title: Crystal structure of δ-isobutoxypentabromo-cyclododecanes, kinetics and selectivity of their isomerization during thermal treatment of flame-proofed polystyrenes.
abstract: 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 α-, β-, γ-, δ-, ϵ-, ξ-, η-, and θ-iBPBCDs with proportions of 10%, 5%, 2%, 21%, 11%, 11%, 12% and 28% were found in technical HBCD. Herein the crystal structure of racemic δ-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 δ-iBPBCDs decrease and those of other stereoisomers increase. Evidence was found that δ-iBPBCDs isomerize stereo- and regioselectively to β-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 0.012min(-1) are found for the δ- to β-iBPBCD isomerization at 120, 130, and 140°C, respectively, corresponding to half-lives of 360, 140, and 56min. These transformations also occur during the production of XPS, which predominantly contain β-iBPBCDs, whereas δ-iBPBCDs dominate in materials experiencing lower thermal stress, e.g. expanded polystyrenes (EPS). The relative configurations of δ- and θ-iBPBCDs are TtCtCt, like the one of γ-HBCDs. γ-HBCDs are the kinetically and α-HBCDs with a TcCtCc configuration the thermodynamically favored products. In analogy, β-iBPBCDs are assumed to have a TcCtCc configuration like α-HBCDs because they are formed from δ-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.
authors.affiliation: Swiss Federal Institute for Materials Science and Technology, Laboratory of Analytical Chemistry, Überlandstrasse 129, Dübendorf, Switzerland. norbert.heeb@empa.ch
authors.forename: Norbert V
authors.initials: NV
authors.lastname: Heeb
authors.forename: Heidi
authors.initials: H
authors.lastname: Graf
authors.forename: W Bernd
authors.initials: WB
authors.lastname: Schweizer
authors.forename: Meret
authors.initials: M
authors.lastname: Heeb
authors.forename: P
authors.initials: P
authors.lastname: Lienemann
mesh_terms: D003460:Crystallization; D005411:Flame Retardants / Q000032:analysis; D006361:Heating; D006842:Hydrocarbons, Brominated / Q000737:chemistry*; D007536:Isomerism; D007700:Kinetics; D011137:Polystyrenes / Q000737:chemistry*
chemical_list: D005411:Flame Retardants; D006842:Hydrocarbons, Brominated; D011137:Polystyrenes; C089796:hexabromocyclododecane