 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing about olefin polymerization in the last few lectures and in this regard we have been particularly looking at the catalyst development that happened during the course of evolution of olefin polymerization, particularly for polyethylene and polypropylene polymerization and we have noted that this olefin polymerization started with Ziegler-Nutters discovery of heterogeneous TICL4 diethyl aluminum chloride being able to polymerize ethylene at ambient conditions, room temperature and one ball pressure producing high density polyethylene and subsequently the Ziegler-Nutters catalyst also could polymerize propylene to give isotactic polypropylene which is a strictly regular propylene with all the methyl room pointing in the same direction of the backbone and then we have focused on evolution of the catalyst as a course of evolution of the field as well and what we had observed that the focus shifted from heterogeneous catalysis to homogeneous catalysis the reason being though heterogeneous catalysis was extremely good in terms of exhibiting very high activity for the polymer but one major drawback to the heterogeneous catalysis for olefin polymerization was the fact that these are multi site catalysis as a result very broad distribution of the molecular weights of the polymer were obtained and consequently the polymers were not really well behaved or the property wise there was a need to make a more narrow or distribution polymer for polyethylene as well as for polypropylene and as the need required the shift then the research focus then shifted from multi site catalyst to single site catalyst and that is where the homogeneous catalysis came in picture now the metallocene complexes of titanium and zirconium did a great job in terms of finding a homogeneous solution to poly ethylene polymerization in a multi single site catalysis manner and as a result with the advent of MAO as well as boron acidity triphenyl borane reagents what we saw was the evolution of metallocene type catalyst for which are homogeneous in nature and they are also single site catalyst for producing poly ethylene as well as polypropylene in high activity so this is what we had been discussing about and for poly a propylene the following three catalysts that we had spoken in the previous lecture is worth mentioning as they give propylene of different structures. For example, the first to note was this zirconium with MAO could give this is of C to V symmetry gave propylene to etactic polypropylene etactic meant means that the methyl groups are randomly oriented something like something like that then the next one that we had spoken about is this Bridszinger compound this with MAO this is C2 symmetric that a propylene to isotactic polypropylene which is stereoregular and then we had also looked into this famous zirconium catalyst zirconium catalyst with MAO that gave propylene to syndiotactic propylene I will draw this one properly and what is that meant was so alternating methyl group and so this is a very interesting observation and that can give through insight into the type of polypropylene that could be obtained by changing the symmetry from going from C to V and this one the last one is Cs symmetry this one is Cs symmetry so one can see that on going changing the symmetry from C to V one gets a tactic from C to symmetric one gets isotactic and from C to Cs symmetric one gets syndiotactic polymer so this is a finest example of how designing of catalyst can lead to polymer of very a high activity with very extremely narrow distribution range and extremely a good properties that one sort of wants to have in the polymer and this has all been achieved using homogeneous single site catalysis catalysis so this is the strength of organometallic chemistry at particularly the homogeneous catalysis in being able to produce so many different types of polypropylene polymers by on the basis of symmetry catalyst structure and so forth. So, based on this we are going to now focus on another extremely a nice example by a professor Weymouth in 1995 by which he could make a stereo block polymers and this is a beautiful elucidation of rationalization chemical modification and its resultant output in terms of the polymer that one could produce now stereo block polypropylene means that this is two polymer segments of polypropylene one may be this the other in cartoon I am giving of another color so this is a stereo block polypropylene and the idea is that these two are sort of a combination of either of either of the three so it can be a one block can be a let us say isotactic other can be syndiotactic or it can be atactic so a combination of two different chains of any of this type that would allow formation of stereo block of polypropylene a very nice example of this as given by professor Weymouth in 1995 stereo stereo block polypropylene by nobility and this was a fine demonstration of polymerization was carried out the using this catalyst and this is illustrated by the equation this ligand is called two phenyl indene or two page ind this ligand complex of zirconium m a o actually resulted in two types of complex zirconium of this in which these two phenyl moiety are in trans conformation and it would have these would react with propylene would give one side a polymer chain and other side a vacant site and there can be two possible conformations present of these which can be in mutual equilibrium and the other conformation would be this so what is literally happened is that there is a conformational rotation that happens between these two ligands along the zirconium axis such that these two phenyl rings can be trans as well as these two phenyl ring can be in the cis disposition as it is shown over here and the corresponding the polymer chain would be as here this being the vacant site now these two change is dependent on temperature k1 and k2 now what is important over here is to note that when one goes from cis to trans the symmetry of the complex changes to the extent that this one remains a c2 symmetric chiral and this one becomes a chiral and it and since there is no answer bridging a ligand between the two zirconium moiety ligands so this free rotation is very much possible going to two conformation in which when it is cis it is a chiral when it is trans it is chiral and and as a result when the polymerization occurs in this conformation then atactic polypropylene is obtained so atactic block of polypropylene obtained and when the conformation polymerization occurs in this conformation KPI then isotactic polypropylene block is obtained and as a result the overall polymer which comes out of this is something like that and then the other block so this is a so this is atactic block this one over here is isotactic block so what is important over here is that stereo block polymer containing isotactic atactic block polypropylene can be obtained and now this is very interesting because point to note over here is that the conformational mobility that may exist between two forms the transform and the cis form that is taken that has been exploited or taken advantage of to produce polypropylene having two blocks one is isotactic and the other is atactic and that is done in a controlled fashion so by raising or changing the temperature concentration one can control this equilibrium of the weight of exchange between the cis and the trans and then can control the length of the stereo block as per will and also one should take note of the fact that only one of the conformation is chiral which is giving this isotactic block and the other is a chiral which is giving this atactic block so this seminal beautiful elegant example that take advantage of this conformational mobility was reported by professor bob waymouth in 1995 which took advantage of this conformational mobility for producing a stereo block polypropylene and these two blocks these two isomers are of comparable energy so they could be varied by changing the temperature pressure and the R groups and property wise this isotactic portion of the block isotactic portion responsible for are more responsible for thermoplastic properties properties which is more crystalline whereas etactic portion of the block is more rubber like rubber like properties and this so also this is sort of the power of homogeneous single site catalysis been being able to produce polymers of two different tacticity in the same polymer chain and this was sort of recognized by the first fact that the first industrial production of LLDPE using metallocene single site homogeneous catalysis was reported by Exxon in 1991 so first industrial production of LLDPE linear load density using metallocene homogeneous single site catalysis reported by Exxon in 1991 so you know this is a significant discovery in the sense that we are one is more familiar with heterogeneous catalysis being used for industrial scale application but here catalysis based on homogeneous catalyst which is not often the case in most of the situations was even built by Exxon for producing LLDPE so this is tremendous success for homogeneous catalysis there are further modification for example ELF-ALTO came in 1999 used metallocene catalyst and caught on to heterogeneous process heterogeneous surface bed for producing this poly for polymerizing olefin and this is the case where homogeneous catalyst has been heterogeneous in the for the industrial scale production so with these we come to an end of today's discussion where we have looked into a nice example by Bob Weymouth that showed how conformational mobility by suitable choice of ligand could be exploited for producing stereo block isomers on the same polypropylene backbone chain one of a and by varying the temperature concentration pressure one could model to the modulate the chain length such that two stereo blocks of different tacticity can be obtained in the same polymer chain and this takes advantage of this phenomenon of confirmation changes from cis to trans that occur in the catalyst in which one gives a chiral environment the other becomes a chiral resulting in isotactic and atactic stereo block polymer and this isotactic stereo block polymer is more crystalline and result thermoplastic properties to the polymer whereas atactic is more rubber like so two different opposing properties present in the same polymer and this is a beautiful elucidation of the chemistry that takes advantage of conformational mobility so with these I come to the conclusion of today's talk we are going to look at more on this catalyst development aspect that we are looking in Ziegler-Natter catalyst in olefin polymerization starting from the Ziegler-Natter catalysis when we meet next till then goodbye and thank you