The mechanism of the anionic polymerization of methyl methacrylate. I. Quantitative determination of active chains using radioactive terminators

Abstract

AbstractThe polymerization of methyl methacrylate in toluene solution at temperatures in the range of −50 to −78°C. has been studied using 9‐fluorenyllithium as an anionic initiator. The strong ultraviolet absorption of the fluorenyl group has been utilized to calculate number‐average molecular weights after comparison with osmotic pressure measurements had shown that one fluorenyl group becomes chemically attached to each chain. Under the conditions of these polymerizations, the chains are not rapidly self‐terminating. Termination of the reaction with C14‐labelled carbon dioxide or tritium‐labelled acetic acid produces radioactive polymers. Comparison of the activity of a polymer with its fluorenyl content determined from ultraviolet measurements gives the percentage of chains which were active when the terminator was added. For example, polymerizations reaching 70% conversion after one hour still had from 80 to 100% of the chains active. The magnitude of a possible tritium‐hydrogen kinetic isotope effect in the acetic acid termination has been estimated by comparing the specific activity of fluorene prepared under the polymerization conditions by reacting 9‐fluorenyllithium with tritium‐labelled acetic acid at −78°C. in toluene with that of the acetic acid. Appropriate correction factors were therefore applied in calculating the specific activities of the polymers. Isotope dilution studies which detect unreacted fluorenyllithium indicate that the initiation reaction is very rapid.