Types of polymers and polymerization methods used for preparation of polymers

  A polymer:
Is a molecular compound distinguished by a high molar mass, ranging into thousands and millions of grams, and made up of many repeating units.

Polymers (Greek: poly 1 meros, many parts): 
Are long-chain molecules synthesized by linking monomers (Greek: mono 1 meros, single part)

a. Naturally occurring polymers include: proteins, nucleic acids, cellulose (polysaccharides), and rubber (polyisoprene).

b. Synthetic polymers (mostly organic compounds):

Nylon, poly(hexamethylene adipamide); Dacron, poly(ethylene terephthalate); and Lucite or Plexiglas, poly(methyl methacrylate).

Methods used for preparing polymers:

Polymer molecules are made of simple repeating monomer units.Repeating units are joined by:

1. Addition reactions
2. Condensation reaction

a. Addition reactions: 

 This method is used with unsaturated compounds that contain double or triple bonds.

•         Example: preparation of Polyethylene:

Polyethylene is made up of ethylene monomers joined together via addition-reaction

1- Initiation step:

R2→2R^.

Heat monomer produce two radicals 

2- Propagation step: 

      

Two radicals attack methylene molecules to generate new radical:

    New radical reacts with another ethylene molecule (propagation reaction):

3- Terminated reaction:

b. Condensation reactions:

Condensation polymerization takes place by removing minor molecule such as; water, ammonia, and hydrogen chloride

The monomers used in condensation polymerization are characterized by their polar character (polar-containing monomers)

Condensation polymerization is used to prepare:

Polyesters: such as PET (Polyethylene terephthalate)

Polyamides: such as Nylon-6,6

Polyester polymer (Dacron -PET (Polyethylene terephthalate):
Condensation polymerization between terephthalic acid 1,2-ethylene glycol:

Polyamide polymer:
Condensation polymerization between diacyl chloride or Hexanedioyl dichloride with hexamethylene diamine: