What is the most confirmative way for the molecular structure identification of very high viscosity polysiloxane polymers i.e. Poly (dimethyl) co-methyl phenyl siloxane?
If the polysiloxanes are not crosslinked they can be solved in tetrachlorocarbon, benzene, toluene or (slightly worse) in chloroform. Using these solvents (the latter three in deuterated form) you can messure a simple 1H-NMR or 13C-NMR. If you are sure that there are only dimethyl- and methylphenyl-siloxane repeating units present in the polymer also a elementar analysis (C, H) is sufficient to determine the ration between both units.
Actually I know that it is a methyl phenyl siloxane. I want to ascertain the ratio of methyl to phenyl group in the polymer and also the environment of the silicon atoms in the backbone. I want to know the substitution groups on individual silicon atoms like whether there are one methyl and one phenyl/two methyls and two phenyls on silicon atoms. Basically the nature of substitution of silicon atoms. I am doing 1H and 13C NMR of the sample. I will come to it when it will be done.
The best would be to use substances for comparison. Poly(methylphenyl)siloxane can be bought from Sigma-Aldrich and Polydimethyl-co-diphenylsiloxane from Shin-Etsu-Silicone.
Do a 29Si-NMR (~5% of Si-Atoms). With DEPT/INEPT 3JSi-H coupling should be observed. Dimethyl, diphenyl and Phenyl/Methyl substitution should give very different chemical shifts. If you pulse long enough you might even be able to integrate the signals and get a Substitution distribution which you can crosscheck via 1H-NMR.
I think that 29Si NMR is one of the best method to identification of molecular structure organosilicon compounds including polysiloxanes. Using Inverse Gated Decoupling gives a quantitative information. 29Si NMR shifts of many compounds can found in good databases e.g. Marsmann H. ,,29Si- NMR Spectroscopic Results", in NMR Basic Principles and Progress (Diehl P., Fluck E., Kosfeld R., eds.). Vol 17, Springer-Verlag, Berlin 1981
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