Ring-Opening Polymerization of Lactones Using RuCl 2 (PPh 3 ) 3 as Initiator: Effect of Hydroxylic Transfer Agents Jose ´ Luis Mata-Mata, Jose ´ E. Ba ´ez, Jose ´ Alfredo Gutie ´rrez, Antonio Martı ´nez-Richa Facultad de Quı ´mica, Universidad de Guanajuato, Noria Alta S/N, 36050 Guanajuato, Gto. Me ´xico Received 22 December 2004; accepted 20 May 2005 DOI 10.1002/app.22963 Published online 19 December 2005 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: -Caprolactone and -valerolactone were poly- merized in bulk at 150°C using the ruthenium(II) complex RuCl 2 (PPh 3 ) 3 as initiator in the presence of 1,3-propanediol (PD) with a series of alcohols as coinitiators. Polymerization of lactones proceeds via ruthenium(II) alkoxide active centers. 1 H-NMR analysis revealed that the ruthenium complex re- acted with the alcohol, generating in situ a ruthenium alkoxide. This species became a more active initiator of ring-opening polymerization than was RuCl 2 (PPh 3 ) 3 . The obtained polylac- tones were characterized by 1 H- and 13 C-NMR and matrix- assisted laser desorption ionization time-of-flight (MALDI- TOF). The results showed the formation had occurred of ,- telechelic PCL and PVL diols, in which PD had been incorporated into the polymer backbone. Depending on the nature of the alcohol used as coinitiator, PCLs with different end groups could be synthesized. Insertion of an alcohol as an end group (benzyl alcohol, n-octanol, or isopropanol) or into the polymeric backbone (propanediol) provided support for the conclusion that a classical coordination–insertion mecha- nism was operating during lactone polymerization. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2737–2745, 2006 Key words: polyesters; ring-opening polymerization; initia- tor; metal-organic catalysts INTRODUCTION In the last few decades, the need for materials with biodegradable properties has increased the interest in biodegradable polymers such as polylactones. 1 Poly(- caprolactone) (PCL) and poly(-valerolactone) (PVL) are two important polylactones. They are currently obtained by ring-opening polymerization (ROP) of their corresponding precursors, -caprolactone (CL) and -valerolactone (VL), respectively. The initiators most commonly used in ROP of cyclic esters are metal alkoxides. Many groups of research- ers 2–6 have reported using alkoxides derived from alkaline metals and rare earth elements containing unoccupied p, d, and f orbitals (such as Mg, Zn, Al, Sn, Ti, Zr, Sm, and Y alkoxides) as initiators in the poly- merization of cyclic esters. The effectiveness of these compounds has mainly relied on the ability of the particular alkoxide to induce formation of the initia- tion species, which involves the complexation of the monomer with the initiator via free p or d orbitals, followed by cleavage of the lactone acyl-oxygen bond. The final morphology and other properties of low- molecular-weight polylactones are closely related to the end-group functionalities. The architecture of the end groups allows versatility in the chemistry of these polymers, as many derivatives with potential new properties can be obtained by proper derivatization. The most common route for obtaining functionalized polylactones involves use of a primary or secondary alcohol as coinitiator. Some recent reports have taken up this approach, and functionalized PCL 7–9 and PVL 9,10 can be obtained in quantitative yields. Ruthenium derivatives have found widespread use as catalysts in the hydrogenation and oxidation of olefins, in the decarbonylation and dehydration of alcohols, and in hydrogen transfer reactions, among others. 11 Ruthenium catalysts mainly have been used for ring-opening metathesis polymerization of cyclic olefins. 12–15 The main oxidation states for ruthenium are II and III. Ruthenium complexes have a variety of useful characteristics including high electron transfer ability, high Lewis acidity, low redox potentials, and stabilities of reactive metallic species such as oxomet- als, metallocycles, and metal– carbene complexes. Thus, a large number of novel, useful reactions have begun to be developed using both stoichiometric and catalytic amounts of ruthenium complexes. Catalytic systems for ROP of lactones are generally composed of derivatives of the transition metals that are on the left side of the d block of the periodic table. Correspondence to: A. Martinez-Richa (richa@quijote. ugto.mx). Contract grant sponsor: Consejo Nacional de Ciencia y Tecnologı ´a (CONACYT). Contract grant sponsor: Universidad de Guanajuato Journal of Applied Polymer Science, Vol. 99, 2737–2745 (2006) © 2005 Wiley Periodicals, Inc.