Cationic Polymerization of Hexamethylcyclosiloxane (D3): Kinetics and Mechanism of Cyclics Formation

In cationic polymerization of hexamethylcyclotrisiloxane (D3) in methylene chloride, a control of the mol wts could be observed with various initiators such as RCOCl/ SbCl5 or trifluoromethanesulfonic acid (TfOH) and its derivatives. But an important difference with usual living polymerizations is the simultaneous formation of large amounts of cyclic oligomers D3x, their weights increasing linearly from the origin. A second population of much larger DPn has been shown to consist of macrocycles (MC). It was concluded that while MC result from end‐to‐end ring closure of a fraction of linear macromolecules, the small D3x cycles (essentially D6) are formed independently by a selective back‐biting reaction involving oxonium end‐groups. However, it has been proposed again in recent publications in which various initiators were used (including TfOH) that D3x cyclics of all sizes result from end‐to‐end ring closure reactions between the (electrophilic) active site for propagation and a silanol end group. In the present paper, the initiator used was the mixed anhydride of trifluoroacetic and trifluoromethanesulfonic acid, which give non‐nucleophilic CF3CO2(CH3)2Si end‐groups. The residual acid present in the anhydride was neutralized by varying amounts of 4‐methyl,2,6‐di‐t‐butylpyridine (MDTBP). The linear increase of the HP molecular weight and the formation of large amounts of D3x oligomers were observed again. The weight ratio of D6 was larger than for TfOH initiation (1 < D6/HP < 1,5) and when a large excess of MDTBP on the acid was used, D6/HP was even higher but MC formation was completely suppressed. This confirms the difference in the mechanisms giving MC and D6 and agrees with small D3x cyclics formation involving the silyltriflate end‐groups alone (and probably the derived oxonium sites).