 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing about olefin polymerization in the last few lectures. In this regard, we have covered several topics including polyolefin classifications from a material perspective, like types of different types of polymers that can be obtained from polyolefin polymerization based on the material properties like something which is elastic, something which is plastic, something which depends on dura plus, depends on temperature, so on and so forth. And we have also looked into polymer characterization based on the processes. For example, the condensation polymers or addition polymers, the reaction they use for making these polyolefin polymers. As well as we have distinguished or classified polymers based on the mechanisms, step growth mechanism and chain growth mechanism. And we have also looked into the polyethylene from the perspective of their different types, classes of polyethylene polymers that are known to the historical perspective of their development from the process onwards to nickel effect and then subsequent discovery by Ziegler to produce high density polyethylene at room temperature under 1 bar pressure using titanium as catalyst. Now, moving on, we have also looked into polypropylene polymers and polypropylene was a polymerization of propylene, which was mainly been attributed to the development by Julia Nata. And we have seen that how polypropylene polymers are different from the polyethylene polymers, mainly from the orientation of this methyl side chains of the propylene group, which can be atactic, syndiotactic, stereoblock, hemi, isotactic, so on and so forth. So, today, we are going to go deep into this polypropylene polymerization, particularly from the mechanism and the catalyst development point of view. Now, polypropylene, this is Ziegler Nata. Now, the catalyst gives isotactic polymers. Now, this has two main issues with it. The first is that why isotactic polymer, which means that all the methyl groups are on the same side, stacked on the same side, they all appear in the same side. Overall, all the carbons have the same configuration, and this led to two questions regarding the mechanism of polymerization with Ziegler Nata catalysis. The first question being, why is polypropylene produced in Ziegler Nata process isotactic? Now, this is an important question, because it says that how come a polymer, which is chiral in nature and isotactic in nature, produced from a catalyst, which is a chiral or non chiral. So, there is an element of suspicion as to how come a chiral catalyst produces a chiral product. So, this is a counter intuitive question, which does not satisfy the intuition that one can obtain isotactic polymer from just TICL4 and diethylaluminium chloride, which is the Ziegler Nata catalyst, which itself is a chiral. So, this is the important question, which throws out some more insight into the process of polypropylene polymerization under Ziegler Nata condition. The second thing, which comes out of it, is how does head-to-tail linking of propylene monomer occur? This also is an important question, because head-to-tail linking means that all the monomers are linked in the same fashion, so if this becomes the head and this becomes the tail, so this is head, this is tail, this is head, this is tail. So, head-to-tail linking happens all the time, resulting in a polymer of this type, whereas there can be possibility of tail-to-tail, head-to-head, other possibilities do not occur. So, these questions throw out important challenges on the mechanism, the mechanistic part that should explain why such thing is observed. Now, the mechanism for polyolefin is best given by a mechanism, which is called Orlman-Cosse mechanism, which was given in 1964, and which states that a free coordination site sees titanium-carbon bond crucial for the polymerization. Now, at this juncture with this information, this Orlman-Cosse reaction mechanism does not explain both the questions we had posed earlier about the isotacticity of the polypropylene or about the head-to-tail, but this sort of explains the second part, the head-to-tail mechanism arising out of this process. It says that a sequence of head-to-tail linking results in chain growth, beta hydride elimination, which causes chain termination. So, this Orlman-Cosse conveniently explains that the presence of a vacant site next to a growing alkene chain is essential for the head-to-tail insertion resulting in the polymer growing polymer chain. But what it does not still yet explain is the reason for isotactic polypropylene, that is observed during the Ziegler-Nurther polymerization. So, let us just see the first, second aspect first that head-to-tail alignment through the mechanism, as is shown over here. So, this is the vacant site, which is occupied by an olefin. So, this is this insertion step on this vacant site occupied by this ethylene, resulting in a polymer chain on this arm and the vacant site on top, as is shown here. So, now the polymer chain has moved and there is a vacant site over here, and that then gets occupied by another polyethane molecule, and the reaction then proceeds further. So, the point to note is two things. One is that first thing is that there is a vacant site C2, the titanium carbon bond, followed by this vacant site being occupied by this propylene, and then the insertion of this propylene unit into the bond. As a result, this alkene chain now shifts to the site, and another vacant site is created to which this olefin now comes and bind. So, this is called migratory insertion. There is a migration of the alkene chain, which happens from this to this. So, and as a result, if one were to take a look that because of this oscillatory migratory insertion, there is a head-to-tail joining of the propylene unit. So, all the methyl are pointing towards the same. So, this explains the second question that this propylene is formed in a head-to-tail fashion, but still a lot more remains to be answered. The first question is that why is it isotactic? Isotacticity would depend on the stereochemistry of this carbon. These are all carbon attached in a head-to-tail fashion propylene, head-to-tail linking of propylene, and then the isotacticity would depend on the configuration of this carbon. For that to happen, then the catalysts have to be chiral in nature. That can be explained by the fact that the stereoselectivity of this Ziegler-Norder catalyst is enhanced in magnesium support. Which are to be considered in more details. So, to explain the chirality, one needs to consider the fact that there is a huge increase in activity and stereoselectivity by anchoring the Ziegler-Norder catalyst to magnesium fluoride support. Now, this is something which is an important observation that Ziegler-Norder catalyst, when attached to magnesium fluoride provides isotactic polymers, which indicates that some amount of chirality in the supported catalyst is brought about by anchoring the Ziegler-Norder catalyst onto a magnesium support. We are going to explain that. The second thing, which also comes into play is that agrostic C-alpha H2-titanium. So, agrostic interaction between C-alpha proton to titanium interaction may promote stereochemical fixing of the polymer chain and bond reorganization during the insertion step. The bond reorganization during the insertion step. This more specifically is explained by the fact that the origin of stereoselectivity arise from chiral titanium site allows enantiofacial differentiation coordination of propene. This chiral occurs by alkyl-titanium unit in octahedral environment MgCl2 crystal surface. This is a very powerful statement. It explains the nature of isotacticity. It says that the origin of stereoselectivity arises from chiral titanium site, which allows enantiofacial differentiation during the coordination of propene and that arise by the titanium alkyl unit present in octahedral environment in a magnesium Cl2 crystal. It says that when titanium alkyl unit of Ziegler-Nard catalyst with a growing polymer chain is present on a magnesium Cl2 crystal, then the overall catalytic site becomes chiral and results in isotactic polymer. This can be best explained by the orientation as is shown over here. The chiral titanium Mg is a chlorine Mg. This is the polymer chain and here is the vacant site. This is the vacant site and this is the polymer chain. Now, let us draw this configuration similarly as is shown here. This is the polymer and this is the vacant site as is marked over here. This is the polymer chain and this is the vacant site. This is how the titanium is supported in the magnesium Cl2 crystal. These are chiral titanium sites. Now, when the olefin comes, then the binding of olefin happens in this fashion because of obvious reasons. This one is favorable binding. This methyl group and this polymer chain are further apart, unfavorable binding because now the methyl chain is sterically hindering. As a result, these results in facial discrimination in propene binding coordination leading to giving isotactic polymer. This is a very important concept, which is shown that this binding is unfavorable. This is a special binding that leads to interfacial discrimination whereas this interaction between these is not favorable and that gives rise to isotactic polypropylene. With this, we come to the conclusion of today's lecture in which we have explained the result of two things which are not very obvious from the Arlman-Cossey reaction. First is the head-to-tail linking of polypropylene polymer. This is explained by the fact that there is a vacant site to the growing titanium-carbon bond of the polymer chain. The second thing is the isotactic polymer, the chiral polymer being obtained from Ziegler-Nurter catalyst. This has been attributed to the chiral site by the formation of the titanium on the magnesium chloride crystal which results in the chiral environment around the catalyst and that successfully carries out the enanti-oficial discrimination of the binding propene which results in isotactic formation of the polypropylene. With this, we come to the end of today's discussion on polypropylene more on the perspective of this interesting chemistry of polypropylene as we meet in the next class. I once again thank you for being with me in this next class. More of the exciting stuff coming up in the next class where we see the catalyst development and walk you through the various aspects of polypropylene chemistry in terms of catalyst design and polymer activity when we meet next. Till then, goodbye and thank you.