Mechanism of Octamethylcyclotetrasiloxane Polymerization in the Presence of Siloxanediols

✍ Scribed by Bischoff, Rémy; Sigwalt, Pierre

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 868 KB
• Volume: 40
• Category: Article
• ISSN: 0959-8103
• DOI: 10.1002/(sici)1097-0126(199606)40:2<99::aid-pi540>3.0.co;2-3

✦ Synopsis

Polycondensation of tetramethyldisiloxanediol (HD,OH) by triflic acid (TfOH) in the presence of octamethylcyclotetrasiloxane (D,) has been studied at 30°C in CH,Cl, solution and in bulk at 80°C. The condensation follows a similar pathway, at a similar rate, in the presence or in the absence of D,. Larger oligosiloxanediols and D, are formed first, and high polymer only after several hours. At 30"C, no polymerization of D, occurs, even 24 h after the total conversion of HD,OH. In bulk at 80"C, polymerization of D, takes place when silanol concentration becomes very low at the end of the polycondensation, and is assumed to result from a reaction of D, with ester end-groups activated by hydrated acid at this temperature. Indeed, these results show that silanol groups activated by TfOH are not responsible for D, polymerization. Polymerization made in the presence of longer-chain oligosiloxanes HD,OH (x -15) indicates that the presence of silyltriflates is a prerequisite of the propagation reaction. Polymerization of D, alone at 80°C also occurs in the presence of water in situ (0.9 mol litre-') after addition of pure triflic acid, and water cocatalyses the propagation reaction. However, the initiation reaction is almost completely inhibited when triflic acid is prehydrated.