Racemization kinetics of bridged binaphthyls do not reflect microviscosities of rubbery polymer matrices

The racemization kinetics of a series of atropisomeric 1,1′-2,2′-bridged binaphthyls with and without appended oligophenyl "paddles" were studied in decalin and in a number of rubbery polymers as a function of temperature. The racemization has an energy barrier of about 33 kcal/mol and a half-life of the order of hours. For the unsubstituted bridged binaphthyl, BNO, the rate was very similar in decalin and the polymeric matrices. In decalin, the attachment of the paddles elicited a ponderal effect, reducing the racemization rate by a factor inversely proportional to the square root of the mass of the group moving in the conformational transition. In the polymers, the temperature dependence of all rates followed the Arrhenius relation rather than the WLF equation, showing that the process was not coupled to the local polymer dynamics. The paddles reduced the rate by a factor that depended on the nature of the paddle and on the polymer but not on temperature and did not, therefore, reflect the "microviscosity" of the polymeric matrix.