An Introduction to Karachi: Its Environs and Hinterland by Manekji Bejanji Pithawalla

By Manekji Bejanji Pithawalla

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For an ideal polymer, Mw /Mn would be equal to one. However, in practice, this ratio is always greater than one, due to the distribution of molar masses of real polymers. Because the number average molar mass depends on the number of molecules with a given molar mass it can be measured on the basis of colligative properties of the polymer in solution. Colligative properties include lowering of vapour pressure, elevation of boiling point and depression of freezing point, all of which will occur upon addition of polymer solute into a solvent.

1 Isomerism At a local scale, the conformation of a polymer molecule depends on rotations about the bonds that make up the polymer backbone. Many synthetic polymers have backbones made from C C bonds and the conformations that can be adopted by rotation are similar to those of alkanes. The carbon atoms are sp3 hybridized and so there is a tetrahedral arrangement of substituents around each one. Conformations can then conveniently be represented by Newman projections, such as the staggered arrangements for butane shown in Fig.

1. Step-growth polymerizations, as their name suggests, occur by stepwise reaction between any two molecular species, which must contain at least two functional groups. e. they are dimers). The product has two different functional groups, but as it is still difunctional it can be involved in further steps of the polymerization. Step polymerizations involving species with functionality greater than two often lead to the formation of networks of highly branched chains. Chain polymerizations can broadly be divided into free radical addition polymerizations and ionic polymerizations.

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