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  Condensation Step-growth Polymerization Reactions, Chemistry tutorial

Introduction 

Step reactions are included that are independent of each other and doesn't include no chain-carrying free radicals or ions as cations and anions. The monomers have more than a functional group. During the polymerization small molecules these as NH3, HCl, CO2 or H2O is lost. We will also see that properties of the polymers are function of the arrangements and links/ bonding in it, determining also its shapes. 

Main content 

The condensation step-growth reactions are self-governing of each other. Chain-carrying free radicals or ions as cations and anions aren't included. Small molecules as NH3, HCl, CO2 or H2O are released during condensation.

Condensation polymers are shaped when the monomers react through each other so that a small molecule, these as H2O, CO2, NH3 or HCl, is abolished as a product each time a monomer reacts by another monomer. This is where the name of condensation polymerization is obtained. There must be at least 2 functional groups on each reacting monomer in order for the reaction to continue to form a chain; therefore a linear polymer is shaped as in nylon and dacron. For monomers by more than 2 functional groups for example through more than 2 sites for reaction, an extremely cross-bonded polymer is shaped as in resins like glyptal polyester. After every elimination, reacting groups are available for more condensations. Other instances of condensation polymers are polyurethane foam, epoxy glue and resins, peptides and proteins. The configuration of nylon (polyamide) and dacron (polyester) are following.

1671_Condensation polymers.jpg

Fig: Condensation polymers

1198_dacron.jpg

Fig: Dacron

Condensation polymers include polycarbonate, polyethylene terephthalate and polyurethane. Polycarbonates are significant as shatterproof eyeglass lenses, CDs, DVDs, bulletproof windows and green houses. Polyethylene terephthalates are polyesters, significant as apparels, soft-drink bottles, tire cords and magnetic tapes. Polyurethanes are an additional condensation polymer used for foam furniture stuffing, spray-on insulation, automobile parts, footwear and water-protective coatings. They enclose urethane groups {RNHCOOR'}, which are formed from the reaction of isocyanate {RN=C=O} and an alcohol {R'OH}. For example reaction of toluene-2, 6-diisocyanate and ethylene glycol in the presence of a blowing agent {N2 or CO2} provides polyurethane foam.

This condensation reaction is unique because there is no liberation of small molecule here, unlike others. They are as well instances of block copolymers. Polyurethanes have the following general structure: 

465_polyurethane.jpg

Polymers are created as long chains that sometimes have connections between them due to involvements of wider diversity of functional groups on the monomer. If the chains aren't attached to one another, the polymer typically can be melted and reshaped constantly. Such polymers are termed thermoplastics. Instances are polyethylene, nylon, and polyester, polyvinyl chloride, and polystyrene. But if the long chains are linked mutually by cross-links, the material becomes extremely rigid; once formed, these polymers don't soften or melt when heated. The polymers are thermosets. Bakelite and related polymers of the condensation polymerization of phenol and methanal [by loss of water molecules] are thermo sets of quite old synthetic polymers, but still have extremely significant utilizes. More instances are the plastics employed to manufacture bowling balls, handles on kitchen utensils, Formica table tops, and several modern auto bodies to lessen weight, compared to if it had been completely made of metals. 

In the formation of thermoplastics, they have individual chain arrangements that find out their physical properties. It could be in both ordered crystallizable [arrangements are more orderly] and amorphous noncrystallizable forms. Regularity through close fittings of the molecules of the polymer allows the crystallizables to have strong forces as hydrogen bondings, dipole-dipole attractions and van der Waals forces by less heat content (low enthalpy). But lack of randomness constitutes unfavorable entropy for the polymer. A balance is strike between this 2, therefore polymers form solids made of regions of crystallinity termed crystallites that are embedded in amorphous materials. They are liable to cracking. At room temperature the plastics are hard, but soften on heating so that it can be shaped. The chains can simply slip over each other at fairly high temperature to provide dissimilar shapes of most of the materials we constantly utilize for example toys, plates of switches, combs, casings and so on.     

Thermo set polymers are extremely rigid through much cross-links. The cross-bonds are dependable for its rigidity, the more the harder. 

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