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Chapter 52 Suggested solutions for Chapter 52 509 sometimes called a polymer' since it is the next step that links these units together in the final polymer. 0 H OCN N 0 0 0 0 N NCO po A H 0 polymer B Now the diamine reacts with the free isocyanates by forming a urca at each end of the chain to link them together. It is possible to show only a small fragment of this massive polymer but that is enough to see the block of polyethylene glycol, the block of (one) diarylmethane, and the double urea linkage beyond. There is more detail of this sort of reaction in the chapter (p. 1458). RNH 0 0 H HN N 0 0 0 0 N NH H 0 0 NH polymer C NH 0 0 H 0 0 N NH HN N 0 H n 0 RNH 0 Problem 11 Why does polymerization occur only at relatively low temperatures, often below What occurs at higher temperatures? Formaldehyde polymerizes only below about but ethylene still polymerizes up to about 500 C. Why the difference? Purpose of the problem Going back to the fundamentals of polymerization, particularly the thermodynamics. Suggested solution Polymerization is favourable if the conversion of many bonds into bonds is sufficiently favourable to make up for the inevitable negative entropy of the reaction. Polymerization always involves many molecules giving one so the entropy is always large and negative. The vital equation is (p. 313) RT In K = TAS Thus there must be a compensating gain in All that is, in the stability of the bonds in the Depolymerization a ways polymer compared with those in the monomer, to offset the loss of entropy. The relative occur as the polymer may contribution made by entropy increases as the temperature goes up polymerization tends to be decompose in some other way. favoured at lower temperatures. At higher temperatures depolymerization tends to occur. Formaldehyde contains a very strong double bond while polyformaldehyde has C single bonds instead. The single bond is only slightly stronger than two double bonds (in other carbonyl compounds such as ketones, C-O is more than twice as strong as C-O). Low temperatures are needed for entropy not to play too big a part in the thermodynamics. Ethylene (ethene), on the other hand, has a weak C=C double bond while the polymer has C-C single bonds. These are much