Addition polymerisation is the assembly of simple monomer units to form a larger molecule called a polymer.

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Addition polymerisation is the assembly of simple monomer units to form a larger molecule called a polymer. These monomers join together in a long chain under the correct conditions of temperature and pressure to become a single polymer chain. The monomers in the chain may also contain cross-links, which alter the properties of the polymer molecules. Different temperature and pressure conditions can cause monomers to join together differently, resulting in different polymers even when the same monomers are used. [1]

Low density poly(ethene) (ldpe) is an extensively branched polymer, made of ethene monomers, its low density a result of the branching. The formative polymerisation reaction is carried out under high pressure and temperature with radicals, these conditions being responsible for the branching of the chains. The growth of the ldpe polymer chains is due to a radical mechanism and consists of 3 stages. At the initiation stage, the radical reduces the C=C double bond in an ethene molecule to a single C–C bond and becomes itself part of the polymer chain. Now one of the CH2 molecules in the chain is the radical. During propagation the radical causes other ethene molecules to add to the chain by splitting the C=C bonds within the molecule to form a C–C in its place and another C–C bond which attaches to the chain. Termination comes last, removing the radical by adding another radical and ceasing the growth of the polymer chain. [2]

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Different structures of poly(ethene) and poly(propene) give the respective polymers different physical properties. The physical characteristics of ldpe are a result of the extensively branched structure of the polymer chains. Ldpe has a low density, low melting point and low tensile strength because the chains on the polymer mean that the polymer chains cannot lie close together. As a result, the intermolecular forces between the chains are weak giving properties as those described above. [2]

In hdpe (high density poly(ethene)) the chains lie much closer together as the chain is not branched but is said to be crystalline. ...

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