Conformational analysis of stereoisomers of styrene trimers by NMR spectroscopy

Conformational analysis of stereoisomers of 1-phenyl-4-(1'-phenylethyl)-tet ralin (1,4-PPET) and 1,3,5-triphenylcyclohexane (1,3,5-TPCH) were investiga ted using H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopy, among several styrene trimers that remained in polystyrene products for food con tainers. Four stereoisomers of 1,4-PPET were isolated from a mixture of sty rene oligomers formed during the thermal polymerization of styrene by emplo ying a recycling method of preparative high-performance liquid chromatograp h!, (preparative HPLC). As for 1,3,5-TPCH, two kinds of stereoisomers were obtained by means of two kinds of synthetic methods with different sterosel ectivities. We confirmed that the four stereoisomers of 1,4-PPET are (1-eq, 4-eq), (l-ax, 4-eq), (l-ax, 4-ax) and (1-eq, 4-ax) conformers (ax: axial, eq: equtorial), in order of their gas-chromatographic (GC) elutions, respec tively, based on the relationships of the vicinal coupling constants of tet ralin protons. This result was also supported by the H-1 and C-13 NMR chemi cal shifts (delta(H), delta(C)) of 1', 2'(CH3) protons and 2'(CH3) carbon. As for 1,3,5-TPCH, we obtained (Irans, cis) and (cis, cis) isomers as two k inds of stereoisomers in order of their GC elutions using two kinds of synt hetic methods with different stereoselectivities, and concluded that they w ere (1-ax, 3-eq, 5-eq) and (1-eq, 3-eq, 5-eq) conformers, respectively, bas ed on the relationship among the typical coupling constants of cyclohexane protons and the corresponding H-1 and C-13 NMR chemical shifts of the cyclo hexane rings. The characterization of the conformation of styrene trimers s eems to be significant for future investigations of endocrine-disrupting me chanisms.