Quantitative Synthesis of Exo-Olefin-Terminated Polyisobutylene: Ether Quenching and Evaluation of Various Quenching Methods
Polymers and High Performance Materials
Addition of excess dialkyl ether, e.g., diisopropyl ether (DIPE) or di-sec-butyl ether (DSBE), to a TiCl4-co-initiated, living carbocationic polymerization of isobutylene (IB) results in capping of the chain ends with oxonium cations, as evidenced by cessation of polymerization. Structure of PIB diisopropyl oxonium cations was determined using 500 MHz H-1 NMR, by introducing DIPE to a mixture of TiCl4 and 2-chloro-2,4,4-trimethylpentane (TMPCl), a model for the PIB chain end, at -70 degrees C in CS2/CD2Cl2 (50/50, v/v). One-pot, two-step quantitative synthesis of mono- and telechelic (difunctional) exo-olefin-terminated polyisobutylene (PIB) was achieved by quenching living PIB with DIPE, followed by termination with methanol at -60 degrees C. TMPCl and 1,3-bis(1-chloro-1-methylethyl)-5-tert-butylbenzene (bDCC) were used as mono- and difunctional initiators, respectively. H-1 NMR spectroscopy was used to characterize end-group composition of PIBs. Percent formation of exo olefin was directly related to the steric bulk of the ether, i.e., sec-butyl (100%) approximate to isopropyl (100%) > n-alkyl (68.5-81.5%). Coupled PIB and tert-chloride PIB were the principle side products observed when exo olefin was <100%. To prevent coupling, molar ratio of Lewis acid/chain ends ([TiCl4]/[CE]) should be at least about 2.2. To prevent tert-Cl PIB, [TiCl4] <= [CE] + [ether]. For [TiCl4]/[CE] = 2.2 and [DIPE]/[CE] >= 3, quantitative mono- and telechelic (difunctional) exo-olefin-terminated PIB was observed at [CE] <= 0.1 M, for reaction volumes about 1 dL. However, the end group composition of difunctional PIB was 96.2% exo olefin and 3.8% coupled, when the reaction was up-scaled to about 4 L (0.66 kg of PIB). Study of various quenching methods for production of exo-olefin-terminated PIB, including ether, sulfide, hindered base, and methallyltrimethylsilane, showed that ether and sulfide quenching were superior to other methods with regard to maximizing exo-olefin end groups at high [CE].
Storey, R. F.
(2013). Quantitative Synthesis of Exo-Olefin-Terminated Polyisobutylene: Ether Quenching and Evaluation of Various Quenching Methods. Macromolecules, 46(6), 2049-2059.
Available at: https://aquila.usm.edu/fac_pubs/7720