 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been talking about olefin polymerization in particularly polyethylene and polypropylene polymerization and we have been looking at it from the perspective of catalyst development for this polymerization process as a course of time. In this context, we have observed that for both polyethylene and polypropylene, for this how the polymerization process moved from heterogeneous catalysis, homogeneous catalysis and each has its own advantages and in the course of the struggle what we had observed that heterogeneous catalysis though exhibited very high activity that is to its advantage, but on the same time exhibited broad molar mass distribution wide polydispersity index or broad distribution. So, this is seen as a disadvantage and hence because of this the efforts were on to move on to homogeneous catalysis and in the also this broad poly polymer mass distribution has been attributed to the presence of multi site catalysis and hence the effort were on to move on to the homogeneous catalysis system and which meant that the exactly opposite attributes were wanted for example, narrow pdi so this turns out to be advantage. So, this is how this disadvantage can be converted to advantage narrow pdi. Similarly, the thing which this offered is single site catalysis and this is also considered to be advantage whereas what in the earlier days what homogeneous catalysis struggled with was a low polymerization activity and this is supposed to be disadvantage. So, we saw how on moving from homogeneous to heterogeneous how this disadvantages like single site multi site catalysis could be converted to the respective advantages and this advantage which was of the high performance becoming disadvantage in case of the homogeneous catalysis. Now, in our study we had also seen that this disadvantage was finally overcome by the work of Kaminsky, who brought about the advent of methyl aluminoxene NaO and then subsequent improvisation by Marx using a boron C6 F5 whole 3 reagent this disadvantage of low activity was finally overcome to get homogeneous catalyst with extremely high activity. So, we had seen the beauty about organometallic chemistry as to how the things evolved from something heterogeneous to homogeneous where it had its advantages in terms of narrow pdi single site catalysis, but its sole disadvantage was low activity and which was also finally overcome by the advent of NaO by Kaminsky followed by using boron tri pentaforofenyl methyl boron by Marx which sort of gave a highly active single site catalyst of the structure something like Cp2 zirconium methyl CH3 B C6 F5 whole 3. So, this is a del positive del negative. So, this is the catalyst which sort of became the final evolved structure through all of these processes and we had discussed this in great detail in our previous class. In this context also what is important is the fact that this ansa breached tetrahydroindenyl zirconium catalyst which could give a tactic polymer. So, the catalyst which is drawn over here this presence catalyst this is called resmic en for ethylene breached. This ligand is T-H-I-N-D tetrahydroindenyl whole twice zirconium dichloride this with NaO in 1 is to 300 ratio could give polymerized propylene at fairly low temperature of 60 degree centigrade in toluene to give itactic polypropylene isotactic polypropylene or IPP. Now, what is important over here is this the question of how and this is extremely high active catalyst up to up to forth 3000 kilogram of polypropylene produced per mole of zirconium per hour. So, this extremely high active highly active catalyst which was producing isopropyl isotactic polypropylene and the question is how the question with regard to this is how is it possible that when we take a resmic catalyst that one gets isotactic polypropylene. So, that is the main intriguing feature about it and the answer to that is guided by the C2 symmetry the answer is provided by the C2 symmetry nature of the catalyst C2 symmetry of catalyst that produced this isotactic for producing isotactic polypropylene the in this thing it is to be noted that if that is the thing the rational then the it is to be noted that a catalyst which is this Cp zirconium similar catalyst produced with a Mayo gave itactic polypropylene. So, this has C2 v symmetry. Now, this is best explained by the presence of C2 symmetry in the catalyst which renders chirality to the complex, this C2 symmetry renders the catalyst and this catalyst which is responsible for the stereo specific insertion of propene and this is best explained in the diagram shown in the next page. The catalyst as is given below the catalyst this zirconium chloride can have when viewed from this side can sort of be represented by the following two diagram as is shown below and so this is bound to zirconium and it can have sites maybe I will draw with the different ink it can have two sites one something like this and the other site is a vacant site where the olefin can come and bind and I draw the same structure over here as is drawn over here this again will have polymer as is shown over here and a vacant site now the question is that a polypropylene when is approaches this catalyst how would it occupy or bind at this site so possibility is there are two possibilities one possibility is when a propylene comes it will approach in this fashion and would sit in the vacant site in the fashion shown as opposed to it can also approach in this fashion and occupy the vacant site and in a fashion like this now a careful look would as quickly reveal that this orientation is less favorable because of steric repulsion between the methyl group and the polymer chain rendering this approach to be unfavorable whereas compared to relative to this this approach where the methyl groups are further apart this becomes the favorable orientation of approach in terms of the energy and hence these results in discrimination of the phases enantiophases of propylene of propylene giving isotactic polypropylene so this is how this racemic catalyst with C2 symmetry steers the approach of propylene to give isotactic polypropylene now this is based summarized by a mechanism which is popularly called as a windshield wiper mechanism which is popularly called as windshield wiper mechanism and is illustrated below this is the catalyst bound to zirconium this is how the two ligands are connected now on one side I will use a different color is bound to zirconium with the methyl moiety pointed in this direction and other side it is bound to a polymer chain which exhibit c alpha H agrastic interaction so there is a special interaction over here which is called this interaction is called c alpha H zirconium agrastic interaction now when this catalyst when olefin is bound in this fashion the first the migratory insertion of propylene occurs and the whole chain shifts to the other side and a molecule of propylene then binds on the opposite side to what is shown over here this is best illustrated by the diagram shown over here zirconium as is shown and then and then again we will illustrate now the polymer chain has shifted to the other side and the insertion has happened as it shown over here and in this side second propylene is bound so what one sort of notices over here that as the insertion and binding of propylene happens the polymer moves from one side to other and this is what is called the windshield wiper mechanism and subsequently again the another insertion happens with the propylene being bound and the catalyst as is shown here with zirconium the ethylene again comes to this side and in this side the polymer chain grows and this is how the insertion the reaction prepares to give isopolypropylene so this windshield wiper mechanism is very much evident by the fact that the stereochemistry of the polymer chain goes from one side of the catalyst to the other and the configuration sort of remains the same as it flips over so this is this windshield wiper mechanism and this is stabilized or this is propagated by the presence of this zirconium alpha ch agrostic interaction which helps it propagate to give isotaptic polymer so polypropylene so with this we have seen how the single side catalyst is improved homogeneous to give polypropylene of high specificity and how symmetry plays a great role in distinguishing between the face enantiophase of the propylene that alpha aliphane that approaches and results in highly a regular stereoregular structure producing poly isopropylene in high stereo specificity so with this we have also seen the role of ansa ligand which has a bridging ligand how it helps in opening up the cyclopentadienyl ring say for better approach of the polymer chain and the catalysis so more of this and this is a very nice demonstration of good thinking in chemistry how it can lead to catalyst improvement so more of this catalyst development on the polymerization mechanism of our polypropylene as we take this topic in bit more detail when we meeting the next class once again thank you for being with me in this class where we discussed the various stereo specific addition of propylene to give polypropylene and how there has been improvement in the catalyst structure as one went about correcting things in terms of polymer requirement and demand and the properties and more of this discovery on polypropylene would continue as we take this topic up in bit more detail when we meet in the next class till then goodbye and thank you