The ring-opening polymerization of octamethylcyclotetrasiloxane, D₄ , in the presence of bis(α,ω-aminopropyl)-1,3-tetramethyldisiloxane using potassium siloxanolate and tetramethylammonium siloxanolate catalysts has been investigated. The use of reversed-phase high performance liquid chromatography (HPLC) and capillary gas chromatography (GC) allowed the disappearance of the starting materials to be monitored as a function of reaction temperature, time, targeted molecular weight, catalyst type and concentration. Due to electronegativity differences, the cyclic tetramer was found to react more quickly than the disiloxane under all conditions studied. This work was extended to the study of polydimethyl-co-diphenylsiloxane oligomers, prepared by the ring-opening copolymerization of D₄ with octaphenylcyclotetrasiloxane, D₄". Reversed-phase HPLC was used to study the disappearance of the cyclic starting materials. Due to volatility considerations these oligomers were not analyzed by capillary GC. ²⁹Si NMR was used to determine the number-average sequence length of each type of siloxane unit as a function of reaction conditions. The co-oligomer composition played the greatest role in determining the average sequence lengths. Oligomers with close to a 50/50 molar composition of dimethyl and diphenyl units showed a tendency towards an alternating distribution, while oligomers with an ≃27/83 molar composition displayed a tendency towards blockiness. A series of polyester-siloxanes was prepared using both bulk and solution polycondensation techniques. Copolymers based on polybutylene terephthalate were highly insoluble, due to the presence of crystallinity in the systems. The incorporation of some isophthalate linkages increased the solubility of the polymer products, making the solution polymerization technique possible. / Ph. D.