the %VBur around the metal center led to a higher mmmm% in the resultant polymer.
Catalysis
Laura Boggioni; Incoronata Tritto
Abstract
Highly active metallocenes and other single site catalysts have opened up the possibility of polymerizing cycloolefins such as norbornene (N) or of copolymerizing them with ethene (E) or propene (P). The polymers obtained show exciting structures and properties. E-N copolymers are industrially produced ...
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Highly active metallocenes and other single site catalysts have opened up the possibility of polymerizing cycloolefins such as norbornene (N) or of copolymerizing them with ethene (E) or propene (P). The polymers obtained show exciting structures and properties. E-N copolymers are industrially produced materials, with variable and high glass transitions depending on the wide range of their microstructures. By realizing the possibility in great variety of stereoregularity of propene and norbornene units and the difference in comonomer distribution, P-N copolymers were expected to have fine tuned microstructures and properties. Moreover, P-N copolymers should be characterized by higher Tg-values than E-N copolymers with the same norbornene content and molar mass. A review of the state of the art of P-N copolymerization by ansa-metallocenes of C2 symmetry, namely rac-Et(Ind)2ZrCl2 (I-I) and rac-Me2Si(Ind)2ZrCl2 (I-II), and rac-Me2Si(2-Me-Ind)2ZrCl2 (I-III), and of catalysts of Cssymmetry, namely (tBuNSiMe2Flu)TiMe2 (IV-I) and derivatives, is given here. Special emphasis is given to microstructural studies of P-N copolymers, including stereo- and regioregularity of propene units as well as of comonomer distribution, stereoregularity of norbornene units, and the structure of chain end-groups. This information allows us to find a rationale for the catalytic activities and the copolymer properties.
