Copolymerization of tetraethoxysilane and dimethyl(diethoxy)silane studiedby Si-29 NMR and ab initio calculations of Si-29 NMR chemical shifts

Copolymerization of tetraethoxysilane (TEOS) and dimethyl(diethoxy)silane ( DMDEOS) was studied by means of the Si-29 NMR spectroscopy and ab initio qu antum chemical calculations. Si-29 NMR spectra of reaction mixtures of TEOS and DMDEOS were effectively measured by a slightly modified DEFT experimen t, in which the pulse sequence is divided into two parts and all pulses on protons are replaced by selective pulses. Quantum chemical ab initio calcul ation of structures and Si-29 NMR chemical shifts of some reaction products were used in signal assignment. The assignment of the signals to correspon ding structure units is not yet unambiguous; however, a semiquantitative an alysis of relations in the reaction mixture of TEOS and DMDEOS could be don e. Although the reactivity of the DMDEOS monomer is much higher in comparis on with TEOS, the arising gel is not strictly phase separated and copolymer ization of both monomers occurs. In the first stages of the process, result ing oligomers are composed of 70% of DMDEOS structure units. In the final s tages of polycondensation, domains of the TEOS structure units are formed, which can act as linking units between cyclic oligomers. (C) 1999 Elsevier Science Ltd. All rights reserved.