Crystal structure of δ-isobutoxypentabromo-cyclododecanes, kinetics and selectivity of their isomerization during thermal treatment of flame-proofed polystyrenes

Hexabromocyclododecanes (HBCDs) are persistent organic pollutants now ubiquitous in the environment. Technical HBCD mixtures and with it flame-proofed polystyrenes (FP-PS) also contain isobutoxypentabromocyclododecanes (iBPBCDs) as minor constituents, which are possibly released together with HBCDs. So far, eight diastereomeric pairs of enantiomers named as alpha-, beta-, gamma-, delta-, epsilon-, xi-, eta-, and theta-iBPBCDs with proportions of 10%, 5%, 2%, 21%, 11%, 11%, 12% and 28% were found in technical HBCD. Herein the crystal structure of racemic delta-iBPBCD, the second most prominent diastereomer, is presented and assigned to (1S)-1-isobutoxy-(2R,5R,6S,9S,10S)-2,5,6,9,10-pentabromocyclododecane and its enantiomer. During thermal treatment of FP-PS, e.g. the production of extruded polystyrenes (XPS), proportions of delta-iBPBCDs decrease and those of other stereoisomers increase. Evidence was found that delta-iBPBCDs isomerize stereo- and regioselectively to beta-iBPBCDs. Based on structural and kinetic data, a transformation mechanism was proposed. Apparent first-order rate constants (k(iso)) of 0.0019, 0.0050, and 0.012 min(-1) are found for the delta- to beta-iBPBCD isomerization at 120, 130, and 140 degrees C, respectively, corresponding to half-lives of 360, 140, and 56 min. These transformations also occur during the production of XPS, which predominantly contain beta-iBPBCDs, whereas delta-iBPBCDs dominate in materials experiencing lower thermal stress, e.g. expanded polystyrenes (EPS). The relative configurations of delta- and theta-iBPBCDs are TtCtCt, like the one of gamma-HBCDs. gamma-HBCDs are the kinetically and alpha-HBCDs with a TcCtCc configuration the thermodynamically favored products. In analogy, beta-iBPBCDs are assumed to have a TcCtCc configuration like alpha-HBCDs because they are formed from delta-iBPBCDs under thermodynamic control. In conclusion, HBCD- and iBPBCD-patterns in flame-proofed polystyrenes vary substantially, reflecting the thermal stress these materials have experienced. When released to the environment, these patterns might further change, as observed for HBCDs. (c) 2011 Elsevier Ltd. All rights reserved.