Investigation of novel quasiliving polyisobutylene chain-end functionalization (quenching) methods
This volume recounts efforts toward the development and understanding of chain functionalization techniques involving the direct addition of nucleophiles to quasiliving polyisobutylene (PIB). Nucleophiles included in the study were sterically hindered organic bases, (di)sulfides, N -substituted pyrroles, and alkoxybenzenes. A kinetic investigation of the end-quenching of TiCl4 -catalyzed quasiliving PIB with sterically hindered amines was used to determine the mode of interaction with TiCl4 and the active species responsible for β-proton abstraction. 2,5-disubstituted- N -hydropyrroles formed pyrrole-TiCl3 adducts that were active in formation of exo-olefin chain ends; whereas, with other sterically hindered amines, only an equilibrium fraction of the amine that did not complex with TiCl4 remained available for proton abstraction. Low-temperature-stable sulfonium ion adducts were generated by addition of mono- and disulfides to TiCl4 -catalyzed quasiliving PIB. At temperatures less than or equal to -60°C, quantitative 1:1 adducts were formed between the (di)sulfides and the oligo-isobutylenes. When a more reactive nucleophile such as an alcohol or amine was added to the reaction, the adducts were destroyed, and both elimination and substitution products were obtained. N -(2-tert -Butoxyethyl)pyrrole was used to end-quench TiCl4 -catalyzed quasiliving PIB and resulted in near quantitative end-capping, except for the formation of <5% exo -olefin chain ends, with alkylation occurring in both the C-3 (57%) and C-2 (38%) position on the pyrrole ring. Further treatment with acids and warming resulted in alkylation via the residual olefin and rapid cleavage of the terminal tert -butyl group of the N -(2-tert -butoxyethyl)pyrrole-capped PIB to provide hydroxyl end group functionality in situ. Alkoxybenzenes were also used to end-quench TiCl4 -catalyzed quasiliving isobutylene polymerizations. Successfully alkylated alkoxybenzenes included those with alkyl tethers, such as anisole and isopropoxybenzene, those with haloalkyl tethers, such as (3-bromopropoxy)benzene and (2-chloroethoxy)benzene, and even those with hydroxyl and amine functionality, such as 4-phenoxybutanol and 6-phenoxyhexylamine. Alkylation occurred exclusively in the para position of alkoxybenzenes, and multiple alkylations were not observed. The alkylation reactions were tolerant of temperatures ranging from -70 to -30°C and were unimpeded by the presence of endo - or exo -olefin termini. Terminal ether cleavage for polyisobutylenes capped with anisole and isopropoxybenzene allowed single-pot synthesis of phenol telechelics.