Sorption and addition of bromine from aqueous solutions on cis-1,4-polyisoprene

The sorption of bromine from bromine water on cis-1,4-polyisoprene film and the initial stages of bromination are studied at concentrations of 0.002–0.1 mol/l and temperatures of 15–35°C. The diffusion coefficient of bromine into natural rubber (NR) is 1.3–2.0 × 10⁻⁶ cm²/sec for the total sorption and 5–13 × 10⁻⁷ cm²/sec for the irreversible sorption. The partition coefficient of bromine between water and rubber increased from 17.3 at 15°C to 37.1 l/kg at 35°C. The chemical potential, enthalpy and change in entropy of partition are, at 25°C, respectively: −1.9 kcal/mol, 6.6 kcal/mol and 28.4 cal/mol. K. The irreversible sorption is due to a charge-transfer complex between bromine molecules and double bonds of the rubber. The complex is the first stage of the addition reaction, which becomes noticeable at concentrations above 0.012 mol/l. With increasing bromine concentration the concentration of the complex decreases and the added bromine increases. The charge transfer complex appears to change the conformation of the cis-NR chains so that the bromine addition occurs in the trans-conformation, as shown by FT–IR spectra. The bromination is accompanied by a marked crystallization effect as illustrated by thermal analysis and WAXS measurements.