Constitution moleculaire d'α,ω-difluoropolydimethylsiloxanes equilibres obtenus par une reaction de redistribution

Abstract

α,ω-Difluoropolydimethylsiloxanes F[(CH 3) 2SiO] n (CH 3) 2SiF have been prepared by reaction of (CH 3) 2SiF 2 with cyclic dimethylsiloxanes [(CH 3) 2SiO] n ( n = 3 or 4). An equilibrium is reached in the redistribution of fluorine vs. oxygen atoms after ca. 24 h at 200°C or 15 days at 150°C when catalysed by AlCl 3. 16 equilibrated samples of varied ( R = F/Si) overall compositions have been analysed by 1H and 19F NMR and by VPC. The presence of rings has been established for the lower values of R. The molecular constitution of the equilibrated samples is described by a set of constants. The abundance of the end units ▪is slightly larger than expected from random distribution. The good resolution of the spectra permits the distinguishing of the 9th order of environment of the end units and the 5th order for the middle units and the verification that there is no preferential association among the building units, and thus the proportion of chains of various lengths obeys Flory's random distribution mode. The equilibrium molecular constitution of these α,ω-difluorosiloxanes is close to that of their dichloro analogs; the substitution of chlorine by fluorine atoms at the end of the chains thus appears to cause little perturbation. On the other hand, the kinetic evolution of the reaction appears to be different, since the first species which forms is always the pyro molecule ▪instead of long chains and large rings as in the case of the chloro analogs.