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Again, the use of a second medium (usually water) facilitates rapid heat transfer between the ingredients of the reaction vessel and a circulating coolant. Reactions can be carried out rapidly (as in the case of suspension polymerization). However, a subtle difference exists between these two types of processes. The reaction occurs within submicron micelles or clusters whose size and number are controlled by the concentration of the emulsifier. These micelles provide a much larger surface area than is offered by the monomer droplets in suspension polymerizations; hence, emulsion polymerization also exhibits a high reaction rate at temperatures comparable to those for sus­ pension polymerization.

The carbanion on the end of the chain is still reactive because in an inert medium there is no termination step. For this reason, these materials have been aptly termed "living polymers" by Szwarc (1956, 1968). However, impurities will "kill" or terminate the "living" end, or the end may be connected to other reactive chains or groups, thus providing tremendous versatility in polymer synthesis. In practice, because of the MORPHOGENESIS OF SOLID POLYMER MICROSTRUCTURES 39 chain-terminating effect of impurities and other experimental difficulties, these extreme or ideal conditions are not met; still it has been practical to produce polymers with relatively narrow molecular weight distributions and molecular weight as high as 107.

The essence in complexity in this regard is to be found in the natural proteins (Fig. 4), where as many as 19 different types of units are involved within a given polymer. Arg -Leu"·— Asp-*— 35 23 pNH2 Glu 122 94 Hi s |-NH2 |-NH2 Γ ΝΗ 2 Lys-*-Asp—-Ala-*-Glu-·— Thr ♦-Thr-·— Lys-·—Tyr-*— Ala-— Cys-·—Asp -*- - P r o - — T y r - 50 Cys * 123 124 HOOC-j |-NH2 Ile --^Val —Ala —Cys —Glu —Gly —Asp —Pro —Tyr —Val —Pro —Val —His —Phe —Asp —Ala—Ser —Val 54 Tyr Cys Arg 115 22 \ \ I■NH 21 33 Fig. 4. Chemical structure of ribonuclease (from Brown, 1791).

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