Anionic polymerization of cyclosiloxanes with cryptates as counterions: New results

Abstract

A kinetic study of the anionic polymerization of hexamethylcyclotrisiloxane ((CH3)2Si0)3 (D3) was carried out in toluene with the cryptate Li+ + 12111 as a counterion. The kinetic order with respect to the living end concentration was found to be equal to 1, and the propagation rate constant relative to cryptated ion pairs was determined at several temperatures between -20 and +20°C. The corresponding activation parameters were calculated : Ep = 9.8 kcal.mo1-1 andThe reaction of 1,3,5,7-tetramethyl-l,3,5,7-tetravinylcyclotetrasiloxane ((CH3)(CH2=CH)Si0)4 (D4*) was studied under the same conditions, the reactivity towards silanolate active centers was shown to be very close to that of D3. The rate constants of propagation and depropagation of D4* and of larger cycles D5* and D6* were determined from polymerization experiments on D4*. The propagation rate constants were found to be nearly the same for D4* and D5* : 1.2-1.3 l.mo1-1.s-1, D6* being the less reactivemonomer : 0.4 l.mo1-1.s-1. The differences observed in equilibrium constants of the cycles are essentially due to the rate constants of their formation which were found to be 0.42, 0.23, and 0.006 s-1 for D4*, D5* and D6*,respectively. ASo# = -20 cal.mol.-lK-l.

Preparation of functionalized block copolymers based on a polysiloxane backbone by anionic ring‐opening polymerization

“…With the less polar solvent, toluene, the propagation started with the addition of [12]crown‐4 but proceeded slowly (28% conversion after 108 h). This was contrary to the results of Hubert et al,19 who reported propagation rate constants of the same magnitude for this initiating system for D 3 and D. The polymerization rates of n ‐BuLi and (CH 3 ) 3 SiCH 2 Li with crown ether in toluene are comparable.…”

“…Therefore, the commercially available D was also chosen as the monomer for the PMVS block and compared with D. Because of the smaller ring strain in D, the ring‐opening reaction needs additional activation with respect to cyclotrisiloxanes 19, 20. For the anionic ring‐opening of cyclosiloxanes, typically strong bases such as organolithium compounds (e.g., butyllithium) and THF as a solvent to minimize ion–ion interactions between the lithium cation and the silanolate anion are used 21.…”

Multiblock silicones, 1 Vinyl functionalized polydimethylsiloxane a

“…2 in comparison with the 4-units monomer. On one hand, propagation rate constants increase with the number of units in the cycles, which is the exact opposite of the trend observed in bulk [24]. A possible explanation lies in the preferential partitioning of large cycles at the interface, where propagation occurs.…”

Creation of a PDMS Polymer Brush on SiO2-Based Nanoparticles by Surface-Initiated Ring-Opening Polymerization

“…Because the propagation of the chains is reversible also backbiting of active centers with its own chain is possible. Thus, the reaction is a thermodynamically controlled ring-chain equilibration reaction between active linear polymer chains (Cn*) and ring species (Mx) with x monomeric units as described by the Jacobson-Stockmayer theory [33,50,[52][53][54] and shown in Scheme 2. Numerous earlier works [21,26,33,53] have demonstrated that the anionic ROP of cyclosiloxanes can be seen as an excellent model system for this theory.…”