Chemical reaction Kinetics, Slides of Chemical Kinetics

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Polymerization

mechanisms

Textbooks

• (^) Basics of Polymers: Fabrication and Processing Technology. By: Muralisrinivasan Natamai Subramanian. Published by Momentum Press, 2015
• (^) Introduction to Polymer Chemistry, 3 rd Edition. By: Charles E. Carraher Jr. Published by CRC Press, Tailor and Francis, 2014

• (^) From the figure 1, the bond lines extending at the ends in the formula of the product indicate that the structure extends for many units in each direction.
• (^) Note that all the atoms—two carbon atoms and four hydrogen atoms—of each monomer molecule are incorporated into the polymer structure. Because the structure it displays, such as the one above are usually cumbersome, the polymerization is often abbreviated as follows:
• (^) nCH 2

=CH

2

→ [ CH

2

CH

2 ] n

• (^) During the polymerization of ethene, thousands of ethene molecules join together to make poly(ethene) - commonly called polythene. The reaction is done at high pressures in the presence of a trace of oxygen as an initiator.

• (^) A polymer may be a chain of thousands of monomers , and so it is impossible to draw the entire polymer. Rather, the structure of a polymer can be condensed and represented as shown in Figure 2.
• (^) The monomer is enclosed in brackets and the n represents the number of repeating units (the saturated form of the monomer) in the polymer, where n is any whole number.
• (^) What this shows, is that the monomer is repeated an indefinite number of times in a molecule of polyethene.
• (^) Figure 2: A simplified representation of polyethene.

• (^) Once initiated, repeating elementary steps continue until the reactants are exhausted.
• (^) When repeating steps generate more chain carriers, they are called chain branching reactions , which leads to expanded fire outbreak or explosions.
• (^) If the repeating elementary steps do not lead to the formation of new product, they are called chain inhibition reactions.
• (^) Addition of other materials in the reaction mixture can lead to the inhibition reaction to prevent the chain propagation reaction. When chain carriers react with one another forming stable product, the elementary steps are called chain termination reactions.

Mechanism of Chain Reactions

• (^) The elementary steps used for mechanisms of chain reactions can be grouped into the following categories:
• (^) initiation step
• (^) chain propagation steps
• (^) chain branching steps
• (^) chain inhibition steps
• (^) chain termination steps

Example of Chain Initiation Step

Chain Propagation Step

• (^) In this step, the chain carrier makes another carrier
• (^) Elementary steps in which the number of free radicals consumed is equal to the number of free radicals generated are called chain propagation steps. Once initiated, the following chain propagation steps repeat indefinitely or until the reactants are exhausted.
• (^) Example (using CH 4 )
• (^) It’s only once the free radicals are present that our substrate (CH 4 ) gets involved. Chlorine radicals are highly reactive, and can combine with a hydrogen from methane to give the methyl radical, •CH 3
• (^) If you count the number of free radicals in this equation, you’ll note that there’s one in the reactants and one in the products. So there is no net increase in the number of free radicals.
• (^) This type of step is referred to as “propagation”.

• (^) In each of these steps, a radical is consumed, and another radical is generated. Thus, the chain reactions continue, releasing heat and light. The heat and light cause more radicals to form. Thus, the chain propagation steps cause chain branching reactions.

There Are Two Propagation Steps In Free-Radical Substitution

• (^) We can do “propagation” step this way: Take the methyl radical, and it reacts with the Cl 2 still present. This gives us CH 3 Cl and the chlorine radical. Note that there has been no net change in the number of free radicals, so this is still a “propagation”.

Chain Branching Steps

• (^) Branching reactions are elementary steps that generate more free radicals than they consume. Branching reactions result in an explosion. For example, in the reaction between hydrogen and oxygen, the following reaction may take place:
• (^) Hence, In this step, one carrier makes more than one carrier. H.+O 2 → HO. + .O.
• where ⋅O ⋅ is a di-radical, because the O atom has an electronic configuration 2s^2 2px^2 2py^1 2pz^1. In this elementary step, three radicals are generated, whereas only one is consumed.
• (^) The di-radical may react with a H 2 molecule to form two radicals. .O.+H 2 → HO. + H.
• (^) Thus, together chain branching reactions increase the number of chain carriers. Branching reactions contribute to the rapid explosion of hydrogen-oxygen mixtures, especially if the mixtures have proper proportions.

Chain Inhibition Steps

• (^) The steps not leading to the formation of products are called inhibition reactions or steps. For example, the following steps are inhibition reactions. Cl. + ClH 2 CCH 3 → H 3 CCH 2 + Cl 2. Cl. + HCl → H. + Cl 2. H. + ClH 2 CCH 3 → H 3 CCH 3 + Cl.
• (^) Furthermore, sometimes another reactive substance ⋅A may be added to the system to reduce the chain carriers to inhibit the chain reactions. Cl. + .A → ClA (not reactive) The species A ⋅ is often called a radical scavenger. In food industry, radical scavengers are added to prevent spoilage due to oxidation; these are called biological oxidants. The mechanisms in chain reactions are often quite complicated. When intermediates are detected, a reasonable mechanism can be proposed. Adding radical scavenger to prevent food spoilage is an important application in food chemistry. This application came from the application of the chain reaction model to natural phenomena.

Example of Chain Termination Steps

• (^) If the concentration of Cl 2 is low relative to CH 4 (in other words, Cl 2 is our limiting reagent) then the rate of Propagation Step #2 will slow down as its concentration decreases.
• (^) Without any Cl 2 to react with, our •CH 3 radicals can just combine with another free radical (such as •Cl) to give CH 3 Cl, for example. There is essentially no barrier to this reaction.
• (^) Note that here the number of free radicals decreases from 2 to zero. This is called termination.

Polymerization techniques

• (^) The mechanism of polymerization used in the synthesis of polymers allows for the classification of the latter into two major classes, namely, addition and condensation polymers.
• (^) The former kind is produced by the repeated and sequential addition of monomers without the loss of a smaller molecule during the process.
• (^) Therefore, no by-product is formed and the repeating unit of additional polymers has the same formula as the alkene or functionally-substituted alkene monomers used to make them.
• (^) These addition reactions follow a stepwise mechanism that involves reactive intermediates such as radicals or ions that help in the conversion of a pi bond in the monomer into a sigma bond in the polymer.
• (^) The four different polymerization techniques used in the synthesis of addition polymers are (i) radical polymerization, (ii) cationic polymerization, (iii) anionic polymerization, and (iv) coordination catalytic-polymerization