Polycondensation Kinetics: 3. Time-Dependent 29Si NMR Spectra of Poly(methyltrimethoxysilane)

As an example of polycondensation of trifunctional monomers (PC-3), PC-3 of methyltrimethoxysilane to high conversions at pH 2.9 and 22 degrees C has been studied by Si-29 NMR. The relative concentrations of nonbonded Si atoms (T-0) and those bonded by one (T-1), two (T-2), and three (T-3) Si-O-Si bonds have been determined from the intensities of multiplets assigned to linear and branched oligomers and tri- and tetraatomic rings that are similar to the crystaline lattice facets. A chain growth model is proposed that takes into account the interconversions of these elements, in which cyclization increases the critical conversion at the gel point from the value of 0.5 expected for acyclic chains to similar to 0.80-0.85.