 Welcome to this course on transition metal organometallics in catalysis and biology. We have been talking about olefin polymerization particularly in the context of polyA ethylene and polypropylene polymerization and from the perspective of the catalyst development that we have been talking about in the last few lectures and in that regard we have been talking about this zirconium catalyst C2 symmetric zirconium catalyst which could give isotactic polypropylene and the catalyst that we have been discussing is this zirconium ethylene bridge complex as is shown over here. The catalyst that we have been discussing is this zirconium anser complex as is shown over here. This has this anser bridge which was playing an important role. Now, the way this catalyst is progressed is this the function of this anser bridge which had two binding sites, the one that contained the polymer site and the other that contained the vacant site, where the propylene coordinated with facial selectivity. And this is a C2 symmetric catalyst. Now, what is important over here is this the role of this anser bridge or the bridging ring clone. What is important is the role of an anser bridge. So, what is its role? Now, the way it works is this anser bridge establishes trans configuration of sandwich structure rendering its chirality. And what is that supposed to mean is if we look at the catalyst and if we depict that the taller upper Cp ligand as this, then zirconium and so this is the top Cp ligand and this is the bottom Cp ligand, which right now is in trans configuration, imparting C2 symmetry with one side occupied by a polymer chain and the other side with a chiral pocket, which can carry out facial selectivity of propylene. Now, what we have discussed which is an important point that as a part of insertion via windshield wiper mechanism, what we had seen is that when the insertion happens, then the polymer moves on to the other side, to the other side, polymer moves on to the other side with this side having the chiral pocket. What is interesting to note is that for C2 symmetric catalyst, the chirality of these two pockets are same. As a result, they are of same chirality because of the C2 symmetric nature of this catalyst. As a result, the propylene which is obtained is isotactic something, all the methyl groups reside in the same side, isotactic polypropylene, because the origin is that the chirality is the same. So, this is an important role of played by this Ansel ligand, this is number one role which we have discussed that it keeps the ligand in the trans configuration as is shown over here and because of windshield wiper mechanism as the polymer chain goes from one side to another and the pocket moves from another to another. However, the chirality of the pocket remains unchanged. The chirality of the pocket remains unchanged resulting in similar kind of insertion. Now, the second function is to increase the tilt angle, the second function of this Ansel ligand to increase tilt angle. So, what is tilt angle? Tilt angle is the angle between two siperings. For example, if this is the top sipering, this is the bottom sipering, they are joined by a ligand and this is where the metal is bound, one side polymer chain, other side vacant side. So, this angle is called the tilt angle. So, this bridge opens up the angle between the metal center or make it the larger tilt angle, which helps in polymerization. For example, replacement for example, replacing over in this catalyst, replacing CH2 linker as is shown with dimethyl SiMe2 increases the tilt angle further and result in isotactic polymer. Now, when one goes back to the drawing board, and one can conjecture that if this is the configuration of the top metallocene ligand bound to a metal, and if there is another metallocene ligand over here, and in one side is a polymer chain, and other side is a chiral pocket. So, if through windshield wiper mechanism, if one builds the polymer such that the top ligand remains the same, metal and the bottom ligand remains the same, however, now the polymer has changed sides and moved on to the other side, and the chiral pocket 2 has moved side and moved to the other side, but the symmetry of the ligand system is such that these two pockets are of opposite chirality, then the facial binding of the propene would be through different faces at each of these pockets, and that should give the product as syndiotactic polypropylene or SPP, which means the methyl groups would be alternatingly incorporated. So, this is a very nice conceptual extension, where the principle which is said is that this ligand is chosen as such with a different symmetry that the chiral pockets are of opposite chirality, resulting in the binding of propylene through different faces, and as a result alternating incorporation of the methyl group, one top or the bottom again top bottom would occur, resulting in syndiotactic polypropylene using this configuration. Indeed, the nice demonstration of this syndiotactic polypropylene was given by even in 1988, where CS symmetric catalyst was used for producing syndiotactic polypropylene, and this is best illustrated below. For example, for this complex CP zirconium, this with two arms, for example polypropylene is in another case, and this being bound to zirconium, which would undergo one insertion and second propylene binding that would give catalyst as is shown here. Now, the polymer arm has moved on to the other side with two methyl alternatingly occupying the position, and then subsequently another insertion with propylene would occur, and that would give the catalyst as shown over here. The polymer chain again moving to the other side as is shown here, and the olefin insertion happening and this perpetually propagating to give syndiotactic polypropylene. So, what is important over here is that the symmetry, the defect of CS symmetry results in two different kinds of chiral pockets of different chirality, opposite chirality. As a result, the polypropylene produced has the signature of alternating methyl group placed around each other, and that arises because selective differentiation of the two phases of the polypropylene being produced resulting in syndiotactic polypropylene. The syndiotactic polypropylene is very useful because they are more ductile materials, and transparent than isotactic polyethylene, than isotactic polyethylene. They are less stiff and less stiff and hard, and these are more suitable for applications in films and sheets, and these are more suitable for applications of films and sheets. So, this was a nice demonstration where one could control the microstructure of this polymer insertion using symmetry. So, what we saw is that C2 symmetric catalyst produced isotactic polypropylene, whereas CS symmetric catalyst produced syndiotactic polypropylene, and then the catalyst development moves on to another level, whereby as per using different strategies, one could prepare polymers of use, and weimouth in this context weimouth had shown that conformational flexibility could give rise to formation of stereoblock polypropylene. Single site catalyst metallocene catalyst to be valuable for producing different types of polypropylene. For example, C2 symmetric metallocene catalyst produced isotactic polypropylene, or IPP, and CS symmetric metallocene catalyst produced syndiotic polypropylene, or STP. So, with this, this was a nice demonstration of the scope of symmetry, modulation of symmetry resulting in incorporation of polymer in a stereoregulate fashion. The top one showed incorporation of polymer through one particular phase of the propene, whereas the bottom one showed incorporation of propylene through two simultaneously alternating between two different phases of the propene. So, with this, we come to an end of today's lecture, where we had seen the effect of C2 symmetry in producing isotactic polymer, as well as a nice work by events, which showed the effect of CS symmetry producing alternating syndiotactic polymers. Syndiotactic polymers are more soft, more ductile and transparent than isotactic polymers, and have relevant applications for films and shades, and these are made by metallocene-based homogeneous catalysis bearing CS symmetry. So, with this, I come to the end of today's lecture. We are going to be looking at the effect of catalyst design on the polymer properties in bit more detail, the catalyst evolution of single-site metallocene-based catalyst, starting from Ziegler data heterogeneous catalyst for propylene polymerization. The story continues, and we are going to be looking at this catalyst development in bit more detail when we take up this discussion again in the next class. So, till then, thank you and goodbye.