Depolymerization in Acidic Solution

A study of depolymerization of low polymers suggests that the degree of internal condensation within three-dimensional polymers depends on the temperature at which they were made or to which they have been heated. Even more importantly it 

Depolymerization of polysilicic acid was studied by Stade and Wieker (109), who prepared a solution of polysilicic acid containing a minimum of monomer by dissolving (LijSiOj) powder in dilute acid at 0°C. Samples were then diluted to 12 ppm SiOj in water at pH 1.3-5.5 and the depolymerization to monomer followed at different pH values and temperatures. The rate of depolymerization was followed by removing samples periodically and analyzing for monomer in solution by the molybdate method of Alexander (24a). At the same time the rate at which the residual polymer reacted with molybdate was also measured. 

When polysilicic acid is formed at low pH at 0 C, polymer particles are formed, but the inner SiOH groups arc not all fully condensed. However, when such a sol is warmed and/or the pH is raised above 2, further internal condensation occurs. It is therefore logical to expect that if the sol is diluted at 0 C, only depolymerization will occur. However, if the temperature is raised, or if the pH is also raised, two processes occur. The particle starts to depolymerize or dissolve, but also it condenses internally with formation or more siloxane bonds that must later be broken to form monomer for reaction with molybdic acid. Thus the reaction rate with molybdic acid is greatly reduced. 

Evidence for this is that the reaction velocity constant for the polymer made and diluted at 0°C was 0.046 min , but w hen it was diluted at 25 C the constant dropped to 0.03 min after 5 min. Additional evidence is that when depolymerization was followed at 0 C where no internal changes occur in the polymer and only depolymerization occurs, the rate of depolymerization in the range from 3 to 5 was exactly proportional to the OH ion concentration. 

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