The preparation of isobutylene-based ionomers through displacement of halide from
brominated poly(isobutylene-co-isoprene) (BIIR) by triphenylphosphine (PPh3) and N,N-dimethyloctylamine (DMOA) is demonstrated. While the resulting phosphonium and ammonium bromide salts (IIR-PPh3Br and IIR-NR3Br, respectively) possess dynamic mechanical properties that are comparable to
thermoset vulcanizates, the elastomeric network is the result of ion-pair aggregation. Evidence of intra-
and intermolecular aggregation provided by dynamic mechanical and dilute solution viscosity analyses
is reinforced by studies of 1H NMR spin−spin relaxation, which demonstrate the reduced mobility of
those polymer chain segments proximal to the phosphonium bromide functionality within IIR-PPh3Br.
The highly electrophilic borane B(C 6 F 5 ) 3 reacts with n-octadecanol (n-C 18 H 37 OH) and n-octadecanethiol (n-C 18 H 37 SH) to form the 1:1 adducts (n-C 18 H 37 EH)B(C 6 F 5 ) 3 (E ϭ O or S). The latter are acidic and react with Cp*TiMe 3 in methylene chloride and toluene to give methane and the complexes [Cp*TiMe 2 ][(n-C 18 H 37 E)B(C 6 F 5 ) 3 ], which are very good initiators for the carbocationic polymerization of isobutene (IB) from Ϫ40 to Ϫ20°C. High conversions to high molecular weight polyisobutene (PIB) in methylene chloride and moderate conversions to high molecular weight PIB in toluene are observed and are consistent with the anions [(n-C 18 H 37 E)B(C 6 F 5 ) 3 ] Ϫ being very weakly coordinating. Although polymerization in methylene chloride is too rapid for the temperature to be controlled, polymerization in toluene is slower, and the temperatures can be controlled so that Arrhenius-type plots of the logarithm of the number-average molecular weight versus T Ϫ1 ϭ 1/T may be obtained. Activation energies for the degree of polymerization in these polymerization reactions and similar polymerizations carried out with n-C 18 H 37 EH:borane ratios of 1:2 and with the activators [Ph 3 C][B(C 6 F 5 ) 4 ] and Al(C 6 F 5 ) 3 range from Ϫ11 to Ϫ27 kJ mol Ϫ1 , values comparable to those for most conventional IB polymerization initiators. However, the values of the weight-average and number-average molecular weights are unusually high for the temperatures used, and this is consistent with current theories of the role of weakly coordinating anions.
An unusual cross-linking of brominated poly(isobutylene-co-isoprene), BIIR, has been shown
to occur in the presence of the normally nonnucleophilic amidine bases 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN). Model compound studies using brominated
2,2,4,8,8-pentamethyl-4-nonene (BPMN) have provided structural information which indicates that this
cross-linking most likely occurs through a sequence of reactions which involves the following: (1)
N-alkylation of the amidine base to an amidinium salt, (2) hydrolysis of the salt to an ammonium lactam,
(3) deprotonation of the ammonium lactam to an amino-lactam, and (4) a second amine N-alkylation.
The ability of DBU and DBN to undergo N-alkylation with BIIR has been confirmed by reaction of BIIR
in solution with an excess of each of these bases to give N-alkylation products that have been characterized
spectroscopically.
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