Monomer, Initiator and Catalyst

MONOMER

Low molecular weight compounds having a functionality of two or more from which polymers are formed are called monomers. To polymerise them, it only renders a suitable reaction conditions. Then, these monomer molecules add each other to form fewer but higher molecular weight compound.

The functionality of a monomer depends on the number of reactive sites it has. A compound assumes functionality because of the presence of reactive functional groups like -OH, -COOH, - NH2 etc. The number of functional groups per molecule of the compound defines its functionality.

Some compounds which do not contain any reactive functional group but the presence of double or triple bonds in molecule bestows poly-functionality on them are called vinyl monomers. For example, ethylene which can take on two atoms of hydrogen, because of the presence of a double bond and hence, functionality two.
CH2=CH2 + H2 = CH3-CH3
Similar is the ease with other homologous of ethylene and vinyl compounds such as methacrylic acid [H2C=C(CH3)COOH], acrylamide [H2C=CH-CONll2], methyl methacrylate [H2C=C(CH3)COOCH3], methyl acrylate [H2C=CHCOOCH3], acrylonitrile [CH2=CH-CN], etc. Acetylene has a functionality of four i.e. [HC  CH].
There are some other compounds in which the presence of easily replaceable hydrogen atoms contributes functionality. Phenol is an example of such type.

INITIATIOR

The initiation reaction of polymerisation is carried out by initiators. Initiators are thermally unstable compounds and decompose into products called free radicals. These free radicals can attack monomers and initiate the polymerisation reaction.
If R- R is an initiator and the pair of electrons forming the bond between two ‘R’ can be represented by dots, the initiator can be written as R:R. Initiators are decomposed by homolytic dissociation and are splited into two symmetrical components. Each component carries with it one of the electrons from the electron pair, is called free-radieal, i.e. R – R  2R•. The decomposition of the initiator to form free radicals can be introduced by heat energy, light, catalyst etc. Potassium persulphate (K2S2O8), azo compounds, peroxides, hydroxides, peracids and peresters are useful initiators.

CATALYST

The catalyst may be responsible for peculiar chain initiation and possess a metal atom with some + ionic character. The reason for their presence varies from reaction to reaction. For instance, in the vinyl type reactions, the catalysis are required to produce initiating centres. Many metallic salts such as FeSO4, FeCl3, Fe2(SO4)3, CuSO4, ZnSO4, K2SO4, CoSO4 , H2SO4 etc. are used as catalyst for graft copolymerisation of cellulosic materials.