Infrared spectroscopic studies on polyethylene, 3. The bromination of unsaturated groups

Abstract

AbstractResults for the rates of bromination of vinyl, vinylidene, and trans vinylene groups in various types of polyethylene are reported and interpreted. It is shown that for two Phillips type vinyl‐containing polymers, one near‐linear and the other ethyl branched, the uptake of vapour phase bromine at ambient temperature is rapid, because the unsaturation lies largely in amorphous regions. Plots of log (At/A0) against time, where A0 and At are the absorbances at 910 cm−1 initially and after time t, are linear until bromination is almost complete, indicating that it is a diffusion controlled process. The subsequent slower bromination is tentatively interpreted in terms of vinyl groups sited in defective surface layers of the crystalline lamellae. Both types of bromination occur more readily with the ethyl branched polymer than with the linear material. Bromination of the residual unsaturation in wholly crystalline regions occurs only in the molten state. X‐ray diffraction measurements confirm that the majority of the vinyl groups are sited in amorphous regions.Similar conclusions are drawn for vinylidene and trans vinylene groups, but the rate of bromination of polymers containing the latter type of unsaturation does not wholly correlate with the amorphous content. From this evidence and from measurements on a thicker film of the ethyl branched vinyl polymer it is concluded tentatively that there are differences in the amorphous regions which lead to varying rates of diffusion of bromine vapour.