16.4 Synthetic Polymers
The polymers which are synthesized in the laboratory are called as synthetic polymers, based on the method of its preparation they can be further classified into addition polymers and condensation polymers.
Addition polymers: Addition polymers are formed by the sequential addition of the monomer units with the help of a reactive in termediate such as free radicals, cation or anions without loss of small molecules. The addition polymerization generally involves three steps called initiation, propagation and termination. These steps apply to all types of addition polymerization such as free-radical, cation and anion. In this process alkenes are typically used as monomers and polymerization results by successive additions across the double bonds.
- Radical polymerization results when a suitable alkene is heated with a radical initiator.
- Initiation step: On heating the initiator molecule decomposes into a radical which combines with the monomer and forms a new radical for the chain propagation.
- Propagation step: The formed radical reacts with another monomer and generate a new radical which propagates the chain. The chain propagation process determines the length of a polymer chain.
- Termination step: There are three ways to terminate the propagating chain either by coupling with another monomer radical or by disproportionation reaction or by reaction with an impurity (such as oxygen).
- In cationic polymerization, a cation intermediate is generated by the addition of a Lewis acid such as BF3, AlCl3, etc. with the monomer typically alkene. The cation formed in the initiation step reacts with another monomer and generate a new cation, this process is repeated until chain termination occurs.
- Chain propagation can be affected by three ways either by loss of a proton or by addition of a nucleophile or by reacting with the solvent molecule.
- Anionic polymerization occurs by the addition of a nucleophile initiator with the monomer alkene and generates a new anions, this process occurs only when the nucleophilicity of the initiator nucleophiles are strong enough to attack the electron rich olefins. Similarly, if electron withdrawing substituents are attached to the olefin bond increases the rate of addition. Here the chain propagation step was terminated by reaction of generated nucleophiles with impurity or solvent molecules.
