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  Addition Chain-growth Polymerization Reactions, Chemistry tutorial

Introduction

We have earlier conversed that Polymers are 'giant molecules' formed from repeating units (poly = many and mer = parts). The individual units are termed monomers; thousands of such are linked together to form a polymer molecule. 'Giant molecules' have high molecular weight [103s to 106s amu].

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It would be significant to write out equations to demonstrate addition chain-growth polymerization reactions as monomers add-up with no loss of molecules that includes the double bonds on the monomers. Vinyl derivatives are extremely suitable for such.

In calculation polymerization reactions the monomers are unsaturated [through double or triple bonds]. During the course of the reaction, for instance in the case of polymerization reaction of ethylene, at the double bond the pi electrons are liberated to form new C-C bonds by 2 other molecules of ethylene. [for instance the double bond initially through pi and sigma bonds, is broken so that the 2 electrons in the pi are now utilized in shaping new C-C single bonds by 2 other ethylene molecules, leaving only the sigma bond of the initial double bond in place. This is repeated on every unsaturated bond; therefore all appear as single bond in the polymer product, as a chain. Addition chain-growth polymerization provides increase to direct integral numerous of the monomer units by similar percentage composition of elements in the total monomers that make it up.   

Monomers are generally made of carbon and hydrogen atoms. Hetero atoms as oxygen, nitrogen and halogen atoms can be present. Good instances of monomers are ethylene molecule, CH2 = CH2, vinyl chloride, CH2 = CHCl, vinyl acetate and CH2 = CHCO2 CH3.  Addition polymers are formed through monomers just "adding on" to each other with no loss of atoms. All monomers forming calculation polymers have C=C bonds (carbon-carbon double bonds). In addition polymerization, the unsaturated monomers are coupled through their numerous bonds. Such double bonds open up as the monomers join in chains 1000 to 10,000 units long. For instance, ethylene molecules can connect mutually (under the proper conditions of temperature, pressure, and catalyst) to form polyethylene (polyethene): 

                   O2 initiator          

nCH2= CH2   460_aro.jpg [CH2-CH2]n       [for instance n=103s-104s]         

                  1500atm, 2000C                                  

Table: Instances of vinyl monomers

Derivatives of ethylene provide as monomers in many chain-growth polymerization reactions. They are termed vinyl compounds with general formula: 

2267_vinyl compounds.jpg

Fig: vinyl compounds

*whole structure is presented CH2=CCH3COOCH3

Activity 1: (a) Illustrate the addition chain-growth polymerization reactions of 2 of the above vinyl monomers. (b) What are the general utilizes the polymer is applied for?   

The reaction gets initiated when reactive molecules are 1st formed via either formation of a radical or ion (cation or anion). Each of such will be observed in the subsequent chapter 3. More instances of addition polymers comprise polymethyl methacrylate (Lucite or Plexiglass), Teflon, polyvinyl alcohol, polystyrene, polyvinyl chloride (PVC), polyacrylonitrile (Orlon), and natural rubber. 

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