Anionic Polymerization of Cyclosiloxanes with Cryptates as Counterions

Abstract

The anionic polymerization of cyclosiloxanes was examined in benzene and toluene with lithium cryptates as counterions. Only one type of active species is observed in the case of Li + + [211]; thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied in detail for the first time. The reactivity of cryptated silanolate ion pairs toward the ring opening of D 3 is greatly enhanced compared to that of other systems. The amount of cyclic by-products is very low when the polymer yield reaches the maximum value. PDMS of narrow molecular weight distributions were obtained using a cryptand larger than the [211].The anionic polymerization of cyclosiloxanes has been studied for a long time. However the knowledge of the mechanism of the process is still unfortunately rather limited (1^2) · This is partially due to the fact that the polymerization is of the equilibrium type and besides linear polymers, cyclic structures exist in solution. It has been shown that under certain conditions, hexamethylcyclotrisiloxane (D3) polymerizes to give linear polymers of negligible cyclosiloxane content and of narrow molecular weight distribution at a much more rapid rate than octamethylcyclotetra siloxane (D4) (3)· This can be explained by the ring strain of D3 which enhances the reactivity of the Si-0 bonds. A suitable initiator such as butyllithium can be mixed with D3 in hydrocarbon solvents to form Bu(-Si (CH3)2 _ 0)3Li (4). No polymerization occurs even in the presence of excess D3 until a donor solvent like THF (3,5), diglyme (6), triglyme (5), DME (7^), HMPA (7) or DMSO (3) is added; then a reasonably rapid polymerization starts to give near monodisperse polymers.Only few kinetic investigations have been performed on these systems though the side reactions which involve intra and intermolecular attacks of the chains by living centers can be neglected in the first stage of polymerization (5,7). The nature of the active propagating species as well as their relative proportion is

Ring-opening polymerization by lithium catalysts: an overview

“…However, with the Li based catalysts, the distribution was significantly narrower. 99 The use of cryptand lithium as a counterion for the anionic polymerization of hexamethylcyclotrisiloxane (D 3 Me) 100 and hexaethylcyclotrisiloxane (D 3 Et) 101 has proven to be a successful catalyst for the preparation of polysiloxanes. A lithium silanolate intermediate is formed by addition of an organolithium compound to the monomer.…”

A Block Copolymer from Polystyrene and Columnar Liquid-Crystalline Poly(diethylsiloxane)

“…In first instance, a nonreactive lithium silanolate results, which can be activated for polymerization by addition of a lithium complexing compound, e.g. the cryptand [211] ( 1 ) . The reaction is terminated by the addition of a chlorosilane.…”

Base catalyzed proton transfer polymerization of 1-hydroxypentamethylcyclotrisiloxane. Comparison of hyperbranched polymer microstructure and properties to those of highly regular linear analogs

“…[26,27] This reaction can occur faster than base catalyzed equilibration due to relief of ring strain in the cyclotrisiloxane. Monosubstituted pentamethylcyclotrisiloxanes undergo AROP with a high degree of chemoselectivity and regioselectivity.…”