Supported neodymium catalysts for MMA polymerization: on the origin of surface-induced stereoselectivity† Iker Del Rosal, a Mathieu J.-L. Tschan, b R egis M. Gauvin, * c Laurent Maron * a and Christophe M. Thomas * b Received 7th October 2011, Accepted 22nd November 2011 DOI: 10.1039/c2py00472k The polymerization of MMA by a supported neodymium borohydride catalyst proceeds with moderate tendency toward isoselectivity, whereas the molecular precursor generates a syndiotactic-rich polymer. DFT studies reveal that initiation proceeds via borohydride attack on the first coordinated monomer followed by BH 3 trapping by the silica surface. The selectivity of the next two insertions was scrutinized, demonstrating that interactions between the growing chain’s oxygens and the metal enforce conformations that lead to favoured formation of an isotactic polymer chain. Introduction One of the major advances in catalytic polymerization is the discovery that polyolefin tacticity can be controlled by the propagating center geometry: most particularly, the use of group 4 metallocene catalysts of either planar or axial symmetry affords syndio- or isotactic polyolefins through enantiomorphic site control. 1 Fine tuning of the ligand impacts on both activity and selectivity. However, transposition of this success story to the polymerization of functionalized olefins such as methyl- methacrylate is difficult. In this very case, stereocontrol of the polymerization is of high importance, as tacticity affects key mechanical properties of polymethylmethacrylate (PMMA), such as crystallinity, glass transition temperature, and brittle- ness. 2 Examples of enantiomorphic site control for this reaction have been provided, though other experimental factors come into play to modulate the stereo-enchainments. 3 As shown by mech- anistic studies, the two reactions proceed via markedly distinct pathways: as coordination-insertion is operative for a-olefins, in the case of MMA, the chain growth proceeds via enolate formation and subsequent Michael addition-type attack of this nucleophile on a metal-coordinated monomer. 3 Immobilization of a molecular catalyst is often seen as interesting in terms of process efficiency, especially when thinking about Ziegler–Natta-type processes. 4 Along these lines, some of us have provided several examples of well-defined silica-supported catalysts based on rare-earth and alkaline-earth metals. 5 These species were immobilized by following the surface organometallic chemistry approach, where the host support is considered as a ligand. 6 Control of the grafting chemistry allows access to supported species of well-understood structure in terms of metal–ligands and metal–support covalent bonds. Interestingly, whereas the molecular precursors were non- or slightly specific, their immobilization has a dramatic influence on their stereo-directing abilities. This holds true for styrene calcium-mediated syndiospecific polymerization or b-butyrolactone neodymium-initiated ring opening polymeri- zation. 5,7 Along with other examples, this demonstrates that the coordination sphere of the supported propagating center impacts on catalytic performances just as molecular counter- parts do. Rationalization of this selectivity improvement upon immobilization of a molecular catalyst/initiator is still an open question. Results To expand upon these promising results, we were interested in investigating the potential of grafted neodymium derivatives in MMA polymerization. Indeed lanthanoid precursors have shown interesting performance for the polymerization of MMA, allowing the preparation of syndiotactic-enriched PMMAs. 3 In particular well-defined rare-earth borohydride complexes (i.e., Y, La, Lu) were reported to similarly favor the syndiotactic enrichment of PMMA. On the other hand, isotactic-enriched PMMAs prepared from related rare earth metals are much less common. Therefore we chose to target our efforts on the [Nd (BH 4 ) 3 (THF) 3 ] complex (1), easily accessible from the corre- sponding chloride on a large synthetic scale (Scheme 1). As we reported, the molecular precursor 1 reacts with a non-porous silica support dehydroxylated at 700  C to selectively afford a Universit e de Toulouse, INSA, UPS, CNRS-UMR5215, LPCNO, 135 Avenue de Rangueil, 31077 Toulouse, France. E-mail: laurent.maron@ irsamc.ups-tlse.fr b Chimie ParisTech, UMR CNRS 7223, 75005 Paris, France. E-mail: christophe-thomas@ens.chimie-paristech.fr; Fax: +33(0)143260061; Tel: +33(0)144276721 c Universit e Lille Nord de France, CNRS UMR8181, Unit e de Catalyse et de Chimie du Solide, UCCS USTL, F-59655 Villeneuve d’Ascq, France. E-mail: regis.gauvin@ensc-lille.fr; Fax: +33 (0)320436585; Tel: +33 (0)320436754 † Electronic supplementary information (ESI) available: Experimental and DFT methodological details, and optimized structures of the second propagation step of the MMA polymerization with (BH 4 ) 2 La@ c-1. See DOI: 10.1039/c2py00472k 1730 | Polym. Chem., 2012, 3, 1730–1739 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Polymer Chemistry Cite this: Polym. Chem., 2012, 3, 1730 www.rsc.org/polymers PAPER