Linear and hyperbranched poly(silyl ester)s: Synthesis via cross-dehydrocoupling-based polymerization, hydrolytic degradation properties, and morphological analysis by atomic force microscopy
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Poly(silyl ester)s with differing compositions and architectures were synthesized via cross-dehydrocoupling polymerizations of AB and AB2 monomers. Three poly(silyl ester)s-linear poly(dimethylsilyl benzoate), linear poly(diisopropylsilyl benzoate), and hyperbranched poly(dimethylsilyl benzoate)-were synthesized through the dehydrocoupling polymerization of 4-(dimethylsilyl)benzoic acid, 4-(diisopropylsilyl)benzoic acid, and 3,5-bis(dimethylsilyl)benzoic acid, respectively, as 10 M solutions in ethylene glycol diethyl ether in the presence of 0.25 mol % of 10 wt % palladium on activated carbon at 100 C under argon for 1-3 days. The characterization of each polymer included infrared (IR), 1H NMR, 13C NMR, and 29Si NMR INEPT spectroscopies, size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The degree of branching of hyperbranched poly(dimethylsilyl benzoate) was 51%, as determined by 1H NMR spectroscopy, with spectral analysis confirmed through comparison with model compounds. The hydrolytic degradation properties of the polymers in solution and in the solid state were studied quantitatively by monitoring the molecular weight' reduction over time using SEC. In addition, the erosion processes of solvent-cast polymer films upon hydrolytic degradation were investigated by monitoring the changes in the surface topographies and morphologies using atomic force microscopy (AFM). The rate of polymer degradation was reduced for the isopropyl-substituted silyl ester linkages in comparison to those bearing methyl silicon side chain substituents. Hydrolysis of each of the linear polymers gave smooth molecular weight loss over time, whereas the hyperbranched material exhibited a two-stage degradation profile.
