2043 Full Paper Macromolecular Chemistry and Physics wileyonlinelibrary.com DOI: 10.1002/macp.201300279 Low-Temperature Polymerization of ε-Caprolactone Catalyzed by Cerium Triflates Steffen M. Weidner, Felix Scheliga, Hans R. Kricheldorf* Using 22 metal triflates as catalysts, ε-caprolactone is polymerized at 22 °C in bulk. Only five relatively acidic triflates prove active. Three triflates, including the neutral Sm 3+ , are active using water as initiator. A very low content of cyclics is found in all the experiments. With Ce 3+ and Ce 4+ , polymerizations are performed in CH 2 Cl 2 and in bulk at 2 °C and 22 °C. Low dispersities (down to 1.1) are obtained. At 22 °C, Ce 4+ and, even better, Ce 3+ also catalyze syn- theses of CO 2 H- and CH 2 OH-terminated polycaprolactones, whereby higher dispersities and larger fractions of cyclics are obtained. Further polymerizations and polycondensations are catalyzed with protic acids. The results can be explained by a proton-catalyzed activated monomer mechanism. Dr. S. M. Weidner BAM – Federal Institute for Materials Research. and Testing, I-3 Structure Analysis, Richard-Willstätter Str. 11, D 12489, Berlin, Germany Dr. F. Scheliga, Prof. H. R. Kricheldorf Institut für Technische und Makromolekulare Chemie der Universität, Bundesstr. 45, D 20146, Hamburg, Germany E-mail: kricheld@chemie.uni-hamburg.de All these initiators have in common that highly reactive end groups are formed, which, in turn, are capable of forming cyclic oligomers by “back-biting.” The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spec- trum of a Bu 2 Sn(OMe) 2 -initiated polymerization of ε-CL at 22 °C presented in Figure 1 is a typical example. In a recent publication, [1] we reported that bismuth triflate slowly catalyzes water or methanol-initiated polymerizations of ε-CL at 21–22 °C in bulk. Gel-perme- ation chromatography (GPC) measurements obtained after prolonged reaction times also suggested that bis- muth triflate catalyzed condensation steps of CH 2 OH and CO 2 H end groups. Furthermore, a very low content of cyclics was observed. Yet, it was also found that bismuth triflate, when applied around or above 100 °C, catalyzes side reactions altering the polymer structure. [2] In this context, the present work serves three purposes. First, the reactivity and usefulness of various metal triflates as catalysts for the low temperature polymerization of ε-CL should be compared. Second, more evidence in favor (or against) polycondensation reactions at room temperature should be elaborated. Third, catalysts yielding poly( ε-CL) free of cyclics should be found. A convenient synthesis of poly( ε-CL) free of cyclics by means of commercial cata- lysts is of interest because it allows one to study physical properties of perfectly linear poly( ε-CL), such as rate of crystallization or degree of crystallization. 1. Introduction Most polymerizations of ε-caprolactone ( ε-CL) (and other lactones) reported in the literature were performed at temperatures around or above 80 °C, partially because the moderate activity of the catalysts requires such tem- peratures, and because temperatures above the melting point of the polylactones allow for rapid polymerizations in a homogeneous phase, when the polymerizations are conducted in bulk. Highly reactive catalysts or initiators allowing for polymerizations at room temperature (or below) are also known. These catalysts and initiators may be subdivided into three categories: i. Ionic metal alkoxides (e.g., sodium methoxide) initiating ani- onic polymerization ii. Covalent (heavy) metal alkoxides and phenoxides, such as Bu 2 Sn(OMe) 2 or Y trisphenoxide iii. Metal alkyls or hydrides such as BuLi, MgBu 2 , ZnEt 2 , or NaH, which generate reactive metal alkoxides chain ends in situ. (CH 2 ) 5 O CO x + ROH (Ce 3+ /Ce 4+ ) HO (CH 2 ) 5 CO OR x Macromol. Chem. Phys. 2013, 214, 2043−2053 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim