Nucleophilic Catalysis of Brominated Butyl Rubber Substitution Reactions
Malmberg, Sean Magnus
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The allylic bromide functionality within brominated poly(isobutylene-co-isoprene), or BIIR, is amenable to substitution by a wide range of nucleophiles. The objective of this work was to gain insight into the dynamics of these substitution reactions, and to develop methods for accelerating these processes. Of particular interest was the reactivity of exomethylene (Exo-Br) and bromomethyl (BrMe) isomers found within BIIR toward various nucleophiles, and catalytic techniques for affecting the proportion of these isomers. BIIR isomerization can be catalyzed through ionic chemistry involving soluble Lewis acids such as zinc stearate and through a nucleophilic SN2’ rearrangement with soluble bromide salts such as tetrabuylammonium bromide (TBAB). The compatibility of TBAB with other nucleophiles makes it a preferable choice, but further rate enhancements can be realized using the corresponding iodide salt (TBAI). TBAI serves not only as nucleophilic isomerization catalyst, but also accelerates halide displacement from BIIR by an in-situ formation of an allylic iodide intermediate. Studies of BIIR isomerization and substitution reactions involved solvent-borne reactions of tetrabutylammonium acetate (TBAAc) and solvent-free reactions with PPh3 and 2-[2-(dimethylaminoethoxy)ethanol]. In all cases, the BrMe isomers were more reactive to nucleophilic substitution than Exo-Br. Since the esterification of BIIR with TBAAc generates TBAB as a reaction by-product, displaced bromide catalyzes the isomerization of Exo-Br to the more reactive BrMe isomers. As a result, these esterifications exhibit auto-accelerating dynamics. Similar behaviour is observed for solvent-fee alkylations of PPh3 and tertiary amines, since the resulting onium bromide salts are effective isomerization catalysts. All reactions show some increase in rate with the addition of TBAI, supporting the concept of nucleophilic catalysis.