Twisting with Fewer Breaks

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Science  29 Sep 2006:
Vol. 313, Issue 5795, pp. 1855
DOI: 10.1126/science.313.5795.1855c

In polymer synthesis, chiral solvents can be used to increase the stereoregularity, or tacticity, of polymer chains. However, in the synthesis of some polymers, the alternation between helical senses also affects the distribution of chain lengths. Holder et al. point out that in the Wurtz-type reductive coupling of dichloro-diorganosilanes to form polymers on supported sodium metal, regions of the chain where the helical sense reverses (switching from P to M screw sense) are prone to backbiting reactions that terminate chain growth. The authors succeeded in raising the proportion of longer chains in the product distribution by running the reaction in enantiomerically pure limonene, a relatively inexpensive and unreactive chiral liquid. This effect increased at higher reaction temperatures: At 90°C, the weight-average molecular weight, measured using size exclusion chromatography, more than doubled when the optically pure solvent was used in place of racemic limonene, presumably because of a reduced number of reversal sites. Investigation by optical absorption and circular dichroism spectroscopy supported a mechanism in which the chiral solvent stabilized a particular helical conformation of the growing polymer chain. — PDS

J. Am. Chem. Soc. 128, 10.1021/ja064587e (2006).

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