 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing the evolution of olefin polymerization catalysts and in that way, we have drifted quite a bit. We started with Ziegler-Natta heterogeneous titanium, zirconium group for transition metal based catalyst, which are multi-site catalyst with extremely high activity for ethylene as well as propylene homopolymerization and then gradually drifted towards group for transition metal based metallocene type single site catalyst, which were extremely well behaved in terms of being able to polymerize, homopolymerize ethylene as well as propylene and also show extremely high activity depending on the modification of the transition metal group for metallocene catalyst. And then subsequently we went towards looking for copolymerization of various kinds of olefinic monomer and subsequently copolymerization with monomer bearing functional groups. Now in that aspect too, we had moved over from group for transition metal catalyst, zirconium and then went on to discuss about other metals for example lanthanide based olefin polymerization catalyst and which we had seen that even though they can polymerize olefins but they cannot compare in terms of activity with respect to group for transition metals. Now in this comparison of looking for other non group transition metal matching up to the activity of the group for transition metal contribution by Brookhart and Gibson was discussed in the last lecture in which iron catalyst which is a non group for transition metal showed extremely high activity in terms of olefin polymerization and it could give a range of products depending on the substituents going from oligomer all the way to the polymer and this was a tremendous discovery in terms of putting iron back into the polymerization map of in olefin polymerization and iron being the most abundant metal on the earth being cheap and economically viable. This is a great news for people who are exploring iron based catalysis. In the last class, we had seen the catalyst which is drawn over here, ethylene when R equals methyl and R dash equals hydrogen, oligomer alpha olefin C10 to C20 fraction produce high density polyethylene. Now this and the activity comparable to Ziegler Nata catalyst. So, this is a great news in terms of the chemistry and in terms of developing iron as a catalyst. This in short is designated by NNFECL2 slash MAOO catalyst. Now, the interesting thing is that the chemistry that chain termination competes with the chain growth and that is shown over here. So, NNN Fe plus polymer with ethylene gives NNN Fe polymer and ethylene with iron in the plus state. Then, this will come and insert, which will then give NNN Fe polymer with Fe plus. So, this is a chain growth, whereas this has a three chain termination pathways as well. So, this is a chain growth reaction and it has a three chain termination pathways as is shown here. The first one is this will bind to the ethylene and one can have NNN iron polymer bound to ethylene. It has a hydrogen and it can beta hydrogen can eliminate and become a hydrogen hydride and that can insert into the thing. So, that can eliminate propene this moiety and then it can give NNN Fe plus hydrogen, which can again undergo ethylene to give chain growth. So, this mechanism is sort of a chain termination mechanism, but giving rise to chain growth. There are other possibilities as well, which is shown here. So, this with Me3Al, Trimethyl Aluminum or MAO would give this complex NNN Fe plus polymer Al Methyl. So, in this case, what this is that trifluoro trical aluminum unit this is in methyl is interacting with iron and as a result there is a chain transfer that happens chain transfer and this kicks out polymer Al Me2 and this methyl gets transferred to iron from aluminum and the unit which goes out is this. As a result, what stays behind on the catalyst is Fe plus methyl and that with ethylene again goes to chain growth. So, what is interesting over here that here we have chain growth, which leads to the growth of the polymer chain and here we have this chain termination mechanism. So, this is mechanism number one, this is mechanism number two, both also leading to chain growth. So, as a result, the catalyst gives very high yield molecular weight polymers and then this is not the second one. There is also a third one, which is a possible third pathway chain termination and that is given by this. So, this gives NNN Fe A plus hydrogen polymer that undergoes associative exchange. As a result, what happens is olefin comes in and propane goes out as is shown to give NNN Fe plus hydrogen in this and this in presence of olefin again gives chain growth. So, what we have over here are three chain termination pathways. This is pathway number one, this is pathway two and pathway three. All the termination pathway eventually in presence of olefin giving chain growth and this is in addition to the normal chain growth pathway. So, this is a wonderful catalyst, which because of these reasons could match up to the activity of that Ziegler-Nurter system and could carry out polymerization and we have seen that how chain growth helps in producing longer catalysts. Now, we have moved from now, we move and go beyond iron and we move to nickel from here and we had seen that how the story sort of started with nickel, how the story sort of started with nickel in terms of the nickel effect, which sort of helped in discovery of the Ziegler-Nurter catalyst and then it went back a complete circle from nickel to titanium, zirconium to lanthanum to iron and then it again goes back to nickel and now we are going to be talking about how nickel can be used for producing polyolefin polymers and the credit goes to Professor Maurice Brookhart, who used a bulky system of the type stone that could produce polymer and the ligand are of two types, one is this or this. So, this is a bulky system and we can in short we can designate this as this ligand, what this ligand does is that in its nickel complex in ether at minus 78 degree centigrade in presence of 8 protonating agent F4, it eliminates methane to give this compound and then this is the ether solvent etherated air4. So, this is the counter ion and with propene it could give a polypropylene and the same he could also start instead of the methyl, he could also start it from the nickel dibromide precursor using MAO 1000 equivalent toluene with isopropene or it can be hydrogen, methyl, butyl it could also produce the same same pathway. So, this is one can think of this as pathway one and this is pathway two, the polymer produced by pathway one are high branching, whereas polymer is produced in pathway two gives a high grade linear polymer and one can control the grades of the polymer depending on the conditions used or pathway follows. In one case one can get high grade linear polymer, in another case one can get highly branched polymer. So, this was a nice work where Brookhart also put a nickel in the map for making polymers because nickel the whole business started off with nickel in the nickel effect setting it up and then nickel known in shop where it was mainly used for oligomerization catalyst it is however professor Brookhart's work which made nickel to come into the map of olefin polymerization and it also convincingly demonstrated that how nickel can be used even to make a polymers and nickel being a late transition metal it has a propensity to chain walk during the polymerization and that is what is explained by branching that is observed. Now, continuing further a nice work by professor Bazan took a nickel to a much higher heights where he used two catalyst working in tandem to produce a high molecular with polypropylene. So, this is a property which uses two catalyst transition metal one is nickel and another is titanium to produce a polymer in a tandem fashion and this is nicely demonstrated illustrated over here. So, Bazan in 2000 demonstrated used nickel in combination with constrained geometry titanium catalyst to produce polyethylene copolymer of one butene the good thing is that this is a copolymer process with only one monomer which is ethylene. So, this is a kind of a phenomenal work. Let me illustrate why first of all that it uses two catalyst one is nickel another is constrained geometry titanium catalyst two catalyst producing a copolymer of ethylene and one butene but the catch is that there is only one monomer which is ethylene. So, one catalyst is used first to convert ethylene to butene and then the second catalyst the first catalyst is nickel which used ethylene to butene and then the second catalyst titanium sort of polymerizes the butene and ethylene to give this copolymer ethylene co one butene. So, this is a beautiful elegant demonstration of organometallic chemistry it also shows how clear thinking can lead to wonderful catalyst development where two catalysts are made to work in a tandem. Let me illustrate this more specifically with the catalysts these were being used the first catalyst is obviously a nickel catalyst which is of the formula this is a nickel allyl complex that is activated with boron C6F5O3. The catalyst converts ethylene to one butene now this one butene then reacts with maybe I will do it with the different color one butene then reacts with another molecule of ethylene and undergoes a copolymerization using a constraint geometry catalyst titanium methyl plus and this is a boron C6F5O3 minus. So, it does this does the copolymerization to produce MnLLDP. So, the good thing is that this in this case two catalysts titanium and two catalysts titanium and nickel work in tandem in same medium on single substrate on one substrate which is ethylene to produce LLDP which is this. So, this is a tremendous demonstration of the capabilities of organometallic chemistry in which this LLDP was produced and depending on the variation of the conditions one can control the chain length and the molecular weight and depending on the and the incorporation extent also of two different by varying the nickel to titanium ratio. It should also be mentioned that these two catalysts nickel and titanium are not interfering with each other. So, with this we come to the end of today's lecture where we had seen the catalyst development from iron Gibson's catalyst to Brookhurst's nickel the focus shifting from iron to nickel and what it says that there are a lot of chemistry even left with the other transition metal other than non-group 4 transition metal and in this case nickel and iron indeed showed did prove their strength in terms of doing wonderful chemistry and the last example by Bazan where tandem reaction between nickel and the constrained geometry titanium group 4 catalyst took to polymerization to a different level. So, with this we come to an end of today's lecture we are going to dwell a bit more on non-metal non-group 4 transition metals when we take up the topic in more detail in the next class. Till then thank you and goodbye.