SYNTHESIS, CHARACTERIZATION AND DEGRADATION OF POLY(SILYL ESTER)S

To investigate the effects of sterics and polymer backbone composition upon the rates of hydrolytic degradation of polymers containing trial kyl-substituted silyl ester Linkages, six poly(silyl ester)s with mole cular weights typically ranging from 5000 to 15000 Da were synthesized through transsilylation ester exchange reactions of alkyl or aryl bis (trimethylsilyl)esters with methyl- and isopropyl-substituted bis(chlo rosilyl)hexanes to give trimethylsilyl chloride as the condensate. Cha racterization of the poly(silyl ester)s included infrared spectroscopy (IR), H-1 NMR, C-13 NMR, and Si-29 NMR INEPT spectroscopies, size-exc lusion chromatography (SEC), differential scanning calorimetry, and th ermogravimetric analysis. Both 29Si NMR and SEC were employed to monit or the polymerization and the degradation experiments. As expected, th e relative stability of the silyl ester bonds toward hydrolysis increa sed with increased steric hindrance of the substituents attached to th e silicon atoms. The incorporation of p-phenylene groups into the poly mer backbone allowed for the formation of crystalline domains, which i n turn greatly increased the stability toward hydrolytic degradation o f the polymers in the solid state. Hydrolysis of the polymers containi ng diisopropyl-substituted silyl esters gave small molecules; however, degradation of the dimethyl-substituted poly(silyl ester)s as solids in air resulted in the formation of poly( dimethylsilylhexylene siloxa ne) from the spontaneous condensation of 1,6-bis(dimethylhydroxysilyl) hexane.