Polymerization that makes addition polymers general joins the monomer molecules end to end. For example, in the addition polymerization of ethylene:
etc. + CH2=CH2 + CH2=CH2 + CH2=CH2 + CH2=CH2 + CH2=CH2 + etc.
through addition polymerization gives:
Chemicals called promoters initiate polymerizations, and pieces of their molecules become joined to the ends of the polymer systems (where "etc." has been written).
Polyesters and polyamides are condensation polymers. Condensation polymers are copolymers, which have monomers having functional groups that can react with each other, usually spitting out water. The reaction between a carboxylic acid and an alcohol creates an ester. If the carboxylic acid and the alcohol were the monomers of the polymer, during polymerization, they would create polyester, and spitting out water in the process. The polymerization of a carboxylic acid and an amine similarly creates polyamides.
Effect of Size and Geometry on Properties
Polymers are very useful because they are very chemically inert. They are not attached by air or its pollutants. They are stable toward water. They must keep these properties under high temperatures, as Teflon does on a non-stick frying pan. Polyesters and polyamides have functional groups that water can attack, but this does not happen due to the enormous size of the molecule. They are completely insoluble in water and therefore they make a good storing container for food but bad for waste disposal since it does not decay.
Polymers and Physical Properties
Physical properties of polymers are the ones that are most required for pratical use. For example, teflon (a polyolefin) has a slipperiness toward almost anything. Nylon (a polyamide) isn't eaten by moths and has superior tensile strength and the ability to be made into fibers and fabrics. Dacron (a polyester) does not mildew, and when made into fibers, it is superior to cotton, with greater strength and lower mass, and it doesn't stretch as much.
Polyolefins are polymers of alkenes and their halogen derivatives. They include saran, teflon, polystyrine, and many others.
Polyacrylates are like polyolefins, but made from derivatives of acrylic acid.
Methyl -cyanoacrylate is an unusual monomer in that its polymerization is promoted by water. "Superglue" contains this monomer, and since there is a film of moisture on almost any surface wherever superglue would be used, the water promotes polymerization, making the substances it is bonding together stick.
Ďaľšie referáty z kategórie
|Referát vhodný pre:||Gymnázium||Počet A4:||2.5|
|Priemerná známka:||2.99||Rýchle čítanie:||4m 10s|
|Pomalé čítanie:||6m 15s|