Constitution moléculaire d'α,ω-difluoropolydiméthyl-N-méthylsilazanes. Equilibres obtenus par une réaction de redistribution

Unlike the α,ω-dihalogenopolydimethylsiloxanes, the α,ω-dichloropolydimethyl-N-methylsilazanes show a net preference for cyclic species with respect to linear structures at equilibrium. The aim of this study is to evaluate the perturbations in the molecular constitution of these α,ω-dihalogenopolydimethyl-N-methylsilazanes resulting from the substitution of the terminal chlorine atoms by fluorine atoms. This polymeric family was prepared by reacting (CH₃)₂SiF₂ with nonamethylsilazane [(CH₃)₂SiNCH₃]₃. The redistribution of the fluorine atoms with the bridging methylimino groups reached an equilibrium after about 5 months' heating at 150°C for all the samples prepared. The relative abundance of the various molecular species and fragments at equilibrium was deduced from the quantitative analysis of the proton nuclear magnetic resonance (NMR) spectra. The molecular constitution at equilibrium is described by two constants. The first, K = [neso] [middles in chains]/[terminal moieties]² = (2.8 ± 0.8) 10^?2, shows that the presence of terminal fluorine atoms is unfavorable to the formation of short chains. On the other hand, the trimeric cyclic species [(CH₃)₂SiNCH₃]₃ are found to be highly favored (K°₃ = 550 ± 100 mole/liter). These observations further confirm that the equilibrium constants which control the noncyclic part of polymeric families depend little on the functionality of the substituents exchanged [for example, on changing from ? N(CH₃)₂ to ? NCH₃? ] when the reorganization heat order is one.