The shape of flow is described as follows:
Figure2:Model of Elongation flow
Deformation : 
Tensile Stress: 
Viscosity is ability of material to flow (easy or difficult):
> High viscosity > it is difficult to flow
> Low viscosity > Its easy to flow
For examples:
| fluid | Unit[N.sec/m2] |
|---|---|
| Water | 10^-3 |
| Glycerin | 10^0 |
| syrup | 10^2 |
| Polymer molten | 10^2-10^7 |
Table 1: Viscosity of some fluids.
There are other types of addition polymerization which do not fit into the categories of polymerization that were mentioned before. It is not within the scope of this course to mention all of the types available.
However, one of the most important types of catalyst is the so-called "Ziegler-Natta" catalyst discovered by Karl Ziegler in 1953.
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These catalysts can be prepared by mixing
of an element from Groups 1 – 3
with a halide (Group 7) of a transition element, as below
Before this new catalyst had been invented polyethylene had been produced by free radical polymerization at
However, this method had resulted in branched polymers being formed.
Question – Why do you think the free radical type of polymerization
Giulio Natta realized the potential of this new type of polymerization for the production of stereospecific polymers. This means that polymer chains could now be produced that had high degrees of regularity in their tacticity. How? Let us look at the mechanism.
Taking propylene as an example, the monomer forms a TT-complex with the titanium active site (a vacant d-orbital).
After the coordination of the monomer at the active site, there is a
transition state with a ring structure involving Ti and C atoms. The new polymer chain then moves back to its original position, which means that the stereochemistry is kept the same as the polymer
In summary, the propagation step can be written as
This discovery had enormous academic and industrial importance and in 1963 Ziegler and Natta were jointly awarded the Nobel Prize for Chemistry.
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In Anionic Polymerization, for the monomer CH2=CHX, the chain carriers are carbanions. Monomers which contain electronegative substituents (groups) such as
When the initiator, potassium amide breaks up into ions the NH2- anion reacts with the monomer (in this case the vinyl monomer) to form the chain carrier the carbanion. The positive counter ion in this example is K+ ion.
No Termination- Anionic polymers are living polymers
Unlike the other types of addition polymerization we have studied
anionic polymerization does not have a termination step. If an
inert (unreactive) solvent and a pure monomer are used, polymerization only stops when the monomer molecules are used up.
Such polymers are known as(living polymers) However, traces of water or other impurities will terminate the chains.
Polymer science is an interdisciplinary field that involves aspects of chemistry (physical and analytical), physics, materials science as well as chemical and mechanical engineering.
The main focus of polymer science is to study the structure of polymeric materials and its effect on their physical and mechanical properties. So let’s start by trying to understand the nature of these materials and define what they actually are.
A polymer is a very large molecule, often referred to as a macromolecule, which is made up of small repeating units.
The word Polymer is made up of two components: “Poly” which means “many”, and “mer” which means “unit”.
A polymer molecule could be made up of hundreds of thousands of these small units (also called monomers) to form a very long chain.
The chemical nature of these small units, as well as the various types of arrangements that the chains can be organized into give polymers a wide range of properties and make them very unique compared to other materials.
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Addition polymerization can also occur by other than a free-radical mechanism. Chain carriers (instead of being free-radicals) can be either
• Cations (used in cationic polymerization)
• Anions (used in anionic polymerization)
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When the monomer CH2=CHX is polymerized by cationic polymerization the chain carrier is a carbocation. Usual cataly~ts for this type of polymerization are • Lewis acids (electron pair acceptors). • Friedel-Craft catalysts. • Strong acids. Free-radical polymerizations usually take place at higher temperatures. Cationic polymerizations proceed best at low temperatures. Eg. Isobutylene (2-methylpropene) polymerizes very rapidly at -100c. Solvent is more important in ionic polymerizations than in free radical reactions since the mechanisms involve charged species (ions) whereas free-radicals are generally neutral.
In reactions where acid is the catalyst, the initiation can be represented as follows, where HA is an acid molecule such as hydrochloric acid (HCI) or sulphuric acid (H2S04), 
A H+ ion (a proton) is transferred from the acid to the monomer producing a carbocation.
In the propagation step the monomer is added to the carbocation produced in the initiation step and the process is repeated many times. Each time a
Termination
Termination can occur by several processes. The simplest way is the combination of the carbocation and its counter ion (from the acid) as below.
1 – Combination
Termination can also take place by chain transfer to monomer (see below). Since the concentration of monomer is much higher than the concentration of ions then this type of termination is more common.
2 – Transfer to Monomer
Termination can also take place by an ions to give a double bond in the polymer chain (leaving it dead) and the acid.
3 - Replacement