 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. In this course, we have been discussing an important topic, which is olefin polymerization, and in this context over the last few lectures, we were particularly focusing our discussion on polyolefin, particularly polyethylene, and various processes that were prevalent in the beginning for preparing various grades of polyolefin, which can be LDPE load density polyethylene, or LLDPE linear load density polyethylene, or HDPE high density polyethylene. In our discussion, we have also looked at several industrial processes starting from that of ICI, or radical based polymerizations, to produce LDPE to heterogeneous transition metal based chromium centric polymerization method developed by Philips, and properly known as Philips process in 1956, to be more precise, for producing HDPE high density polyethylene, and also we have looked at how improvisation of the process by replacing the initial high valent chromium trioxide as the precursor with low valent chromocene by union carbide led to accessing these polymers in a more efficient way. We have also seen how union carbide by control the chain length by varying the introduction of hydrogen gas in the polymerization chamber, thereby carrying out modification of the Philips process for producing HDPE, and successfully use this Philips process to produce LLDPE or linear load density polyethylene. In this context, we have also noted the contribution of union carbide in a process called Unipole process, where they did away with conventional reactor and used fluid based reactor for producing both LLDPE and high density polyethylene. Now, in and around at this time, we have also discussed about the development of signal Nata catalysis or the origin of signal Nata catalysis, and what we have noted that signal Nata catalysis started from an investigation on an aluminum based catalysis that would produce long chain compounds of a chain length of up to C200, and which was commonly called as the off-power reaction. The off-power reaction in short is given by 100 bar pressure produced, and then there is this competing reaction called dehydroalumination that resulted in aluminum hydrate compound plus alpha olefin. Now, it is while observing the production of long chain aliphatic compounds that Ziegler observed an opposite trend, and the opposite trend was that instead of obtaining polymers, what was obtained was two molecules of ethylene gave away to one butene in quantitative yields, and this was a very chance discovery, which showed that something opposite to what was expected was long chain polymers, long chain linear long chain aliphatic compounds. This was what was expected from this reaction, but what was found was one butene, which is just a dimer of ethylene, and this is sort of a dimerization process, and that was sort of a chance discovery, which was later attributed to nickel impurities in the reaction vessel, and this in short is called nickel effect, and this was in short called the nickel effect. Now, this nickel effect is what later on was the origin or source for the discovery of this famous Ziegler-Natta catalyst. So, in search of other transition metals that give premature termination of off bow reaction, this is exactly similar to what was the effect of the nickel effect. Another unexpected observation was made. So, this is kind of very interesting because if you look, so it is sort of like two certain deputies led to one big Nobel Prize winning discovery. The first one itself was the nickel effect, because people were studying off bow reaction, and was trying to figure out ways to increase and make linear long chain polymers. However, they came across this nickel effect, which led to just the dimer. Now, once the nickel effect was found, they were looking for other transition metal that would behave similar to that of nickel effect, and they were looking for other transition metal that would have similar effect like nickel effect on the off bow reaction, and which would also give dimerized or oligomerized product. However, the other expected observation was that during this search for other metal, they found another metal which could even give much larger polyethylene polymer chains, which was again like going back to what they had really wanted in the first place. So, the way we say that two left brings us back to the same position, similarly two errors in the way of process of discovery led to exactly the same thing what they had wanted in the initial stages that wanted to make long chain polymers, and that was indeed the case, but that has to be found by first the nickel effect, and then another unexpected observation that was made, which led to the Ziegelnatter polymerization. So, let us see this unexpected polymer. So, it was found TiCl4 and Et2AlCl gave HDPE high density polyethylene at low pressure of one bar. So, this is indeed a significant discovery, because now unlike in radical polymerization, one could, where very extremely high pressures are needed, pressure of around 2000 PSI were needed to produce polymers, which are not really very long in chain like LDPE, this TiCl4 triethyl ethyl aluminum chloride efficiently gave HDPE high density polyethylene at a very low pressure of one bar. So, this was indeed a radical discovery at that point of time, because now we have a catalyst, which is titanium based catalyst that can polymerize ethylene to give long chain linear polyethylene at very low pressure of one bar, and these are highly crystalline materials without much branching. So, that is the uniqueness about this Ziegelnatter polyethylene discovery, and this is how it came into being. First, the observation of nickel effect by looking at by getting one butene from of power reaction, and then the second thing was they wanted to see what other transition metal can give similar premature termination of above reaction, but instead they ended up finding a transition metal, which could give extremely long chain polyethylene, then even what the above reaction can give. So, with this, let me just give the reaction HDPE Dalton. So, this is about 10000 to 10000 Dalton molecular weight, and this HDPE obtained were almost linear chains with no branching. So, this was a very important discovery at that point, which was made by Ziegler, that now he has found a system, which is titanium tetrachloride with diethyl aluminium chloride at very ambient condition 25 degree centigrade and one bar pressure, and could produce high density polyethylene of very high molecular weight ranging from 10000 to 100000 Dalton, and that produced linear polymer chains with no branching. So, this was indeed a great discovery by Ziegler, and what he found that actually the catalyst is beta surface alkylated beta TiCl3, which is obtained from this. The real catalytic species is beta form of TiCl3, which is surface activated beta TiCl3. Now, this was the catalyst, which was produced, which was producing high density polymer. Now, for this process was initially called as Ziegler, 1955, discovered this polyethylene or HDPE and this molar mass 10 to the power 4 to 10 to the power 5 Dalton, and this process was that time known as Mulheim normal pressure polyethylene process, Mulheim normal pressure polyethylene process. So, what Ziegler found that actually this is surface alkylated beta TiCl3 catalyst was carrying out the process, and they were obtained from TiCl4 and diethyl aluminum chloride, and for obtaining the lower molecular weight hydrogen was added, H2 was added to control the chain length polymer chain length. So, this is similar to what we had observed in case of the philip process as well, where hydrogen was added to control the chain length, and then philip process was used for making LLDPE. Now, what were the advantages? Advantages of Ziegler-Nut polymerization, let me just highlight. The first thing is that it could produce at very low pressure, this is the number one advantage. Second thing is that the temperature is also at room temperature ambient condition. So, this is the second advantage. Third advantage is it was producing high density polyethylene, which is almost linear and without almost no branching, and fourth advantage is the control of polymer chain length by increasing by adding hydrogen in variation. Now, there are also limitations associated with the Ziegler process, limitation says that being heterogeneous, the Ziegler process is a large broad distribution of molecular weight of polymer mass was observed. So, that means a broad distribution, the chain lengths are varied a lot as we have seen from 10 to the 4 to Dalton, and that means that these are multi-site catalysis. So, to overcome this limitation, the efforts were put into develop single-site catalysis to overcome this disadvantage, single-site catalysts were developed. These single-site catalysts would give a narrow distribution of molecular weight, and they would give a pure or more narrower and well-behaved type of polymer molecular weight. So, this was the story about the first half of the Ziegler-Natta catalysis. Now, as you can see, the Ziegler-Natta catalysis bears the name of two contributors, one is called Ziegler, and the second is Julio-Natta. The first half of the story is about the ethylene polymerization, which was identified and first developed by Ziegler. So far, we have spoken about the Ziegler portion of the Ziegler-Natta catalysis. Now, we are going to move on to the second half of the Ziegler-Natta catalysis, which is about the Natta portion of the Ziegler-Natta catalysis. Now, in terms of chemical point of view, Ziegler-Natta catalysis uses polymerization of ethylene to give polyethylene, which was Ziegler's contribution, and Natta's contribution had been extending Ziegler's contribution to polypropylene. So, in that way, one can say that the real glory about the discovery of polymerization actually lies with Ziegler, because Ziegler is the one who had first observed the polymerization of ethylene to HDPE under ambient condition and one bar for ethylene. So, extending the same method to propylene would be an obvious extension of the existing knowledge, but the way things spanned out is that Professor Natta came and worked in the lab of Professor Ziegler in one of the summers, and he understood the implication of propylene that might have in the same catalysis, and so he went and extended the polymerization reaction to propylene, and he very well knew that propylene having a methyl group would have a stereoregular properties in the polymer, and that is he developed that aspect. Polyethylene, on the other hand, does not have any stereoisomerism issues or stereoregularity issues, because it is just an ethylene backbone, whereas polypropylene had this stereoregularity of the methyl group, and that is another dimension to the polymer that Professor Natta conceived and that he applied successfully in extending the Ziegler's discovery to propylene substrates. So, today with that we come to the conclusion of this lecture, in which we have looked into the historical perspective of the development of Ziegler-Natta catalysis right from the time of the Philip process, where the process industrial processes were being developed for producing various kinds of polyethylene from LDPE, LLDPE, or HDPE, and then we had also noted that how the discovery of how the study of Abbao reaction to produce long chain polymers led to the observation of simple quantitative dimerization of ethylene, and that was later accounted for the presence of impurity of nickel in the reactor, and that is what was called nickel effect. Subsequent study of the nickel effect for finding another or other transition metal, which would give now the premature termination of Abbao reaction to give one butene, however led to an opposing trend or opposing discovery that in presence of early transition metal like titanium tetrachloride and diethyl aluminum chloride, extremely linear long chain high density polyethylene were obtained at room temperature and one bar pressure. So, this was a great discovery again which was unexpected and that is what led to polyethylene polymerization. So, this is the story which was developed by Professor Karl Ziegler and the limitation about the advantage of the Ziegler-Natta polymerization or Ziegler polymerization is that this produces HDPE high density polyethylene at room temperature one bar pressure, and also the chain length can be controlled by metered addition of hydrogen, and the limitation what it had is that it has large distribution of polymer molar mass, the huge distribution of the molecular weight, and that suggested that this is a multi site catalysis, and hence to improve on that the focus changed to synthesizing single site catalysts for making more well behaved polymer with narrow polydispersity index. So, with this we come to end of the Ziegler story of the Ziegler-Natta polymerization, and we are going to be taking up Natta's story the next as a part of the Ziegler-Natta polymerization, and we would see how Professor Julia Natta translated the knowledge developed by Professor Ziegler for polymerizing polyethylene, and used the same on propylene to produce polypropylene, and how that was developed in subsequent ways. So, with that I again thank you for being with me in this class, and we are going to be discussing more about this exciting story of Ziegler-Natta catalysis as we meet next in the next lecture. Till then, goodbye and thank you.