Polymerization of Cyclosiloxanes. I. Kinetic Studies on Living Polymer—Octamethylcyclotetrasiloxane Systems

Abstract

AbstractThe kinetics of the anionic polymerization of octamethylcyclotetrasiloxane (D4) initiated by α‐methylstyrene living polymer in tetrahydrofuran was studied. The following kinetic scheme was postulated: Initiation: equation image Propagation: equation image where S‐ and M represent the initiator and D4, respectively. At a living end concentration of 0.0377 mole/l. and a monomer concentration of 1.5 mole/l. in tetrahydrofuran at 25°C. the following kinetic data were obtained: k1 = 2.3 × 10−4 l./mole‐sec., k2 < 2.3 × 10−5 sec.−1, k3 = 2.75 × 10−2l./mole‐sec. k4 ≈ 1.17 × 10−2 sec.−1, K1 > 10 l./mole and K2 ≈ 2.35 l./mole. The rate constants k1 and k3 were found to be dependent on the concentration of anions. This is attributed to the dissociation of ion pairs to free ions at lower concentration. Under the experimental conditions studied the majority of the anions were present in the form of ion pairs. The reactivity of the free ions is about 100 times greater than that of ion pairs. There is no temperature effect on K2, indicating zero ΔH and positive ΔS in the propagation reaction.