Types of polymerization
Sol. Types of polymerization: polymerization is a process in which the joining together of many small molecules to form very large molecule called polymer. Polymers are formed in different ways:
1 Addition or chain reaction polymerization: monomaniac units contain one or more double bonds which by intermolecular rearrangement may provide the molecule bifunctionality.
2 condensation or step reaction polymerization: in condensation polymerization reaction take place in step wise manner and the polymer build up is slow. Step reaction polymerization reactions are mostly accompanied by the elimination of small molecules.
3 Ionic Polymerization mechanisms: it involves an attack on the electron pair of the monomer either by a positive or negative ion. Ionic polymerization mechanism is of two types.
(1) Cationic mechanism: in this initiation is done by a proton and the propagation carried out by a harmonium ion. Examples of monomers that can undergo cationic polymerization are methyl viny1 etc.
(2) Anionic mechanism: in this the attack on the electron pair of the monomer is done by an anion. The harmonium thus formed now propagates the chain growth by attack in the second monomer unit. Alkali metal amides alkaloids and hydroxides are used as initiators monomers capable of undergoing anionic polymerization include butadiene isoprene styrene acrylonitrile etc.
4 Coordination polymerization or Ziegler Natta polymerizations: in such polymerization reactions especially of olefins and dines are catalyzed by organ metallic compounds. The first step in this process in the formation of a monomer catalyst complex between organ metallic compound and the monomer.