Kinetics and Mechanism of End-Quenching of Quasi living Polyisobutylene with Sterically Hindered Bases
Polymers and High Performance Materials
Kinetic investigation of end-quenching (beta-proton abstraction to form exo-olefin) of quasiliving polyisobutylene (TiCl(4), -60 degrees 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 beta-proton abstraction. Strongly basic, sterically hindered amines such as 2TBP, PMP, and TMP formed reversible 1:1 complexes with TiCl(4). A minor fraction of the base remained uncomplexed and was the active species responsible for proton abstraction. For these bases, the rate of beta-proton abstraction decreased in the order PMP > TMP > 2TBP and was directly proportional to [TiCl(4)]. The weakly basic 25DMP behaved differently. NMR spectroscopy showed that it reacted quantitatively and irreversibly with TiCl(4) to form the Ti(eta(5)-2,5-dimethylpyrrolyl)Cl(3) coordination complex, which was the active species responsible for beta-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 [TiCl(4)](2). The behavior of 25DMP was predicted to extend to other 2,5-disubstituted-N-hydropyrroles, such as 2,3,4,5-tetramethylpyrrole.
Morgan, D. L.,
Storey, R. F.,
Harrison, J. J.,
Stokes, C. D.,
Storey, R. F.
(2011). Kinetics and Mechanism of End-Quenching of Quasi living Polyisobutylene with Sterically Hindered Bases. Macromolecules, 44(8), 2438-2443.
Available at: http://aquila.usm.edu/fac_pubs/517