 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing olefin polymerization over the last few lectures. In particular, we have been focusing about various topics, various aspects of propylene polymerization or polypropylenes. In this regard, we have noted that in the previous lecture that Ziegler-Narder catalyst can also polymerize propylene. One of the main questions that remained at that time about Ziegler-Narder polymerizing propylene is the fact that the polymers obtained were isotactic in nature and the polymers were connected in a head-to-tail linkages. Let me illustrate this as follows. What is important to note is that propylene were polymerized with aluminum chloride or Ziegler-Narder catalyst to give isotactic polypropylene, which is a polypropylene or IPP. The second thing, which at that time was difficult to understand is this head-to-tail linkages of propylene units. These can be illustrated. This is one unit, this is another unit. These are head-to-tail linkages of this propylene unit. These are the two aspects that needed explanation in trying to understand the Ziegler-Narder polymerization of catalyst. The explanation came in the way of two things that these TiCl3 crystals are supported by MgCl2. The active catalyst actually is chiral in nature, where it has a sigma vacant site adjacent to a transition metal polymer bond. This can be explained as such. Fluoride is supported on a magnesium chloride crystal, where there are two sites. One is the polymer chain, which is bound to titanium. The other is the vacant site. Overall, what is to note is that overall that is chiral site that allows for enantiofacial discrimination of propylene to give isotactic polypropylene. This is the first explanation to the observation that polypropylene is isotactic in nature. This is illustrated by the approach of olefin, which can be either this with the methyl group on this side, or it can be this with the two possibilities of which one possibility is favored. This one is favored, and this approach is not favored, resulting in room for enantiofacial discrimination of propylene to give isotactic polypropylene. The other thing that we had discussed that leads to the formation of head-to-tail linkages is the fact that there is a vacant site present to the polymer titanium-carbon bond. There is a presence of a free coordination site to titanium-carbon bond, and this is what allows for this head-to-tail orientation. Head-to-tail linkages is dictated by that. One thing about Ziegler-Natta polymerization is that the polymerization happens on TICL food supported over MgCl 2 crystals, and hence this is Ziegler-Natta polymerization one attribute it, this is the multisite catalyst. That means that there are many active catalytic centers which are present, which carry out polymerization. As a result of this presence of multisite catalyst, the polymer so obtained become broad molar mass distribution or high in short high Pdi, polydispersity index, so that means that the polymer has a wide range of molecular weight distribution, that is what it means in short. To have more well behaved polymers of uniform property, then the need became for developing a single site catalyst for obtaining Pdi, polydispersity index, single site catalyst were developed for obtaining a narrow distribution, a single site catalyst were developed. That lead to another set of discoveries geared towards developing single site olefin polymerization catalyst. In this context, the Nata system of this titanium dichloride is what provided the first initiation towards this end. The catalyst was titanium with Al2, Et2, AlCl, so this was provided by Nata in 1957 and this is the first single site homogeneous catalyst for ethylene polymerization. This catalyst had low activity for ethylene polymerization and could not match up to the heterogeneous TiCl4 diethylaluminum chloride catalyst, which is the Ziegler Nata original system. It has low activity and second thing is that this also could not polymerize alpha olefins. This thing was like no match up for the Ziegler Nata heterogeneous TiCl4 diethylaluminum chloride catalyst, but nonetheless there are two advantages to this system, which are to be noted here. The first is that this is the single site catalyst, this is the advantage number one, and the advantage number two is that this is also a homogeneous system, so that means that the polymer of low Pdi and much well behaved. So these are the two advantages of this system, despite the fact that it had its own problem like the activity was very low and could not polymerize alpha olefins, these are sort of the drawback for this system. Now then another following Nata's observation, there was another noted contribution was made by Kaminsky in 1980, who reported high activity with MAO or methyl alumina alumino oxane. So at that time the structure of MAO was not known and this MAO was obtained by partial hydrolysis of non-uniform and structurally it was not uniform, non-uniform structure of MAO arises from partial hydrolysis of triethylaluminum. At that time this is what led to enhancement of reactivity for homogeneous catalysis, which is originally reported by Nata. Now following this discovery or MAO applications can be very much engaged from the fact that the corresponding zirconium dimethyl catalyst produced 500 kg of polyethylene mole of zirconium per hour of catalysis. So this is a highly active catalyst with MAO, the zirconium dimethyl thus became highly active catalyst that could produce 500 kg of polyethylene mole of zirconium per hour, that is a huge activity. This was even tremendously exceeded the results under zirconium conditions, conventional conditions in contrast this conventional exist under zirconium nada conditions. So here now we have a which is a heterogeneous system, so here we have a homogeneous system catalysis which could beat the heterogeneous system, so this zirconium MAO dimethyl could beat these conventional zirconium nada conditions under homogeneous system. Now this catalyst was also moderately active for alpha olefins and the product was atactic. The catalyst also moderately active for alpha olefins and produced atactic polymer and this was not surprising given the fact that this is a chiral molecule. The next important discovery was made by Bilsinger as well as even who produced isotactic polymer using chiral resolution one at low temperature and other using chiral derivative. So, that was the first development towards using homogeneous catalysis to produce isotactic polymer as was obtained using heterogeneous zirconium nada system, so isotactic polypropylene was obtained by first even in 1984 in presence of diphenyl titanium myocatalyst at minus 30 degree centigrade, so the low temperature was given, so that more ordered formation could be formed and the second by Bilsinger in 1985 using chiral and zirconium derivative, this is given as polypropylene in toluene 60 degree centigrade and the catalyst is this is a ansa catalyst zirconium is chloride and this gave isotactic polypropylene with the catalyst is to MO 1 is to 300 ratio and MO is given as Al methyl N, so with this we come to conclusion of today's talk where we saw that the development of chiral homogeneous metallocene catalyst that could give isotactic polypropylene under homogeneous condition in presence of this chiral catalyst and MO in large excess of MO about 1 is to 300 ratio and could be used to make this isotactic polypropylene, isotactic polypropylene could also could be obtained using this catalyst under very low temperature as reported by even, so with this we come to the end of today's discussion on the development of catalyst for producing polypropylene, what we had seen that which started off from a simple basic question as to why the heterogeneous zyglarnut and titanium tetrachloride diethylaluminum chloride was producing an isotactic polyethylene to the understanding of which lead to the understanding that it is a multi site catalyst and that a result the polymer obtained were broad and less well we had which lead to the focus on a single site catalyst and the first single site catalyst for ethylene though of low activity was reported by Julio Nart in 1957 using titanium dichloride and ethylaluminum chloride which could polymerize ethylene but could not polymerize any alpha olefin subsequent contribution by Kamensky which who came up with this aluminum oxane reagent by this is a non uniform structure obtained by partial hydrolysis of diethylaluminum lead to high activity reported for ethylene polymerization using zirconium B.C.P. zirconium dimethyl which could even surpass the results obtained from homogeneous heterogeneous zyglarnut conditions and they could also polymerize alpha olefin now subsequent to that further development as reported by even for propylene polymerization at minus 30 degree centigrade or the famous Bridszinger's 1985 chiral and zirconium catalyst which in conjunction with MO in 1 is to 300 ratio could give isotactic polypropylene similar to that with obtained with zyglarnut system so these catalyst is extremely active and could produce about 43000 kilogram of polypropylene per mole of zirconocene per hour per hour and as a result about 50,000 Dalton polymer could be grown in 3.8 seconds so this is a tremendously active catalyst which was producing so huge amount of polypropylene under such conditions so with these we come to an end on today's discussion where we have seen how the evolution of single site catalysts from multi site catalysts to place during a polypropylene polymerization how the focus shifted and how eventually the homogeneous catalyst could make a more well behaved narrow distributed polymer with much more activity higher than that of the heterogeneous conditions using suitable modifications on the metallocene system so more on the catalyst development this will highlight some of the interesting struggles that have been taken up by organometallic chemist in coming up with ideas that could lead to further enhancement of this polypropylene catalysis attributes which will be discussed in the subsequent class so more of these interesting stuff ahead as we go on discussing about the catalyst development in polypropylene polymerization chemistry so with this I once again thank you for being with me in this lecture and I look forward to have some more interesting discussion on propylene polymerization when we meet next till then goodbye and thank you