Synthesis of poly(silyl ester)s via AB monomer systems
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The polymerization of AB-type monomers was investigated for the preparation of hydrolytically labile poly(silyl ester)s. AB-type polymerizations were based upon either transsilylation or hydrosilylation reactions, and each case gave polymers of higher molecular weight in shorter polymerization times than the previously studied transsilylation polymerizations of AA/BB monomer systems. The transsilylation reaction between the chlorosilane and trimethylsilyl ester functional groups of trimethylsilyl 5-(chlorodimethylsilyl)pentanoate as a neat sample in the presence of DMF catalyst and heated to 100C under argon for 8 days gave poly(5-dimethylsilyl pentanoate) (Mw = 12400, Mw/Mn = 3.8). The strategy using hydrosilylation chemistry has the additional advantage of elimination of monomer preparation difficulties and polymerization at lower reaction temperatures. Polymerization of (dimethylsilyl) 4-pentenoate, (dimethylsilyl) 4-vinyl benzoate, and (dimethylsilyl) 4-pentynoate as 10 M solutions in dry tetrahydrofuran afforded poly(5-dimethylsilyl pentanoate), poly(p-dimethylsilylethyl benzoate), and poly(dimethylsilyl 4-pentenoate) respectively, by hydrosilylation in the presence of 0.5 mol% dichloro-(1,5-cyclooctadiene)platinum(II) catalyst. The temperature was optimized for each polymerization, to yield the poly(silyl ester)s having degrees of polymerization of approximately 100 in 120 min. 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). Comparisons were made between the degradative properties of polymer materials based upon different backbone compositions. It was found that the incorporation of a phenyl moiety to the carbonyl and the attachment of a vinyl group to the silicon atoms increased the susceptibility of the silyl ester linkage toward nucleophilic attack and thereby decreased the hydrolytic stabilities of the poly(silyl ester)s. In addition, several small molecule model studies were performed to help understand the degradation behavior of the polymer materials.