 This lecture is regarding the reactions of olefins or the reactions where olefins are the prime actors in the whole scene. There are two major reactions that we will be talking about. One is the oligomerization, the other is the polymerization of olefins. Both of them are probably multi billion dollar industries. There is a large demand for products that are generated from simple olefins like ethylene and propylene. So, it is important that we look at the chemistry behind these systems. Oligomerizations are basically talking about systems where the molecule for example, you have an alkene. If you take something like 4 alkenes and if you end up with this product, you have a system where you have oligomerized or you have combined together several of these olefins. Then it becomes a more useful product than ethylene itself, which is useful, but not necessarily in the way a tetramer of the ethylene is useful. So, it is important to look at these molecules or the molecules, the metal complexes which will carry out such oligomerizations. A large number of molecules appear to do this and the challenge is to do it specifically, meaning you want only the octene, you want only the one octene. So, you do not want this and you also want, you do not want the molecule to proceed to hexene when you are trying to synthesize octene. So, this type of specificity in the position of the double bond and in the oligomerization number that makes the whole chemistry challenging. We have already looked at one type of chemistry where multiple alkenes were stitched together to generate different molecules. The oligomerization that we discussed in detail, just to remind you of what is possible was the cobalt catalyzed cyclotrimerization of acetylene to give you arenes. Arenes for the product, simple alkenes were starting materials. So, you stitched together three alkenes in order to generate an arene. This was done by an oxidative addition mechanism, where the cobalt which is a cobalt one species oxidatively added and at the same time coupled to carbon fragments. A similar reaction could be envisaged with alkenes. So, you can take alkenes, put two alkenes and then you would get a metallocyclopentane and that could be used to stitch together a variety of alkenes. So, the two processes that can happen are one direct transfer of hydrogen from the beta position to the alpha dash carbon. The other is a transfer of hydrogen from the beta position, which is this position to the metal and then a second reductive elimination to the alpha dash carbon. So, in either mechanism it is the beta hydrogen, which is transferred to the alpha dash carbon. Now, it turns out that butene is a useful molecule, but it would be valuable to have hexene as a product. So, that means you need to trimerize an olefin and the synthesis of alpha olefins. That means where the double bond is present in the alpha carbon is very important for the production of a variety of materials used in bulk quantities in the industry. These are listed here. That is low density polyethylene, plasticizers, detergents and so on. So, it was only recently in 1997 that a very clean catalyst was discovered by this person Jolly, who figured out that chromium is a suitable metal for carrying out this oxidative coupling of three ethylenes to produce hexene. The mechanism is very similar to what we had seen earlier. You combine two alkenes. In this case, two ethylenes as the most popular and most cheaply available alkene that people use to synthesize a metallocyclopentane into to insert another ethylene molecule to generate a metallocycloheptane. Now, the generation of a metallocycloheptane can result in the formation of an alkene, which will be in the alpha position due to beta hydride, beta hydrogen elimination. This beta hydrogen transfer is identical to what we had discussed earlier. It can happen directly or it can happen through a metal hydride intermediate and result in this olefin complex. Now, we know that substituted olefins are less stable than unsubstituted olefins. So, referentially you would have exchange of this hexene with these ethylene molecules in order to generate this catalytically active species which is written here. In this particular case, M equals chromium is the best option and a variety of chromium complexes have been tested for this particular reaction. You will notice that in the case of nickel, where you had a D 8 system, you are able to couple 2 ethylenes. In the case of a D 6 system, you are able to couple 3 alkenes. So, there seems to be a very strong electronic effect in the coupling of alkenes to generate the final product. You just keep this in mind as we go ahead with the discussion. So, the obvious extension for this would be to proceed to a D 4 system. In fact, if you use a D 4 system, what you expect is a tetramerization, but it turns out as late as 2002, it was discovered that the tetramerization of titanium is tetramerization of ethylene is not what you observe, but you end up with a trimerization, a very efficient trimerization of ethylene in order to generate hexene. In all these cases, we are talking about the formation of one hexene which is the more useful product. Now, there are couple of more things that we need to emphasize. This is not the only complex that has been tried. I have picked up the examples which are most efficient and probably recent in the literature. In this case, the design involves several important factors. One of them is the presence of this benzene ring in a position suitable for having this weak interaction, which is marked here with a dotted line. This stabilizes the catalytically active intermediate. Although it is a weak bond, it is a titanium 3 plus complex that is involved. You have a weak bond, but that interaction is responsible for the formation of a catalytically active species. The other important point to note is that the bridge size here, the number of carbons and the substituents present on this bridge play an important role. The best catalyst was in fact with two methyl groups at this position and also with a single carbon, so that this interaction is not too strong and it is not too weak either. I have to explain also what this particular molecule MAO is. MAO is a mysterious aluminum oxide. It is methyl aluminum oxide and it is a polymeric compound, which has been used extensively in the polymer industry. It has got a methyl aluminum bond. This methyl aluminum bond is capable of carrying out a reduction of this titanium 3 species to a lower valence species. So, this lower valence species is presumably responsible for this oxidative coupling reaction, which leads to very high selectivity, very efficient selectivity for the trimerization of ethylene. Now, let us talk a little bit more about MAO because this is a molecule, which we are going to talk about more extensively. It is basically a polymeric species, where you have a methyl aluminum bond. What does it do? It essentially removes an X minus group to the saturated aluminum, which is co-ordinatively unsaturated and which can accept an X minus to form an anionic species. Here is a possible anionic species that would be formed if the aluminum picks up an X minus from the catalytically active species. But, if it is just picking up an anionic species, one would need only one aluminum per X group that you need to remove. So, in the previous example, we had three chloride groups, which are present on the titanium. So, depending on the number of titanium chloride bonds that you want to reduce, you would need one methyl alumoxane. But, you remember that the number of methyl alumoxane molecules that are usually added to the catalytically active species is very large. This is a problem that chemists have been trying to solve and are struggling with in the polygomerization and polymerization reaction. But, let us continue with this species. What else does it do? Because you can abstract an X group and form an anion, it forms a very active species, which is stabilized because the large anion, the polymeric anion that is formed is non-coordinating. It basically prevents other anions from reaching the titanium 3 plus site. Finally, because of the methyl aluminum bond, it is highly reducing. So, that is a point that has to be added. It is highly reducing. So, it would convert the titanium 3 plus to a titanium plus or titanium zero species even. The presence of methyl alumoxane can reduce the titanium completely. So, let us just take a minute and look at the possible catalytic cycle that would be involved in the selective trimerization of polyphins. So, the mechanism that we have written here is using the titanium 1 plus species, which we have indicated here as plus 1. The oxidation state is plus 1 here. We have reduced titanium from plus 3 to plus 1. After oxidative addition, it becomes plus 3. That is the plus 3 state that is involved in this reaction. Then, back at the end of this reaction, it would come back to plus 1. After oxidative addition becomes plus 3. So, if you add 3 alkenes, so with every step you would add an ethylene molecule. You would end up with a metallocycloheptane. This metallocycloheptane has got 6 carbons, 3 ethylenes joined together. This is indicated by the 3 ethylenes, which are coming together to form a metallocycloheptane. This can now by abstraction of the beta hydrogen. The beta hydrogen elimination has been already explained previously. An identical reaction happens. Now, I have transferred it to the metal titanium here. This species can undergo a reductive elimination of these 3 of these 2 units. That gives you the olefin complex, which is the titanium 1 complex. The titanium 1 complex because it has got a hexene coordinated. It prefers the ethylene, exchanges for the ethylene and the catalytic cycle continues. So, the reaction apparently goes through this type of a catalytic cycle, which is very similar to what we have noticed for the chromium and the nickel. Now, let us just take a brief minute to ask this question. Can we avoid the MAO or the MAO? The MAO is used in very large amounts. Usually it is used in a ratio of 1 is to 1000. If the catalytically active metal is chromium, the ratio of aluminum to chromium would be at least 1000. So, this type of a excess use of MAO has been found to be essential. So, that leads to very little molecular understanding. Usually if you need a molecular understanding, one should have some stoichiometric reaction of the 2 entities that are involved. Here chromium and aluminum are present in such a large ratio that a molecular understanding is avoided. Now, the 2 important functions that we have just reviewed are that it is a good reducing agent and we have seen that it protects the active site. So, recently it has been possible as very recently it has been shown that MAO can be avoided and it has been substituted by 2 different strategies. One is to use aluminum trimethyl aluminum as a reducing agent. The other was to use a very large bulky anion. We will show this in the next slide, but basically the use of a large very large amount of aluminum has been avoided and stoichiometric amounts or close to stoichiometric amounts of trimethyl aluminum are used in this whole reaction. In this specific case, about 10 equivalents of trimethyl aluminum were added and this is a far cry from the 1000 equivalents which is usually used in order to achieve very large efficiencies of the order that is shown here. So, let us see what happens in this reaction. Here is a chromium complex that was used. It is a chromium dichloride, a dichlorocomplex of chromium with a net charge of plus 1. The net charge of plus 1 indicates that chromium is now in the plus 3 oxidation state. The bulky anion that was used was this perfluorinated butoxide ion. So, perfluorobutoxide anion tetrakis perfluorobutinoid is what you have here. Four of these coordinated to the aluminum, so much so that the single negative charge is there on the aluminum resulting in a very large bulky anion. At the same time, you have a chromium 2 plus species as a starting material. If it is treated with this activator, which is aluminum trimethyl aluminum, this is a lower case here, trimethyl aluminum, then 10 equivalents was sufficient to bring about a substantial productivity. This is the grams of polyethylene, grams of the oligomerized ethylene per gram of the chromium that is used. So, this is how the efficiency is measured. So, this is grams of ethylene oligomerized for each gram of chromium that is utilized in the reaction mixture. So, the ligand that we have here, we have not expanded it, but this is a special ligand, which has got the methoxy group on the aryl moiety, which is attached to the phosphorous. This AR has got OME group and that turns out to be critical, because it stabilizes the chromium after it is activated by the trimethyl aluminum. For the sake of comparison, the researchers have also carried out the same reaction with Mao and they have noticed that you need close to 300 equivalents of Mao in order to do carry out the same reaction in the absence of this bulky anion and in the presence of the methyl alumoxane as a reducing agent. But you will notice that the activity that you get with this bulky anion is not as good as what you would get with trimethyl aluminum oxane, but the selectivity for hexane, which is one of the important parameters is still retained. You will also realize that we have moved from a chromium 3 plus complex to a chromium probably a chromium 0 species, which is carrying out the actual reaction. One very important criterion in this whole process is that the recovered catalyst. The recovered catalyst can be used again and it has almost as good an activity when you use this ligand as the starting material itself. So, let us take a look at the recovered catalyst that they have characterized, which is I think a very significant, this is a very significant observation that they have made during the course of this reaction. They have shown that the chlorides have been removed and a co-ordinatively unsaturated species of chromium 2 plus is generated in the reaction medium. They have isolated crystalline material, which they have characterized. I have shown the crystal structure here and you will notice that the anion that you have that is the tetra alkoxy species, aluminate species, which is behaving as anion is shown here. The green fluorescent atoms are all fluorines and you can see that the whole aluminum is completely encapsulated by this fluorous sphere. So, it becomes a very large stabilizing anion and the chromium species is also covered by this very bulky phosphinoamine that we have used as a ligand. This oxygen that is there in the ortho position of the aryl group is responsible for stabilizing the chromium in its co-ordinatively unsaturated state. If you look at the crystal structure, which is freely available on the website of the journal, you would be able to understand the interactions a little more, but I have shown you one interaction on the screen right here. Using this catalyst, it was possible to catalyze the reaction very efficiently and generate one hexane in a very selective fashion. You will notice that the tendency to trimerize is fairly significant and what that tells us is, when the molecule oxidatively adds and inserts ethylene, it tends to form a cycloheptane, metallocycloheptane molecule and that has got a beta hydrogen extremely close to the metal and bringing the C1 close to the metal becomes more favorable when you have this ring size. The metal size also is important. Most of these oligomerization reactions have been carried out with 3D metal ions. 3D metal ions have been used for making the oligomers and it is not the 4D and we can see that the chances of biting the CH bond changes when you have a 3D versus 4D. So, the 3D metal turns out to be good for making oligomers. Now, in all the cases that we have mentioned including the one on titanium, where we expected a tetramerization, we saw only trimerization of the ethylene. So, it is indeed gratifying to note that eventually chemists have solved this puzzle and they have discovered a catalyst which can do tetramerization. That means, if you tetramerize ethylene, you would get octene and if you can make specifically one octene, it would be an extremely good achievement. This is exactly what the researchers have discovered and they have published this work in the Journal of American Chemical Society in 2004. So, let us just take a look again. It is a phosphinoamine that has been used and in this particular instance, they have used a very common ligand which is this diphenyl phosphino compound, where you have an isopropyl group on the nitrogen. So, you just take isopropyl amine and treat it with PPH2Cl and you would get this very easy to synthesize ligand. That ligand is abbreviated in this pink form here and the complex that they isolated and characterized is shown here and that is the complex that they used as a starting material. But in this case also, MAO was used as an activating agent. MAO was the best activating agent that has been used and they have used about 300 equivalents of MAO in a toluene solvent in order to achieve this very high selectivity for tetramerization. What is interesting is that both C6 and C8 are formed in this reaction. So, obviously that is a tendency for the chromium to add another ethylene and form a cyclononane and it is the cyclononane which has the right orientation in order to do the CH abstraction. You will notice that in the case of this ligand which they have this PNP ligand, it is possible to achieve very high selectivity for C8. What is written here is the C6 selectivity, C6 versus C8 selectivity can be seen here. So, that is 17 percent versus 68 percent C6 versus C8 in the case of DPPA and within that you will see that one hexene turns out to be about 70 percent. So, that means the remaining 30 percent of the hexene are internal olefins. Similarly, out of 70 percent or 68 percent of the C8 octene that is formed 98 percent is one octene. So, that is excellent selectivity for the one octene which is what we really need in the industry. If you look at the remaining amount it is mostly polyethylene. So, this is the polymeric species that is formed which is also formed in small amounts. So, we do not have to go through the catalyst cycle again, but suffice it to say that in the presence of large amounts of methyl alumoxane the chromium which is present here in the 3 plus state. If you draw a line here you will realize that the dimer has got 3 clorins for each chromium. So, it is a chromium in the plus 3 state in the starting material is reduced in the presence of mauve to the chromium 1 plus or chromium 0 species which then carries out this catalytic function. Now, it was also possible to use chromium ACAC directly as a precursor and that makes the reaction fairly easy to handle in the laboratory. A very simple complex called DPPE which is the best diphenyl phosphino ethane which has got 2 phenyl groups on each phosphorous can also be used as a catalyst. Surprisingly that gives you significantly less activity, but the C8 selectivity is also poor. So, clearly the PNP ligand is a much better ligand if you want to synthesize the optin. So, it is now it is clear that the electronic effect of the PNP and also as we shall see in a minute the steric effect is an important factor. Now, these researches although I have shown you only 2 ligands these researches have applied a large library of PNP ligands as well some of them as exotic as a thynyl substituent on the phosphorous, but these substituents although the one that is pictured for you here is sufficiently reactive. It is not necessary to use this exotic ligand and the ligand that we saw earlier the bis diphenyl phosphino amine is itself quite useful for carrying out selective octamerization of the ethylene to produce octene. So, now we have seen that chromium which we had originally touted as a main metal which would do trimerization is useful for tetramerization. So, chromium was a classic trimerization metal. So, how is it that we are able to do tetramerization? So, more research has gone on in this area and a very recent paper published in the American chemical society journal called catalysis tells us about the selectivity between trimerization versus tetramerization. What they have shown is that the steric bulk on the ligand which we use. So, the PNP ligand that you have the steric bulk on this ligand is responsible for the selectivity between trimerization and tetramerization. The more bulky the ligand it is likely that will give you the tetramerization catalyst. So, not only electronics steric effects are also very important in order to decide between the trimerization or the tetramerization. So, far we have talked about trimerization, tetramerization and so on. We can also do reactions with multiple ends which means dienes and prions and variety of reactions are known, but the principles are the same. In order to do a coupling reaction you usually carry out an oxidative coupling reaction and that leads to the coupling of two dienes. Here I have shown you two dienes which can couple in a linear fashion. This is a linear product and this is a branched product. So, these dienes are not so readily available as the ethylene is. So, these reactions are less studied than the ethylene reactions that we have just discussed, but suffice it to say these reactions are also possible. Now, let us take a look at polymerization. Why is polymerization so important? Because the polymerization of ethylene to give you polyethylene is a very useful reaction polyethylene both low density polyethylene and high density polyethylene have got extensive industrial uses. So, it is important that we take some time and examine this particular process. Let me give you a brief history of the polymer catalysis and I have listed here a series of events that are critical for the polymerization for understanding polymerization history. First of all radical cationic and anionic polymerizations were known, but the molecular weights of the polymerized species were not as high as they were when Ziegler and Nata discovered metal catalysts in 1955. So, polyethylene and polypropylene were first discovered and made popular essentially in the 1950s until then only polymers which were not having high molecular weights were known and they were also not reproducibly made. What Ziegler and Nata discovered was that not only are the metals titanium zirconium group metals are important, they also showed that the support on which these catalysts are kept in this particular case I have shown you magnesium chloride and secondly the discovery of Mao. These things made a very big change in the catalysis industry. So, 1980s another very significant discovery was made and that was the fact that instead of using the group 4 transition metals one can also use metallocenes which could perform as single side catalysts when you suspend the catalysts on or support the catalysts on magnesium chloride the nature of the catalyst is not very clear. However, metallocene catalysts are single side catalysts meaning they have a well characterized structure and there is only one type of an active species. We will see how this makes a difference a little later. Then more recently in 1995 it was discovered that late transition metals can also do catalysis of ethylene. Let us proceed with Ziegler and Nata. We will not be talking about the properties of polymer but essentially the mechanisms of polymerization and how the metals play an important role in this polymer synthesis. So, Ziegler was the first one to show that polyethylene can be generated and very low pressures compared to what was done earlier that very low pressures they can be polymerized to generate useful polymeric materials. It was Nata who extended it to polypropylene and that made a very big change in the polymer industry. As a result of their 1955 discovery they were together awarded the Nobel Prize in 1963. Now you might be wondering what is so great about polymerization of ethylene or propylene and what are the advantages. There are significant advantages and disadvantages of the Ziegler Nata system and we will take a look at it in the following slides. First of all radical polymerization as I had mentioned earlier of ethylene requires a very high pressure of ethylene and it also required a high temperature and it led to significant amount of branching and the polymers that were made using these procedures were not very useful. But when Nata polymerized propylene which is what is shown here he obtained what is called isotactic polymers. Isotactic polymers are those where the where the alkyl groups are organized in same on the same side of the carbon in a polymer chain. I have pictured for you here the polymer chain which is as if it is in the plane of the screen. Then the methyl groups are going inside the screen whereas the hydrogen which is present here is coming towards you. So if you keep all the carbons in a plane then the methyl groups are going away from you and since all of them are going away from you is this called an isotactic polymer. Now this turns out that the polymer characteristics are very important and these are dependent on the way in which these polymers pack in the solid state and the way they pack depends on how these methyl groups are oriented. If they are isotactic they have excellent properties. So not only that the polymer molecular weights are very high it is about 10 power 5 to 10 power 6 Dalton's they have high density etcetera and all of these are significant advantages. Propylene polymerization was not even possible although ethylene could be polymerized propylene was not polymerizable at all and even if it was polymerized it generated polymers with not so clear stereochemistry. It would either be a syndiotactic which means they alternate the methyl groups alternate or they would be totally etactic which means they were randomly oriented along the polymer chain. Sometimes you can generate a metal complex which can give you stereo blocks but alternating but regular orientation of the methyl groups. All of these polymers they have different properties and so they are useful in their own right except when you have a random orientation in which case the usefulness decreases. So this was the most significant improvement that Nata made and now we ask the question why is the Nata catalyst so specific and how is it so active and at room temperature and low pressures how are we able to carry out polymerization so efficiently. Let us first look at the basic mechanism for polymerization it involves an extremely simple insertion reaction as opposed to the oxidative coupling that we have been talking about these are insertion reactions carried out on the titanium center. Now if you have a methyl group from the mauve or an alkyl group from the A L E T 3 which is usually used to activate the titanium you will have an alkyl group on the titanium in the place where I have marked it with a growing polymer chain. So in this position you will realize that you have this group which is attached to the titanium which can now undergo an insertion reaction and we will mark this insertion reaction like this. If this anionic group inserts into the ethylene and a new carbon titanium bond is formed you would get this particular product and that product has got a vacant site here this is a vacant site here that we have marked it with V and you have the growing polymer chain in the cis position. Now it is very easy to see that you can add an ethylene again into the vacant coordination site and the growing polymer chain can do another insertion reaction this time between this carbon and the alkene carbon like this and so the growing polymer chain now has got second ethylene attached to it. So we started out with a magenta carbon being attached to the titanium and we have added a blue ethylene just for the sake of identification I have color coded it. So we have added a blue ethylene and a red ethylene to the titanium. So you can have a very rapid polymerization of this ethylene notice that we are not changing the thermodynamics of this whole process. The thermodynamics is still in the same way as in the uncatalyzed process carbon carbon bond formation releases a lot of energy because you are converting a double bond to a single bond. But you lose entropy the net gain is the strong carbon carbon bond which leads to forward reaction polymerization is feasible in these cases. Now let us take a look now at stereo regularity. If you take propylene and if you look at the polymerization in the same catalytic cycle that we have just discussed you will notice that because these two groups are oriented in a cis position it is important to orient them away in such a way that the methyl group which is the bulky group on the propylene methyl group is the bulky group on the propylene. The growing polymer chain is the bulky group on the alkyl chain which is attached to the titanium both of them will have to be oriented away from each other. And because of this requirement you will always end up with the chain inserting in such a way that the methyl groups end up in the same position or in the same direction along the polymer chain because you want to always orient the methyl group away from the growing polymer chain. So this very simple fact and the fact that in the resting state of the catalyst this vacant site has a beta hydrogen interaction as a weak interaction with the beta hydrogen on the growing polymer chain you end up with a very strong stereo regularity in the polymerization process. Now it is been possible to have polymerization of ethylene with a variety of non groups for metals and this has led to a situation where you might call it the iron age of polymerization where even metals like iron have been activated using mauve again you need a reducing age with a large anion so that the iron is protected and only the ethylene is able to approach the metal center. And when you have ethylene coordinated to the iron and you can have insertion reaction and you can have oligomeric oligomerization of olefins to give you alpha olefins or high molecular weight polymers like HDP when you use these catalysts. So the mechanism of the reaction is assumed to be the same and it is because of these bulky groups that are present on the iron that approach of other anions which would poison the catalyst are prevented especially water will be a serious poison for the catalyst so that is prevented by the presence of a large excess of mauve. Now the chain growth is what needs to be very efficient in order to generate the polymer and this chain growth process as we have mentioned would be identical to what we showed on the titanium except that now you have the iron which is in the plus 1 oxidation state and it is undergoing an insertion reaction with the ethylene. But at the same time it has to compete with other processes which will stop the chain from growing what are these processes now you can see them in this slide that I have pictured for you here you have several processes instead of having insertion reaction you can have an exchange with simple ethylene which would regenerate the catalyst instead of continuing the polymer growth you can have an exchange reaction where ethylene comes in and the bulky polymer alpha olefin goes away so that is called an exchange reaction you can also have a reaction with trimethyl aluminum if you have trimethyl aluminum as a reducing agent then it tends to have a chain transfer reaction where a methyl group is transferred to the growing chain and the reaction stops and this is pictured here where once again a methyl iron bond is formed and the chain growth can continue but never the less you have stopped the main chain from growing at this particular point a third possibility is the beta hydrogen abstraction if beta hydrogen abstraction becomes more favorable than the alkene insertion then of course you would end up with a short polymer chain rather than a very long high molecular weight polymer in the whole reaction process. So, any polymerization catalyst has to compete with these two three possibilities so this is the chain growth mechanism that would favor high molecular weight and all these processes these three processes which I am labeling as one two and three these three have to compete with the polymerization process which is going on in the in the upward direction. So, this is usually favored by bulky groups on the metal atom which is coordinating to the ethylene and growth is favored when you have unfavorable energetics for the beta hydrogen abstraction. So, people have from the iron that I have just described to you it is also been possible to have nickel complexes and here is a nickel complex that is pictured once again you have a bulky ligand which is present on the metal that seems to be a key factor and methyl allomoxane is again a key factor that is necessary for reducing the nickel from the 2 plus state to the 0 plus state and it is possible to generate high molecular weight polymers using this particular catalyst as well. So, you can see that the principles are very similar to what is happening on the titanium titanium is not unique you need a low valent metal center which can carry out the polymerization very effectively, but it needs to be prevented from anions attacking the metal center and stopping the reaction. It not only 3 D metals here is an unusual example of a 4 D metal or a 4 D metal which is capable of carrying out this reaction. In this case very large tetra aryl borate anion has been used as a stabilizing anion to carry out this polymerization process and this polymerization is carried out at not too high pressures, but the main the important difference and advantage is that they have co polymerized 2 species in order to generate a very interesting polymer which is having the substituent on the polymer chain and this allows for speciality polymers to be synthesized. So, the polymer industry is in fact having a gamut of transition metals and metal complexes available for it to make very interesting polymers. Here is one more example where a palladium center has been used for generating alternating co polymer of carbon monoxide and ethylene. So, this generates polymer where the repeat unit has got CO and ethylene. So, the ethylene has come from these 2 carbon centers and carbon monoxide was also a reactant. So, you can make this high molecular weight thermoplastic using this palladium complex. Finally, we look at some of the products the early and properties of the early and late transition metals and the type of catalysts that have been used. The important point is to generate an active site that would not be blocked by an anion and in most cases, MOA was used as the very large blocking anion that is very weakly interacting with the metal center. So, you have a weakly or non coordinating species that is important for both centers. You can have soft ligands in the case of late transition metals, but you need an oxo group or an X minus on the titanium and the vanadium and the early group early transition metals which have to be activated with MOA and usually the coordination geometry around the early transition metals are tetrahedral whereas the late transition metals prefer a square planar intermediate. Now, the tolerance of functional groups is one place where the two differ very significantly. In the case of late transition metals, we can have tolerance of functional groups whereas in early transition metals only ethylene can be polymerized. Finally, I would end with a very recent example where a chromium catalyst has been used for synthesizing ultra high molecular weight polyethylene.