Polymer Chemistry by Alka L. Gupta

By Alka L. Gupta

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The process involves in follOWing way : It is interesting to note that intermediate products also contains a double bond between CH, ~+CH' ~ ~. CR, ~-CH ~ + CH, ~ ! 2Molecules of Styrene CH, ~_CH' ~-CH ~ + nCR, ~ 36 • POLYMER CHEMISTRY carbon atoms and, hence, have same reactivity and structural similarity as the starting monomer. Unlike chain polymerisation, in polyaddition reaction, the intermediate product is quite stable and can be isolated from the reaction mixture. In the polyaddition reaction a pair of monomers containing reactive functional groups forms a polymer, without liberating any simple molecule in the following manner: o O=C N-R-N C 0 + OH-R'-OH Di isocyanate I H o +O=C=N-R-N=C=O -------~) + HO-R':""OH H I " O=C=N-R-N-C-O-R'-O-C-N-R-N=C=O o ----~) " --+ O=C=N-R-N-C-O-R'-OH diol HI 0" H 0 I" " O=C=N-R-N-C-O-R'-O-C-N-R-N-C-O-R'-OH I H "I 0 H Polyurethane In above case, the hydrogen atom from the hydroxyl group of the diol migrates and adds on to the nitrogen of the isocyanate group.

In the reaction (2), one H-atom from one chain is abstracted by the other chain, and utilised unpaired electron, whereas the chain which has donated the H-atom, gets stabilised and form a double bond. In this reaction, two polymer molecules are formed. Both the molecules consist of shorter chain length as against a single molecule of a longer chain length. ' ---+ 28 • POLYMER CHEMISTRY Thus, in above cases, the product molecule is formed known as polymer. This product does not consist of any free-radical site and, hence, cannot grow any further.

This type of copolymerisation is called an 'ideal copolymerisation' or 'azeotropic copolymerisation' because the copolymer thus formed will have the same ratio of the monomeric units as that of the monomer reaction mixture. The two monomer units arranged in the chain remain in a purely random sequence. (3) When rl > 1, and r2 < 1 In this case, the propagation reaction types 11 and 21 will be preferred to types 12 and 22, and consequently the probability of MI entering, into the copolymer chain will be higher as compared to M2 monomer.

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