Neutralization of residual catalyst in polydimethylsiloxane. effect of neutralization on the thermal stability of the polymer

It is known that the thermal stability of polydimethylsiloxane is adversely affected by the presence in the siloxane polymer of active chain ends (i.e., by residual catalyst). The presence of catalyst causes the degradation reactions to proceed through an ionic mechanism (as opposed to the free‐radical mechanism typical of the “pure” polymer). Since ionic reactions take place at temperatures at least 100°C. below those at which free‐radical reactions occur, the thermal stability is thus strongly impaired. Degradation reactions occurring in polydimethylsiloxanes exposed to elevated temperatures fall roughly into two groups: ( 1 ) depolymerization (i.e., reactions involving the cleavage of the siloxane bonds) and ( 2 ) degradation proper, which results in profound changes in the chemical structure of the polymer (i.e., reactions involving carbon substituents on the Si atom, their splitting off, oxidation, etc.). Depolymerization of polydimethylsiloxane at temperatures of 150°C. and higher has been studied in vacuo and in air. The rate of depolymerization was followed quantitatively by means of weight losses; changes in the molecular weight of the polymer were taken as indicative of changes in polymer structure. The dependence of the rate of depolymerization of polydimethylsiloxane on the catalyst (KOH) concentration in the polymer was measured. It was found that the residual basic catalyst can be neutralized by acids. The neutralized polymer is characterized by a high resistance to depolymerization. The kind of acid used for the neutralization has a strong influence on the resistance of substituents toward high temperature and oxidative atmosphere. Benzoic acid and H 2 SO 4 , suppress the depolymerization almost completely, but strong mineral acids considerably decrease the resistance of substituents toward oxidation. It was found that very advantageous stabilizing properties are displayed by hydroxides of the Al(OH) 3 type. The amphoteric nature of these hydroxides makes them suitable for stabilization of polydimethylsiloxane produced by anionic as well as cationic polymerization.