Polymerization of Oxetanes with Organoaluminium Catalysts. II. Analysis of Molecular-Weight Distributions in the Polymerization of 3-Methyl-3-chloromethyloxetane with the Al(i-C4H9)3–H2O Catalyst System

Abstract

Molecular-weight distributions and polydispersity parameters of the polymers formed in the polymerization of 3-methyl-3-chloromethyloxetane with the Al(i-C4H9)3–H2O catalyst system have been investigated. It was shown that the main feature of this process is the ability of macromolecules to regulate their own size in the formation of a polymer of high molecular weight (∼106) and narrow molecular-weight distribution (Mw/Mn=1.30±0.05) without any dependence on conversion, and the initial monomer and catalyst concentrations. Proceeding from the suggestion that the ratio of the termination to propagation-rate constants (kt/kp) depends on the size of the macromolecules, equations for the unnormalized weight molecular-weight distribution (MWD), and polydispersity parameters were derived. The experimental and calculated values of weight MWD and polydispersity parameters are in satisfactory agreement. On the basis of these data, a model for polymerization is proposed, assuming the formation of an active centre in the form of an ion-pair of the polymer zwitter-ion solvated by its own polymer chain. This model explains adequately the dependence of the kt/kp ratio on the degree of polymerization and the observed molecular-weight characteristics of the polymer.