Synthesis, kinetics and supercritical fluid fractionation studies of functional organosiloxanes and their incorporation into segmented copolymers

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Virginia Polytechnic Institute and State University

The synthesis of organosiloxanes via equilibration processes is fairly well known, but relatively little is known about the detailed kinetics and mechanisms involved, particularly in the presence of functional endblockers. A major focus of the present study was to investigate aspects of the kinetics of siloxane equilibration polymerizations in the presence of 1,3-bis(3-aminopropyl) tetramethyldisiloxane. Catalysts studied included the commonly used potassium siloxanolate catalyst, as well as the analogous tetramethylammonium and tetrabutylphosphonium siloxanolate catalysts. The reactions of the quaternary ammonium and phosphonium systems were limited to about 80°C due to the known transient nature of those species at elevated temperatures. The rates of disappearance of the cyclic tetramer D₄ and aminopropyl disiloxane were monitored through the use of HPLC and GC techniques. The results indicated that the ammonium and phosphonium catalysts were much more efficient at early incorporation of the aminopropyl disiloxane into the oligomer. By contrast, studies using the potassium siloxanolate catalyst were less effective, even at much higher temperatures such as 160°C. The explanations for this behavior may be related to the higher degree of dissociation and possibly enhanced solubility of the ammonium and phosphonium systems relative to the more studied potassium system. The significance of these results with respect to functional siloxane systems has not been previously appreciated.

Kinetic results indicated that the apparent rates are proportional to the square root of the catalyst concentration, in agreement with earlier studies on the potassium catalyst in nonfunctional siloxane systems. The reaction order with the bulky tetrabutylphosphonium catalyst was slightly higher, again indicating a more dissociated active siloxanolate species. Despite the rate differences, the same equilibrium number average molecular weight was obtained, regardless of catalyst concentration. The effect of catalyst is manifested mainly in the rate at which equilibrium is reached.

Supercritical fluid fractionation studies of the functional siloxanes were demonstrated to be quite feasible. Relatively narrow molecular weight distribution fractions were obtained when compared with polysiloxane standards synthesized via the organolithium initiated polymerization of the cyclic trimer. Excellent agreement was realized between the molecular weight values determined by titration of the amine endgroups and those calculated by GPC.

Additional aspects of this study included the preparation of alkyl-substituted polyarylesters and polyformals, and modification of these materials with polydimethylsiloxanes. Tough transparent films were readily solution cast from these materials, which may have potential use as permselective gas separation membranes. Due to the intentionally short segment lengths, evidence of significant amounts of phase mixing was observed by DSC and TEM.