 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing a very important topic, which is a Nobel Prize award winning topic Olefin Polymerization in the past few lectures. In this regard, we have approached this topic from two perspectives, to start with we have looked into it from the perspective of application point of view, particularly we started talking about polymer classification from the application perspective, and we have spoken about various kinds of polymers like thermoplastic, duroplastic, elastomers, elastoplastic materials or thermoplastic elastomers and reversible duroplastic. So these were all classified based on the properties of the polymer, then in the same topic we have looked into polymer classification based on the chemistry perspective from a chemistry perspective based on the process by which they are synthesized, and in this context we have looked at two kinds of classification, one is condensation polymerization, the other is addition polymerization depending on the reaction by which they are formed. If they use a condensation reaction, where a small molecule is eliminated as a path to polymerization, then these are condensation polymers, whereas when the monomers are just added without elimination of any molecule, they are called addition polymers, and for one main feature of addition polymer is that the composition of the monomer and the composition of the repeat unit of the polymer are the same, they are constant. So we have done that, and again we have also looked into the classification of polymer from the perspective of the mechanism point of view, and these were from the mechanism by which these polymers are formed. In this context, these polymers can be divided into two types based on the pathways they take, one is step growth polymerization, the other is chain growth polymerization, and we had seen what are their characteristic molecular weight profile of the polymers, so formed in this process as a function of person conversion of the reaction, polymerization reaction, and what we had seen that in the step growth process the molecular weight sort of increases drastically significantly towards the end of the polymerization, that means towards the end of at higher person conversion, whereas molecular weight does not have a bearing in case of chain growth polymerization on the percent of the conversion of the polymerization reaction. So in this context, we have also discussed about the various types of polyethylene, which are obtained and the types of processes which are synthesized, and polyethylene are of three types based on their densities. The first one is LDP, these are synthesized by these are called low density polyethylene. Polyethylene can be classified into three parts, the first is the LDP or low density polyethylene. The next one is HDPE or high density polyethylene, the other one may be I write it later, the next one after low density polyethylene is LLLDP, this is linear and then HDPE is high density polyethylene HDPE, this is there are three types of polyethylene depending on the branching, this polyethylene has different material properties, some are soft, some are hard and brittle, some are transparent, some are opaque. Now in the last class, we have also discussed about the industrial processes that are used for preparing this kind of polyethylene, and the first one that we have spoken about was this ICI, Imperial Chemical Industries, first developed this process for preparing LDP linear low density in 1993, so this is probably the oldest polyethylene polymer to have been prepared industrially, and presently these LDP are prepared radical polymerization using oxygen initiators, so like peroxo compound. Now this radical polymerization is not good, because it has several limitations requires high pressure of around 2800 bar, so and also the reaction is uncontrolled as a result polyethylene obtained are branched, has significant branching and they are soft material without non-crystallinity, so the next process are the process which is used for preparing high density polyethylene HDPE is the Philip process, this was discovered by Hogan in 1956, and this is used for making this high density polyethylene, and it was using chromium trioxide as the catalyst, as a pre-catalyst, but the real chemistry was done, and this was done on a heterogeneous fashion on a silicon surface, done on a heterogeneous fashion in a silicon surface, so this Philip's process actually uses chromium II and chromium IV as the active metal centers, which carry out this polymerization process. We have also seen that the improvement of the Philip's process was reported by Union Carbide, which is a chromocene, which is a chromocene low valent, low valent chromium compound as the precursor for this HDPE. Now there is another variant of the Philip's process, which uses hydrogen to control the chain length, so chain length under Philip's process is controlled by metered addition of hydrogen, chain length is controlled under metered addition of hydrogen, and that gives rise to this low density polymer, and this is how this LLDP is produced. Now in context of this, there was a significant discovery, which is another process called Unipole process from Union Carbide, which uses fluid bed reactor, they have been used for producing both LLDPE as well as HDPE, so this is an interesting process. What we see if we sum up that there are several industrial processes involved for producing various grades of polyethylene, to start with it was the ICI or Imperial Chemical Industries, currently also radical polymerization is used with oxygen initiators for producing LDP, low density polyethylene for HDPE, it was a heterogeneous chromium trioxide process in the 50s by called Philip's process, which was producing HDPE, and then Union Carbide comes into play and produces HDPE with chromocene, which is a low valent chromium precursor, and finally, by a controlled metered addition of hydrogen to Philip's process, one can get LLDP as well as LLDPE, so Philip's process can give both, it can give HDPE, and Philip's process with hydrogen can also give LLDPE, and after this there was a technological breakthrough in form of this Unipole process, which uses fluid bed reactor, and then this can be used for producing both LLDPE and HDPE, so this is a technological breakthrough from Union Carbide, where they have gone from conventional reactor to fluid bed reactor, a new technological development for producing this high molecular weight polyethylene, so this shows the chemical reactivity space from an industrial perspective for producing the various grades of polyethylene, so what we can see that this low density polyethylene is the one, which is produced by radical polymerization, whereas both LLDPE and LLDPE are produced by ionic or cationic coordination in solution polymerization, so now we are going to move on to something more interesting that during the course of the development of all these Philip's process and then Unipole process, there was a parallel development on Ziegler development towards Ziegler data catalyst was taking place simultaneously, and the studio starts with this off bow reaction or chain growth reaction on aluminum, and so we are going to take a look at this exciting development as we proceed further in this lecture. The off bow reaction is this reaction is primarily used for the formation of linear aliphatic compounds with a maximum chain length C200, so at that time this was a sort of polymerization on the back of aluminum of bow reaction, which is called chain growth reaction at the back of aluminum, and which could give linear aliphatic compound of maximum length of C200, so that is really long chain compounds, and the reaction of this is given as R2Al ethyl N-1CH2CH2 at 90 to 120 degree centigrade 100 bar gives R2Al CH2CH2N, now what is happening is these CH2CH2 unit is getting added on this metal ethyl bond in high pressure, and this off bow reaction, this is what is called off bow reaction, and this reaction competes with another reaction called hydroalumination, and this is given as R2Al CH2CH2RD, giving dehydroalumination, sorry this is dehydroalumination R2Al H plus CH2 double bond CHRD, and these can be explained as a beta hydride elimination, and this is explained as such, so one gets aluminum hydride and this alpha olefin, now these are really a long chain products, long chain polymer, and Carl Ziegler was observing this formation of these long chain polymers in his laboratory during systematic study of off bow reaction Ziegler observed opposite effect, that is quantitative conversion of ethylene to one butene, so instead of observing a very high polymer chain with Ziegler observed that in one cases he had observed a quantitative conversion to just oligomerized product or dimerization of ethylene to one butene, and this was a surprising observation and was later attributed to the presence of impurity in the reactor, and this is supposed to be nickel impurity reactor, and that was later as the famous nickel effect, so while pursuing this off bow reaction under different condition, carrying out a systematic study a professor Ziegler called Ziegler observed that in certain cases exactly opposite was happening, that is a conversion of quantitative conversion of ethylene instead of making polymer was making one butene, and this was sort of contrasting and surprising observation at the time of the study, where the focus of the off bow reaction was to make long chain polymer, and here they were quantitatively getting dimer, so this surprising observation was later attributed to the presence of impurities in the reactor particularly the nickel impurities and which was famous which was later came to be known as the famous nickel effect that actually triggered the discovery of Ziegler Nata polymerization, so with this I stop come to an end of today's lecture, in today's lecture we have looked at the various industrial processes that are prevalent or were prevalent in producing or accessing various kinds of polyethylene ranging from LDPE, LLDPE and HDPE, and we are also put in a perspective various discoveries and changes that were affected on individual processes to access each of these polymers, we have also looked into the beginning of the discovery of the Ziegler Nata catalysis starting from off bow reaction and how the opposing effect of nickel impurities led to dimerization of ethylene into one butene, so that was later recognized as the famous nickel effect and which even triggered the discovery of Ziegler Nata catalysis, so with this I thank you for being with me in this lecture, we are going to discuss further from the point of nickel effect to Ziegler Nata and various types of other Ziegler Nata catalysis examples as we take this topic up in the next class, till that thank you and goodbye.