Facile synthesis of poly(isobutylene-co-isoprene) (IIR) carboxylated derivatives by thiol–ene click chemistry

The thiol–ene click reaction was applied to modify poly(isobutylene-co-isoprene) (IIR) by two different strategies using carboxylated thiols (mercaptanoic acids) with different alkyl spacer chain lengths. In the first approach, the direct radical-promoted addition of thiols across the 1,4-isoprene units proceeded with 80–90% conversion of the internal double bonds. An alternate method involving the conversion of sterically hindered 1,4-isoprene units to pendant double bonds through epoxidation and hydrolysis prior to the click reaction proceeded with 90–98% conversion of the alkene moieties. The carboxylated IIR derivatives were characterized by 1H NMR, FT-IR spectroscopy, and gel permeation chromatography analysis. Irrespective of the synthetic strategy used, the yields of the reactions decreased as the length of the alkyl spacer in the mercaptanoic acid increased. The outcomes of the reactions also depended on the amounts of solvent and free radical initiator used. Carboxylated butyl rubber derivatives were obtained by reacting the isoprene units in the isobutylene copolymer with alkylmercaptanoic acids comprising alkyl spacers of different lengths. The yield of the reactions varied from 80 to 90% for direct reaction of the copolymer containing 1,4-isoprene units, but increased to 90–98% if the 1,4-units were isomerized to terminal alkenes prior to the reactions.