Kinetics and mechanism of end-quenching of quasiliving polyisobutylene with sterically hindered bases

摘要

Kinetic investigation of end-quenching (β-proton abstraction to form exo-olefin) of quasiliving polyisobutylene (TiCl4, -60 °C, 60/40 hexane/methyl chloride) with sterically hindered amines, 2-tert-butylpyridine (2TBP), 1,2,2,6,6-pentamethylpiperidine (PMP), 2,2,6,6-tetramethylpiperidine (TMP), and 2,5-dimethylpyrrole (25DMP), was conducted to determine the mode of interaction of these quenchers with TiCl_4 and identify the active species responsible for β-proton abstraction. Strongly basic, sterically hindered amines such as 2TBP, PMP, and TMP formed reversible 1:1 complexes with TiCl4. A minor fraction of the base remained uncomplexed and was the active species responsible for proton abstraction. For these bases, the rate of β-proton abstraction decreased in the order PMP > TMP > 2TBP and was directly proportional to [TiCl4]. The weakly basic 25DMP behaved differently. NMR spectroscopy showed that it reacted quantitatively and irreversibly with TiCl_4 to form the Ti(η5-2,5- dimethylpyrrolyl)Cl3 coordination complex, which was the active species responsible for β-proton abstraction. In the presence of a stronger base, e.g., 2,6-lutidine, the complex was rapidly regenerated and maintained at a constant concentration. With increasing [25DMP]_0, the rate of quenching increased to a maximum at [25DMP]_0 = ([TiCl _4]~0 - [2,6-lutidine])/6 and then decreased. The rate of quenching with 25DMP was proportional to [TiCl4]2. The behavior of 25DMP was predicted to extend to other 2,5-disubstituted-N- hydropyrroles, such as 2,3,4,5-tetramethylpyrrole.