Cationic polymerization of siloxanes kinetically controlled oligomerization in hexamethylcyclotrisiloxane – linear dimethylsiloxane systems

The kinetically controlled distribution of linear oligomers in the cationic solution polymerization of hexamethylcyclotrisiloxane (D3) in the presence of hexamethyldisiloxane (MM), octamethyltrisiloxane (MDM), and decamethyltetrasiloxane (MD2M) as molecular chain stoppers was investigated by gas chromatography. At an early stage of the process the distribution was practically unaffected by consecutive reactions involving original stable products of oligomerization, which renders the distribution an important tool in the studies of the mechanism of cationic polymerization of siloxanes. Triple pattern distribution (the excess of every third homologue of the MDnM series) is observed even at nearly full monomer conversion. The distribution of this excess oligomers at low monomer conversion is highly specific as the amounts of each subsequent homologue is decreased by a constant factor. The factor is related to the relative reactivity of the siloxane bonds in the chain stopper and in the monomer. In principle, the results agree with the chain mechanism of polymerization involving kinetic chain transfer to the molecular chain stopper. Studies of the kinetically controlled distribution of linear oligomers in the oligomerization involving a chain stopper of unsymmetrical structure with respect to the reacting siloxane bond allow to determine the specificity of the combination of stopper fragments with a linear molecule. Low specificity showed by MDM constitutes evidence against the mechanism involving a silicenium ion intermediate.