Metal-Free Activation in the Anionic Ring- Opening Polymerization of Cyclopropane Derivatives Nicolas Illy, Sylvie Boileau, Jacques Penelle, Valessa Barbier* Introduction In the past few years, the polymerization of dialkyl cyclopropane-1,1-dicarboxylates has been developed as a route to carbon-chain polymers that carry ester side groups located on every third carbon along the polymethylene backbone. [1–3] A few studies are available on the physico- chemical properties exhibited by this class of unusual polymers. [4,5] The anionic ring-opening polymerization (ROP) of these monomers was performed in dimethyl sulfoxide (DMSO) using sodium or potassium thiopheno- lates as initiators. The livingness of the process was observed under appropriate experimental conditions, up to temperatures as high as 200 8C. [2] This remarkable selectivity results from the high stabilization of the propagating carbanion R–C(COOR ´ )  2 by two electron-with- drawing ester groups (malonate substitution). From that perspective, dialkyl cyclopropane-1,1-dicarboxylate mono- mers resemble an epoxide more than a classical cyclopro- pane. Unfortunately, the excellent selectivity displayed by the polymerization that prevents side-reactions and makes the polymerization living is not complemented by an equally satisfying reactivity (low k p ), which limits the range of accessible molecular weights. Tetrameric phosphazene base Bu t P 4 is a very strong, sterically hindered, and non-charged base because of the nitrogen atoms doubly bounded to a pentavalent phos- phorous. [6] It can be used as an activating ligand for metallic cations involved as counter-ions in anionic polymeriza- tions. [7] In association with an alcohol, it has been used to initiate the polymerization of ethylene oxide, [8,9] cyclosi- loxanes, [8,10,11] and cyclic esters. [12] The very soft large counter-ion formed by deprotonation of the alcohol (methanol or 1-pyrene butanol) involves a fast ROP of those monomers. In this communication, the successful activation observed when using Bu t P 4 phosphazene base and different thiols for the anionic ROP of di-n-propyl cyclopropane-1,1- dicarboxylate is described (see the scheme in the summary). Communication N. Illy, S. Boileau, J. Penelle, V. Barbier Institut de Chimie et Mate ´riaux Paris-Est (ICMPE), Universite ´ Paris Est, CNRS, 94320 Thiais, France Fax: 0033 1 4978 1208.; E-mail: barbier@icmpe.cnrs.fr The successful activation observed when using Bu t P 4 phosphazene base and thiophenol or bisthiols for the anionic ring opening polymerization (ROP) of di-n-propyl cyclopropane-1,1- dicarboxylate is described. Well-defined monofunctional or difunctional polymers with a very narrow molecular weight distribution were obtained through a living process. Quantitative end-capping of the propagating malonate carba- nion was accessible by using either an electro- philic reagent such as allyl bromide or a strong acid such as HCl. Kinetics studies demonstrated a much higher reactivity compared to the conven- tional route using alkali metal thiophenolates. Macromol. Rapid Commun. 2009, 30, 1731–1735 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.200900303 1731