Silyl ether-coupled poly(epsilon-caprolactone)s with stepwise hydrolytic degradation profiles

Silyl ether-coupled poly(epsilon-caprolactone)s (PCLs) with stepwise degradation profiles were synthesized via the cross-dehydrocoupling polymerizations between 1,4-bis(dimethylsilyl)benzene (BDSB) and telechelic, diol-terminated PCL macromonomers. With the presence of 10 wt % palladium on activated carbon as the catalyst, the condensations between BDSB and diol-terminated PCL macromonomers having molecular weights of 1200, 2010, and 5500 g/mol were performed in toluene at 100 degrees C under argon. Hydrogen was eliminated as the condensate upon the formation of silyl ether bonds linking the PCL blocks, yielding within 24 h, silyl ether-coupled PCLs of molecular mass 7590, 29,900, and 29,500 g/mol, respectively. The characterization of each polymer included (1)H NMR, (13)C NMR, and (29)Si NMR spectroscopies, size exclusion chromatography (SEC), and differential scanning calorimetry. The hydrolytic degradation properties of the polymers in solution were studied, and the molecular weight reductions over time were monitored by SEC. The silyl ether linkages of the polymers underwent hydrolysis in the presence of mineral acids, whereas the PCL segments released from the cleavage of the labile silyl ether coupling unit did not undergo detectable molecular weight reduction over 15 days. In the presence of acetic acid, the silyl ether functionalities were cleaved with a half-life of 3 days; however, the PCL chain required reaction with trifluoroacetic acid to give a number-average molecular weight loss half-life of 4 days. The silyl ether-coupled PCLs underwent degradation in a gradient fashion, therefore, by a protocol that involved the addition of acetic acid for cleavage of the silyl ether functionalities, followed by further addition of trifluoroacetic acid to bring the hydrolysis of the silyl ether functionalities to completion and to trigger the degradation of PCL segments.