Anionic polymerization of octamethylcyclotetrasiloxane in aqueous emulsion I: Preliminary results and kinetic study

Abstract

The synthesis of poly(dimethylsiloxane) (PDMS) by anionic polymerization of octamethylcyclotetrasiloxane (D4) in aqueous emulsion using an emulsifying agent acting also as initiator (benzyldimethyldodecylammonium hydroxide) is described. Stable emulsions of α,ω‐dihydroxy PDMS with an easily controlled molar mass (up to 15 000), a low polydispersity index and high yields were obtained. The amount of cyclics formed (essentially D4 to D7) is lower than that observed in bulk. Kinetics analysis and computerized simulations are in good agreement with a reaction scheme involving simultaneous polyaddition/polycondensation processes. At high monomer consumption, polycondensation predominates leading to a broadening of the molecular weight distribution (MWD). The apparent rate constants of initiation, propagation and condensation at various temperatures and the corresponding activation energies were determined.

Kinetic Study of Thermal Degradation of Polydimethylsiloxane: The Effect of Molecular Weight on Thermal Stability in Inert Atmosphere

“…As it was stated by De Gunzbourg [3] and Zhang [7] and confirmed by others [12,24,25] all three reactions occur at the particle/water interface, where also backbiting reactions take place. The latter are responsible for low final conversions [12,24,25].…”

“…Comparing the ROP polymerization in emulsion to bulk or solution ROP, the ROP in emulsion gives higher monomer conversion and products of higher molecular weight with narrower distribution [3][4][5]. Moreover, by emulsion ROP method polymer of designed molecular weight distribution can be obtained.…”

“…On the other hand, condensation reactions are desirable when polymer of higher average molecular weight is preferred. The condensation reactions were proven by a study of De Gunzbourg et al [3,13], who observed a significant increase in average molecular weight and polydispersity index above 70 and 60% of monomer conversion.…”

“…In reversible termination reaction, the active oxygen of the growing PDMS chains reacts with water to give silanol end-group. As mentioned before, when such terminated polymer chain reaches a critical degree of polymerization, the chain penetrates into the particle interior [3,5], where condensation and redistribution reactions take place, which affect the polymer molecular weight and distribution. The condensation reactions are responsible for further increase of the PDMS molecular weight at unchanged conversion.…”

“…The extents of condensation and redistribution reactions affect the average molecular weight and distribution of the polymer drastically. In order to achieve a narrow molecular weight distribution, much attention to control of backbiting, condensation and redistribution reactions has to be payed [3,5,12]. On the other hand, condensation reactions are desirable when polymer of higher average molecular weight is preferred.…”