Ziegler-Natta Polymerisation
This time, we will see another method of polymerisation which use transition metal complex chemistry in the process. This polymerisation is called Ziegler-Natta (ZN) polymerisation and it is named after German chemist Karl Ziegler and Italian chemist Giulio Natta who developed this polymerisation initially. Ziegler and Natta were awarded 1963 Nobel Prize in chemistry.
ZN polymerisation is the only way to polymerise propylene and the best polymerisation for ethylene. Besides that, it is also very important industrially as it made Karl Ziegler a millionare from his patent and the annual production of polypropylene and polyethylene from this process is 35 million and 60 million tons in 2004. Besides that, the main advantage of this process is it produces highly stereoregular polymer, an isotactic or a syndiotactic polypropylene and linear polyethylene, which cannot be achieved in chain polymerisations.
ZN polymerisation consists of Ti(IV)-based catalyst and Al-based co-catalyst such as AlMe3 or (AlMeO)n.
The initial step of ZN polymerisation is transmethylation between Ti catalyst and Al co-catalyst which then followed by the abstraction of Me group. This methyl group is removed by Al co-catalyst which is a Lewis acid to create a vacant site. This vacant site then can be used to coordinate the ethylene (or propylene) monomer via π bond complex. A migratory insertion than happen with the attack of Me group to π-bound ethylene to create again a vacant site. This process is repeated to form a long chain polypropylene which then terminated via β-elimination chain transfer.
However, this polymerisation is so unefficient as only 1-6 mol. % is catalytically acitve and heterogenous ZN catalyst is so ill-defined and very impure. The mechanism is still a matter of debate due to its difficulty to study. In term of polydispersity, this polymerisation produce a broad MWD with Mw/Mn between 5 to 30.
ZN polymerisation can be carried out in two ways, homogenous solution or heterogenous mixture. In homogenous solution, such as TiCl4/AlEt3, gives syndiotactic polypropylene at low temperature but an atactic polypropylene at higher temperature. At heterogenous mixture, the reaction occurs at the interface such as the polymerisation of acetylene. Heterogenous ZN polymerisation can also give isotactic polypropylene but normally it is expected to see a syndiotactic polymer. This might be due to a surface effect which make the preference to form an isotactic polymerisation. Furthermore, using Ti catalyst with chiral ligands can produce isotactic polymer in homogenous solution.
Another example of important ZN polymerisation is polymerisation of acetylene which give polyacetylene. This polymer can be oxidised using halogen to form a conducting polymer with conductivity up to 105 S cm-1; conductivity of copper is around 108 S cm-1.
This ZN polymerisation of acetylene was pioneered Shirakawa, MacDiarmid, and Heeger led to a new class of polymer which is conducting polymers and it was awarded Nobel Prize in chemistry in 2000.
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| K. Ziegler (left) and G. Natta (right) |
ZN polymerisation is the only way to polymerise propylene and the best polymerisation for ethylene. Besides that, it is also very important industrially as it made Karl Ziegler a millionare from his patent and the annual production of polypropylene and polyethylene from this process is 35 million and 60 million tons in 2004. Besides that, the main advantage of this process is it produces highly stereoregular polymer, an isotactic or a syndiotactic polypropylene and linear polyethylene, which cannot be achieved in chain polymerisations.
ZN polymerisation consists of Ti(IV)-based catalyst and Al-based co-catalyst such as AlMe3 or (AlMeO)n.
 |
| ZN polymerisation using Ti(IV) catalyst analogue |
The initial step of ZN polymerisation is transmethylation between Ti catalyst and Al co-catalyst which then followed by the abstraction of Me group. This methyl group is removed by Al co-catalyst which is a Lewis acid to create a vacant site. This vacant site then can be used to coordinate the ethylene (or propylene) monomer via π bond complex. A migratory insertion than happen with the attack of Me group to π-bound ethylene to create again a vacant site. This process is repeated to form a long chain polypropylene which then terminated via β-elimination chain transfer.
However, this polymerisation is so unefficient as only 1-6 mol. % is catalytically acitve and heterogenous ZN catalyst is so ill-defined and very impure. The mechanism is still a matter of debate due to its difficulty to study. In term of polydispersity, this polymerisation produce a broad MWD with Mw/Mn between 5 to 30.
ZN polymerisation can be carried out in two ways, homogenous solution or heterogenous mixture. In homogenous solution, such as TiCl4/AlEt3, gives syndiotactic polypropylene at low temperature but an atactic polypropylene at higher temperature. At heterogenous mixture, the reaction occurs at the interface such as the polymerisation of acetylene. Heterogenous ZN polymerisation can also give isotactic polypropylene but normally it is expected to see a syndiotactic polymer. This might be due to a surface effect which make the preference to form an isotactic polymerisation. Furthermore, using Ti catalyst with chiral ligands can produce isotactic polymer in homogenous solution.
Another example of important ZN polymerisation is polymerisation of acetylene which give polyacetylene. This polymer can be oxidised using halogen to form a conducting polymer with conductivity up to 105 S cm-1; conductivity of copper is around 108 S cm-1.
This ZN polymerisation of acetylene was pioneered Shirakawa, MacDiarmid, and Heeger led to a new class of polymer which is conducting polymers and it was awarded Nobel Prize in chemistry in 2000.
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