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Cationic Polymerization of Hexamethylcyclosiloxane (D 3 ): Kinetics and Mechanism of Cyclics Formation
Author(s) -
Toskas G.,
Moreau M.,
Sigwalt P.
Publication year - 2006
Publication title -
macromolecular symposia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.257
H-Index - 76
eISSN - 1521-3900
pISSN - 1022-1360
DOI - 10.1002/masy.200650809
Subject(s) - cationic polymerization , polymer chemistry , oxonium ion , chemistry , trifluoroacetic anhydride , polymerization , ring opening polymerization , silanol , nucleophile , ring (chemistry) , methylene , medicinal chemistry , organic chemistry , catalysis , ion , polymer
In cationic polymerization of hexamethylcyclotrisiloxane (D 3 ) in methylene chloride, a control of the mol wts could be observed with various initiators such as RCOCl/ SbCl 5 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 D 3x , 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 D 3x cycles (essentially D 6 ) 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 D 3x 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 CF 3 CO 2 (CH 3 ) 2 Si 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 D 3x oligomers were observed again. The weight ratio of D 6 was larger than for TfOH initiation (1 < D 6 /HP < 1,5) and when a large excess of MDTBP on the acid was used, D 6 /HP was even higher but MC formation was completely suppressed. This confirms the difference in the mechanisms giving MC and D 6 and agrees with small D 3x cyclics formation involving the silyltriflate end‐groups alone (and probably the derived oxonium sites).