Polymerization of methyl methacrylate by anions–II. Influence of initiation procedures on the kinetics of the subsequent polymerization in tetrahydrofuran-toluene solution initiated by disodium salt of tetrameric α-methyl styrene

Abstract

Conditions for optimum efficiency of initiation ( ƒ) by the disodium salt of α-methyl styrene tetramer in toluene-tetrahydrofuran (THF) solution are established. Under these conditions, ƒ = 0·35–0·45. The propagating species are stable < 270 K but decay at ambient temperature. The decay product appears to be an alkoxide. Methods of assaying the concentration of propagating species are compared and assessed. The rates of polymerization depend strongly on the method of initiation. The standard two-step method adopted gave values of k p which fitted the Arrhenius equation ( A p ⋍ 10 2 M −1 sec −1 , E p ⋍ 16 KJ mol −1 ) and depended linearly on [THF]. If, however, the whole aliquot of monomer was added to initiator in one step at ca. 200 K, the rate is > 10 3 times that observed after a two-step initiation in which a trace of monomer is added some few minutes before the bulk. At higher temperatures there is no such discrepancy. It is postulated that the reactive species is a rare, solvated ion-pair in equilibrium with a less reactive, less solvated, contact ion-pair. The high rates observed after single step initiations at low temperatures are attributed to the slow relaxation of propagating ion-pairs which initially contain a non-equilibrium excess of the reactive form. On the basis of this hypothesis, the propagation coefficient for contact ion pairs: k c = 1·2 × 10 −2 M −1 sec −1 at 283 K while that of the solvated (or solvent-separated) ion pairs: k s > 25 M −1 sec −1 at ca. 200 K. The polymer produced was essentially stereo-random. Molecular weight distributions were broad and bimodal, showing a high mol. wt. peak in the 10 5 range and a low mol. wt. in the 10 3 range. The polymers produced in rapid polymerization after single step initiation at low temperatures had narrower high mol. wt. peaks than those prepared under equilibrated conditions.