Kinetics and mechanism of oxetanes polymerization catalyzed by AlR3. Part II. Non‐stationary polymerization and chain transfer to polymer

Polymerization of 3.3‐bis(chloromethyl)oxetane induced by (iso‐C4H9)3Al/H2O catalytic system proceeds in chlorobenzene solution at 55–95°C in homogeneous conditions. Dilatometric study in a high vacuum system permitted to establish the following main features of the process under study : irrespectively of the [(iso‐C4H9)3Al]/[H2O] ratio or [catalyst]/[monomer] ratio a gradual acceleration at the start of reaction was observed, followed by a “linear” portion, which in turn is followed by a deceleration. The whole process can be described as the non‐stationary reaction with slow initiation, fast propagation and slow degradative chain transfer to polymer. Polymerization is externally first order on [monomer] (when the influence of initiation was excluded), first order on [H2O] until [H2O] ≤ [(iso‐C4H9)3Al]. The sigmoidal dependence with saturation of rate and degree of polymerization on [(iso‐C4H9)3Al] shows, that for [(iso‐C4H9)3Al] ≥ [H2O] the excess of (iso‐C4H9)3Al has no important influence on both polymerization kinetics as well as on the polymer P̄. The low ratio of P̄w/P̄n (lower than 1.1) indicates that the acceleration period is not due to the consumption of impurities. Approximated 10 molecules of H2O participate in the formation of one active center. This value was found by extrapolation of the plot of {Pn (theoretical)/Pn (experimental)} on [H2O] to [H2O] = 0. With increasing the ratio of [catalyst]/[monomer] the percentage of the unused catalyst also increases. Solution of the kinetic scheme as described above, with a degradative chain transfer to polymer treated as an irreversible process, led to the following absolute values of the apparent rate constants, being at 70°C equal to : ki = 1.6·10‐3 mole‐1·1·sec‐1, kp = 8.5 mole‐1·1·sec‐1, and ktr = 1.2·10‐3 mole‐1·l·sec‐1. The ktr was compared with the rate constants of the chain transfer to low molecular ethers (ktE). The ktr value is close to ktE found for β‐chloroethyl ether of a structure close to this of a polymer segment. The plot of ktE/kp on – pKa is linear with a slope expected for a cationic process.