Half-sandwich ruthenium complexes for the controlled radical polymerisation of vinyl monomers Oscar Tutusaus a,1 ,S ebastien Delfosse b , Franc ßois Simal b,2 , Albert Demonceau b, * , Alfred F. Noels b , Rosario N u~ nez a , Clara Vi ~ nas a , Francesc Teixidor a,3 a Institut de Ci  encia de Materials, CSIC-UAB, Campus de Bellaterra, Cerdanyola, 08193 Barcelona, Spain b Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Li  ege, Sart-Tilman (B.6a), B-4000 Li  ege, Belgium Received 26 July 2002; accepted 23 August 2002 Abstract Ruthenium complexes of the type ½RuXðCp # ÞðPPh 3 Þ 2  (X ¼ Cl and H; Cp # ¼ Cp, Cp*, indenyl, and carboranyl) catalyse the radicalpolymerisationofstyreneand n-butylacrylate,andboththecatalystactivityandthedegreeofcontrolofthepolymerisation strongly depend on the Cp # ligand and the monomer. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Homogeneous catalysis; Olefins; Polymerisations; Radical reactions; Ruthenium and compounds Itwasin1995thatSawamotoandcoworkers[1]and Matyjaszewski and Wang [2] independently reported their seminal papers on atom transfer radical polymer- isation (ATRP) [3a,3b]. This process takes advantage of the redox properties of transition metals, and is based on a dynamic equilibration between active propagating radicals and dormant species. This equilibrium is es- tablished through the reversible transition metal-catal- ysedhomolyticcleavageofthecovalentcarbon–halogen bond in the dormant species (Scheme 1). Catalyticengineeringatthemetalcentreaimstoshift this equilibrium toward the dormant species. Thus, the concentration of propagating radicals remains low thorough the whole polymerisation process and a high degreeofcontrolensues,allowingnotonlythesynthesis of polymers of predictable molecular weights and low polydispersities, but also the preparation of novel, functionalised, block copolymers amenable to further transformations. Amongthenumerouscatalyticsystemsdevelopedfor ATRP, ruthenium play a prominent role [1,3a,4a,4b]. We recently reported on the exceptional efficiency and versatility of new catalysts based on ½RuCl 2 ðp-cymeneÞ ðPR 3 Þ [5] (p-cymene is 4-isopropyltoluene) and ½RuCl 2 ð¼ CHPhÞðLÞðL 0 Þ [6](L,L 0 arePR 3 and N-heterocyclic carbenes) for promoting the ATRP of vinyl monomers. Later on, Sawamoto and coworkers [7] and Simal et al. [8] independently discovered that half-sandwich ruthe- nium complexes 1–3 were efficient and markedly active catalysts for the controlled radical polymerisation of vinyl monomers and the Kharasch addition as well. Nowadays,thefactorsgoverningcatalystactivityand the mechanism by which half-sandwich ruthenium cat- alysts perform radical reactions are not known. Inves- tigations were undertaken to address both of these topicsinthefollowingway:byvaryingtheligandsphere around the ruthenium catalyst, we wished to determine how the electronic and steric properties of the ligands affect catalyst activity. Ruthenium complexes 1–3 and ruthenacarboranes [9] 4 (Scheme 2) were compared for two reasons: (1) the cyclopentadienyl anion (Cp), its derivatives (Cp* and Ind), and the carboranyl anion present in complexes 4 are isolobal and uninegative Inorganic Chemistry Communications 5 (2002) 941–945 www.elsevier.com/locate/inoche * Correspondingauthor.Tel.:+32-4-366-34-195;fax:+32-4-366-3497. E-mail addresses: A.Demonceau@ulg.ac.be (A. Demonceau), teix- idor@icmab.es (F. Teixidor). 1 O.T. is enrolled in the PhD program of the UAB. 2 Present address: UCB Chemicals, Research and Technology, Anderlechtstraat 33, B-1620 Drogenbos, Belgium. 3 Also corresponding author. Fax: +34-93/580-5729. 1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S1387-7003(02)00608-1