KINETIC ANALYSIS OF THE GEL EFFECT IN RADICAL COORDINATION POLYMERIZATION OF METHYL METHACRYLATE IN THE PRESENCE OF ZIRCONOCENE DICHLORIDE
Authors:   Kazan National Research Technological University, OSIPOV A.A. , TERESHCHENKO K.A. , SHIYAN D.A. , BONDARENKO V.P. , SABITOVA E.M. , ULITIN N.V. ,   Kazan National Research Technological University
Published: December 2024

Using a kinetic model, an analyzis of the coordination active sites functioning in radical coordination methyl methacrylate polymerization in the presence of initiating system “benzoyl peroxide + zirconocene dichloride” at different methyl methacrylate conversions was carried out. The purpose of the work was to establish the reasons for the manifestation of the gel effect in this process despite the fact that, firstly, a significant role in it is played by coordination chain propagation, where the gel effect is not characteristic, and, secondly, there are reactions with reversible-deactivation polymerization that the gel effect should suppress. As a result of the computational experiment, the following findings were demonstrated (experimental conditions: polymerization temperature T = 333 K, initial concentrations of benzoyl peroxide and zirconocene dichloride 1 mmol/L, initial concentration of methyl methacrylate 9.4 mol/L). In the reaction system, there are the coordination active sites of two types (adducts containing a zirconium atom with ligands: a cyclopentadienyl ring, a polymer chain, and one or two chlorine atoms). Each type of coordination active sites has three forms: active, intermediate and “dormant”. Most of the coordination active sites are in a “dormant” form. The mechanism of transition between active site forms is similar to the mechanism of Organometalic Mediated Radical Polymerization (OMRP). Unlike OMRP, which involves two forms of radical active site and one reversible transition reaction between them, the mechanism of transition between coordination active sites contains three forms of the coordination active site and two reversible reactions between them. The difference is due to the fact that coordination chain propagation runs in two stages (includes the stage of coordination of the monomer and the stage of its incorporation into the chain), and radical chain propagation runs in one stage. The multisite of coordination active sites is associated with the possibility of the existence of coordination active sites with different numbers of chlorine atoms as ligands of zirconium atom. Transition reactions between active sites of different types are polymerization reactions with the transfer of a chlorine atom (Reverse Atom Transfer Radical Polymerization, RATRP). Thus, all transition reactions between forms and types of coordination active sites are reversible. When diffusion limitations arise, the apparent values of the rate coefficiens of direct and reverse reactions decrease in different ways, which leads to a change of the proportions between the concentrations of both forms and types of coordination active sites. As a result, after 400 min of polymerization, a sharp increase of the concentration of coordination active sites is observed. Because of this, a sharp increase of the coordination chain propagation rate and an S-shaped time dependence of the conversion methyl methacrylate which is polymerized at coordination active sites are observed. In this case, the external manifestation, the S-shaped time dependence of methyl methacrylate conversion, is similar to the manifestation of the gel effect. But the internal reason for this external manifestation is different from that of the gel effect. In case of the gel effect, the increase of the concentration of radical active sites is caused by a decrease of the bimolecular chain termination rate, and in the case under consideration, the increase of the concentration of coordination active sites is caused by the transition of their “dormant” form to the active form. Thus, in this work we described a new phenomenon, identical to the gel effect in radical polymerization by external manifestation, still different by internal reason, which was proposed to be named the pseudogel effect.