The pyrolysis of some methylchlorosilanes
By analogy with the established mechanism for the low pressure pyrolysis of tetramethylsilane (TMS)1, the decomposition of trimethylchlorosilane (3MCS) would be expected to proceed thus, The dimeric product, if formed, could be useful as a precursor to long chain polymers containing (-Si-C-)n units. Thus, 3MCS was pyrolysed, at low pressure in a static system, using quadrupole mass spectrometry as the principal method of analysis. First order Arrhenius parameters given by log10 k = (17. 03 +.34) - (366.5 + 7.2 KJ mol-1)/2 . 303RT were measured for the decomposition, the activation energy being assigned to the silicon-methyl bond strength. The dimeric product proposed above however, was not formed, dimethyldichlorosilane (2M2CS) being the major silicon-containing product. Since a small quantity of hydrogen chloride was invariably present in the experimental system, owing to some hydrolysis of the reactant by background water, it was suggested that the addition process, could prevent reaction (4). A series of experiments was devised to test this idea and it was shown to be a possible explanation. The decomposition of 1,1-dimethyl-l-silacyclobutane (DMSCB) is well known2. From the co-pyrolysis of DMSCB with hydrogen chloride3 it was found that the addition is a very efficient process, reaction (7) being suppressed until most of the hydrogen chloride had been used up. Since no competition between processes (7) and (8) could be observed, an unusual analysis method was devised, which involved a comparison of the "on-set time" for reaction (7) with the calculated amount of DMSCB decomposed. Computer simulation of the co-pyrolysis mechanism yielded values of log10 k7 = (7.5 + .3) - (12 + 5 KJ mol-1)/2.303 RT. The addition of hydrogen bromide to 1,1-dimethylsilaethene was also investigated. Finally, the decomposition of dimethylchlorosilane (DMCS) and of methyldichlorosilane (MDCS), was shown to proceed via a radical route, consistent with the proposed pyrolysis mechanism of trimethylsilane1. The co-pyrolysis of DMCS and of MDCS with sulphur hexafluoride provided evidence for the presence of silicon radicals, the hexafluoride being a good "trap" for such species.