 Welcome to this course on Transition Metal Organometallics in Catalysis and Biology. We have been discussing olefin polymerization in the last few lectures, and the main focus of our discussion had been on the classification of olefin polymers that one can obtain through this olefin polymerization process. One thing for sure, which is very much evident in our discussion so far, that polymers of different properties can be obtained through this process, and these properties can be very diverse. For example, they can be something very soft, they can be something materials which are soft, there will be materials which are hard, the materials which can be deformed, material which will be resistant to deformation, there will be materials which once deformed will retain the new shape, and then the materials which once the deformation is released, stress is released, it goes back and has a memory to its initial state. What we had come across is a wide variety of properties that can arise in polymers, and based on these properties and their subsequent applications, these polymers are classified. In this context, in our previous class, we have looked into three types of polymers. The first that we looked into, discussed was thermoplastic materials. These are the materials that exhibit stability under short-term strain, however, upon warming they transform into a plastic, that means they are easily deformed, and they retain the deformed shape. Then we had discussed about dura plastic materials, and these are materials which maintain their shape upon extended period of strain or high temperatures. They are usually formed by cross-linking pre-polymers by heating, and this cross-linking formation is usually reversible, and this cross-linking is very fine meshed, and they have very low segmental mobility, and as a result of cross-linking, dura plastic is rarely crystalline. In our earlier class, we had also discussed another new type called elastomers, and these elastomers are the materials which can be easily deformed, however, once the deformed stress is removed, they go back to their original shape. That means they have a memory of their initial shape, even though they are deformed, once the deformation stress is gone, they go back to the original stress. These are elastic materials or rather properly known as elastomers. They also are made up of cross-linking of pre-polymers, and these pre-polymers are however long-chain, and they have wide mesh. These are the three materials that we had discussed in the previous class, and today we are going to start with the fourth one, which is called elastoplastic material or thermoplastic elastomers. The fourth one that we are going to be talking about are elastoplastic materials or thermoplastic elastomers. Along with the same line, we are going to describe another form of polymer, which are called reversible duroplasts. Today, we are going to focus on these two polymers, number 4 and 5, which are elastoplastic material and reversible duroplasts, before we move on to various classification of polyethylene polymer. Let us begin by discussing elastoplastic material or thermoplastic elastomers, or thermoplastic elastomers. Lies somewhere between non-cross-linked and thermoplastic, and these polymers lie in between non-cross-linked thermoplastic, which we had discussed earlier, and cross-linked elastomers, and these are achieved in copolymers or in blends. They are usually achieved in two polymers of two different types in the same chain or in blends that have both duroplastic as well as elastomeric domains that have both duroplastic and elastomeric domains. This work in such a way that at low temperature, the mechanical properties of elastomer contain dominate, and that however at higher temperature, the cross-links of duroplastics domains are cleaved, and the material becomes thermoplastic. This is interesting that these elastomeric materials or thermoplastic elastomers lie in between cross-linked thermoplastics and non-cross-linked thermoplastic and cross-linked elastomers, and they usually are synthesized by or they are achieved in copolymers or in blends that have both duroplastic and elastomeric domains. At low temperature, the mechanical properties of the elastomer dominate, whereas at high temperature, the cross-links of the duroplastic material domains are cleaved, and as a result the material becomes thermoplastic and behaves as a thermoplastic material. This is an interesting classification of a new kind of polymeric material. The last in this discussion before we move into polyethylene is that of reversible duroplasts. Now reversible duroplasts are the exhibit properties which are both duroplastic and thermoplastic materials. Their primary difference from duroplasts is that unlike genuine duroplasts, they are reversibly cross-linked. In contrast to genuine duroplasts, they are reversibly cross-linked. In this case, usually this chemical cross-linking is often a result of coordination of ionic polymers to metal ions. These are not covalent cross-links, where there is overlap of orbitals to make a bond. These are mainly of ionic cross-link type. The chemical cross-linking arises from coordination of ionic polymers to metal ions. This is unlike the chemical cross-linking, which involves a chemical bond cleavage. This is more like a co-ordination bond between the ionic polymer and the metal ion, which gets reversibly cleaved during its application purpose. The other properties of reversible duroplasts include partially crystalline thermoplastic materials, thermoplastic materials, and also the reversible physical linking cross-linking is affected by lattice energy of crystalline domain. Now, this is an important property that this reversible physical cross-linking is affected by lattice energy as well. This points to the fact that non-covalent interactions also play an important role in the reversible duroplasts. So far, we have discussed five classes of materials starting from thermoplastic, duroplasts, elastoplastic materials, and reversible duroplasts based on the type of applications and the type of their material properties. Now, with regard to polyolefins, the versatility of polyolefins can be gauged by the fact that there are polyolefins for each of these categories have been prepared, and that is why they are so very important. The versatility about polyolefins is evident from the fact that polyolefins have been prepared for each of the categories mentioned above. That shows how important these polyolefins are. Now, with these having defined what are the categories of polyolefin classifications of polyolefins, we are going to in a bit more detail and look into the type of classification that exists for polyethylene. So, polyolefins are classified according to their density, according to their density arise from their nature of crystalline extent of crystallinity, then number and nature of cross linking the number nature of crosslinking present between polymer chains. Polyolefins are classified according to the density, and this is something that arise from crystallinity as well as number of nature of crosslinking present between polymer chains. Now, based on this, we are going to now look into various classification of polyethylene, which is kind of important from our perspective. Now, polyethylene are usually classified into three types, the first being LDPE, so this is called low density polyethylene, they are usually flexible materials and transparent as a film, their density varies from 0.90 to 0.925, and their property is that they would have long chains with small branches, and the branch chain will have smaller branches as is shown here, also over here, the branch side chains will have branches, and these are called low density polyethylene. Next variation is called LDPE, these are linear low density polyethylene, they are also fairly flexible transparent, and their density would vary from 0.925 to 0.94, and they would also have branches on the main chain, the side branches are less, so they mainly have branches in the main chain, so that is why they are called linear low density polyethylene, and the third of this type are called HDPE or high density polyethylene, and they would vary from 0.94 to 0.97, they are usually rigid and often cloudy to even opaque, and they are just polymer chains without any branching as is shown over here. With this, we come to the end of today's discussion, where we have looked into various the remaining two varieties of polymer classification, including elastoplastic material or thermoplastic elastomers and reversible duroplast, and finishing that we looked into classification of polyethylene, which are of mainly three types, first is LDPE, low density polyethylene, that has a density of around 0.90 to 0.925, and they are long chain polymers with branching on the side, then we have linear low density polyethylene, where density is from 0.925 to 0.94, they are also flexible and transparent material, here also these have very short branches around the main chain, but not really big chain branches, and last we had this high density polyethylene having density from 0.94 to 0.97, and these are rigid and cloudy to opaque materials, which does not have at all any branching. With this, we come to the end of today's discussion on polyethylene classification, we are going to look into this polymer classification in a bit more detail, when we meet next for ethylene as well as for polypropylene, and I once again thank you for being with me in this class, and we would have more interesting discussion about this polyethylene classification, when we take up the topic in the next class. Till then, goodbye and thank you.