 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been talking about olefin polymerization in the last few lectures, and in this regard, in the immediate past few lectures, we have focused on the development of Ziegler-Nurter catalyst particularly with respect to polyethylene as well as polypropylene. Let me just briefly talk about the highlights of what we have covered so far. In this context, the first we have spoken about is ethylene, TICL 4, 25 degree centigrade, one bar pressure. This is Ziegler-Nurter catalyst, gives a polyethylene, and this is under heterogeneous conditions, produces HDP of molar masses 10 to the power 4 to 10 to the power 5 Dalton. Subsequently, we saw that for polypropylene, it gave isotactic polypropylene or IPP, and this is also heterogeneous, this also has head to tail linkages. In this advance, focusing more on polypropylene, the first catalyst for polypropylene was CP2 titanium dichloride with Et2 AlCl, this is for polyethylene, sorry ethylene, this catalyst is homogeneous, but lower activity for ethylene, and no activity for alpha olefin, and this was reported by Natta. The first homogeneous catalyst was reported by Natta, then subsequently Kaminsky produced CP2 zirconium ME2 NaO, so for polyethylene homogeneous, and extremely active 500 kg of PE polyethylene per millimole of zirconium per hour, so post this, what was reported is the fifth one, which is CP2 zirconium PAH2, CP2 titanium PAH2 NaO, and this ethylene EN THIND zirconium Cl2 NaO, this we have discussed, that is again homogeneous and was producing 43,000 kg of polypropylene per mole of zirconium per hour, so the point to note is that this is sort of a timeline of development, so the first is heterogeneous for polyethylene, then heterogeneous for polyethylene, and then first homogeneous came with this BCP system, homogeneous for polyethylene, now this BCP system with NaO gave extremely high activity of polyethylene per millimole per hour, and then subsequently BCP system with NaO and this ANSA system with NaO gave extremely high isotactic polyethylene per hour, and we had looked at this system, this ANSA system in much more detail in our previous discussion, so and what we saw that as we move from homogeneous to heterogeneous in these cases the catalysis are as we move from homogeneous to heterogeneous, this is multisite for this heterogeneous and heterogeneous, and for the homogeneous ones these are all homogeneous, these are single site catalysis, and this single site catalysis means that polymer has narrow polydispersity index and low PDI, and polymers are much more well behaved in terms of their properties, so let us now focus on this ANSA complex that we had discussed which is extremely good activity for polypropylene, so this ANSA complex Enthind zirconium Cl2 NaO, structure wise the complex looks something like as it is shown over here, maybe the structure as shown over here, this is bound to zirconium with NaO, so this is the structure of this complex, and the ratio of these two NaO is about NaO can be represented as AlO methyl, so this ratio is 1 is to 300 means large excess large excess of NaO is required, and then the question comes that what is the role of NaO, the question subsequently comes is what is the role of NaO, it seems like that NaO is a magical reagent which enhances the activity of this catalyst, this homogeneous catalyst to the extent that it even surpasses the original heterogeneous Ziegler-Natter catalyst, the answer to that is that NaO has three functions, NaO has three functions that NaO acts as a methylating agent, that NaO acts as a methylating agent, that means that these halides get replaced to becoming methyl, so they convert zirconium halides to zirconium methyl, and second function is that it has a Lewis acid carbonic acceptor Lewis acidic acceptor, this is an important function of NaO, the first is it is methylating it, and then making it dimethyl, and still there are large excess of NaO which are highly electron deficient Lewis acidic species, now after they have methylated the halide they abstract one of the methyl, so it is not only a methylating agent, but also it is a methyl anion acceptor, so that is why it is called this carbon anion acceptor, so not only it is making methylating it, converting halide to methyl, and then subsequently it is also accepting, abstracting the methyl, snatching the methyl, and the third role of NaO is that and serve as a counter anion zirconium cation, so it has three role, and this can be suitably explained in this series of chemical equation as is shown below, for example Cp2 zirconium chloride methyl NaO, so methyl, so this is the first function that we had seen the methylating methylating agent, now the second function is that it acts as a methyl abstraction, so this can be represented as methyl with NaO giving Cp2 zirconium, one of the methyl gets abstracted, the other is a vacant site plus, so only one methyl gets accepted and then ends up making a vacant site over here, and then the anion, so this is the second role, this is the what portrayed out here is the second role of methyl abstraction, and then this methyl group ends up in the carbon ion of NaO as is shown here Al Me O N Me, so this methyl group which gets abstracted from here ends up on the aluminum as is shown here, and then the both the second and then subsequently this acts as a counter ion to this zirconium cation, and what happens subsequently is in this vacant site the polypropylene lands up occupies polypropylene comes, and Cp2 zirconium methyl this cation and NaO-anion which undergoes insertion subsequently with many propylene as is shown to give isotactic polypropylene, so the role of NaO is greatly explained that it has three, first is it works as a methylating agent, so this gets converted to methyl, second it has an an abstractor, so one methyl gets converted gets abstracted, and then there is a vacant site hole, and then subsequently to this hole the olefin comes binds, and so this now serves as a non-coordinating anion with low charge density, so this is an interesting observation that this NaO has several roles and the after abstraction what is possible is that it stabilizes this zirconium two species cation, zirconium cation being a non-coordinating anion with low charge density, now what it sort of implies that this can be achieved with any other non-coordinating anion with low charge density and that should also be equally effective, and the ingenuity of organometallic chemist comes into play where indeed such well behaved catalyst were developed with other non-coordinating anion, and which were extremely good for polymerization, so this shows how the structure activity relationship how understanding helps in developing the catalyst or helps in improving the catalyst, and this is done by Tobin Marx professor of northeastern university who had used a non-coordinating anion to develop this zirconium species which is extremely active, so what Marx did is he took this Cp2 zirconium dimethyl and used this Lewis acid of boron, so now instead of aluminium it is boron C6F5O3 in pentane gave Cp2 zirconium methyl of the loosely interacting with CH3 Bc6F5O3, so this is ion pair separated species with zirconium being in positive and this boron being negative after abstracting this methyl from zirconium methyl, and this was contribution this famous nice example was reported by Marx in 1991, and this complex this cation take complex resembled the MAO analogs in catalytic performance, and this was structurally the catalyst was structurally characterized is rightfully designated as a single site catalyst, so this was a fantastic improvisation made by Marx where he developed a replacement of MAO using this boron trifluorofenyl reagent which abstracted one of the methyl and also behaved as a non-coordinating anion in stabilizing this zirconium Cp methyl cation which carried out the polymerization of polypropylene with equal efficiency as that of the MAO counterpart, so that this shows how understanding a suitable proper understanding of the catalyst mechanism helps in improving the catalyst greatly, and that is why the structural activity relationship is so important in developing the efficiency of the catalyst, so with this I come to the conclusion of today's lecture where we have seen how these catalysts for polypropylene improved from the heterogeneous Ziegler-Natta titanium tetrachloride diethyl aluminum chloride based catalyst, a multi site catalyst to the focus shifted towards developing single site catalyst that were advantageous, but the inherent difficulty of this homogeneous single site catalyst was their activity was inherently low, which was however overcome by advent of MAO by Kaminsky in 1981, and subsequently even MAO got replaced by contribution from Marker who could develop better similar catalyst bearing boron based non-coordinating anion, where the boron trifluorofenyl C6F5 moiety C6S5 moiety could serve as a methyl abstraction as well as non-coordinate in anion and stabilize this single site catalyst, so with this I come to a conclusion of today's lecture we are going to be looking at this catalyst development and some more interesting aspects about how to achieve the syndiotacticity isotacticity at will and the rational that went in developing this catalyst as we discuss the topic in more detail in the next lecture till that time goodbye and thank you for being with me in this class see you in the next lecture