Lactide Loops...

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Science  16 Mar 2007:
Vol. 315, Issue 5818, pp. 1467
DOI: 10.1126/science.315.5818.1467b

Selective routes to cyclic polymers must overcome the dual challenges of enthalpic strain and unfavorable entropy. Culkin et al. have found that an N-heterocyclic carbene substituted with two bulky mesityl groups can catalyze the polymerization of lactide to yield macrocycles with molecular weights on the order of 20 kD and polydispersities of ∼1.2 to 1.3. The authors had previously shown the effectiveness of this catalyst for generating linear poly(lactide) in the presence of alcohol initiators; the cyclic products result when the initiators are omitted. Polymerization of optically pure lactide proceeds with retention of stereochemistry. The narrow polydispersities and observation of a product molecular weight increase with reaction time suggest that propagation outpaces the macrolactonization step that liberates the carbene catalyst. — JSY

Angew. Chem. Int. Ed. 46, 10.1002/anie.200604740 (2007).

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