Polymerisation

A chemical reaction in which similar molecules join in long chain is called polymerization and resulting product is a polymer. All of plastics are polymers. It was stated that a molecule of ethylene can polymerise when many of these molecules join in a chain structure as illustrated in Figure. Ethylene (C₂H₄) is an unsaturated molecule since it has a double bond among two atoms of carbon. This double bond is again illustrated in Figure (a). In the presence of heat, pressure or a catalyst the carbon-carbon double bond is obtained and opens up for bonding with another activated molecule of ethylene .This process of joining might continue until a few hundred to a few thousand molecules have joined in chain. The number of units (mers) that join throughout polymerisation is known as degree of polymerisation (DP). The polymerisation chain of ethylene is shown in Figure (c) which is same as illustrated in previous Figure. This polymer is named as polyethylene and has average DP of 3500 to 25000. The molecular mass of any polymer is the product of DP and molecular mass of one unit of the polymer. For instance, the molecular mass of C2H4 is 2 × 12 + 4 × 1= 28 units. Therefore the molecular mass of polyethylene of DP of 5000 is 28 × 5000 = 140000 units.

                                 (a)                                    (b)                                                  (c)