Anionic polymerization of cyclosiloxanes with cryptates as counterions: New results

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Abstract
A kinetic study of the anionic polymerization of hexamethylcyclotrisiloxane ((CH3)2SiO)3 (D3) was carried out in toluene with the cryptate Li+ + [211] 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.mol−1 and Δ S°t = −20 cal.mol.−1K−1.The reaction of 1,3,5,7‐tetramethyl‐1,3,5,7‐tetravinylcyclotetra‐siloxane ((CH3)(CH2=CH)SiO)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* : 1.2–1.31.mol−1. s−1, D6* being the less reactive monomer : 0.41.mol−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.